Evolution below the species level results from variation and from natural selection
and the action of other forces upon this variation. Genetic drift consists of random
changes in gene frequencies resulting from sampling error, especially in small populations.
Selection and restricted gene flow can lead to geographic variation,
which is often the prelude to the formation of new species.
Genetic variation in populations (See previous outline for details)
- Hardy-Weinberg equilibrium
- Departures from Hardy-Weinberg:
- mutations (unbalanced)
- migrations
- nonrandom mating (assortment, inbreeding)
- genetic drift
- natural selection
Microevolution is evolution below the species level.
It results from:
- Variation and its origins (sources of variability):
- Mutations in single genes
- Chromosomal rearrangements: inversions, translocations, etc.
- Variations in phenotypic expression
- Chromosome number changes
- Genetic recombination through mating
- Sexual & asexual reproduction
- Factors tending to reduce variation:
- Selection (see below)
- Genetic drift, including bottleneck and founder effects
- Factors that protect and preserve variation (persistence of variability):
- Inefficient selection against recessive traits, especially uncommon traits
- Frequency-dependent selection by predators and pathogens; also "rare male" effects
- Negative assortment in mating
- Balanced polymorphism (e.g., sickle-cell anemia)
- Gene flow between populations.
- Restricted gene flow, including reproductive isolation.
Selection often reduces variation. Genotypes contribute genes unequally to the next generation.
- Natural selection occurs by natural processes.
- 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— both alleles persist
indefinitely.
Example: Sickle-cell anemia
- Forms of selection for quantitative traits:
- Directional selection shifts the population mean.
examples: Aristelliger, corn, mice, humans
- Stabilizing (centripetal) selection: reduces variance
examples: Bumpus's sparrows, Aristelliger
- —often results in a compromise (e.g., human birth weight)
- Disruptive (centrifugal) selection (rare): increases variance
- Agents of selection:
- Artificial selection (by human agency)
- Natural selection (by natural agency)
- by predators
- by pathogens, parasites, & other diseases
- by starvation
- by environmental (climatic) extremes (of temperature, etc.)
- other physical agents: fire, landslides, etc.
- sexual selection
Genetic drift ("Sewall Wright effect"): In smaller populations,
gene frequencies can fluctuate
randomly in either direction simply by chance.
This also occurs for rare alleles in larger populations.
Important subtypes include:
- "Bottleneck effect" in populations temporarily small
- "Founder effect" among founders of a new population (e.g., Dunkers)
Microevolution *
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.
REVIEW:
Study guide and vocabulary
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