The Lone Ranger
08-05-2008, 01:02 PM
An Introduction to Zoology
Chapter Five: Animal Reproduction:
Why Reproduce?:
It might seem like a strange question to ask, but why do organisms reproduce? Well, it might make more sense to ask, “Why do organisms die?” Those two questions are much more closely-linked than you might suspect at first.
Suppose that organisms were born “immortal” – that is, suppose living things didn’t grow old and eventually die. In such a world, would reproduction be necessary? Yes it would. The fact of the matter is that true immortality simply isn’t possible. Even if organisms didn’t age and die, they’d still be susceptible to disease, predators, and accidents. So, for that reason alone, reproduction is necessary. The genes of any organism that doesn’t reproduce will soon be eliminated from the gene pool by virtue of the fact that true immortality simply isn’t possible.
Genes that promote reproduction can persist indefinitely, since they’ll be passed down from one generation to the next. On the other hand, any genes that have the effect of preventing their possessors from reproducing will soon be eliminated from the gene pool, no matter how beneficial they might be in other ways.
Another reason why reproduction is necessary is that it’s necessary in order for evolution to occur. The genetic makeup of an individual does not change during its lifetime. This severely limits its ability to change in response to changing environmental conditions. But because of reproduction – especially sexual reproduction – populations can and do experience genetic change over time, and so they change in response to their environments. That is, populations evolve in response to their environments.
Odd as it might seem at first, it appears that organisms are, in effect, genetically “programmed” to grow old and eventually die. Why is this advantageous? Well, the brute fact of it is that once you’ve successfully reproduced and raised your offspring to independence, from an evolutionary perspective you have no further “purpose.” What’s more, by surviving past the point where your children need your support, you’re consuming resources that they could be using. You’re thus competing with your own offspring for limited resources and thus reducing the probability of their survival and reproduction. In other words, weird as it might seem, there comes a time when it’s in your best genetic interest to die and thus improve your offspring’s chances of survival.
Sexual vs. Asexual Reproduction:
Normally, cellular division results in diploid cells that are genetically-identical. This form of cellular division is known as mitosis, and this is how normal body cells reproduce. There is a second form of cellular division, however, known as meiosis. Meiotic cell division results in haploid cells that have only half the normal number of chromosomes and that are not genetically-identical. This is how the sex cells are produced.
In sexual reproduction, males and females produce haploid sex cells known as gametes through meiosis. The male gametes (spermatozoa) are small and mobile. The female gametes (ova) are much larger and are non-mobile. A haploid spermatozoan and a haploid ovum unite at fertilization to produce a zygote that is diploid (that is, it has a complete set of chromosomes). The zygote ultimately grows into a new individual that is genetically distinct from either of its parents.
A sexually-reproducing species in which individuals can produce both spermatozoa and ova simultaneously is said to be monoecious or hermaphroditic. A species in which each an individual can produce either spermatozoa or ova, but not both at the same time, is said to be dioecious.
In asexual reproduction, there is no union of spermatozoan and ovum, so only one parent is involved. Usually, asexual reproduction involves reproduction through mitosis and the production of an offspring that is genetically-identical to its parent – that is, it is a clone of its parent.
[b]Asexual Reproduction:
Many simple animals can reproduce asexually through the process of fission. Fission occurs when the animal simply divides into two, and each half then re-grows whatever organs it needs in order to make itself whole.
http://www.freethought-forum.com/forum/gallery/files/5/0/fission.jpg
A planarian (Phylum Platyhelminthes) can
reproduce asexually through fission. The
animal splits in two, then the anterior half
grows a new tail while the posterior half
grows a new head.
Another form of asexual reproduction that is common in simple animals is budding. Budding occurs when a “bud” grows on the animal’s side and grows into a smaller, genetically-identical version of the “parent.” When it has become large-enough, the smaller animal breaks free of its “parent” and becomes independent.
http://www.freethought-forum.com/forum/gallery/files/5/0/budding_27295.jpg
A hydra (Phylum Cnidaria) reproducing through budding.
[break=Parthenogenesis]
Some animals can reproduce through parthenogenesis. Parthenogenesis occurs when a female produces an ovum (it can be either haploid or diploid) that can grow into a new individual (which may or may not be genetically identical to its mother) without being fertilized by a spermatozoan.
In some insects (Phylum Arthropoda), for example, a fertilized ovum develops into a diploid female, while unfertilized ova develop into haploid males. Animals with this sort of genetics are referred to as haplodiploid, because individuals of one sex are haploid and individuals of the other sex are diploid. Honeybees are an example of insects that are haplodiploid. Male honeybees, since they’re haploid, produce spermatozoa that are genetically-identical to themselves; female honeybees, since they’re diploid, produce ova that are not genetically-identical to themselves. (This has important consequences for honeybee behavior.)
Parthenogenesis is not limited to “simple” animals. It has been observed in fishes, reptiles, and birds. In fact, there are some lizard species in the genus Cnemidophorus in which males are not known to occur. Every member of the species is a female, and all reproduction is through parthenogenesis. (This means that every daughter is a clone of her mother.)
http://www.freethought-forum.com/forum/gallery/files/5/0/cnemidophorus.jpg
Two Cnemidophorus uniparens engaging in “pseudocopulation.” Males are not known to
occur in this species and reproduction is through parthenogenesis. “Copulation” with
another lizard seems to stimulate a female to ovulate. This vestigial sexual behavior
provides strong evidence that C. uniparens evolved fairly recently from a sexually-reproducing species.
[break=Sexual Reproduction]
[b]Sexual Reproduction:
The great majority of animals reproduce through sexual reproduction, though there are some species that can switch between sexual and asexual reproduction as circumstances demand. That raises an interesting question: given the obvious benefits of asexual reproduction, why do most animals reproduce sexually?
Natural selection is all about getting as many of your genes into the next generation as possible. As such, sexually-reproducing animals would seem to have an obvious disadvantage compared to asexually-reproducing animals. After all, if you’re reproducing asexually and your offspring are clones of you, then each offspring carries 100% of your genes. But if you’re reproducing sexually, each offspring carries only 50% of your genes. Asexual reproduction, then, would seem to be the way to go, since you can get twice as many genes into the next generation per offspring if you reproduce asexually.
So, to repeat the question: Why do the great majority of animals reproduce sexually?
When asked this question, my students occasionally suggest that sexual reproduction is more fun. That may be, but the answer doesn’t really address the question.
[b]The Advantages of Sexual Reproduction:
Asexual reproduction, of course, generally results in offspring that are genetically identical to their parent. Sexual reproduction, on the other hand, since it occurs through the mixing of genes from two different parents, results in offspring that are genetically distinct from both of their parents. Herein lies the key to why most animal species reproduce sexually.
