16.4 Evidence of Evolution Notes

16.4 Evidence of Evolution • How does the geographic distribution of species today relate to their evolutionary history? • Patterns in the distribution of living and fossil species tell us how modern organisms evolved from their ancestors. • Biogeography is the study of where organisms live now and where they and their ancestors lived in the past. • Two biogeographical patterns are significant to Darwin’s theory. • The first is a pattern in which closely related species differentiate in slightly different climates. • The second is a pattern in which very distantly related species develop similarities in similar environments. Closely Related but Different • To Darwin, the biogeography of Galápagos species suggested that populations on the island had evolved from mainland species. • Over time, natural selection on the islands produced variations among populations that resulted in different, but closely related, island species. • For example, natural selection produced variation in shell shape among the giant land tortoises that inhabit the islands. Distantly Related but Similar • On the other hand, similar habitats around the world are often home to animals and plants that are only distantly related. • Darwin noted that similar ground-dwelling birds (rheas, ostriches, and emus) inhabit similar grasslands in Europe, Australia, and Africa. • Differences in body structures among those animals provide evidence that they evolved from different ancestors. • Similarities among those animals, however, provide evidence that similar selection pressures had caused distantly-related species to develop similar adaptations. The Age of Earth and Fossils • • How do fossils help to document the descent of modern species from ancient ancestors? • Many recently discovered fossils form series that trace the evolution of modern species from extinct ancestors. The Age of Earth • Evolution takes a long time. If life has evolved, then Earth must be very old. • Hutton and Lyell argued that Earth was indeed very old, but technology in their day couldn’t determine just how old. • Geologists now use radioactivity to establish the age of certain rocks and fossils. Radioactive dating indicates that Earth is about 4.5 billion years old—plenty of time for evolution by natural selection to take place. Recent Fossil Finds • Darwin’s study of fossils had convinced him and other scientists that life evolved, but paleontologists in 1859 hadn’t found enough fossils of intermediate forms of life to document the evolution of modern species from their ancestors. • Since Darwin, paleontologists have discovered hundreds of fossils that document intermediate stages in the evolution of many different groups of modern species. • One recently discovered fossil series documents the evolution of whales from ancient land mammals. Several reconstructions based on fossil evidence are shown on the following slides. The exceptions to the reconstructions are the modern Mysticete and Odontocete. All historical records are incomplete, and the history of life is no exception. The evidence we do have, however, tells an unmistakable story of evolutionary change. Comparing Anatomy and Embryology • What do homologous structures and similarities in embryonic development suggest about the process of evolutionary change? • • • Evolutionary theory explains the existence of homologous structures adapted to different purposes as the result of descent with modification from a common ancestor. By Darwin’s time, scientists had noted that all vertebrate limbs had the same basic bone structure. Homologous Structures • Darwin proposed that animals with similar structures evolved from a common ancestor with a basic version of that structure. • Structures that are shared by related species and that have been inherited from a common ancestor are called homologous structures. • Biologists test whether structures are homologous by studying anatomical details, the way structures develop in embryos, and the pattern in which they appeared over evolutionary history. • Homologous bones, as shown by color-coding, support the differently-shaped front limbs of modern vertebrates. • These limbs evolved, with modifications, from the front limbs of a common ancestor whose bones resembled those of an ancient fish. • Similarities and differences among homologous structures help determine how recently species shared a common ancestor. • For example, the front limbs of reptiles and birds are more similar to each other than either is to the front limb of an amphibian or mammal. This similarity—among many others—indicates that the common ancestor of reptiles and birds lived more recently than the common ancestor of reptiles, birds, and mammals. Analogous Structures • The clue to common descent is common structure, not common function. A bird’s wing and a horse’s front limb have different functions but similar structures. • Body parts that share a common function, but not structure, are called analogous structures. The wing of a bee and the wing of a bird are analogous structures. Vestigial Structures • Not all homologous structures have important functions. • Vestigial structures are inherited from ancestors, but have lost much or all of their original function due to different selection pressures acting on the descendant. • The hipbones of bottlenose dolphins are vestigial structures. In their ancestors, hipbones played a role in terrestrial locomotion. However, as the dolphin lineage adapted to life at sea, this function was lost. • Why would an organism possess structures with little or no function? One possibility is that the presence of a vestigial structure does not affect an organism’s fitness. In that case, natural selection would not eliminate it. Embryology • Researchers noticed a long time ago that the early developmental stages of many animals with backbones (called vertebrates) look very similar. • Recent observations make clear that the same groups of embryonic cells develop in the same order and in similar patterns to produce many homologous tissues and organs in vertebrates. • Similar patterns of embryological development provide further evidence that organisms have descended from a common ancestor. • Evolutionary theory offers the most logical explanation for these similarities in patterns of development. • Similar patterns of embryological development provide further evidence that organisms have descended from a common ancestor.