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The Evolution of Populations
Chapter 23 The Evolution of Populations
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Overview: The Smallest Unit of Evolution
One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve
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Genetic variations in populations
Contribute to evolution Figure 23.1
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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
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The Modern Synthesis Population genetics
Is the study of how populations change genetically over time Reconciled Darwin’s and Mendel’s ideas
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The modern synthesis Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection Focuses on populations as units of evolution
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Gene Pools and Allele Frequencies
A population Is a localized group of individuals that are capable of interbreeding and producing fertile offspring MAP AREA ALASKA CANADA Beaufort Sea Porcupine herd range Fairbanks Whitehorse Fortymile NORTHWEST TERRITORIES YUKON Figure 23.3
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The 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
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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
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Mendelian inheritance
Preserves genetic variation in a population Figure 23.4 Generation 1 CRCR genotype CWCW Plants mate All CRCW (all pink flowers) 50% CR gametes 50% CW Come together at random 2 3 4 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions
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Preservation of Allele Frequencies
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
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Hardy-Weinberg Equilibrium
Describes a population in which random mating occurs Describes a population where allele frequencies do not change
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A population in Hardy-Weinberg equilibrium
Figure 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 64% CRCR 16% CRCW 4% CWCW qp Eggs pq If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: q2 64% CRCR, 32% CRCW, and 4% CWCW Gametes of the next generation: 64% CR from CRCR homozygotes 16% CR from CRCW homozygotes + = 80% CR = 0.8 = p 16% CW from CRCW heterozygotes 20% CW = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: 64% CRCR, 32% CRCW and 4% CWCW plants 4% CW from CWCW homozygotes
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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
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Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem Describes a hypothetical population In real populations Allele and genotype frequencies do change over time
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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
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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
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Two processes, mutation and sexual recombination
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
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Mutation Mutations Cause new genes and alleles to arise
Are changes in the nucleotide sequence of DNA Cause new genes and alleles to arise Figure 23.6
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Point Mutations A point mutation Is a change in one base in a gene
Can have a significant impact on phenotype Is usually harmless, but may have an adaptive impact
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Mutations That Alter Gene Number or Sequence
Chromosomal mutations that affect many loci Are almost certain to be harmful May be neutral and even beneficial
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Gene duplication Duplicates chromosome segments
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Mutation Rates Mutation rates Tend to be low in animals and plants
Average about one mutation in every 100,000 genes per generation Are more rapid in microorganisms
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In sexually reproducing populations, sexual recombination
Is far more important than mutation in producing the genetic differences that make adaptation possible
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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
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Differential success in reproduction
Natural Selection Differential success in reproduction Results in certain alleles being passed to the next generation in greater proportions
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Statistically, the smaller a sample
Genetic Drift Statistically, the smaller a sample The greater the chance of deviation from a predicted result
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Genetic drift Describes how allele frequencies can fluctuate unpredictably from one generation to the next Tends to reduce genetic variation Figure 23.7 CRCR CRCW CWCW Only 5 of 10 plants leave offspring Only 2 of Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
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In the bottleneck effect
A sudden change in the environment may drastically reduce the size of a population The gene pool may no longer be reflective of the original population’s gene pool Figure 23.8 A (a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent. Original population Bottlenecking event Surviving population
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Understanding the bottleneck effect
Can increase understanding of how human activity affects other species Figure 23.8 B (b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.
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The Founder Effect The founder effect
Occurs when a few individuals become isolated from a larger population Can affect allele frequencies in a population
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Gene Flow Gene flow Causes a population to gain or lose alleles
Results from the movement of fertile individuals or gametes Tends to reduce differences between populations over time
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Concept 23.4: Natural selection is the primary mechanism of adaptive evolution
Accumulates and maintains favorable genotypes in a population
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Genetic Variation Genetic variation
Occurs in individuals in populations of all species Is not always heritable Figure 23.9 A, B (a) Map butterflies that emerge in spring: orange and brown (b) Map butterflies that emerge in late summer: black and white
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Phenotypic polymorphism
Describes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeable Genetic polymorphisms Are the heritable components of characters that occur along a continuum in a population
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A Closer Look at Natural Selection
From the range of variations available in a population Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations
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The phrases “struggle for existence” and “survival of the fittest”
Evolutionary Fitness The phrases “struggle for existence” and “survival of the fittest” Are commonly used to describe natural selection Can be misleading Reproductive success Is generally more subtle and depends on many factors
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The sickle-cell allele
Causes mutations in hemoglobin but also confers malaria resistance Exemplifies the heterozygote advantage Figure 23.13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan)
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Neutral Variation Neutral variation
Is genetic variation that appears to confer no selective advantage
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Sexual Selection Sexual selection
Is natural selection for mating success Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics
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Intrasexual selection
Is a direct competition among individuals of one sex for mates of the opposite sex
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Intersexual selection
Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex May depend on the showiness of the male’s appearance Figure 23.15
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The Evolutionary Enigma of Sexual Reproduction
Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Figure 23.16 Asexual reproduction Female Sexual reproduction Male Generation 1 Generation 2 Generation 3 Generation 4
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If sexual reproduction is a handicap, why has it persisted?
It produces genetic variation that may aid in disease resistance
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