Back in 1930, the geneticist and evolutionary theorist R. A. Fisher noted that in sexually-reproducing populations, the continual production of and recombination of sex cells means that every individual is genetically unique, and that there will be a tremendous amount of genetic variability within a sexually-reproducing population. Since genetic variability provides the “raw material” with which natural selection works, the more genetic variability there is within a population, the greater is its capacity to change in response to a changing environment. Furthermore, the more genetic variability there is in the population, the faster it can evolve in response to a changing environment. These observations have led to two major hypotheses regarding the advantages of sexual reproduction over asexual reproduction.
The first hypothesis regards the advantages of sexual reproduction in fluctuating physical environments. The evolutionary theorist George Williams likened reproduction to a raffle. You can either have many tickets with different numbers (sexual reproduction) or many copies of the same ticket, bearing the same number (asexual reproduction). If you know what number will be drawn in the raffle, it’s in your best interest to have many copies of the winning ticket; that way, you’ll win big. But if you don’t know what number will be drawn in the raffle, it’s obviously in your best interest to have as many different tickets as possible. That way, you maximize your chances of having at least one of them come up a winner.
This is known as the “bet-hedging hypothesis” or the “tangled-bank hypothesis.” The idea is that, in an environment which fluctuates unpredictably, it’s best to produce offspring with as much genetic diversity as possible. In that way, parents maximize the likelihood that at least some of their offspring will happen to inherit gene combinations that allow them to survive.
According to this hypothesis, sexual reproduction should be favored in unstable, unpredictable environments, while asexual reproduction should be favored in stable, predictable environments. Experiments tend to support this conclusion. For example, when species alternate between sexual and asexual reproduction, they tend to reproduce asexually in the Spring and Summer, when conditions are relatively stable, and then switch to sexual reproduction in the Fall and Winter, when conditions are much less stable and predictable.
http://www.freethought-forum.com/forum/gallery/files/5/0/polyarthra_remata.jpg
The rotifer species Polyarthra remata (Phylum Rotifera). Many rotifers
switch between sexual and asexual reproduction in response to
changing environmental conditions.
The second hypothesis regards the advantages that sexual reproduction provides in dealing with other organisms. When two or more species evolve in response to each other, this is known as coevolution. Every species must coevolve with its predators and/or prey and/or hosts and/or parasites. Evolutionary “arms races” result, because predators must evolve to cope with evolutionary changes in their prey, prey species must evolve in response to evolutionary changes in their predators, host species must evolve in response to evolution of their parasites, and parasites must evolve in response to evolution of their hosts.
In other words, every species’ evolution is influenced by the demands imposed by the other species with which it interacts. For example, if a host species evolves a more effective defense against its parasites, that will create selection pressure for the parasites to evolve more effective ways to circumvent their host’s defenses. That, in turn, will create selection pressure for the host to evolve even more effective defenses – which will create selection pressure for the parasites to defeat those defenses. And so on, and so on.
According to this hypothesis, the advantage of sexual reproduction is that it greatly speeds up the rate at which evolution occurs, thus making it far easier for species to respond to evolutionary changes in their prey, predators, parasites, or hosts.
In an asexually-reproducing population, since there is no sexual recombination of genes, the only source of new gene combinations is the occasional mutation. Therefore, asexual populations inevitably evolve far more slowly than do sexual populations of the same species.
Because species must constantly evolve in order to keep pace with other species, they’re caught in a situation similar to that faced by the Red Queen in Alice in Wonderland, who told Alice, “Now here, you see, it takes all the running you can do, to keep in the same place.” So this hypothesis – namely, that sexual reproduction is favored because it allows species to quickly evolve in response to changes in other species – is known as the “Red Queen hypothesis.” The advantage to the individual in producing genetically-diverse offspring is, again, that this maximizes the chances that at least some of those offspring will happen to inherit gene combinations that make them well-adapted to deal with their predators/prey/hosts/parasites.
According to the Red Queen hypothesis, the need to quickly evolve in response to other species, especially parasites, is the driving force behind sexual reproduction. The hypothesis predicts that asexual reproduction should be favored in environments where parasites are rare or absent, whereas sexual reproduction should be favored in environments where parasites are common. Field studies support this hypothesis, too. In populations that can switch between sexual and asexual reproduction, whether they reproduce sexually or asexually tends to be strongly correlated with parasite densities.
http://www.freethought-forum.com/forum/gallery/files/5/0/potamopyrgus_antipodarum.jpg
The freshwater snail species Potamopyrgus antipodarum
(Phylum Mollusca). In parasite-free environments, asexual
populations out-reproduce and replace sexual populations.
In parasite-filled environments, sexual populations out-reproduce
and replace asexual populations.
So it appears that both hypotheses are correct. Under most environmental conditions, sexually-reproducing populations rapidly replace asexually-reproducing populations of the same species. The advantage provided by sexual reproduction is evidently the great genetic diversity that results, making rapid evolutionary change possible.
More to the point, in most environments it is in an individual’s best interest to reproduce sexually, because by producing genetically-diverse offspring, the individual maximizes the chances that at least some of its offspring will survive. In changeable, unpredictable environments, the genetic diversity that results from sexual reproduction maximizes the probability that some offspring will happen to inherit gene combinations that make them well-suited to future conditions – whatever those might be.
In environments where their offspring will have to avoid predators, capture prey, fend off parasites, or find hosts to parasitize – and let’s face it, that’s pretty-much every environment – it’s also in an individual’s best interest to produce genetically-diverse offspring. Again, this maximizes the chances that at least some of those offspring will happen to inherit gene combinations that make them well-suited to dealing with the other species with which they will interact.
[break=Reproductive Cycles and Patterns]
[b]Reproductive Cycles and Patterns:
Most animals reproduce only at certain times of the year. Unsurprisingly, they generally time their reproductive behaviors so that offspring are produced during the time of year when they’re most likely to survive. Humans are most unusual in that they can and do reproduce at any time of the year.
Animals’ reproductive cycles are generally controlled by chemicals known as hormones. A hormone is a chemical that is produced in one part of the body, and that travels through the blood to affect one or more “target organs” in some other part of the body. Production of specific hormones can be regulated by changes in such factors as the day length and the temperature. As hormone levels change in response to changes in an animal’s environment, those hormones, in turn, promote changes in the animal’s sexual behaviors, reproductive behaviors, and physiologies.
Consider a songbird, for example. As the day length changes in the early Spring, this triggers increased production of sex hormones such as estrogen, progesterone and testosterone. These hormones, in turn, trigger sexual behaviors such as courtship and mating, as well as reproductive behaviors such as nest-building.
