The upper forelimbs of humans and bats have fairly similar skeletal structures, whereas the corresponding bones in whales have very different shapes and proportions. However, genetic data suggest that all three kinds of organisms diverged from a common ancestor at about the same time. Which of the following is the most likely explanation for these data? a. Forelimb evolution was adaptive in people and bats, but not in whales. b. Natural selection in an aquatic environment resulted in significant changes to whale forelimb anatomy. c. Genes mutate faster in whales than in humans or bats. d. Whales are not properly classified as mammals.

Key Concepts

Homologous Structures

Homologous structures are anatomical features in different species that share a common ancestry, despite differing in function and appearance. The forelimbs of humans, bats, and whales exemplify this concept, as they have similar bone structures inherited from a common ancestor, yet have adapted to different environments and functions—grasping, flying, and swimming, respectively.

Natural Selection

Natural selection is a fundamental mechanism of evolution, where organisms better adapted to their environment tend to survive and reproduce. In the case of whales, natural selection in an aquatic environment likely led to significant anatomical changes in their forelimbs, optimizing them for swimming, which contrasts with the adaptations seen in terrestrial mammals like humans and bats.

Adaptive Evolution

Adaptive evolution refers to the process by which species evolve traits that enhance their survival and reproduction in specific environments. The differences in forelimb anatomy among humans, bats, and whales illustrate adaptive evolution, as each species has developed unique adaptations that suit their ecological niches, driven by the selective pressures of their respective environments.