Evidence for evolution
- 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
- 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.
The evidence for evolution
Anatomy and embryology
Determining relationships from similar features
- The same genetic material (DNA)
- The same, or highly similar, genetic codes
- The same basic process of gene expression (transcription and translation)
- The same molecular building blocks, such as amino acids
Direct observation of microevolution
- Before DDT was applied, a tiny fraction of mosquitos in the population would have had naturally occurring gene versions (alleles) that made them resistant to DDT. These versions would have appeared through random mutation, or changes in DNA sequence. Without DDT around, the resistant alleles would not have helped mosquitoes survive or reproduce (and might even have been harmful), so they would have remained rare.
- When DDT spraying began, most of the mosquitos would have been killed by the pesticide. Which mosquitos would have survived? For the most part, only the rare individuals that happened to have DDT resistance alleles (and thus survived being sprayed with DDT). These surviving mosquitoes would have been able to reproduce and leave offspring.
- Over generations, more and more DDT-resistant mosquitoes would have been born into the population. That's because resistant parents would have been consistently more likely to survive and reproduce than non-resistant parents, and would have passed their DDT resistance alleles (and thus, the capacity to survive DDT) on to their offspring. Eventually, the mosquito populations would have bounced back to high numbers, but would have been composed largely of DDT-resistant individuals.
- Homologous structures provide evidence for common ancestry, while analogous structures show that similar selective pressures can produce similar adaptations (beneficial features).
- Similarities and differences among biological molecules (e.g., in the DNA sequence of genes) can be used to determine species' relatedness.
- Biogeographical patterns provide clues about how species are related to each other.
- The fossil record, though incomplete, provides information about what species existed at particular times of Earth’s history.
- Some populations, like those of microbes and some insects, evolve over relatively short time periods and can observed directly.