2
the phyla (and of invertebrate classes) appear abruptly in the
fossil record, and their ancestors cannot be traced. Furthermore,
numbers of extinct phyla or other major animal groups also
appear in the fossil record, adding branches to the tree of life
that must be reckoned with, but with no definitive indication
of either their origins or branching patterns. Finally, genetic
changes that have accumulated after branching events sepa-
rated the major groups should have left a pattern of variation
in the structure of genes that may be reconstructed to yield a
phylogeny. If genetic changes have occurred in a reasonably
regular manner, then the amount of divergence in the structure
of genes and gene products, such as protein and ribonucleic
acid (RNA) molecules, should be proportional to the time since
branching occurred. Some genes change so fast that their pat-
tern of divergence is useful only for recent branching events,
whereas others are so conservative that they are used to study
divergences that occurred billions of years ago. These methods
are promising when used in conjunction with other evidence.
Animal Evolution (continued)
Fig. 2: Phylogenetic tree depicting the possible relations among the animal phyla. The
branching pattern is constrained by models of development and of body-plan evolution
and by molecular data. Branch length is not drawn to scale.
(Copyright © McGraw-Hill Education)
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Fig. 1: Evolution of animal groups, showing hypothetical relationships.
(Credit: G. B. Moment)
