Anatomy And Embryology

Descent with modification also predicts the findings of anatomy—the study of the body structure of organisms— and embryology—the study of how organisms develop. Look at the bones in the forelimbs of humans, penguins, alligators, and bats shown in Figure 15-8. These forelimbs are used in different ways in each animal, yet each limb has a similar bone structure. One explanation for the commonalities among the forelimb bones of the four animals is that an early ancestor shared by all these vertebrates had a forelimb with a similar bone structure. As generations passed, different populations of descendants adapted to different environments. Bones inherited from ancestors may have become modified for different tasks.

Biologists define homologous structures as anatomical structures that occur in different species and that originated by heredity from a structure in the most recent common ancestor of the species. Homologous organs often have a related structure even if their functions differ between species. On the other hand, analogous structures have closely related functions but do not derive from the same ancestral structure. For example, even though birds, bats, and moths have wings, their wings have very different underlying structures. Scientists think that wings evolved independently in each of these groups of animals.

figure 15-8

Humans, penguins, alligators, and bats each have forelimbs with homologous parts.

Humans, penguins, alligators, and bats each have forelimbs with homologous parts.

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