The effects of changing hormone levels on an animal’s behavior can be subtle, or they can be quite dramatic. Consider some of the effects of the hormone oxytocin, for instance. In humans, oxytocin is released during orgasm in both males and females. Oxytocin has been shown to promote “bonding behavior” between individuals, so it’s likely that one of the effects of oxytocin release during sexual intercourse is to promote psychological bonding between partners. Needless to say, such bonding is an important factor in successfully raising offspring. Oxytocin, therefore, has been referred to as the “hormone of love.”
Studies in other mammal species have shown that adequate oxytocin levels appear to be necessary for proper bonding between parents and offspring. Accordingly, it has been suggested that conditions such as postpartum depression and failure of mothers (and fathers) to properly bond with their children may be at least partially due to abnormally low oxytocin levels. In the future, such conditions may be treatable through careful manipulation of patients’ hormone levels.
[break=Ovulation]
Ovulation is the release of mature ova, which can then be fertilized by spermatozoa. Normally, ovulation occurs midway through the female’s reproductive cycle. For example, the reproductive cycle in human females averages 28 days in length. Ovulation typically occurs on Day 14 of the cycle. If the ovum is not fertilized and pregnancy does not occur, then on or around Day 28 the reproductive cycle begins again as a new ovum begins to mature in one of the woman’s ovaries.
http://www.freethought-forum.com/forum/gallery/files/5/0/cycle_original.jpg
The reproductive cycle in the human female averages 28 days in length.
It is driven by changes in the levels of four principle hormones:
luteinizing hormone, follicle-stimulating hormone, estradiol,
and progesterone. These hormones influence body temperature,
regulate changes in the ovaries, and cause dramatic changes in the
uterine lining (the endometrium). Ovulation occurs at the midpoint of the cycle.
[break=Finding a Mate]
One of the disadvantages of sexual reproduction is that one requires a mate. In species where individuals tend to be widely-separated and so are unlikely to encounter each other during their fertile periods, hermaphroditism is relatively common. A hermaphrodite is an organism that possesses functional male and female reproductive organs. Many snail species (Phylum Mollusca) are hermaphroditic, for example, as are earthworms (Phylum Annelida).
http://www.freethought-forum.com/forum/gallery/files/5/0/wormporn.jpg
Earthworms (Phylum Annelida) mating. Earthworms are hermaphrodites,
and when they mate, each worm inseminates the other.
[break=Hermaphrodites]
Needless to say, if a species is hermaphroditic, this makes it considerably easier for individuals to find mates, since any member of the same species is a potential mate. Another way in which animals can cope with a lack of suitable mating partners is by changing sex. In many fish species, for instance, individuals can change from males that produce spermatozoa to females that produce ova or vice versa, depending upon which sex is rarer and more in demand by potential mates. In fact, among coral-reef fish species, those that cannot change sex are the exception – most species can change sex in one or both directions.
http://www.freethought-forum.com/forum/gallery/files/5/0/labroides_dimidiatus_original.jpg
The reef fish Labroides dimidiatus (Phylum Chordata). In this species, males are dominant.
If a dominant male dies, a female can change sex and take his place.
[break=Bringing Sperm and Ova Together]
[b]Bringing Sperm and Ova Together:
The female gamete, as we’ve mentioned, is the ovum. The terms “ovum” and “egg” are often used as if they’re interchangeable, but this really isn’t the case. An egg is an ovum (especially a fertilized ovum – a zygote), plus any supporting cells or tissues that may be present in addition.
Fertilization can occur either inside the female’s body or outside of it. If the ova are fertilized outside the female’s body, this is known as external fertilization. Typically, the female lays her eggs and the male sheds sperm over them as they’re laid or shortly afterwards. This is a quick and easy way to get eggs fertilized, but it has a number of disadvantages. The most obvious disadvantage is that external fertilization must take place in a wet environment, else the eggs will dry out and die.
http://www.freethought-forum.com/forum/gallery/files/5/0/frogporn_original.jpg
Most amphibians such as these Wood Frogs (Phylum Chordata) have external fertilization.
As the female expels her eggs, the male sheds sperm onto them.
If the ova are fertilized inside the female’s body, this is known as internal fertilization. One advantage of internal fertilization is that the eggs are better-protected than are those of animals that practice external fertilization – up until the moment of fertilization, at least. Another advantage of internal fertilization is that the sperm can more easily fertilize the ova. Probably not coincidentally, animals that practice internal fertilization tend to produce fewer eggs than do closely-related species with external fertilization. If fertilization is internal, it isn’t such a random process, and the chances of an egg being successfully fertilized are higher.
In many species, after the egg is fertilized, the female secretes a protective shell around it that helps protect the developing embryo inside. This means that the eggs can then be deposited in relatively dry environments. Obviously, this is possible only if fertilization is internal, since the egg can’t be fertilized after a shell is secreted around it.
In some species, such as most mammals, the embryo remains inside the mother’s body as it develops. Obviously, in such species, fertilization must be internal.
http://www.freethought-forum.com/forum/gallery/files/5/0/penguinporn.jpg
In birds (Phylum Chordata), fertilization occurs inside the female’s body.
[b]Identifying a Suitable Mate:
Of course, to successfully reproduce, you can’t very well attempt to mate with every animal of the opposite sex that you happen to encounter. That would not be an effective reproductive strategy, to say the least. Therefore, it’s vitally important to make sure that your potential mate is of the same species. Not surprisingly, then, animals have evolved various means of indicating the species to which they belong.
Before we discuss some of those adaptations, it’s worth considering which sex is the one that has the most to lose by making a poor choice when selecting a mate. If the members of one sex have more to lose from choosing an unsuitable partner, you’d expect selection to favor more “choosy” behavior on the part of that sex. This indeed seems to be the case.
[break=Anisogamy]
Anisogamy (or heterogamy) refers to the fact that, in animals, the gametes produced by males and females differ in size. The spermatozoa produced by males are much smaller than are the ova produced by females. And it’s not just the gametes; the nutrients and tissues that surround the ovum and make up an egg further add to the amount of nutrients and energy a female invests in production of offspring. What’s more, in many species (notably mammals), females retain the developing embryos inside their bodies for some period of time. Females may even continue to nourish the young after they’re born (for instance, through production of milk).
What this means is that, in many species, females invest much more in the production of each offspring than do males. In these species, what limits the reproductive capacity of a female more than anything else is the quality of her mate(s). Since the number of offspring she can produce is sharply limited, it’s in her best genetic interest to choose the best mate that’s available, that she will have the healthiest offspring possible.
By contrast, since sperm are “cheap” to produce, what typically limits a male’s reproductive capacity is the quantity of his mates. Since a healthy male is typically more than capable of fertilizing all the eggs of several different females, it’s in his best genetic interest to mate with as many different females as possible.
Of course, this is a generalization. There are plenty of species in which males and females invest roughly equally in reproduction. There are even some species in which males invest more in the production of offspring than do females.
