17.1 Understanding evolution  (Page 7/21)

Organisms in the fossil record represent transitions from an early type of organism to various current ones. An early organism may have diversified over time to lead to many different species today. An example, with a much shorter-time frame, is your great-grandmother. She had one or more children -- one of which was your grandmother. Your grandmother is similar, but not the same as your great-grandmother Your grandmother had one or more children -- one of which was your mother, who is similar, but not the same as her mother. If you imagine instead of individual generations, but millions of years, and instead of single individuals, but whole species, then the concept is very much the same. You and your siblings and your cousins all share a common ancestor. So do individual species. The fossil record can show us species who may be those ancestors, or may be close relatives of those ancestors (to go back to our analogy: like your great-grandmother's sister). A transitional form     , or transitional fossil is an intermediate form of an organism linking two other organisms together. For example, [link] shows transitional forms that link the Hyracotherium to the modern horse. Evolution occurs by gradual change, and therefore many transitional organisms exist between an ancient organism and a living one.

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 the appendages of a human, dog, bird, and whale all share the same overall construction ( [link] ) resulting from their origin in the appendages of a common ancestor. Over time, evolution led to changes in the shapes and sizes of these bones in different species, but they have maintained the same overall layout. Scientists call these synonymous parts homologous structures     .

Figure 18.1

Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. These unused structures without function are called vestigial structures     . Other examples of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales.

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 ( [link] ab ). These similarities occur not because of common ancestry, but because of similar selection pressures—the benefits of not being seen by predators.