Mechanisms of Evolution Microevolution Population Genetics

A Population is a group of interbreeding organisms living together in space and time. (This means they are necessarily all the same species) A Population is The Smallest Unit of Evolution Individual organisms DO NOT evolve (in the Darwinian sense) Natural selection acts on individuals, but populations evolve What changes is the gene pool of the population, from generation to generation

Genetic variations in populations –Contribute to Natural Selection and are what is changed by natural selection Figure 23.1 Cuban Tree Snails

Key Concepts 23.1: Population genetics provides a foundation for studying evolution 23.2: Mutation and sexual recombination produce the variation that makes evolution possible 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition 23.4: Natural selection is the primary mechanism of adaptive evolution

Concept 23.1: Population genetics provides a foundation for studying evolution Microevolution –Is change in the genetic makeup of a population from generation to generation Figure 23.2

The Modern Synthesis Integrates Darwinian selection and Medelian inheritance and focuses on population genetics Population genetics (began in 1930’s) –Is the study of how populations change genetically over time –Reconciled Darwin’s and Mendel’s ideas

At the time The Origin of Species was published, little was known about inheritance. Darwin did not know how variation appeared or how it was transmitted His raw material for selection was variation in quantitative characters (vary along a continuum) Mendel’s characters were discrete Mendel’s inheritance was rediscovered in the early 1900’s, but it wasn’t until the 1930’s that scientists recognized that Darwin’s quantitative characters are genetically inherited

The Modern Synthesis was formulated in the 1940’s by many scientists. Ernst Mayr, biogeographer and systematist emphasized: –The population as the unit of evolution –Natural selection as the primary mechanism –Gradualism as an explanation of large changes resulting from the accumulation of small changes over long periods of time

Evolutionary science continues to develop Current debate focuses on the rate of evolution and on the importance of evolutionary mechanisms other than natural selection

Gene Pools and Allele Frequencies The genetic structure of a population is defined by its allele and genotype frequencies A population –Is a group of individuals of the same species living together in space and time A Species –is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature.

Most species are not evenly distributed over their geographic range They may have several localized population centers MAP AREA ALASKA CANADA Beaufort Sea Porcupine herd range Fairbanks Whitehorse Fortymile herd range NORTHWEST TERRITORIES ALASKA YUKON Figure 23.3 The population centers may be more or less isolated Even when centers are less isolated, individuals are still more likely to mate with others from their population center, so gene flow is reduced by the intermediate range

Population Gene Pool Is the total aggregate of genes in a population at any one time Consists of all gene loci in all individuals of the population Is made up of alleles that are combined to form the next generation –An allele is said to be “fixed” if all members of the population are homozygous for that gene –Normally there will be two or more alleles for each locus, each having a relative frequency in the gene pool

Gene Pool In diploid species, each individual will be homozygous or heterozygous for each locus because each locus is represented twice

The Hardy-Weinberg Theorem Describes a population that is not evolving States that the 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 inheritance Preserves genetic variation in a population Figure 23.4 Generation 1 C R genotype C W genotype Plants mate All C R C W (all pink flowers) 50% C R gametes 50% C W gametes Come together at random Generation 2 Generation 3 Generation 4 25% C R C R 50% C R C W 25% C W C W 50% C R gametes 50% C W gametes Come together at random 25% C R C R 50% C R C W 25% C W C W Alleles segregate, and subsequent generations also have three types of flowers in the same proportions Does not alter the frequency of alleles or genotypes

Example 500 Wildflower plants with two alleles for flower color (C R and C W ) 320 Homozygotes C R C R are red 160 Heterozygotes C R C W are pink 20 Homozygotes C W C W are white This means there are 800 C R alleles and 200 C W alleles

Example 500 diploid wildflowers have 1000 alleles 320 C R C R have 640 C R alleles 160 C R C W have 160 C R alleles for a total of C W C W have 40 C W alleles 160 C R C W have 160 C W alleles for a total of 200 The frequency of the C R allele is 0.8 and the frequency of the C W allele is 0.2

Hardy-Weinberg Equilibrium Describes a population in which random mating occurs Describes a population where allele frequencies do not change Describes a population that is NOT evolving

Hardy-Weinberg equilibrium p is the frequency of one allele and q is the frequency of the other allele If there are only two alleles then p + q = 1 Genotype frequencies are calculated using allele frequencies Figure 23.5 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% C R (p = 0.8)20% C W (q = 0.2) Sperm C R (80%) C W (20%) p2p2 64% C R 16% C R C W 16% C R C W 4% C W qp C R (80%) Eggs C W (20%) pq If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: q2q2 64% C R C R, 32% C R C W, and 4% C W C W Gametes of the next generation: 64% C R from C R C R homozygotes 16% C R from C R C W heterozygotes += 80% C R = 0.8 = p 16% C W from C R C W heterozygotes += 20% C W = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: 64% C R C R, 32% C R C W and 4% C W C W plants p2p2 4% C W from C W C W homozygotes

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then –p 2 + 2pq + q 2 = 1 –And p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype In cases of complete dominance p is usually reserved to represent the dominant allele and q represents the recessive allele

If p is 0.8 and q is 0.2 –Then p 2 is 0.64 –q 2 is 0.04 –2pq is 0.32 Figure 23.5 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% C R (p = 0.8)20% C W (q = 0.2) Sperm C R (80%) C W (20%) p2p2 64% C R 16% C R C W 16% C R C W 4% C W qp C R (80%) Eggs C W (20%) pq If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: q2q2 64% C R C R, 32% C R C W, and 4% C W C W Gametes of the next generation: 64% C R from C R C R homozygotes 16% C R from C R C W heterozygotes += 80% C R = 0.8 = p 16% C W from C R C W heterozygotes += 20% C W = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: 64% C R C R, 32% C R C W and 4% C W C W plants p2p2 4% C W from C W C W homozygotes

Conditions for Hardy- Weinberg Equilibrium The Hardy-Weinberg theorem –Describes a hypothetical population In real populations –Allele and genotype frequencies do change over time

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 Using the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease 1/400 African Americans have sickle-cell disease The frequency of the homozygous recessive genotype is (q 2 ) q = 0.05 is the frequency of the recessive allele p = 0.95 is the frequency of the dominant allele 2pq = is the frequency of the heterozygous carrier genotype in the African American population of the U.S.