[break=Female Choice]
Since members of different species cannot successfully interbreed, it’s in every organism’s best genetic interest to mate only with members of the same species. So you’d expect mechanisms to evolve that allow animals to recognize members of their own species. And, indeed, that is the case.
If one sex invests more in reproduction than does the other, as is frequently the case, the sex that invests more in reproduction has more to lose by choosing the wrong partner. Accordingly, you’d expect selection for more “choosy” behavior when it comes to selecting a mate in the sex that has more to lose. Usually, the female invests more in reproduction than does the male, and indeed, females, on average, seem to be much more careful than males when selecting potential mates.
So prevalent is this that it’s frequently referred to as the “female choice model.”
If selection favors “choosy” behavior in one sex, it’s in the best genetic interest of members of the opposite sex to somehow advertise their suitability as mates. So it’s most-often males who have evolved means of indicating the species to which they belong – and their suitability as mates.
http://www.freethought-forum.com/forum/gallery/files/5/0/cardinals_original.jpg
Male and female Northern Cardinals (Cardinalis cardinalis): In many songbird species, males are
much more brightly-colored than are females. As a rule, the more brightly-colored the male,
the healthier he is and the fewer parasites he has. When given a choice, females generally
prefer more brightly-colored males as mates. Their bright colors allow males to demonstrate
their fitness and, therefore, their suitability as potential mates.
[break=Courtship Displays]
In some species, males directly compete with each other for access to females. (In those species in which males invest more in reproduction than do females, females may compete with each other for opportunities to mate with males.)
http://www.freethought-forum.com/forum/gallery/files/5/0/sparring_original.jpg
Bull Elk (Cervus canadensis) competing for access to females.
Competition for mates need not involve outright combat, however. Very commonly, individuals (usually males, of course) engage in courtship displays. These displays typically do two things: they identify the animal’s species (each species has a distinctive courtship display, which eliminates confusion), and they provide clues to the animal’s overall health and, therefore, it’s suitability as a mate. Females can choose mates based upon the quality of their courtship displays, and there is therefore no need for males to engage in potentially-dangerous combat to prove that they’re strong and healthy.
http://www.freethought-forum.com/forum/gallery/files/5/0/superb_original.jpg
A male Superb Bird of Paradise (Lophorina superba), seen from behind, performs
an elaborate and energetically-demanding courtship display while a female observes.
[break=Courtship Rituals]
Even in species where males and females invest roughly equally in reproduction, courtship rituals are common. In these species, they’re typically performed by both sexes. Again, each species’ courtship ritual involves unique behaviors, and helps to ensure that animals don’t wind up mating with individuals of the wrong species.
http://www.freethought-forum.com/forum/gallery/files/5/0/albatross.jpg
Two Laysan Albatrosses (Phoebastria immutabilis) engage in a courtship ritual.
[break=Pheromones]
Animals don’t always advertise for mates through courtship displays or courtship rituals. Many other methods are available. For instance, many species secrete chemicals known as pheromones that can travel through air or water, and that indicate an individual’s sexual status. In many moth species, for example, sexually-receptive females release pheromones which males can detect and home in on from considerable distances.
http://www.freethought-forum.com/forum/gallery/files/5/0/luna-moth.jpg
The antennae of a male Luna Moth (Actias luna) are extremely
sensitive to pheromones produced by sexually-receptive females.
[break=Song and Light Shows]Many animals produce sounds that identify their species and sexual status. Male songbirds, for instance, are famous for the calls they produce to attract mates. Male frogs and toads also call to attract females. It has been shown that in many frog species, a male’s calls provide clues to his size and health, and that females can identify the largest, healthiest, or least-parasitized males simply by listening to their calls.
Some animal species can even produce flashes of light with which to identify their species and their sexual status. Fireflies are surely the most famous examples of animals that use light to identify themselves and to attract mates.
http://www.freethought-forum.com/forum/gallery/files/5/0/hyla_original.jpg
The Gray Treefrog species Hyla chrysoscelis (left) and Hyla versicolor (right). Their geographic ranges
overlap, and both species can be found in the same habitats. They are “cryptic species”; this means
that they cannot be distinguished from each other morphologically. If you find a Gray Treefrog, the only
way to be sure what species you’ve found is through a genetic analysis. Or by listening to its call.
Cryptic species are much more common than most people would guess.
http://www.freethought-forum.com/forum/gallery/files/5/0/photinus_original.jpg
Fireflies use distinctive flashing patterns to indicate their species and attract mates.
The flashing patterns of six different firefly species in the genus Photinus are shown.
[break=Ensuring the Survival of Offspring]
[b] Ensuring the Survival of Offspring:
Okay, so you’ve managed to locate an individual of the proper species and sex. Even better, this individual proved to be healthy, so you’ve mated. That isn’t going to do you much good if your offspring don’t survive.
Parental investment refers to the amount of resources (of whatever sort) that parent(s) invest in offspring. Of course, it’s frequently (but by no means always) the case that females invest more in their offspring than do males. In any event, even after fertilization has occurred, parents must continue to devote resources to their offspring for some time if those offspring are to have any chance of survival. Broadly speaking, there are two general parental-investment strategies.
The first parental-investment strategy is to invest relatively little in each offspring, but to produce many offspring. This sort of strategy is typical of most insects and of most fishes. Typically, the female lays her eggs then promptly abandons them.
One disadvantage of this strategy is that while many offspring are produced, their survivorship tends to be very low. Consequently, it’s typically the case that very few of the offspring manage to live long-enough to reproduce. On the other hand, in a good environment, this strategy can result in phenomenally-fast population growth.
http://www.freethought-forum.com/forum/gallery/files/5/0/salmon.jpg
Spawning salmon: Each female will lay thousands of eggs. [B]Maybe one or
two of the young that hatch out of them will survive to adulthood.
[break=Invest Much, Produce Few]
The opposite parental-investment strategy is to invest a great deal in each offspring, and to produce only a small number of them. This general strategy is typical of birds and mammals. Typically, one or both parents feeds and protects the offspring for some time after it is hatched or born.
One disadvantage of this strategy is that since so few offspring are produced, population growth tends to be slow, even under ideal circumstances. On the other hand, survivorship of offspring is generally very high, so each offspring has an excellent chance of living long-enough to reproduce.
http://www.freethought-forum.com/forum/gallery/files/5/0/elephant_original.jpg
An African Bush Elephant (Loxodonta africana) and her calf. She has only one calf at a time, or maybe two,
but each calf has an excellent chance of surviving to adulthood.
[break=What Comes Next]
In the next chapter, we’ll return to the subject of animal development. We’ll consider the process in a little more detail, and we’ll look at some of the ways in which development differs between animal taxa (http://www.freethought-forum.com/forum/showthread.php?t=17135&garpg=3#content_start).
