POPULATION GENETICS OVERVIEW THE SMALLEST UNIT OF EVOLUTION

POPULATION GENETICS

OVERVIEW: THE SMALLEST UNIT OF EVOLUTION � Natural � On � only selection acts on individuals phenotypes populations evolve � Genetic variations contribute to evolution � Microevolution � change in the genetic makeup of a population from generation to generation

THE MODERN SYNTHESIS � Population � study genetics of how populations change genetically over time � integrates Mendelian genetics with the Darwinian theory of evolution by natural selection

GENE POOLS AND ALLELE FREQUENCIES • gene pool is the total aggregate of genes in a population at any one time � gene pool consists of all gene loci in all individuals of the population

MAP AREA CANADA ALASKA LE 23 -3 Beaufort Sea T ES S HW RIE RT TO NO RRI TE Porcupine herd range Fairbanks ALASKA YUKON Fortymile herd range Whitehorse

THE HARDY-WEINBERG THEOREM � describes a 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 � Mendelian population inheritance preserves genetic variation in a

LE 23 -4 Generation 1 X CRCR genotype Generation 2 Plants mate CW CW genotype All CRCW (all pink flowers) 50% CW gametes 50% CR gametes come together at random Generation 3 25% CRCR 50% CRCW 50% CR gametes 25% CWCW 50% CW gametes come together at random Generation 4 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

� If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then � p 2 + 2 pq + q 2 = 1 � p 2 and q 2 represent the frequencies of the homozygous genotypes � 2 pq represents the frequency of the heterozygous genotype �p +q=1 �p represents the frequency of one allele (dominant) � q represents the frequency of the other allele (recessive)

� 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

POPULATION GENETICS AND HUMAN HEALTH � We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease

VARIATION THAT MAKES EVOLUTION POSSIBLE � Mutation changes in the nucleotide sequence of DNA � cause new genes and alleles to arise � point mutation is a change in one base in a gene � � usually harmless but may have significant impact on phenotype Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful � Gene duplication is nearly always harmful � Mutation rates are low in animals and plants � � one � � mutation in every 100, 000 genes per generation Mutations are more rapid in microorganisms sexual recombination � far more important than mutation in producing the genetic differences that make adaptation possible

Deletion Duplication Inversion Translocation

A POPULATION’S GENETIC COMPOSITION � Three major factors alter allele frequencies and bring about most evolutionary change: � Natural selection � Genetic drift � Gene flow

LE 23 -7 CWCW CRCR CRCW Only 5 of 10 plants leave offspring CRCR CWCW CRCW CWCW CRCR CRCW CRCR CRCW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW) = 0. 3 CWCW CRCR Only 2 of 10 plants leave offspring CRCR CRCR CRCW Generation 2 p = 0. 5 q = 0. 5 CRCR Generation 3 p = 1. 0 q = 0. 0

LE 23 -8 Original population Bottlenecking event Surviving population

GENETIC VARIATION � Genetic variation occurs in individuals in populations of all species � It is not always heritable

LE 23 -9 Map butterflies that emerge in spring: orange and brown Map butterflies that emerge in late summer: black and white

POLYMORPHISM � Phenotypic polymorphism �a population in which two or more distinct morphs for a character are represented in high enough frequencies to be readily noticeable � Most species exhibit geographic variation differences between gene pools of separate populations or population subgroups

LE 23 -11 Heights of yarrow plants grown in common garden Mean height (cm) 100 50 Altitude (m) 0 3, 000 2, 000 1, 000 Sierra Nevada Range 0 Seed collection sites Great Basin Plateau

SELECTION � Selection favors certain genotypes by acting on the phenotypes of certain organisms � Three modes of selection: � Directional � Disruptive � Stabilizing

� Directional � favors selection individuals at one end of the phenotypic range � Disruptive selection � favors individuals at both extremes of the phenotypic range � Stabilizing � favors selection intermediate variants and acts against extreme phenotypes

Frequency of individuals LE 23 -12 Original population Evolved population Directional selection Original population Phenotypes (fur color) Disruptive selection Stabilizing selection

THE PRESERVATION OF GENETIC VARIATION � Diploidy � maintains genetic variation in the form of hidden recessive alleles � Balancing selection � occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population � leads to a state called balanced polymorphism � Heterozygote Advantage � maintains two or more alleles at that locus � Ex. sickle-cell allele � causes mutations in hemoglobin � also confers malaria resistance

LE 23 -13 Frequencies of the sickle-cell allele 0– 2. 5% 2. 5– 5. 0% 5. 0– 7. 5% Distribution of malaria caused by Plasmodium falciparum (a protozoan) 7. 5– 10. 0% 10. 0– 12. 5% >12. 5%

� In frequency-dependent selection � fitness of any morph declines if it becomes too common in the population

LE 23 -14 On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue a new set of images (a new feeding opportunity). Parental population sample 0. 6 Phenotypic variation Experimental group sample 0. 5 0. 4 Frequencyindependent control 0. 3 0. 2 0 Plain background Patterned background 20 40 60 Generation number 80 100

SEXUAL SELECTION � Sexual selection � natural selection for mating success � can result in sexual dimorphism � marked differences between the sexes in secondary sexual characteristics � Intrasexual selection � competition among individuals of one sex for mates of the opposite sex � Intersexual selection � occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex � Selection may depend on the showiness of the male’s appearance