The Evolution of Populations

Chapter 23

 

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

•         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

Variation Between Populations

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

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

 

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

Point Mutations

•         A point mutation is a change in one base in a gene

•         The effects of point mutations can vary:

–        Mutations in noncoding regions of DNA are often harmless

–        Mutations in a gene might not affect protein production because of redundancy in the genetic code

•         The effects of point mutations can vary:

–        Mutations that result in a change in protein production are often harmful

–        Mutations that result in a change in protein production can sometimes increase the fit between organism and environment

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

 

Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving

•         The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population

 

Gene Pools and Allele Frequencies

•         A population is a localized group of individuals capable of interbreeding and producing fertile offspring

–        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

–        For example, p + q = 1

 

The Hardy-Weinberg Principle

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

·         Hardy-Weinberg equilibrium describes the constant frequency of alleles in such 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

–        where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

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

Applying the Hardy-Weinberg Principle

·         We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:

–        The PKU gene mutation rate is low

–        Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

–        Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions

–        The population is large

–        Migration has no effect as many other populations have similar allele frequencies

·         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

 

Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

–        Natural selection

–        Genetic drift

–        Gene flow

Natural Selection

Genetic Drift

The Founder Effect

·         The founder effect occurs when a few individuals become isolated from a larger population

·         Allele frequencies in the small founder population can be different from those in the larger parent population

The Bottleneck Effect

·         The bottleneck effect is a sudden reduction in  population size due to a change in the environment

·         The resulting gene pool may no longer be reflective of the original population’s gene pool

·         If the population remains small, it may be further affected by genetic drift

·         Understanding the bottleneck effect can increase understanding of how human activity affects other species

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken

·         Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois

·         The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

·         Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck

·         The results showed a loss of alleles at several loci

·         Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

Effects of Genetic Drift: A Summary

1.      Genetic drift is significant in small populations

2.      Genetic drift causes allele frequencies to change at random

3.      Genetic drift can lead to a loss of genetic variation within populations

4.      Genetic drift can cause harmful alleles to become fixed

 

Gene Flow

 

Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

A Closer Look at Natural Selection

Relative Fitness

·         The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals

·         Reproductive success is generally more subtle and depends on many factors

·         Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

·         Selection favors certain genotypes by acting on the phenotypes of certain organisms

Directional, Disruptive, and Stabilizing Selection

·         Three modes of selection:

–        Directional selection favors individuals at one end of the phenotypic range

–        Disruptive selection favors individuals at both extremes of the phenotypic range

–        Stabilizing selection favors intermediate variants and acts against extreme phenotypes

 

The Key Role of Natural Selection in Adaptive Evolution

 

Sexual Selection

 

The Preservation of Genetic Variation

Diploidy

·         Diploidy maintains genetic variation in the form of hidden recessive alleles

Balancing Selection

·         Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes

•         Natural selection will tend to maintain two or more alleles at that locus

•         The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

Frequency-Dependent Selection

•         In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population

•         Selection can favor whichever phenotype is less common in a population

Neutral Variation

•         Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage

•         For example,

–        Variation in noncoding regions of DNA

–        Variation in proteins that have little effect on protein function or reproductive fitness

 

Why Natural Selection Cannot Fashion Perfect Organisms

  1. Selection can act only on existing variations
  2. Evolution is limited by historical constraints
  3. Adaptations are often compromises
  4. Chance, natural selection, and the environment interact

 

You should now be able to:

  1. Explain why the majority of point mutations are harmless
  2. Explain how sexual recombination generates genetic variability
  3. Define the terms population, species, gene pool, relative fitness, and neutral variation
  4. List the five conditions of Hardy-Weinberg equilibrium
  1. Apply the Hardy-Weinberg equation to a population genetics problem
  2. Explain why natural selection is the only mechanism that consistently produces adaptive change
  3. Explain the role of population size in genetic drift
  1. Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection
  2. List four reasons why natural selection cannot produce perfect organisms