Evolution and speciation explain how heritable variation, molecular evidence, selection, isolation, and chromosome change produce population divergence and new species over time.
Nearly all cellular life stores heredity in DNA, uses an almost universal genetic code, translates RNA on ribosomes, transfers energy with ATP and shares core metabolic chemistry. This deep unity is evidence of common ancestry.
| Observation in homologous sequences | Supported inference |
|---|---|
| many conserved positions | inheritance from a shared ancestor |
| fewer sequence differences between two taxa | usually a more recent common ancestor |
| differences accumulated after a split | evidence of divergence, interpreted with the gene and model used |
Sequence similarity is evidence, not a complete history by itself. The sequences must be homologous, sampling must be adequate, and different genes or evolutionary models can yield different estimates.
A breeding population already contains heritable variation produced by mutation and recombination. The breeder does not create the desired allele by wanting it.
Individuals with a desired phenotype are chosen as parents, so alleles contributing to that phenotype are overrepresented among their offspring.
Repeating the choice over generations shifts the population mean and can produce striking breeds or crop forms, as in pigeons, Brassica vegetables and maize.
Artificial and natural selection act on inherited variation, but the selecting agent differs: human preference in breeding, and differential survival or reproduction in the environment.
| Pattern | Origin | Evolutionary process | Example |
|---|---|---|---|
| Homologous structures | one ancestral structure, modified for different functions | divergent evolution | pentadactyl forelimbs for grasping, flight, running or swimming |
| Analogous structures | independent structures shaped by similar selection | convergent evolution | bat and insect wings as flight surfaces |
Similarity of function does not establish close ancestry. Classify evolutionary relationships using homologous features and independent molecular evidence; use analogous features to explain how similar niches can produce similar solutions.
One ancestral species begins as populations exchanging alleles through reproduction; gene flow tends to keep their gene pools similar.
A geographic, ecological, temporal, behavioural or chromosome barrier reduces allele exchange, so the populations no longer evolve as one fully connected gene pool.
Mutation, genetic drift and different selection pressures alter allele frequencies independently on each side of the barrier.
If reproductive isolation becomes strong enough that the lineages no longer exchange genes and produce fertile offspring, one pre-existing species has split into two or more species.