Evolution is supported by many independent lines of evidence that all point to the same conclusion: species share common ancestors and change over time. Fossils show that life has a history, while anatomy reveals patterns of similarity among organisms. Embryology, biogeography, and DNA evidence add more clues from development, location, and molecules.
This convergence matters because strong scientific explanations are supported by evidence from many different sources.
The fossil record preserves sequences of organisms through time, including transitional forms that connect major groups. Comparative anatomy shows homologous structures, such as the bones in human arms, whale flippers, and bat wings, which suggest inheritance from a common ancestor. Molecular biology provides especially detailed evidence because DNA and protein similarities can be measured and compared.
When fossils, body structures, embryos, geographic patterns, and genetic data agree, they form a powerful case for common descent and evolutionary change.
Key Facts
- Evolution means changes in inherited traits of populations across generations.
- Common descent means different species can trace back to shared ancestral populations.
- Homologous structures have similar underlying anatomy because of shared ancestry, even if their functions differ.
- Fossil age can be estimated using radioactive decay: N = N0(1/2)^(t/T), where T is the half-life.
- DNA similarity is evidence of relatedness because mutations accumulate over generations in inherited genetic material.
- A simple molecular clock model is genetic differences = mutation rate x time since divergence.
Vocabulary
- Fossil record
- The collection of preserved remains, traces, and impressions of past organisms found in rock layers.
- Homologous structure
- A body part shared by different species because it was inherited from a common ancestor.
- Vestigial structure
- A reduced or modified structure that has lost much or all of its original ancestral function.
- Biogeography
- The study of where species live now and where their ancestors lived in the past.
- Molecular evidence
- Evidence for evolutionary relationships based on comparisons of DNA, RNA, or protein sequences.
Common Mistakes to Avoid
- Saying individual organisms evolve, which is wrong because evolution describes changes in populations over generations, not changes within one lifetime.
- Treating fossils as the only evidence for evolution, which is wrong because anatomy, embryos, geography, and DNA also provide independent evidence.
- Assuming similar-looking structures are always homologous, which is wrong because some similarities result from convergent evolution rather than shared ancestry.
- Thinking evolution is a straight ladder from simple to complex, which is wrong because evolution is a branching pattern with many lineages adapting in different directions.
Practice Questions
- 1 A fossil layer is dated using a radioactive isotope with a half-life of 5,000 years. If 25% of the original isotope remains, how old is the fossil layer?
- 2 Two species have DNA sequences that differ at 12 positions. If the mutation rate used for this gene is 2 differences per million years, estimate the time since they diverged using genetic differences = mutation rate x time.
- 3 Explain why the forelimbs of a human, bat, whale, and cat are strong evidence for common descent even though they are used for different functions.