Chapter Five: Animal Reproduction:
Why Reproduce?:
It might seem like a strange question to ask, but why do organisms reproduce? Well, it might make more sense to ask, “Why do organisms die?” Those two questions are much more closely-linked than you might suspect at first.
Suppose that organisms were born “immortal” – that is, suppose living things didn’t grow old and eventually die. In such a world, would reproduction be necessary? Yes it would. The fact of the matter is that true immortality simply isn’t possible. Even if organisms didn’t age and die, they’d still be susceptible to disease, predators, and accidents. So, for that reason alone, reproduction is necessary. The genes of any organism that doesn’t reproduce will soon be eliminated from the gene pool by virtue of the fact that true immortality simply isn’t possible.
Genes that promote reproduction can persist indefinitely, since they’ll be passed down from one generation to the next. On the other hand, any genes that have the effect of preventing their possessors from reproducing will soon be eliminated from the gene pool, no matter how beneficial they might be in other ways.
Another reason why reproduction is necessary is that it’s necessary in order for evolution to occur. The genetic makeup of an individual does not change during its lifetime. This severely limits its ability to change in response to changing environmental conditions. But because of reproduction – especially sexual reproduction – populations can and do experience genetic change over time, and so they change in response to their environments. That is, populations evolve in response to their environments.
Odd as it might seem at first, it appears that organisms are, in effect, genetically “programmed” to grow old and eventually die. Why is this advantageous? Well, the brute fact of it is that once you’ve successfully reproduced and raised your offspring to independence, from an evolutionary perspective you have no further “purpose.” What’s more, by surviving past the point where your children need your support, you’re consuming resources that they could be using. You’re thus competing with your own offspring for limited resources and thus reducing the probability of their survival and reproduction. In other words, weird as it might seem, there comes a time when it’s in your best genetic interest to die and thus improve your offspring’s chances of survival.
Sexual vs. Asexual Reproduction:
Normally, cellular division results in diploid cells that are genetically-identical. This form of cellular division is known as mitosis, and this is how normal body cells reproduce. There is a second form of cellular division, however, known as meiosis. Meiotic cell division results in haploid cells that have only half the normal number of chromosomes and that are not genetically-identical. This is how the sex cells are produced.
In sexual reproduction, males and females produce haploid sex cells known as gametes through meiosis. The male gametes (spermatozoa) are small and mobile. The female gametes (ova) are much larger and are non-mobile. A haploid spermatozoan and a haploid ovum unite at fertilization to produce a zygote that is diploid (that is, it has a complete set of chromosomes). The zygote ultimately grows into a new individual that is genetically distinct from either of its parents.
A sexually-reproducing species in which individuals can produce both spermatozoa and ova simultaneously is said to be monoecious or hermaphroditic. A species in which each an individual can produce either spermatozoa or ova, but not both at the same time, is said to be dioecious.
In asexual reproduction, there is no union of spermatozoan and ovum, so only one parent is involved. Usually, asexual reproduction involves reproduction through mitosis and the production of an offspring that is genetically-identical to its parent – that is, it is a clone of its parent.
[b]Asexual Reproduction:
Many simple animals can reproduce asexually through the process of fission. Fission occurs when the animal simply divides into two, and each half then re-grows whatever organs it needs in order to make itself whole.
http://www.freethought-forum.com/forum/gallery/files/5/0/fission.jpg
A planarian (Phylum Platyhelminthes) can
reproduce asexually through fission. The
animal splits in two, then the anterior half
grows a new tail while the posterior half
grows a new head.
Another form of asexual reproduction that is common in simple animals is budding. Budding occurs when a “bud” grows on the animal’s side and grows into a smaller, genetically-identical version of the “parent.” When it has become large-enough, the smaller animal breaks free of its “parent” and becomes independent.
http://www.freethought-forum.com/forum/gallery/files/5/0/budding_27295.jpg
A hydra (Phylum Cnidaria) reproducing through budding.
[break=Parthenogenesis]
Some animals can reproduce through parthenogenesis. Parthenogenesis occurs when a female produces an ovum (it can be either haploid or diploid) that can grow into a new individual (which may or may not be genetically identical to its mother) without being fertilized by a spermatozoan.
In some insects (Phylum Arthropoda), for example, a fertilized ovum develops into a diploid female, while unfertilized ova develop into haploid males. Animals with this sort of genetics are referred to as haplodiploid, because individuals of one sex are haploid and individuals of the other sex are diploid. Honeybees are an example of insects that are haplodiploid. Male honeybees, since they’re haploid, produce spermatozoa that are genetically-identical to themselves; female honeybees, since they’re diploid, produce ova that are not genetically-identical to themselves. (This has important consequences for honeybee behavior.)
Parthenogenesis is not limited to “simple” animals. It has been observed in fishes, reptiles, and birds. In fact, there are some lizard species in the genus Cnemidophorus in which males are not known to occur. Every member of the species is a female, and all reproduction is through parthenogenesis. (This means that every daughter is a clone of her mother.)
http://www.freethought-forum.com/forum/gallery/files/5/0/cnemidophorus.jpg
Two Cnemidophorus uniparens engaging in “pseudocopulation.” Males are not known to
occur in this species and reproduction is through parthenogenesis. “Copulation” with
another lizard seems to stimulate a female to ovulate. This vestigial sexual behavior
provides strong evidence that C. uniparens evolved fairly recently from a sexually-reproducing species.
[break=Sexual Reproduction]
[b]Sexual Reproduction:
The great majority of animals reproduce through sexual reproduction, though there are some species that can switch between sexual and asexual reproduction as circumstances demand. That raises an interesting question: given the obvious benefits of asexual reproduction, why do most animals reproduce sexually?
Natural selection is all about getting as many of your genes into the next generation as possible. As such, sexually-reproducing animals would seem to have an obvious disadvantage compared to asexually-reproducing animals. After all, if you’re reproducing asexually and your offspring are clones of you, then each offspring carries 100% of your genes. But if you’re reproducing sexually, each offspring carries only 50% of your genes. Asexual reproduction, then, would seem to be the way to go, since you can get twice as many genes into the next generation per offspring if you reproduce asexually.
So, to repeat the question: Why do the great majority of animals reproduce sexually?
When asked this question, my students occasionally suggest that sexual reproduction is more fun. That may be, but the answer doesn’t really address the question.
[b]The Advantages of Sexual Reproduction:
Asexual reproduction, of course, generally results in offspring that are genetically identical to their parent. Sexual reproduction, on the other hand, since it occurs through the mixing of genes from two different parents, results in offspring that are genetically distinct from both of their parents. Herein lies the key to why most animal species reproduce sexually.
