What evidence shows that species share common ancestry and change over time?
Describe the lines of evidence that support evolution, including fossils, comparative anatomy, biochemistry and biogeography
Multiple independent lines of evidence support evolution: the fossil record, comparative anatomy (homologous structures), molecular biology (DNA and protein similarity) and biogeography.
Reviewed by: AI editorial process; not yet individually human-reviewed
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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.
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).
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 SACE Stage 22 marksStromatolites are sedimentary rocks formed by primitive photosynthesising prokaryotic cells. Describe how fossils in stromatolites may be used as evidence that prokaryotic cells existed on Earth before eukaryotic cells.Show worked answer →
For 2 marks, link fossil age (dating) to the order of appearance.
Stromatolite fossils containing prokaryotic cells are found in the oldest (deepest) rock layers and can be radiometrically dated to about 3.5 billion years ago.
Fossils of eukaryotic cells appear only in much younger rock layers. Because prokaryotic fossils are older than any eukaryotic fossils, this is evidence that prokaryotic cells existed before eukaryotic cells.
2019 SACE Stage 23 marksEukaryotic cells are thought to have evolved from prokaryotic cells by endosymbiosis. Describe one structural feature of a mitochondrion, and explain how it provides evidence of an endosymbiotic event.Show worked answer →
Choose one feature and connect it to the free-living prokaryotic ancestor.
Feature: a mitochondrion has its own circular DNA (and its own ribosomes), and a double membrane.
Explanation: the circular DNA and ribosomes resemble those of free-living prokaryotes (bacteria) rather than the eukaryotic host. The inner membrane is thought to be the original prokaryote's membrane and the outer membrane from the host engulfing it. This supports the idea that the mitochondrion was once a free-living prokaryote taken up by a larger cell, an endosymbiotic event.
2018 SACE Stage 21 marksA table shows the same part of a short DNA sequence for four related species A, B, C and D, where A and B differ at only one base. Based only on this information, which two species are likely to have shared a common ancestor most recently?Show worked answer →
Species A and B. The fewer the differences between two species' DNA sequences, the more recently they shared a common ancestor, because fewer mutations have accumulated since they diverged. Species A and B differ at only a single base (the smallest difference shown), so they are the most closely related and shared a common ancestor most recently.
2019 SACE Stage 21 marksCopies of DNA from species X were hybridised with homologous DNA from species A, B, C and D, and the separation temperatures were 30, 40, 70 and 80 degrees C respectively. Which statement is supported by the data: A and X have the least percentage difference in DNA; A and B together have the same difference from X as C does; species C is more closely related to D than to X; or D and X have the least percentage difference in DNA?Show worked answer →
The supported statement is that D and X have the least percentage difference in their DNA. In DNA hybridisation, more complementary base pairing means more hydrogen bonds, so more closely related species (more similar DNA) need a higher temperature to separate.
The D-X hybrid separates at the highest temperature (80 degrees C), so D and X have the most matching bases and therefore the least percentage difference, making them the most closely related pair. (The A-X hybrid separates at the lowest temperature, so A and X are the least similar.)