Chapter 6: the ways of change; drift and selection

Polyphenic

a single genotype produces distinct alternative phenotypes in response to environmental conditions

EX) temperature dependent sex determination in reptiles; ants cast differentiation

Polymorphic

presence of 2 or more genetically determined phenotypes. Not primarily induced by environment.

-multiple genotypes producing multiple phenotypes.

—Flower colors

—Blood type in humans

Polygenic

trait that is controlled by 2 or more genes, where each gene contributes a small additive effect to overall phenotype— influenced by environment

EX) human height

Qualitative traits

-very little environmental influence

-ex) shape of the pea, round or wrinkled

Quantitive traits

continues variation

-controlled by multiple genes

-relatively higher influence of environment

EX) human height

Population

group of interacting and potentially interbreeding individuals of the same species

Genetic locus

location of a specific gene or sequence of DNA on a chromosome

Homozygous

individual carries 2 copies of the same allele @ a locus

Heterozygous

individual carries different copies of the same alleles at a locus

Population genetics

the study of the distribution and frequencies of alleles in populations, and how and why allele frequencies change.

Hardy-Weinberg equilibrium

(no evolution)— random mating. no migration, genetic drift, mutation or natural selection.

Result: allele frequencies do not change

-principle stating that genetic variation in a pop. will remain constant from one generation to another in absence of disturbing factors.

no change if (assumptions—incorrect):

-pop. large

-genotypes do not confer differences in fitness

-mating is random

-no migration

—serves as fundamental null model in pop. genetics

—useful because it provides mathematical proof that evolution will not occur in absence of selection, drift,migration, or mutation.

Hardy-weinberg equilibrium -2

p² + 2pq + q² = 1

p= big “A”

q= little “a”

p²= represents AA (homozygous)

q²= represents aa (homozygous)

2pq= Aa (heterozygous)

-The sum of the allele frequencies for all alleles at a locus must be 1 p +q= 1

Genetic drift

a powerful mechanism of evolution in small populations

-smaller sample; higher error—small pop. experiences strong drift

-drift reduces genetic variation

-some alleles become fixed in the pop. while others disappear

Bottleneck effect

results in non representative set of alleles for subsequent pop. even after the pop. size rebounds.

-allele making it thru

—1) frequency of the allele before the bottleneck

—2) the severity of the bottleneck

Relative fitness

contribution of individuals w/ one genotype compared w/ the average contribution of all individuals in a population.

fitness- survival and reproductive success of an individual w/ a particular phenotype.

—components:

-survival to reproductive age

-mating success

-fecundity

Negative frequency-dependent selection

common phenotypes are selected against, rare phenotypes are favored.

—heterozygote advantage: hetrozygosity confers greater fitness than homozygotes

Pleiotropy

mutation in a single gene affects more than one phenotypic trait

—pesticide resistance nad pleiptropy

Ester^1 gene increased in response to peptide use in coastal areas

Antagonistic pleiotropy

beneficial effects for one trait but detrimental effects for other traits

—net effect of fitness determines outcome of selection

EX) carrying this allele father inland proved detrimental in escaping predation

Balancing selection maintains multiple alleles in populations

-Negative frequency-dependent

-Heterozygote advantage