2.5 How do we know evolution has occurred? Comparative anatomy.

Comparative anatomy

Comparative anatomy is a field of evolution in which scientists measure similarities and differences between different species. Related organisms have similar bodily structures, or anatomy. Anatomical similarities are particularly evident in a comparison of the forelimbs of the humans, lizards, whales, birds, and other vertebrate organisms (Figure 2.4). These animals live in different types of environments, and use their limbs for different functions–flying, swimming, or grasping and throwing. Despite these different functions, all of these animals possess limbs with certain shared characteristics: a five-digit structure, a single large bone (humerus), two bones in the forearm (ulna and radius), and several wrist bones. Anatomical similarities like these are used as evidence for common ancestry. In other words, these similarities suggest that all of the species shown in Figure 2.4 share a common ancestor somewhere in their distant past. Learning about shared ancestry provides an important clue to understanding an organism’s evolution.

Homologous traits are features that different organisms share as a result of a common ancestor that possessed that feature. The tetrapod limbs discussed above are homologous traits. Because they evolved in a common ancestor and are currently shared by different organisms, homologies are also called shared derived traits (“derived” indicating that the traits are derived from the common ancestor. Cactus spines, the leaves of a maple tree, and the cup-like “pitcher” of a pitcher plant are all modified from a common structure in an ancestor shared by all leaf-bearing land plants. While these leaves look different, and have evolved to serve different purposes, they are homologous structures and tell important stories about each plant’s history (Figure 2.5).

Figure 2.5 Leaves as homologous structures. Cactus spines, the leaves of a maple tree, and the cup-like “pitcher” of a pitcher plant are all modified from a common structure in an ancestor shared by all leaf-bearing land plants.

Analogous traits and convergent evolution

Analogous traits are anatomical features that different types of organisms share, but NOT as a result of a shared ancestor. Instead, analogous structures are similar because the organisms’ traits have evolved in response to similar environments. When distantly related organisms share features as a result of similar environmental pressures (and not because of common ancestry), we say that they have undergone convergent evolution. For example, arctic mammals such as foxes and snowshoe hares grow white fur during the winter months. White fur allows these organisms to blend into the ice and snow that characterizes their polar home, and presumably protects them from predation. However, foxes and snowshoe hares do not share a common ancestor with white fur. Of course they ultimately share a common ancestor, as do all mammals, but the fox lineage is full of non-white animals, as is the group to which hares belong. The winter white of arctic foxes and snowshoe hares is thus an analogous trait, due to convergent evolution in a white, wintry landscape.

Vestigial traits are anatomical features that are either no longer in use, or their use has been greatly reduced or altered. Vestigial structures, or vestigia, provide clues to an organism’s history by suggesting which features were useful in the past, and by linking an organism to related species. For example, many mammals exhibit piloerection, whereby muscles constrict around the hair follicles and the animal’s hair stands on end. If you’ve ever surprised or otherwise threatened a dog or cat, you’ve probably seen the results of piloerection. Humans are mammals too, but over time we’ve evolved to have only a scant covering of body hair. When we are scared, the muscles around our hair follicles also constrict, but this just gives us goosebumps (Figure 2.6). Goosebumps aren’t very scary, but as a vestigial trait they link us to our mammalian relatives and tell us something about our evolutionary history.