Back in 1930, the geneticist and evolutionary theorist R. A. Fisher noted that in sexually-reproducing populations, the continual production of and recombination of sex cells means that every individual is genetically unique, and that there will be a tremendous amount of genetic variability within a sexually-reproducing population. Since genetic variability provides the “raw material” with which natural selection works, the more genetic variability there is within a population, the greater is its capacity to change in response to a changing environment. Furthermore, the more genetic variability there is in the population, the faster it can evolve in response to a changing environment. These observations have led to two major hypotheses regarding the advantages of sexual reproduction over asexual reproduction.
The first hypothesis regards the advantages of sexual reproduction in fluctuating physical environments. The evolutionary theorist George Williams likened reproduction to a raffle. You can either have many tickets with different numbers (sexual reproduction) or many copies of the same ticket, bearing the same number (asexual reproduction). If you know what number will be drawn in the raffle, it’s in your best interest to have many copies of the winning ticket; that way, you’ll win big. But if you don’t know what number will be drawn in the raffle, it’s obviously in your best interest to have as many different tickets as possible. That way, you maximize your chances of having at least one of them come up a winner.
This is known as the “bet-hedging hypothesis” or the “tangled-bank hypothesis.” The idea is that, in an environment which fluctuates unpredictably, it’s best to produce offspring with as much genetic diversity as possible. In that way, parents maximize the likelihood that at least some of their offspring will happen to inherit gene combinations that allow them to survive.
According to this hypothesis, sexual reproduction should be favored in unstable, unpredictable environments, while asexual reproduction should be favored in stable, predictable environments. Experiments tend to support this conclusion. For example, when species alternate between sexual and asexual reproduction, they tend to reproduce asexually in the Spring and Summer, when conditions are relatively stable, and then switch to sexual reproduction in the Fall and Winter, when conditions are much less stable and predictable.
http://www.freethought-forum.com/forum/gallery/files/5/0/polyarthra_remata.jpg
The rotifer species Polyarthra remata (Phylum Rotifera). Many rotifers
switch between sexual and asexual reproduction in response to
changing environmental conditions.
The second hypothesis regards the advantages that sexual reproduction provides in dealing with other organisms. When two or more species evolve in response to each other, this is known as coevolution. Every species must coevolve with its predators and/or prey and/or hosts and/or parasites. Evolutionary “arms races” result, because predators must evolve to cope with evolutionary changes in their prey, prey species must evolve in response to evolutionary changes in their predators, host species must evolve in response to evolution of their parasites, and parasites must evolve in response to evolution of their hosts.
In other words, every species’ evolution is influenced by the demands imposed by the other species with which it interacts. For example, if a host species evolves a more effective defense against its parasites, that will create selection pressure for the parasites to evolve more effective ways to circumvent their host’s defenses. That, in turn, will create selection pressure for the host to evolve even more effective defenses – which will create selection pressure for the parasites to defeat those defenses. And so on, and so on.
According to this hypothesis, the advantage of sexual reproduction is that it greatly speeds up the rate at which evolution occurs, thus making it far easier for species to respond to evolutionary changes in their prey, predators, parasites, or hosts.
In an asexually-reproducing population, since there is no sexual recombination of genes, the only source of new gene combinations is the occasional mutation. Therefore, asexual populations inevitably evolve far more slowly than do sexual populations of the same species.
Because species must constantly evolve in order to keep pace with other species, they’re caught in a situation similar to that faced by the Red Queen in Alice in Wonderland, who told Alice, “Now here, you see, it takes all the running you can do, to keep in the same place.” So this hypothesis – namely, that sexual reproduction is favored because it allows species to quickly evolve in response to changes in other species – is known as the “Red Queen hypothesis.” The advantage to the individual in producing genetically-diverse offspring is, again, that this maximizes the chances that at least some of those offspring will happen to inherit gene combinations that make them well-adapted to deal with their predators/prey/hosts/parasites.
According to the Red Queen hypothesis, the need to quickly evolve in response to other species, especially parasites, is the driving force behind sexual reproduction. The hypothesis predicts that asexual reproduction should be favored in environments where parasites are rare or absent, whereas sexual reproduction should be favored in environments where parasites are common. Field studies support this hypothesis, too. In populations that can switch between sexual and asexual reproduction, whether they reproduce sexually or asexually tends to be strongly correlated with parasite densities.
http://www.freethought-forum.com/forum/gallery/files/5/0/potamopyrgus_antipodarum.jpg
The freshwater snail species Potamopyrgus antipodarum
(Phylum Mollusca). In parasite-free environments, asexual
populations out-reproduce and replace sexual populations.
In parasite-filled environments, sexual populations out-reproduce
and replace asexual populations.
So it appears that both hypotheses are correct. Under most environmental conditions, sexually-reproducing populations rapidly replace asexually-reproducing populations of the same species. The advantage provided by sexual reproduction is evidently the great genetic diversity that results, making rapid evolutionary change possible.
More to the point, in most environments it is in an individual’s best interest to reproduce sexually, because by producing genetically-diverse offspring, the individual maximizes the chances that at least some of its offspring will survive. In changeable, unpredictable environments, the genetic diversity that results from sexual reproduction maximizes the probability that some offspring will happen to inherit gene combinations that make them well-suited to future conditions – whatever those might be.
In environments where their offspring will have to avoid predators, capture prey, fend off parasites, or find hosts to parasitize – and let’s face it, that’s pretty-much every environment – it’s also in an individual’s best interest to produce genetically-diverse offspring. Again, this maximizes the chances that at least some of those offspring will happen to inherit gene combinations that make them well-suited to dealing with the other species with which they will interact.
[break=Reproductive Cycles and Patterns]
[b]Reproductive Cycles and Patterns:
Most animals reproduce only at certain times of the year. Unsurprisingly, they generally time their reproductive behaviors so that offspring are produced during the time of year when they’re most likely to survive. Humans are most unusual in that they can and do reproduce at any time of the year.
Animals’ reproductive cycles are generally controlled by chemicals known as hormones. A hormone is a chemical that is produced in one part of the body, and that travels through the blood to affect one or more “target organs” in some other part of the body. Production of specific hormones can be regulated by changes in such factors as the day length and the temperature. As hormone levels change in response to changes in an animal’s environment, those hormones, in turn, promote changes in the animal’s sexual behaviors, reproductive behaviors, and physiologies.
Consider a songbird, for example. As the day length changes in the early Spring, this triggers increased production of sex hormones such as estrogen, progesterone and testosterone. These hormones, in turn, trigger sexual behaviors such as courtship and mating, as well as reproductive behaviors such as nest-building.
