The Evolution of Populations and the Origin of Species Chapters 23-24
Evolution happens to populations/ species over time. • Lamarckian evolution - evolution result of change in individual in response to environment (i.e. giraffe stretching its neck to eat) - incorrect hypothesis.
Population composed of many different genotypes, phenotypes because of alleles carried in population. • Sum total of all alleles in population - gene pool; variation in gene pool - variations in individual phenotypes.
Measure of genetic variation in population - allele frequency of gene. • # of copies of allele divided by total # of copies of gene in population.
If population does not change - Hardy-Weinberg equilibrium – no evolution. • Frequencies change - evolution occurring.
Conditions for population to stay at Hardy-Weinberg equilibrium: • 1Random mating occurs. • 2Population large enough to avoid random statistical fluctuations in frequencies. • 3No mutation.
4No migration into/out of population. • 5No natural selection. • Under conditions - free flow of genes between members of same species. • Alleles shuffled up from 1 generation to next.
In wildflower population of 500, 80% (0.8) of flower color alleles are R and 20% (0.2) are r. • Each gamete - 1 allele for flower color; gamete drawn from gene pool at random has 0.8 chance of bearing R allele, 0.2 chance of bearing r allele.
Rule of multiplication - frequencies of 3 possible genotypes in next generation. • RR genotype - probability of picking 2 R alleles is 0.64 (0.8 x 0.8 = 0.64 or 64%). • rr genotype - probability of picking 2 r alleles is 0.04 (0.2 x 0.2 = 0.04 or 4%).
Heterozygous individuals - either Rr or rR - R allele from sperm or egg. • Probability of ending up with both alleles is 0.32 (0.8 x 0.2 = 0.16 for Rr, 0.2 x 0.8 = 0.16 for rR, and 0.16 + 0.16 = 0.32 or 32% for Rr + rR).
p = gene frequency of dominant allele, q = frequency of recessive allele; p + q = 1. • Equation for Hardy-Weinberg principle is: • p2 + 2pq + q2 = 1 • 2pq - # heterozygotes in population.
Hardy-Weinberg equilibrium, frequency of dominant homozygous curly hair (CC) is 64%. Percentage with curly hair? • p = frequency dominant allele (C) q frequency recessive allele (c).
CC frequency 64% so p2 = .64; p = .8. • (p + q = 1) q = 1 - .8 = 2. • Individual with curly hair - either CC or Cc. • Percentage of population with curly hair is p2 + 2pq = .64 + 2(.8*.2) = .96 or 96% of population.
Instabilities in populations • Conditions can change Hardy-Weinberg equilibrium. • Mutations cannot happen in equilibrium; occurs in real world. • Errors in DNA replication accumulate over time as well as mutagenic factors in environment. • Mutations can lead to new alleles not previously in gene pool.
Mutations either neutral or harmful on survival of individual. • New phenotypes in population raw material that natural selection acts on to drive evolution; mutations only source of new alleles. • Migration affects equilibrium. • Different populations have different allelic frequencies in gene pools.
1 population breeds with another population, frequencies of alleles change (gene flow). • Small population more likely to have random event than large population.
Genetic drift - changes in allele frequencies in small population caused by random events. • Even in large population if small # of individuals pass on traits can decrease diversity. • Individuals that do not pass traits on may have harmful alleles - alters gene pool of next generation (2 ways)
1Bottleneck - large population reduced to small # by disease, natural disaster, over-hunting/fishing. • Individuals left eventually reproduce, generations not representative of original gene pool.
Inbreeding usually follows bottleneck; individuals with same recessive genes have more chance of passing harmful gene on. • Population more susceptible to disease/infections that may not have occurred with more diversity in population.
2Founder effect - small # of individuals of species migrate into new habitat. • If only a few individuals colonize new area, new population reflect only their gene pool not larger gene pool where they came from.
Nonrandom mating - equilibrium cannot occur. • Individuals must choose mate randomly without respect to phenotype. • If phenotype influences selection, genotypes and phenotypes of population will be changed.
Self-fertilization in plants has this effect. • Reduces # of heterozygotes in population; increases # of homozygotes. • Many species exhibit sexual selection (form of nonrandom mating)
Natural selection - differential production of offspring based on inherited traits. • Individuals with more favorable phenotypes may survive, reproduce; alters population frequencies. • Fitness - key description of natural selection.
Fitness - organism’s ability to contribute alleles, traits to future generations. • Factors involved - ability to survive to reproductive age, mate and produce offspring, raise offspring to maturity. • Other factors - ability to escape predation, gather food, attract mates, or care provided to offspring.
Individual with long life but few offspring - poor fitness if other individuals have more offspring. • Animals that take care of offspring - greater fitness than those that do not. • Balanced by having more offspring that receive little care/fewer offspring that receive more care.
Three types of selective pressures that affect natural selection over time. • Any given population, trait distribution bell-shaped.
1Stabilizing selection - not change average, tends to sharpen curve. • Newborns can have problems if too large or too small at birth – stabilizing selection pushed average to 8 pounds; perfect size for newborn infant.