Evolution below the species level results from variation and from natural selection
and the action of other forces upon this variation.
Selection occurs whenever the chances of leaving offspring differ among
genotypes.
Microevolution is evolution below the species level.
It results from:
- Variation, brought about by mutation, chromosomal changes, and
genetic recombination through mating.
- Selection, genetic drift, and other forces that act upon variation.
- Reproductive isolation (topic #64).
Selection: Genotypes contribute genes unequally to the next generation,
either by differences in mortality and survival, by differences in mating
success, or by differences in fertility and fecundity (leaving offspring).
- Natural selection is success by natural processes. The peppered moths of
England, selected by predators (birds), are an example.
- Artificial selection is selection of captive species by humans.
- Sexual selection is selection based on success in mating.
- Selection against a dominant trait can eliminate the trait rapidly.
- Selection against a recessive trait works very slowly and becomes
much less effective once the recessive allele becomes rare.
- Selection against heterozygotes can result in either allele becoming
lost and the other taking over 100% of the gene pool.
- Selection favoring heterozygotes over both types of homozygotes
results in balanced polymorphism in which both alleles persist
indefinitely. Sickle-cell anemia is an example of this situation.
- Directional selection shifts the population mean.
- Disruptive selection increases population variance.
- Centripetal or stabilizing selection reduces variance.
Genetic drift: In smaller populations, gene frequencies can fluctuate
randomly in either direction simply by chance.
Geographic variation: Natural selection in different environments
causes populations to differ. Gene flow reduces the opportunities
for populations to differ; restricted gene flow allows enhanced differences.
Populations of some geographically widespread species may differ so much
that they may become unable to interbreed.
- If barriers to breeding accompany differences in visible traits,
the species may become divided into subspecies.
- Continuous geographic variation is usually described in terms
of clines (character gradients across a map).
- Geographic variation is usually the first step in species formation.
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