Outcomes:

• • Evaluate evidence that suggests the organisms living on Earth today have evolved from organisms that existed on Earth a long time ago.

  • Explain the link between evolution and heredity.

Evidence for Evolution

There is a wealth of evidence that supports the theory of evolution, including fossil records, comparative anatomy, and molecular biology. Fossils provide a record of past life on Earth, and they show how different species have changed over time.

Comparative anatomy involves comparing the structures of different organisms to identify common features and evolutionary relationships. Molecular biology looks at the genetic similarities and differences between different species, which can reveal how closely related they are.

All of these lines of evidence support the idea that all living organisms share a common ancestor, and that species have evolved over millions of years through a process of natural selection and genetic drift.

Evidence for evolution comes from many different areas of biology:

• • Anatomy. Species may share similar physical features because the feature was present in a common ancestor (homologous structures).

  • Molecular biology. DNA and the genetic code reflect the shared ancestry of life. DNA comparisons can show how related species are.

  • Biogeography. The global distribution of organisms and the unique features of island species reflect evolution and geological change.

  • Fossils. Fossils document the existence of now-extinct past species that are related to present-day species.

  • Direct observation. We can directly observe small-scale evolution in organisms with short lifecycles (e.g., pesticide-resistant insects).

Evolution happens on large and small scales

Broadly speaking, evolution is a change in the genetic makeup (and often, the heritable features) of a population over time. Biologists sometimes define two types of evolution based on scale:

• Macroevolution, which refers to large-scale changes that occur over extended time periods, such as the formation of new species and groups.

• Microevolution, which refers to small-scale changes that affect just one or a few genes and happen in populations over shorter timescales.

Microevolution and macroevolution aren’t really two different processes. They’re the same process – evolution – occurring on different timescales. Microevolutionary processes occurring over thousands or millions of years can add up to large-scale changes that define new species or groups.

Anatomy and embryology

Darwin thought of evolution as "descent with modification," a process in which species change and give rise to new species over many generations. He proposed that the evolutionary history of life forms a branching tree with many levels, in which all species can be traced back to an ancient common ancestor.

Comparative Embryology

Comparative embryology studies similarities and differences in embryos of different species, providing evidence of common ancestry. Vertebrate embryos share gill slits and tails, which are often lost in adulthood. Comparing organisms in the embryonic stage is valuable due to similarities that may disappear by adulthood. 

See: http://www.pbs.org/wgbh/evolution/library/04/2/pdf/l_042_03.pdf for additional information and a comparative diagram of human, monkey, pig, chicken and salamander embryos.

1. What are the different areas of biology that provide evidence for evolution?

2. How do homologous structures provide evidence for evolution?

3. How does DNA comparison provide evidence for evolution?

4. What do fossils tell us about the existence of past species and their relationship to present-day species?

5. How do similarities in embryos provide evidence for common ancestry?