The effects of changing hormone levels on an animal’s behavior can be subtle, or they can be quite dramatic. Consider some of the effects of the hormone oxytocin, for instance. In humans, oxytocin is released during orgasm in both males and females. Oxytocin has been shown to promote “bonding behavior” between individuals, so it’s likely that one of the effects of oxytocin release during sexual intercourse is to promote psychological bonding between partners. Needless to say, such bonding is an important factor in successfully raising offspring. Oxytocin, therefore, has been referred to as the “hormone of love.”
Studies in other mammal species have shown that adequate oxytocin levels appear to be necessary for proper bonding between parents and offspring. Accordingly, it has been suggested that conditions such as postpartum depression and failure of mothers (and fathers) to properly bond with their children may be at least partially due to abnormally low oxytocin levels. In the future, such conditions may be treatable through careful manipulation of patients’ hormone levels.
[break=Ovulation]
Ovulation is the release of mature ova, which can then be fertilized by spermatozoa. Normally, ovulation occurs midway through the female’s reproductive cycle. For example, the reproductive cycle in human females averages 28 days in length. Ovulation typically occurs on Day 14 of the cycle. If the ovum is not fertilized and pregnancy does not occur, then on or around Day 28 the reproductive cycle begins again as a new ovum begins to mature in one of the woman’s ovaries.
http://www.freethought-forum.com/forum/gallery/files/5/0/cycle_original.jpg
The reproductive cycle in the human female averages 28 days in length.
It is driven by changes in the levels of four principle hormones:
luteinizing hormone, follicle-stimulating hormone, estradiol,
and progesterone. These hormones influence body temperature,
regulate changes in the ovaries, and cause dramatic changes in the
uterine lining (the endometrium). Ovulation occurs at the midpoint of the cycle.
[break=Finding a Mate]
One of the disadvantages of sexual reproduction is that one requires a mate. In species where individuals tend to be widely-separated and so are unlikely to encounter each other during their fertile periods, hermaphroditism is relatively common. A hermaphrodite is an organism that possesses functional male and female reproductive organs. Many snail species (Phylum Mollusca) are hermaphroditic, for example, as are earthworms (Phylum Annelida).
http://www.freethought-forum.com/forum/gallery/files/5/0/wormporn.jpg
Earthworms (Phylum Annelida) mating. Earthworms are hermaphrodites,
and when they mate, each worm inseminates the other.
[break=Hermaphrodites]
Needless to say, if a species is hermaphroditic, this makes it considerably easier for individuals to find mates, since any member of the same species is a potential mate. Another way in which animals can cope with a lack of suitable mating partners is by changing sex. In many fish species, for instance, individuals can change from males that produce spermatozoa to females that produce ova or vice versa, depending upon which sex is rarer and more in demand by potential mates. In fact, among coral-reef fish species, those that cannot change sex are the exception – most species can change sex in one or both directions.
http://www.freethought-forum.com/forum/gallery/files/5/0/labroides_dimidiatus_original.jpg
The reef fish Labroides dimidiatus (Phylum Chordata). In this species, males are dominant.
If a dominant male dies, a female can change sex and take his place.
[break=Bringing Sperm and Ova Together]
[b]Bringing Sperm and Ova Together:
The female gamete, as we’ve mentioned, is the ovum. The terms “ovum” and “egg” are often used as if they’re interchangeable, but this really isn’t the case. An egg is an ovum (especially a fertilized ovum – a zygote), plus any supporting cells or tissues that may be present in addition.
Fertilization can occur either inside the female’s body or outside of it. If the ova are fertilized outside the female’s body, this is known as external fertilization. Typically, the female lays her eggs and the male sheds sperm over them as they’re laid or shortly afterwards. This is a quick and easy way to get eggs fertilized, but it has a number of disadvantages. The most obvious disadvantage is that external fertilization must take place in a wet environment, else the eggs will dry out and die.
http://www.freethought-forum.com/forum/gallery/files/5/0/frogporn_original.jpg
Most amphibians such as these Wood Frogs (Phylum Chordata) have external fertilization.
As the female expels her eggs, the male sheds sperm onto them.
If the ova are fertilized inside the female’s body, this is known as internal fertilization. One advantage of internal fertilization is that the eggs are better-protected than are those of animals that practice external fertilization – up until the moment of fertilization, at least. Another advantage of internal fertilization is that the sperm can more easily fertilize the ova. Probably not coincidentally, animals that practice internal fertilization tend to produce fewer eggs than do closely-related species with external fertilization. If fertilization is internal, it isn’t such a random process, and the chances of an egg being successfully fertilized are higher.
In many species, after the egg is fertilized, the female secretes a protective shell around it that helps protect the developing embryo inside. This means that the eggs can then be deposited in relatively dry environments. Obviously, this is possible only if fertilization is internal, since the egg can’t be fertilized after a shell is secreted around it.
In some species, such as most mammals, the embryo remains inside the mother’s body as it develops. Obviously, in such species, fertilization must be internal.
http://www.freethought-forum.com/forum/gallery/files/5/0/penguinporn.jpg
In birds (Phylum Chordata), fertilization occurs inside the female’s body.
[b]Identifying a Suitable Mate:
Of course, to successfully reproduce, you can’t very well attempt to mate with every animal of the opposite sex that you happen to encounter. That would not be an effective reproductive strategy, to say the least. Therefore, it’s vitally important to make sure that your potential mate is of the same species. Not surprisingly, then, animals have evolved various means of indicating the species to which they belong.
Before we discuss some of those adaptations, it’s worth considering which sex is the one that has the most to lose by making a poor choice when selecting a mate. If the members of one sex have more to lose from choosing an unsuitable partner, you’d expect selection to favor more “choosy” behavior on the part of that sex. This indeed seems to be the case.
[break=Anisogamy]
Anisogamy (or heterogamy) refers to the fact that, in animals, the gametes produced by males and females differ in size. The spermatozoa produced by males are much smaller than are the ova produced by females. And it’s not just the gametes; the nutrients and tissues that surround the ovum and make up an egg further add to the amount of nutrients and energy a female invests in production of offspring. What’s more, in many species (notably mammals), females retain the developing embryos inside their bodies for some period of time. Females may even continue to nourish the young after they’re born (for instance, through production of milk).
What this means is that, in many species, females invest much more in the production of each offspring than do males. In these species, what limits the reproductive capacity of a female more than anything else is the quality of her mate(s). Since the number of offspring she can produce is sharply limited, it’s in her best genetic interest to choose the best mate that’s available, that she will have the healthiest offspring possible.
By contrast, since sperm are “cheap” to produce, what typically limits a male’s reproductive capacity is the quantity of his mates. Since a healthy male is typically more than capable of fertilizing all the eggs of several different females, it’s in his best genetic interest to mate with as many different females as possible.
Of course, this is a generalization. There are plenty of species in which males and females invest roughly equally in reproduction. There are even some species in which males invest more in the production of offspring than do females.
