Chapter 21 - The Evolution of Populations

do organisms evolve during their lifetime?

no they do not, populations of organisms evolve over many generations

• natural selection acts on individuals, evolution is the result of the accumulation of changes made by natural selection to a population over time

microevolution

change in allele frequencies in a population over generations

phenotype

physical expression of the genotype

variation and how it translates to genotype/phenotype

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

how do geneticists measure variation 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 sequencies of pairs of individuals

mutations

changes in the nucleotide sequence of DNA

• cause new genes and alleles to arise

• only mutations in cells that produce gametes can be passed to offspring

  • mutations in somatic cells always happen, but no negative effect

  • silent mutations have no effect

4 sources of genetic vartiation

1. formation of new alleles by mutation

2. altering gene number or position

3. rapid reproduction increases mutation rate

4. sexual reproduction

point mutation and effects

change in one base in a gene

• effects can vary

• if it causes a change in protein function, it is often harmful and usually deleted by natural selection

  • but sometimes this change in protein function can increase the fit between an organism and the environment (and instead preserved by natural selection)

mutations that alter gene number or position

• chromosomal mutations that delete, disrupt, or rearrange many loci are usually harmful

• but duplication of genes can arise from errors in meiosis

• increases in gene number have played a major role in evolution

neofunctionalization and examples

when duplicated genes take on new functions by further mutation

• important source of evolutionary novelty

• ex. gene for luteinizing hormone has been duplicated six times to produce the chorionic gonadotropin gene family

• ex. human chorionic gonadotropin (hCG) is important in the early maintenance of pregnancy

what was the original function of luteinizing hormone

• to maintain early pregnancy by maintaining the corpus luteum (corpus luteum rescue)

luteinizing hormone and neofunctionalization

new copies of the LH beta gene (the chorionic gonadotropins) have new functions:

• control invasion of the placenta into mother’s uterine endometrium during early embryo development in pregnancy

• hCG also regulates maternal thyroid during gestation

  • also key to immunotolerance of the semi-allogenic fetus (immune system regulation)

mutation rates in plants and animals vs. prokaryotes and viruses

• plants and animals

  • low mutation rates (average of 1 mutation in every 100,000 genes per generation)

    • lower than prokaryotes

• prokaryotes and viruses

  • more generations per unit time

  • mutations can accumulate quickly

• ex. HIV

  • 2 day generation time

  • high mutation rate

  • mutations accumulate rapidly making drug treatments ineffective

sexual reproduction and genetic variability

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

Population

localized group of individuals that can interbreed and produce vertile offspring

gene pool

contains all of the alleles for all loci in a population

when is a locus fixed?

a locus is fixed if all individuals in a population are homozygous for the same allele

• no variability

How to calculate the Frequency of an Allele in a Population

For diploid organisms:

total # of alleles at a locus = total # of individuals x 2

total # of dominant alleles at a locus = 2 alleles per homozygous dominant individual + 1 allele per heterozygous individual

^same thing for recessive alleles

Hardy-weinberg Principle

describes a population that is not evolving

• if a population does not meet the criteria of the Hardy-Weinberg principle, then that population is evolving

• states that frequencies of alleles and genotypes in a population remain constant from generation to generation

  • allele frequencies don’t change even in a population where gametes contribute to the next generation randomly

  • Mendelian inheritance preserves genetic variation in a population

Why is the Hardy-Weinberg Theorem important?

it is the entry point to the study of population genetics

• in real populations, allele and genotype frequencies do change over time

• but Hardy-weinberg theorem allows us to determine the cause of changes in gene frequencies

Hardy Weinberg Equation

p² + 2pq + q² = 1

• p - dominant allele frequency

• q - recessive allele frequency

• p² - probability of a homozygous dominant individual

• q² - probability of a homozygous recessive individual

• 2pq - probability of a heterozygous individual

• used to compute genotype frequencies if we know the allele frequencies

• genotype frequencies must add up to 1.0

Steps on how to apply Hardy-Weinberg equation

1. start with genotype frequencies

2. use Hardy-Weinberg equation to help compute the allele frequencies

3. allow random breeding, then use Hard-Weinberg equation to compute genotype frequencies in the next generation

5 assumptions/conditions of the Hardy-Weinberg Equilibrium

1. No mutations

2. Random Mating

3. No natural Selection

4. Extremely large population size

5. No gene flow

PKU and applying the Hardy-Weinberg Equation

We can assume the locus causes Phenylketonuria (PKU) is in Hardy-Weinberg equilibrium since:

