Evolution is a term denoting all genetic changes that have transformed life on Earth from its beginning until today. Small genetic changes may lead to adaptation, microevolution, while big genetic changes may lead to speciation, macroevolution.
Darwinian evolution is evolution by natural selection:
- Genotypes within populations vary and this variability is heritable.
- Biotic and abiotic components of an organism’s environment act as selection pressures.
- Genotypes that are best adapted to these selection pressures leave the most offspring.
Only the mechanism of selection generates adaptation. The other mechanisms generate genetic variation at various levels of taxa, but the genetic variation has no linkage to improved survival in the environment.
Prokaryotes and eukaryotes have partly different mechanisms causing genetic variability:
- Prokaryotes, in contrast to eukaryotes, may transfer genetic material horizontally.
- Prokaryotes are asexual organisms and leave clones, not - as eukaryotes - offspring with sexually recombined genotypes.
Prokaryotes are haploid. Therefore the genotype of an individual reflects the phenotype. Since most eukaryotes are diploid they may carry recessive genes that are not expressed and hence “hidden” from selection pressure.