Mechanism of evolution: mutation, recombination, gene flow and drift
Variation comes from mutation and recombination; gene flow and genetic drift change allele frequencies; natural selection then shapes the population. Together these form the modern synthetic theory.
Darwin's variations vs de Vries' mutations
Later studies in population genetics combined both ideas: mutations supply variation, and selection acts on it.
The modern synthetic theory
The modern synthetic theory (also called neo-Darwinism) joins Darwin's natural selection with Mendel's genetics and population genetics. It sees evolution as a change in allele (gene) frequencies in a population over generations. Your textbook names five factors that change allele frequencies: gene flow (gene migration), genetic drift, mutation, genetic recombination and natural selection. Extra, not in your textbook: many reference books also list changes in chromosome number and structure and reproductive isolation, which keeps new groups apart so they can become separate species.
What changes allele frequencies
| Factor | What happens | Effect |
|---|---|---|
| Mutation | A sudden heritable change in a gene or chromosome | Brings in new alleles; the raw material of evolution |
| Recombination | Crossing over and independent assortment in meiosis, and fusion of gametes | New combinations of existing alleles in each generation |
| Gene flow (gene migration) | Individuals or gametes move between populations many times | Adds alleles to the new population and removes them from the old |
| Genetic drift | A chance change in allele frequency, mainly in small populations | Can fix or lose an allele regardless of its usefulness |
| Natural selection | Better-adapted variants leave more offspring | Raises the frequency of useful alleles |
Gene flow vs genetic drift
From variation to a new species
Microbial experiments show that advantageous mutations already present, when selected, give new phenotypes; over generations this can lead to speciation. Reproductive isolation is an extra step, not named in your textbook.
When a few tigers are moved to start a new population in another Indian reserve, they carry only some of the original alleles: a real founder effect. Wildlife scientists move animals between reserves to keep gene flow going.
2 marks: “What is genetic drift? How does the founder effect arise?” Genetic drift is a chance change in allele frequencies, mainly in small populations. When a small group moves away and starts a new population, its allele frequencies differ sharply: the founder effect.
3 marks: “How did Hugo de Vries' idea of evolution differ from Darwin's?” De Vries (evening primrose): large sudden mutations, random and directionless, saltation, single-step speciation. Darwin: small heritable variations, directional, gradual evolution.
3 marks: “State the modern synthetic theory of evolution and list its main factors.” Evolution is a change in allele frequencies over generations, joining natural selection with genetics. Textbook factors: mutation, genetic recombination, gene flow, genetic drift, natural selection. (Reference books also add chromosomal changes and reproductive isolation.)
MR GG NS: Mutation and Recombination make variety; Gene flow and Genetic drift shuffle it; Natural Selection sorts it.