Evaluate the evidence for evolution from fossils, comparative anatomy, biochemistry and biogeography

What this dot point is asking

SCSA wants you to describe each line of evidence, explain what it shows, and recognise that the strength of the theory comes from many independent lines pointing to the same conclusion. A strong answer links each type of evidence to common descent.

The fossil record

Fossils are the preserved remains or traces of past organisms. Arranged by the age of the rock layers in which they are found, they show that life has changed over time, with simpler forms in older rocks and more complex or modern forms in younger rocks. Transitional fossils, which show features intermediate between groups, document the gradual change from one form to another. The fossil record is incomplete because fossilisation is rare, but the patterns it does show are consistent with evolution.

Comparative anatomy

Comparing the body structures of different species reveals telling patterns.

• Homologous structures have the same basic underlying structure but different functions, such as the bone pattern shared by a human arm, a whale flipper and a bat wing. Their shared plan points to a common ancestor.
• Vestigial structures are reduced remnants of organs that were functional in ancestors, such as the pelvic bones of whales.

Biochemical and molecular evidence

All living things use DNA, the same genetic code, and many of the same proteins, which itself points to a common origin. More precisely, the more closely related two species are, the more similar their DNA and protein sequences. By comparing sequences such as cytochrome c or DNA itself, biologists can measure relatedness and build evolutionary trees. This molecular evidence often confirms relationships first suggested by anatomy.

Biogeography

Biogeography is the study of where organisms live and why. Isolated regions tend to have unique organisms found nowhere else, which makes sense if those organisms evolved in isolation from a few ancestral colonisers.

Why multiple lines matter

No single line of evidence proves evolution on its own; the strength of the theory comes from the agreement of many independent lines. Fossils show change over time, anatomy shows shared ancestry, biochemistry quantifies relatedness, and biogeography explains distribution. When evidence collected by different methods converges on the same conclusion of common descent, the explanation becomes very well supported.

Linking evidence to the mechanisms

The evidence for evolution and the mechanisms of evolution fit together. Natural selection, drift, gene flow and speciation explain how change happens; fossils, anatomy, biochemistry and biogeography are the record of that change having happened. Understanding both means you can explain not only that species share ancestry but how and why their gene pools diverged over time.