EVIDENCE OF EVOLUTION

Key Concepts What evidence supports the theory of evolution? How do scientists infer evolutionary relationships among organisms? How do new species form?

Key Terms Homologous Structures Body parts that are structurally similar in related species.

Key Terms Branching tree A diagram that shows how scientists think different groups of organisms are related.

Identifying Supporting Evidence Identifying supporting evidence helps us understand the relationship between the facts and the hypothesis. Evidence consists of facts that can be confirmed by testing or observation.

Supporting Darwin’s Theory Fossils show that organisms that lived in the past were very different from organisms alive today. Patterns of early development show that some different organisms look similar during their early stages. Similar body structures in different species show that the organisms shared a common ancestor.

Interpreting the Evidence FOSSILS EARLY DEVELOPMENT BODY STRUCTURE

FOSSILS The preserved remains or traces of an organism that lived in the past. Fossils show that organisms that lived in the past were very different than organisms alive today. Scientists use fossils to infer the structures of ancient organisms.

EARLY DEVELOPMENT (embryology) Scientists compare the early development of different organisms to make inferences about evolutionary relationships. Similarities in early development among different organisms suggests that they are related and share a common ancestor.

BODY STRUCTURE (homologous structures) Darwin compared body structures of living species when observing the species on the Galapagos Islands. Body structure is an organism’s body plan, how its bones are arranged. Similarities in body structure provide evidence that organisms evolved from a common ancestor.

Species Relationships Fossils, early development patterns, and body structure provide evidence that evolution has occurred. Scientists also have used these kinds of evidence to infer how organisms are related to one another.

New Evidence DNA Protein Structure

Similarities in DNA Scientists compare the genes of different species to determine how closely related the species are. The more similar the sequence of bases in the DNA, the more closely related the species are. The more similar the order of amino acids (codes for proteins) in the DNA, the more closely related the species are.

Combining Evidence The use of DNA and protein structure has confirmed conclusions that scientists had already based on fossils, embryos, and body structure. The use of DNA and protein structure has also caused scientists to “revise” the branching trees of some species.

Branching Trees Branching trees show common ancestry (phylogeny). Evolution is about gradualism and phylogeny.

How Do New Species Form? (speciation) A new species can form when a group of individuals remains isolated from the rest of its species long enough to evolve different traits. Isolation/Separation River Volcano Mountain range

Convergent Evolution The process whereby organisms not closely related, independently evolve similar traits as a result of having to adapt to similar environments or ecological niches.

Divergent Evolution (adaptive radiation) The process by which related species evolve different traits.

Gradualism Evolution occurs slowly but steadily.