Ch 23 Flashcards
microevolution
evolutionary changes in a population, the smallest scale of evolution
Microevolution occurs when there are changes in:
allele frequencies
Main mechanisms for allele frequency changes
- natural selection
- genetic drift
- gene flow
genetic variation
genetic differences among individuals
discrete characters
possibilities are either/or
quantitative characters
possibilities vary along a continuum
average heterozygosity
average percentage of loci that are heterozygous
alt: average percentage of population that are heterozygous for specific loci
geographic variation
genetic differences between separate populations
cline
graded change in a character along a geographic axis
population
individuals of the same species in the same area
gene pool
population’s genetic makeup
The gene pool contains:
all copies of
all alleles in
all members of a population
The Hardy-Weinburg principle describes:
the gene pool of a population that is not evolving
Hardy-Weinburg equation for two alleles
p^2 + 2pq + q^2 = 1
In the equation, p and q =
frequency of an allele
In the equation, p^2 and q^2 =
frequency of a genotype
Conditions for Hardy-Weinburg
- no mutations
- random mating
- no natural selection
- extremely large population size
- no gene flow
genetic drift
chance events cause unpredictable fluctuation in allele frequencies
Genetic drift has a greater effect on:
small populations
Two types of genetic drift:
founder and bottleneck effects
founder effect
when a few individuals become isolated and establish a new population whose gene pool differs from the source population
bottleneck effect
severe drop in population sizes that causes over-/underrepresentation or loss of alleles
gene flow
transfer of alleles into or out of population
Natural selection is the only mechanism that:
consistently causes adaptive evolution
adaptive evolution
betters match between the organism and the environment
relative fitness
contribution an individual makes to the gene pool of the next generation relative to others’ contributions
Types of selection
- directional
- disruptive
- stabilizing
directional selection
favors one extreme of a phenotypic range
disruptive selection
favors both extremes of a phenotypic range
stabilizing selection
favors intermediate variants of a phenotypic range
sexual selection
individuals with certain inherited traits are more likely to obtain mates
balancing selection
when natural selection maintains two or more forms in a population
heterozygote advantage
being heterozygous at a particular locus makes the organism best fit
frequency-dependent selection
fitness of a phenotype depends on how common it is in the population
Why don’t organisms become perfect through selection?
- selection only acts on existing variation
- limited by historical anatomy
- adaptations are often compromises
- chance and environment
