Evolution of Populations

The Smallest Unit of Evolution

• Natural selection acts on individuals, but only populations evolve– Genetic variations contribute to evolution

Population genetics

• Population genetics – study of how populations change genetically

over time• Mendelian genetics with the Darwinian

theory• populations as units of evolution

Gene Pools and Allele Frequencies

• Population • localized group of individuals capable of

interbreeding and producing fertile offspring

• gene pool – total aggregate of genes in a population at

any one time– all gene loci in all individuals of the

population

The Hardy-Weinberg Theorem

• population that is not evolving• frequencies of alleles and genotypes in a

population’s gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work

• preservation of genetic variation in a population

Hardy-Weinberg Equilibrium

• The five conditions for non-evolving populations are rarely met in nature:– Extremely large population size– No gene flow– No mutations– Random mating– No natural selection

Hardy-Weinberg Equilibrium

• If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then– p2 + 2pq + q2 = 1– And p2 and q2 represent the frequencies of the

homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

LE 23-4

Generation3 25% CRCR

Generation4

50% CRCW 25% CWCW

50% CW

gametes50% CR

come together at random

25% CRCR 50% CRCW 25% CWCW

Alleles segregate, and subsequentgenerations also have three typesof flowers in the same proportions

gametes

Generation2

Generation1

CRCR CWCW

genotypegenotypePlants mate

All CRCW

(all pink flowers)

50% CR 50% CW

gametes gametes

come together at random

X

Evolutionary Change

• Three major factors alter allele frequencies and bring about most evolutionary change:– Mutations– Natural selection– Nonrandom Mating– Genetic drift– Gene flow

Variations that make Natural Selection Possible

• Mutation– changes in the nucleotide sequence of DNA– new genes and alleles to arise– Point Mutations• change in one base in a gene• usually harmless • may impact on phenotype

Mutations

• Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful

• Gene duplication is nearly always harmful

Natural Selection

• Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

• 3 conditions for natural selection to occur and to result in evolutionary change1. Variation must exist among individuals in a

population2. Variation among individuals must result in

differences in the number of offspring surviving in the next generation

3. Variation must be genetically inherited

14

Sexual Recombination

• far more important than mutation • produces the genetic differences that make

adaptation possible

• Nonrandom mating– Assortative mating

• Phenotypically similar individuals mate

• Increases proportion of homozygous individuals

– Disassortative mating• Phenotypically different

individuals mate• Produces excess of

heterozygotes

Genetic Drift

• The smaller a sample, the greater the chance of deviation from a predicted result

• allele frequencies fluctuate unpredictably from one generation to the next

• reduces genetic variation through losses of alleles

Genetic Drift• The Bottleneck Effect– sudden change in the

environment that may drastically reduce the size of a population

– gene pool may no longer be reflective of the original population’s gene pool

Genetic Drift

• The Founder Effect– a few individuals

become isolated from a larger population

– affects allele frequencies

Gene Flow

• genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes

• gain or loss of alleles• reduce differences between populations over

time

A Closer Look at Natural Selection

• From the range of variations available in a population, natural selection increases frequencies of certain genotypes, fitting organisms to their environment over generations

Evolutionary Fitness

• Misleading– “struggle for existence” – “survival of the fittest”

• Fitness – contribution an individual makes to the gene pool of the

next generation, relative to the contributions of other individuals

• Relative fitness – contribution of a genotype to the next generation,

compared with contributions of alternative genotypes for the same locus

Directional, Disruptive, and Stabilizing Selection

• Selection favors certain genotypes by acting on the phenotypes of certain organisms

• Three modes of selection:– Directional

• favors individuals at one end of the phenotypic range

– Disruptive• favors individuals at both extremes of the phenotypic range

– Stabilizing• favors intermediate variants and acts against extreme

phenotypes

The Preservation of Genetic Variation

• Diploidy – maintains genetic variation in the form of hidden

recessive alleles• Balancing selection – natural selection maintains stable frequencies of

two or more phenotypic forms

• Heterozygote Advantage – Some individuals who are heterozygous at a

particular locus have greater fitness than homozygotes

– Natural selection will tend to maintain two or more alleles at that locus

– Sickle cell and malaria

• Sexual selection – natural selection for mating success– sexual dimorphism

• differences between the sexes in secondary sexual characteristics

• Intrasexual selection – competition among individuals of one sex for mates of the

opposite sex• Intersexual selection – individuals of one sex (usually females) are choosy in

selecting their mates from individuals of the other sex

Why Natural Selection Cannot Fashion Perfect Organisms

• Evolution is limited by historical constraints• Adaptations are often compromises• Chance and natural selection interact• Selection can only edit existing variations