Evidence for evolution (SACE Stage 2 Biology)

What this dot point is asking

You need to describe the main types of evidence for evolution and explain how each one supports the idea that species change over time and share common ancestors.

What evolution is

Evolution is the change in the inherited characteristics of populations over many generations, leading to the diversity of life from common ancestors. The strength of the theory comes from many independent lines of evidence all pointing to the same conclusion.

Fossil record

Fossils are preserved remains or traces of past organisms. By dating the rock layers they are found in, scientists can see how life has changed over time. The fossil record shows:

• organisms appearing, changing and going extinct in a time sequence
• transitional forms that share features of older and newer groups (for example, fossils linking fish to early land vertebrates)

This demonstrates that present-day species descended from different earlier forms.

Comparative anatomy

Comparing the body structures of different species reveals patterns of shared ancestry:

• Homologous structures have the same basic structure but different functions, such as the pentadactyl (five-fingered) limb of humans, whales, bats and cats. Their common underlying plan indicates a common ancestor.
• Vestigial structures are reduced remnants of features that were useful in ancestors (for example, the human appendix), consistent with descent with modification.

Molecular (biochemical) evidence

Because all organisms use DNA and the same genetic code, their molecules can be compared:

• The more similar the DNA base sequences or protein amino acid sequences of two species, the more closely related they are.
• Closely related species share more of their genetic code; distantly related species share less.

The universality of the genetic code itself is strong evidence of a single common origin of life. Molecular evidence is especially powerful because it independently confirms relationships suggested by anatomy and fossils.

Molecular clocks and techniques

Two molecular techniques appear in SACE data questions. DNA hybridisation measures relatedness by how strongly the DNA of two species binds: more complementary bases means more hydrogen bonds, so a higher temperature is needed to separate the strands - the higher the separation temperature, the more closely related the species. Sequence comparison counts the differences in DNA or protein sequences directly: fewer differences indicate a more recent common ancestor, because fewer mutations have accumulated since the two lineages diverged. Because mutations build up at a roughly steady rate, the number of differences can act as a rough molecular clock estimating how long ago two species split.

Biogeography

Biogeography is the study of where species live. The geographic distribution of organisms supports evolution: species on isolated islands often resemble nearby mainland species but have diverged, and related species are found where a common ancestor could have spread and then changed in different environments (as Darwin observed with the Galapagos finches).