1. PKU gene mutation rate is low

2. Mate selection is random

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

4. population is large

5. migration has no effect as many other populations have similar allele frequencies

3 factors that alter allele frequencies

1. Natural Selection

2. Genetic drift

3. gene flow

Natural Selection

A successful reproduction results in certain alleles being being passed to the next generation in greater numbers

Genetic drift

Random change in allele frequencies

• tends to reduce genetic variation through losses of alleles

• the smaller a sample, the greater chance of deviation from a predicted result

Founder Effect

when a few individuals become isolated from a large population and start a new population

• allele frequencies in the small founder population can be different from those in the larger parent population

• can also result in loss of allelic diversity due to random sampling effects

Huntington’s disease

• first autosomal dominant disease discovered

• practical application of founder effect

• high frequency of the disease in the Lake Maracaibo region of northwest Venezuela

  • all of those with the disease have ancestry with a European sailor who had the disease in 1800 and had children

Bottleneck effect

sudden reduction in population size due to a change in he environment

• resulting gene pool may no longer reflect the original population’s gene pool

• if the population remains small, it may be further affected by genetic drift

Genetic drift impact on Greater Prairie Chicken

loss of prairie habitat caused a severe reduction in the population

• surviving birds had low levels of genetic variation

• affected reproduction (only 50% of eggs hatched)

Cheetahs and population bottleneck

• underwent a severe population bottleneck during the pleistocene (roughly 10,000 years ago)

• Cheetah’s lost 90-99% of genetic variation during the bottleneck

• there is so little variation that they do not reject skin grafts

Summary of the effects of Genetic Drift

• significant in small populations

• causes allele frequencies to change at random

• can lead to a loss of genetic variation within populations

• can cause harmful alleles to become fixed

Gene flow

the movement of alleles among populations

• alleles can be transferred through the movement of fertile individuals or gametes (ex. pollen)

• tends to reduce differences between populations over time

• is more likely than mutation to alter allele frequencies directly

• can decrease the fitness of a population

what is the ONLY mechanism that consistently causes adaptive evolution

Natural Selection

Natural selection

increases the frequencies of alleles that enhance survival and reproduction

adaptive evolution

when the match between an organism and its environment increases

Natural selection and adaptive evolution

• only natural selection consistently results in adaptive evolution

  • genetic drift or gene flow can cause adaptations sometimes, but not consistently

• causes adaptive evolution by acting on the phenotype of an organism in its current environment

Relative fitness

the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals

3 modes of selection

1. directional selection - favours individuals at one end of the phenotypic range

• ex. NBA makes population of its players towards taller players

2. Disruptive or diversifying selection - favours individuals at both extremes of the phenotypic range

• least common

3. stabilizing selection - favours intermediate variants and acts against extreme phenotypes

• most common

Stabilizing selection for birth weight in humans

very small and very large babies have lower survival rates

• since bigger babies are more likely to get stuck in the birth canal

Balancing selection

Some selection may preserve variation at some loci, thus maintaining two or more phenotypes in a population

• 2 types:

  • heterozygote advantage

  • frequency-dependent selection

Heterozygote advantage

when heterozygotes have a higher fitness than both homozygotes

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

  • ex. sickle-cell allele causes mutations in hemoglobin but also gives malaria resistance

Frequency-dependent selection

fitness of a phenotype declines if it becomes too common in the population

• selection can favour whichever phenotype is less common in a population

Frequency-dependent selection in salmon

• large hooknoses fight to get close to the females and small jacks sneak

• small jacks do better when they are more rare in the population, large hooknoses do better when jacks are common in the population

Sexual selection

natural selection for mating success

• can result in sexual dimorphism

sexual dimorphism

differences between sexual characteristics of males and females

• ex. male red-winged blackbirds are about 1/3 larger than females

2 main types of sexual selection

• intrasexual selection

• intersexual selection

intersexual selection

involves competition among individuals of one sex for mates of the opposite sex

• it is often, but not always, males competing with males

intersexual selection

when individuals of one sex (usually females) are choosy in selecting their mates

• often called mate choice

• males can be showy due to mate choice, which can increase their chances of attracting a female, but also decreasing their chances of survivals

4 reasons why natural selection cannot make perfect organisms

1. Natural selection can only act on existing variations

2. evolution is limited by historical constraints

3. adaptations are often compromises

4. chance, natural selection, and the environment interact