An Introduction to Zoology
Chapter Four: Animal Body Plans and Phylogenies:
Body Plans:
An organism’s
body plan is the set of distinctive
morphological and
developmental traits that is characteristic of the
taxon to which it belongs. It’s important to remember that a particular body plan may have evolved more than once in life’s history though, so it can be a mistake to think that just because two different species have similar body plans that they’re necessarily closely-related.
Extant animal species display a relatively small number of body plans. From this fact, and from examination of the molecular and fossil records of animal evolution, it seems that once a particular body plan has evolved, it tends to remain remarkably stable. For instance, comparisons between species that are separated by hundreds of millions of years of evolutionary history reveal that the molecular control of
gastrulation has not changed in over 500 million years.
Because animals’ body plans are so stable, they have often been used to classify organisms. For example, all animals with
radial symmetry are often assumed to make up a distinct
clade, while animals with
bilateral symmetry are often assumed to make up a different clade.
Nowadays, we have the ability to directly compare organisms’ genetic and biochemical makeups in order to determine their evolutionary relationships. What these molecular techniques sometimes reveal is that just because organisms share similar body plans doesn’t always mean that they’re related.
Introduction
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