The Evolution of Populations Chapter 23 The Evolution of Populations

Overview: The Smallest Unit of Evolution One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but only populations evolve Genetic variations in populations contribute to evolution Microevolution is a change in allele frequencies in a population over generations

Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

Genetic Variation Variation in individual genotype leads to variation in individual phenotype Not all phenotypic variation is heritable Natural selection can only act on variation with a genetic component

Variation Within a Population Both discrete and quantitative characters contribute to variation within a population Discrete characters can be classified on an either-or basis Quantitative characters vary along a continuum within a population

Variation Between Populations Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups

Fig. 23-3 1 2.4 3.14 5.18 6 7.15 8.11 9.12 10.16 13.17 19 XX 1 2.19 3.8 4.16 5.14 6.7 Figure 23.3 Geographic variation in isolated mouse populations on Madeira 9.10 11.12 13.17 15.18 XX

Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels Average heterozygosity measures the average percent of loci that are heterozygous in a population Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

Ldh-B b allele frequency Fig. 23-4 1.0 0.8 0.6 Ldh-B b allele frequency 0.4 0.2 Figure 23.4 A cline determined by temperature 46 44 42 40 38 36 34 32 30 Latitude (°N) Maine Cold (6°C) Georgia Warm (21°C)

Animation: Genetic Variation from Sexual Recombination Mutation Mutations are changes in the nucleotide sequence of DNA Mutations cause new genes and alleles to arise Only mutations in cells that produce gametes can be passed to offspring Animation: Genetic Variation from Sexual Recombination

Mutations That Alter Gene Number or Sequence Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Duplication of large chromosome segments is usually harmful Duplication of small pieces of DNA is sometimes less harmful and increases the genome size Duplicated genes can take on new functions by further mutation

Mutation Rates Mutation rates are low in animals and plants The average is about one mutation in every 100,000 genes per generation Mutations rates are often lower in prokaryotes and higher in viruses

Sexual Reproduction Sexual reproduction can shuffle existing alleles into new combinations In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible

Gene Pools and Allele Frequencies A population is a localized group of individuals capable of interbreeding and producing fertile offspring A gene pool consists of all the alleles for all loci in a population A locus is fixed if all individuals in a population are homozygous for the same allele

Porcupine herd Porcupine herd range Fortymile herd range Fig. 23-5 Porcupine herd MAP AREA CANADA ALASKA Beaufort Sea NORTHWEST TERRITORIES Porcupine herd range Figure 23.5 One species, two populations Fortymile herd range ALASKA YUKON Fortymile herd

The frequency of an allele in a population can be calculated For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

The frequency of all alleles in a population will add up to 1 By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies The frequency of all alleles in a population will add up to 1 For example, p + q = 1

The Hardy-Weinberg Principle The Hardy-Weinberg principle describes a population that is not evolving If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation In a given population where gametes contribute to the next generation randomly, allele frequencies will not change Mendelian inheritance preserves genetic variation in a population

Alleles in the population Frequencies of alleles Fig. 23-6 Alleles in the population Frequencies of alleles Gametes produced p = frequency of Each egg: Each sperm: CR allele = 0.8 q = frequency of 80% chance 20% chance 80% chance 20% chance CW allele = 0.2 Figure 23.6 Selecting alleles at random from a gene pool

Hardy-Weinberg equilibrium describes the constant frequency of alleles in a gene pool 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 AA + Aa + aa = 1 (you can think of it like this) where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) Eggs CW (20%) CR CW Fig. 23-7-1 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) CR (80%) Figure 23.7 The Hardy-Weinberg principle Eggs 64% ( p2) CRCR 16% ( pq) CRCW 4% (q2) CW CW 16% (qp) CRCW CW (20%)

Figure 23.7 The Hardy-Weinberg principle 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) CR (80%) Eggs 64% ( p2) CR CR 16% ( pq) CR CW 4% (q2) CW CW CW (20%) 16% (qp) CR CW 64% CR CR, 32% CR CW, and 4% CW CW Figure 23.7 The Hardy-Weinberg principle Gametes of this generation: 64% CR    +     16% CR    =   80% CR  = 0.8 = p 4% CW      +    16% CW    =  20% CW = 0.2 = q Genotypes in the next generation: 64% CR CR, 32% CR CW, and 4% CW CW plants

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 nonevolving populations are rarely met in nature: No mutations Random mating No natural selection Extremely large population size No gene flow

Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

The occurrence of PKU is 1 per 10,000 births q2 = 0.0001 q = 0.01 The frequency of normal alleles is p = 1 – q = 1 – 0.01 = 0.99 The frequency of carriers is 2pq = 2 x 0.99 x 0.01 = 0.0198 or approximately 2% of the U.S. population