Biological Evidence of Evolution Ch. 6-3

Evidence for Evolution Living species that are closely related share a common ancestor. How close two species are related depends on how closely in time they diverged (split) from their common ancestor. The fossil record contains many fossil sequences showing close ancestral relationships. Living species show evidence of ancestry too!

Comparative Anatomy Common ancestry is not always hard to tell in species. What are some common features of robins, finches, and hawks that tell us they evolved from a common ancestor? But what about a cat and a hawk? Or a cat, hawk, and frog? Scientists can use comparative anatomy and study similarities and differences among structures of living species. Homologous structures Analogous structures

Homologous Structures Humans, cats, frogs, bats, and birds look different and move in different ways. Humans use arms for balance and hands to grasp objects Cats use forelimbs to run, jump Frogs use their forelimbs to jump Bats and birds use their forelimbs as wings for flying

Homologous Structures The forelimb bones of these species show similar patterns. Known as homologous structures Body parts of organisms that are similar in structure and position but different in function What does this suggest? That these species are related. The more similar two structures are to each other, the more likely it is that the species have evolved from a recent common ancestor.

Analogous Structures Birds and flies both have wings that are used for flight. What are some differences? Bird wings are covered in feathers Fly wings are covered in tiny hairs Bird wings and fly wings are known as analogous structures Body parts that perform a similar function but differ in structure These differences indicate that bird and flies are not closely related

Vestigial Structures What do human’s appendix and cormorant’s wings have in common? Examples of vestigial structures Body parts that have lost their original function through evolution Suggests that the species has an ancestral species that used the structure for a specific purpose. Our appendix may have been useful when humans ate foods that were harder to digest

Vestigial Structures in Whales Whales have tiny pelvic bones inside its body. What do you think this suggests? Suggests that whales descended from ancestors that used legs for walking on land. Fossils of whale ancestors show a gradual loss of legs over millions of years.

Embryology Studying the development of embryos can also provide scientists with evidence that certain species are related. Embryology = the science of the development of embryos from fertilization to birth

Pharyngeal Pouches Embryos of different species often resemble each other at different stages of their development. All vertebrate (backbone) embryos have pharyngeal pouch at one stage. Includes fish, reptiles, birds, and humans It develops into different body parts in each vertebrate. The similarity in structure and function suggests a strong evolutionary relationship.

Molecular Biology Study of gene structure and function Studying fossils, comparative anatomy, and embryology provide support of Darwin’s theory of evolution by natural selection. Molecular biology has confirmed this data. Genes provide powerful support for evolution.

Divergence Scientists have found that some stretches of shared DNA mutate at regular, predictable rates. Called “molecular clock” and is used to estimate at what time in the past living species diverged (split) from common ancestors. Molecular data indicate that whales and porpoises are closely related to hippopotamuses

Comparing Sequences Scientists can study relatedness of organisms by comparing genes and proteins of living species. The more closely related two species are, the more similar their genes are proteins are.

How New Species Form Scientists argue on the rate at which new species form. Many think that natural selection produces new species slowly and steadily. Others think species exist stably for long periods, then change occurs in short bursts. Once members of the same species can no longer reproduce with each other, it has become two different species. Variations within the species and the environment can cause this over time.