Ch 23: Evolution of Populations

microevolution

• change in allele frequencies in a population over generations

• caused by natural selection, genetic drift, and gene flow

genetic variation

differences among individuals in the composition of their genes or other DNA segments

average heterozygosity

the average percentage of loci that are heterozygous

geographic variation

• differences in genetic composition of separate populations

• populations separated from one another will evolve independently and accumulate differences

cline

• graded change in characteristics along a geographic axis

  • could be due to environment or natural selection

population

a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring

gene pool

• characteristics in a population’s genetic makeup

• consists of all copies of every type of allele at every locus in all members of the population

Hardy-Weinberg Principle

the frequency of alleles and genotypes in a population will remain constant, provided that only Mendelian segregation and recombination of alleles are at work

genetic drift

• chance events can cause allele frequency to fluctuate unpredictably from one generation to the next

• has significant impact on small populations

founder effect

when some members of a population get isolated from a larger population and establish a new population whose gene pool differs from the source population

bottleneck effect

• by only chance, certain alleles become over represented, under represented, or absent in survivors of a catastrophe

• even if a population recovers in size, it may suffer from low genetic diversity for a long time

• genetic drift suffering more likely

gene flow

• transfer of alleles in or out of a population via the movement of fertile individuals or their gametes

• usually decreases genetic differences between populations

• can increase or decrease adaptation to environment

relative fitness

the contribution of an individual to the gene pool relative to the contributions of others

directional selection

• when conditions favor individuals of one extreme of a phenotype, shifting the frequency curve in one direction

• common when the environment changes, or a population migrates

disruptive selection

favors both ends of a phenotypic range over the intermediate

stabilizing selection

• acts against both extreme phenotypes and favors the intermediate

• decreases variation

sexual selection

individuals with certain inherited characteristics are more likely than others to mate

sexual dimorphism

• there is a difference in secondary sexual characteristics between the two sexes

• a possible consequence of sexual selection

intrasexual selection

a form of sexual selection where individuals of one sex compete directly for mates of the opposite sex

intersexual selection

a form of sexual selection where individuals of one sex are choosy in selecting a mate

balancing selection

• where natural selection maintains two or more forms in a population

• includes heterozygote advantage and frequency-dependent selection

heterozygote advantage

where heterozygotes are at an advantage over both varieties of homozygotes

frequency-dependent selection

when the fitness of a phenotype depends on how common it is in a population

why natural selection cannot perfect organisms

1. it can only act on existing variations

2. it is limited by historical constraints

3. adaptations are often compromises

4. it interacts with chance and the environment

effects of genetic drift

1. significant in small populations

2. causes allele frequency to change at random

3. can lead to a loss of genetic variation

4. can cause harmful alleles to become fixed