# The Evolution of Populations

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The Evolution of Populations
Chapter 23 The Evolution of Populations

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

The Modern Synthesis MAP AREA CANADA ALASKA Beaufort Sea Porcupine herd range NORTHWEST TERRITORIES Fairbanks Fortymile Whitehorse YUKON Population genetics is the study of how populations change genetically over time

The Hardy-Weinberg Theorem
Generation 3 25% CRCR 4 50% CRCW 25% CWCW 50% CW gametes 50% CR come together at random Alleles segregate, and subsequent generations also have three types of flowers in the same proportions 2 1 CRCR CWCW genotype Plants mate All CRCW (all pink flowers) X The Hardy-Weinberg theorem describes a population that is not evolving

Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes a population where allele frequencies do not change p2 + 2pq + q2 = 1 And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Gametes for each generation are drawn at random from the gene pool
LE 23-5 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) p2 pq (80%) CR Eggs 64% CRCR 16% CRCW (20%) CW 16% CRCW 4% CWCW qp q2

Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population

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 a population carrying the allele for an inherited disease

Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible
Two processes, mutation and sexual recombination, produce the variation in gene pools that contributes to differences among individuals

Point Mutations A point mutation is a change in one base in a gene
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful

Sexual Recombination Sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible What are the sources of genetic variation in sexual recombination?

Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow

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

Animation: Causes of Evolutionary Change
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 Animation: Causes of Evolutionary Change

LE 23-7 CRCR CRCR CWCW CRCR CRCR CRCW CRCW CRCR CRCR CWCW CRCR CRCR
Only 5 of 10 plants leave offspring CRCW Only 2 of 10 plants leave offspring CRCR CRCR CWCW CRCR CRCR CWCW CRCR CRCR CRCW CRCW CRCR CRCR CRCR CRCW CWCW CRCR CRCR CRCR CRCW CRCW CRCW CRCR CRCR Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0

The Bottleneck Effect The bottleneck effect is a sudden change in the environment that may drastically reduce the size of a population Original population Bottlenecking event Surviving

The Founder Effect The founder effect occurs when a few individuals become isolated from a larger population It can affect allele frequencies in a population

Gene Flow Gene flow consists of genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes

Map butterflies that emerge in spring: orange and brown
LE 23-9 Genetic variation occurs in individuals in populations of all species It is not always heritable Map butterflies that emerge in spring: orange and brown Map butterflies that emerge in late summer: black and white

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

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

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

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

Plasmodium falciparum (a protozoan)
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%

Sexual Selection Sexual selection is natural selection for mating success It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

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