BIOL 101 Lecture Notes - Tetrodotoxin, Stabilizing Selection, Sexual Selection
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What Are the Mechanisms of Evolutionary Change?
•Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are
•Known evolutionary mechanisms:
Mutations Generate Genetic Variation
•Origin of genetic variation is mutation; mutation is any change in an organism’s DNA
•Most mutations are harmful to their bearers or are neutral, but if environmental conditions
change, previously harmful or neutral alleles may become advantageous
•Mutations can restore to populations alleles that other evolutionary processes have
•Most mutations appear to be random and are harmful or neutral to their bearers.
•Some mutations can be advantageous.
•Mutation rates are low; one out of a million loci is typical.
•Although mutation rates are low, they are sufficient to create considerable genetic
•Rates as high as one mutation per locus in a thousand zygotes per generation are rare; one
in a million is more typical
•One condition for Hardy–Weinberg equilibrium is that there is no mutation.
•Although this condition is never met, the rate at which mutations arise at single loci is
usually so low that mutations result in only very small deviations from Hardy–Weinberg
•If large deviations (from H-W expectations) are found, it is appropriate to dismiss mutation
as the cause and look for evidence of other evolutionary agents.
Gene flow may change allele frequencies
•Gene flow results when individuals migrate to another population and breed in new
•No immigration is allowed for a population to be in Hardy–Weinberg equilibrium.
Genetic drift may cause large changes in small populations
•Genetic drift is the random loss of individuals (and their alleles)-may produce large
changes in allele frequencies from one generation to the next
•In very small populations, genetic drift may be strong enough to influence the direction of
change of allele frequencies even when other evolutionary agents are pushing the
frequencies in a different direction.
•Organisms that normally have large populations may pass through occasional periods
when only a small number of individuals survive (a population bottleneck). Genetic
variation can be reduced by genetic drift.
oPopulation bottlenecks occur when only a few individual survive a random event,
resulting in a shift in allele frequencies within the population
•Founder effect- random changes in allele frequencies resulting from establishment of a
population by a very small number of individuals
oWhen a few pioneering individuals colonize a new region, the resulting population
will not have all the alleles found among members of the source population.
Nonrandom Mating Changes Genotype Frequencies
•Nonrandom mating occurs when individuals mate either more often with individuals of
the same genotype or more often with individuals of a different genotype.
•The resulting proportions of genotypes in the following generation differ from Hardy–
•If individuals mate preferentially with other individuals of the same genotype, homozygous
genotypes are overrepresented and heterozygous genotypes are underrepresented in the
•Conversely, individuals may mate preferentially with individuals of a different genotype