[break=Female Choice]
Since members of different species cannot successfully interbreed, it’s in every organism’s best genetic interest to mate only with members of the same species. So you’d expect mechanisms to evolve that allow animals to recognize members of their own species. And, indeed, that is the case.
If one sex invests more in reproduction than does the other, as is frequently the case, the sex that invests more in reproduction has more to lose by choosing the wrong partner. Accordingly, you’d expect selection for more “choosy” behavior when it comes to selecting a mate in the sex that has more to lose. Usually, the female invests more in reproduction than does the male, and indeed, females, on average, seem to be much more careful than males when selecting potential mates.
So prevalent is this that it’s frequently referred to as the “female choice model.”
If selection favors “choosy” behavior in one sex, it’s in the best genetic interest of members of the opposite sex to somehow advertise their suitability as mates. So it’s most-often males who have evolved means of indicating the species to which they belong – and their suitability as mates.
http://www.freethought-forum.com/forum/gallery/files/5/0/cardinals_original.jpg
Male and female Northern Cardinals (Cardinalis cardinalis): In many songbird species, males are
much more brightly-colored than are females. As a rule, the more brightly-colored the male,
the healthier he is and the fewer parasites he has. When given a choice, females generally
prefer more brightly-colored males as mates. Their bright colors allow males to demonstrate
their fitness and, therefore, their suitability as potential mates.
[break=Courtship Displays]
In some species, males directly compete with each other for access to females. (In those species in which males invest more in reproduction than do females, females may compete with each other for opportunities to mate with males.)
http://www.freethought-forum.com/forum/gallery/files/5/0/sparring_original.jpg
Bull Elk (Cervus canadensis) competing for access to females.
Competition for mates need not involve outright combat, however. Very commonly, individuals (usually males, of course) engage in courtship displays. These displays typically do two things: they identify the animal’s species (each species has a distinctive courtship display, which eliminates confusion), and they provide clues to the animal’s overall health and, therefore, it’s suitability as a mate. Females can choose mates based upon the quality of their courtship displays, and there is therefore no need for males to engage in potentially-dangerous combat to prove that they’re strong and healthy.
http://www.freethought-forum.com/forum/gallery/files/5/0/superb_original.jpg
A male Superb Bird of Paradise (Lophorina superba), seen from behind, performs
an elaborate and energetically-demanding courtship display while a female observes.
[break=Courtship Rituals]
Even in species where males and females invest roughly equally in reproduction, courtship rituals are common. In these species, they’re typically performed by both sexes. Again, each species’ courtship ritual involves unique behaviors, and helps to ensure that animals don’t wind up mating with individuals of the wrong species.
http://www.freethought-forum.com/forum/gallery/files/5/0/albatross.jpg
Two Laysan Albatrosses (Phoebastria immutabilis) engage in a courtship ritual.
[break=Pheromones]
Animals don’t always advertise for mates through courtship displays or courtship rituals. Many other methods are available. For instance, many species secrete chemicals known as pheromones that can travel through air or water, and that indicate an individual’s sexual status. In many moth species, for example, sexually-receptive females release pheromones which males can detect and home in on from considerable distances.
http://www.freethought-forum.com/forum/gallery/files/5/0/luna-moth.jpg
The antennae of a male Luna Moth (Actias luna) are extremely
sensitive to pheromones produced by sexually-receptive females.
[break=Song and Light Shows]Many animals produce sounds that identify their species and sexual status. Male songbirds, for instance, are famous for the calls they produce to attract mates. Male frogs and toads also call to attract females. It has been shown that in many frog species, a male’s calls provide clues to his size and health, and that females can identify the largest, healthiest, or least-parasitized males simply by listening to their calls.
Some animal species can even produce flashes of light with which to identify their species and their sexual status. Fireflies are surely the most famous examples of animals that use light to identify themselves and to attract mates.
http://www.freethought-forum.com/forum/gallery/files/5/0/hyla_original.jpg
The Gray Treefrog species Hyla chrysoscelis (left) and Hyla versicolor (right). Their geographic ranges
overlap, and both species can be found in the same habitats. They are “cryptic species”; this means
that they cannot be distinguished from each other morphologically. If you find a Gray Treefrog, the only
way to be sure what species you’ve found is through a genetic analysis. Or by listening to its call.
Cryptic species are much more common than most people would guess.
http://www.freethought-forum.com/forum/gallery/files/5/0/photinus_original.jpg
Fireflies use distinctive flashing patterns to indicate their species and attract mates.
The flashing patterns of six different firefly species in the genus Photinus are shown.
[break=Ensuring the Survival of Offspring]
[b] Ensuring the Survival of Offspring:
Okay, so you’ve managed to locate an individual of the proper species and sex. Even better, this individual proved to be healthy, so you’ve mated. That isn’t going to do you much good if your offspring don’t survive.
Parental investment refers to the amount of resources (of whatever sort) that parent(s) invest in offspring. Of course, it’s frequently (but by no means always) the case that females invest more in their offspring than do males. In any event, even after fertilization has occurred, parents must continue to devote resources to their offspring for some time if those offspring are to have any chance of survival. Broadly speaking, there are two general parental-investment strategies.
The first parental-investment strategy is to invest relatively little in each offspring, but to produce many offspring. This sort of strategy is typical of most insects and of most fishes. Typically, the female lays her eggs then promptly abandons them.
One disadvantage of this strategy is that while many offspring are produced, their survivorship tends to be very low. Consequently, it’s typically the case that very few of the offspring manage to live long-enough to reproduce. On the other hand, in a good environment, this strategy can result in phenomenally-fast population growth.
http://www.freethought-forum.com/forum/gallery/files/5/0/salmon.jpg
Spawning salmon: Each female will lay thousands of eggs. [B]Maybe one or
two of the young that hatch out of them will survive to adulthood.
[break=Invest Much, Produce Few]
The opposite parental-investment strategy is to invest a great deal in each offspring, and to produce only a small number of them. This general strategy is typical of birds and mammals. Typically, one or both parents feeds and protects the offspring for some time after it is hatched or born.
One disadvantage of this strategy is that since so few offspring are produced, population growth tends to be slow, even under ideal circumstances. On the other hand, survivorship of offspring is generally very high, so each offspring has an excellent chance of living long-enough to reproduce.
http://www.freethought-forum.com/forum/gallery/files/5/0/elephant_original.jpg
An African Bush Elephant (Loxodonta africana) and her calf. She has only one calf at a time, or maybe two,
but each calf has an excellent chance of surviving to adulthood.
[break=What Comes Next]
In the next chapter, we’ll return to the subject of animal development. We’ll consider the process in a little more detail, and we’ll look at some of the ways in which development differs between animal taxa (http://www.freethought-forum.com/forum/showthread.php?t=17135&garpg=3#content_start).