35 Macroevolution
Evidence of Evolution
The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species, to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader.
Fossils
Fossils provide solid evidence that organisms from the past are not the same as those today, and fossils show a progression of evolution. Scientists determine the age of fossils and categorize them from all over the world to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years ((Figure 1). For example, scientists have recovered highly detailed records showing the evolution of humans and horses (Figure 1). The whale flipper shares a similar morphology to bird and mammal appendages (Figure 2) indicating that these species share a common ancestor.
Anatomy and Embryology
Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in human, dog, bird, and whale appendages all share the same overall construction (Figure 2) resulting from their origin in a common ancestor’s appendages. Over time, evolution led to changes in the bones’ shapes and sizes different species, but they have maintained the same overall layout. Scientists call these synonymous parts homologous structures.
Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. We call these unused structures without function vestigial structures. Other examples of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales.
As explained in Determining Evolutionary Relationships, when similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin are completely different. These are analogous structures (Figure 3).
LINK TO LEARNING
View this video to learn which bone structures are homologous and which are analogous, and see examples of evolutionary adaptations to illustrate these concepts.
Another evidence of evolution is the convergence of form in organisms that share similar environments. For example, species of unrelated animals, such as the arctic fox and ptarmigan, living in the arctic region have been selected for seasonal white phenotypes during winter to blend with the snow and ice (Figure 4). These similarities occur not because of common ancestry, but because of similar selection pressures—the benefits of predators not seeing them.
Embryology, the study of the anatomy