chapter 21
microevolution
a change in allele frequencies in a population over generations
evolution is observable
true
phenotype
the physical expression of the genotype
variation in an individual leads to
variation in an individual's phenotype
all phenotypic variation is heritable
false
natural selection can effect what
genetic components
variation within a population
how much genetic variation there is between individuals
population geneticists do what
measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels
the different measures of genetic variability
average heterozygosity
nucleotide variability
average heterzygosity
average percent of loci that are heterozygous in a population
nucleotide variability
comparison between the dna sequences of pairs of individuals
mutations
changes in nucleotide sequence of dna
mutations do
cause variability
only mutations in cells that produce gametes can be passed to offspring
sources of genetic variation
formation of new alleles by mutation
altering gene number or position
rapid reproduction increases mutation rate
sexual reproduction shuffles existing genetic variation
point mutation
a mutation that changes a single base in a gene
point mutations are often
harmful and result in deletion by natural selection
chromosomal mutations that alter or delete many loci are
usually harmful
neofunctionalization
duplicated genes that can take on new fuctions
mutation rates in plants and animals tend to be
low
mutation rates in prokaryotes tend to be
very low, but their rapid reproduction makes them mutate quickly
genetic drift
random change in allele frequencies in a population
genetic drift effects
significant on smaller populations
causes allele frequencies to change at random
can lead to a loss of genetic variation
can cause harmful alleles to become fixed
founder effect
when a few individuals get isolated from a larger population
bottleneck effect
when a population suddenly decreases is size due to a change in the environment
gene flow
alleles moving among populations transferred by movement of fertile individuals or gametes
3 modes of selection
1) directional
2) disruptive or diversifying
3) stabilizing
directional selection
favours individuals at one end of the phenotypic range
bell curve shifts over to one side
disruptive or diversifying selection
favours individuals at both extremes of the phenotypic range
bell curve splits into 2
stabilizing selection
favours intermediate variants and acts against extreme phenotypes
bell curve heightens
balancing selection
selection that preserves variation at some loci, maintaining two or more phenotypes in a population
heterozygous advantage
a way for balancing selection to arise
when heterozygotes have a higher fitness than either homozygote
frequency dependent selection
the fitness of an allele depends on its frequency in a population (declines when becomes too frequent)
sexual dimorphism
marked differences between the sexes in secondary sexual characteristics
types of sexual selection
intra and inter
intrasexual selection
competition among the same sex for opposite sex
intersexual competiton
individuals of one sex are choosy in their mate
4 reasons why natural selection cannot create a perfect organism
1) selection can act only on existing variations
2) evolution is limited by historical constraints
3) adaptations are often compromises
4) chance, natural selection, and the environment interact