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

Hardy-Weinberg equation