Chapter 23- Evolutionary Processes

  • Evolution is now understood to be driven by four processes: 
    • Niαtural selection increases the frequency of certain alleles, the ones that contribute to reproductive success in a particular environment 
    • Genetic drift causes allele frequencies to change randomly. In some cases, drift may cause alleles that decrease fitness to increase in frequency
    • Gene flow occurs when individuals leave one population, join another, and breed. 
    • Mutαtion modifies allele frequencies by continually introducing new alleles. 

23.1 Null Hypothesis: The Hardy-Weinberg Principle 

  • They imagined a hypothetical scenario in which all of the alleles from all the gametes produced in each generation go into a single group called the gene pool
  • Hardy-Weinberg principle which serves as a mathematical null hypothesis for the study of evolutionary processes.
  • If biologists observe frequencies that conform to 1e Hardy-Weinberg predictions, the frequencies are said to be in Hardy-Weinberg equilibrium. 
  • The mathematical model underlying the Hardy-Weinberg principle is based on five important assumptions about how populations and alleles behave:
    • Random mating
    • No natural selection
    • No genetic drift(random allele frequency changes)
    • No gene flow
    • No mutation

23.2 Nonrandom Mating

  • The most intensively studied form of nonrandom mating is called inbreeding, the mating between relatives.
  • Deleterious alleles are alleles that lower fitness
  • Inbreeding depression is the decline in average fitness that may take place when homozygosity increases and heterozygosity decreases in a population. 
  • Sexual selection is fundamentally different from inbreeding because it does lead to changes in allele frequencies in 1e population and increases fitness, and us it is a form of natural selection

23.3 Natural Selection 

  • Traits that enable organisms to survive in their physical and biological environments, are sometimes called ecological ( or environmental) selection.
  • Genetic variation is the number and relative frequency of alleles that are present in a particular population.
  • Natural selection occurs in four main patterns, or modes, each affecting genetic variation in different ways: 
    • Directionαl selection changes the average value of a trait.
    • Stabilizing selection reduces variation in a trait. 
    • Disruptive selection increases variation in a trait. 
    • Bαlαncing selection maintains variation in a trait.
  • When directional selection occurs, the average phenotype of a population changes in one direction.
  • When deleterious alleles decline in frequency, purifying selection is said to occur.
  • Disruptive selection eliminates phenotypes near the average value and favors extreme phenotypes 
  • Stabilizing selection favors phenotypes near the average value and eliminates extreme phenotypes
  • Balancing selection occurs when no phenotype has a distinct advantage. 
  • Of the various mechanisms of balancing selection, the following are the most common:
    • Heterozygote advantage occurs when heterozygous individuals have higher fitness than homozygous individuals do. 
    • Frequency-dependent selection occurs when certain alleles are favored when they are rare, but not when they are common
  • Intersexual selection is the selection of an individual of one sex for mating by an individual of the other sex.
  • Individuals of the same sex sometimes compete with one another to obtain mates is referred to as intrasexual selection. 
  • A territory is an actively defended area where the owner has exclusive or semi“ exclusive use. 
  • Both intersexual and intrasexual selection help to explain why some aits differ between the sexes of the same species, a difference called sexual dimorphism
  • ==Natural selection is the only evolutionary process that results in adaptation, but it is not the only evolutionary process that violates the Hardy-Weinberg assumptions.== 

23.4 Genetic Drift

  • Genetic drift is defined as a change in allele frequencies in a population that is due to chance.
  • Sampling error occurs when the allele frequencies of a chosen subset of a population (the sample) are different from those in the total population, by chance
  • Key information to remember about genetic drift are:
    • Genetic drift is random with respect to fitness
    • Genetic drift is most pronounced in small populations
    • Over time, genetic drift can lead to the random loss or fixation of alleles
  • A genetic bottleneck is a sudden reduction in the diversity of alleles in a population

23.5 Gene Flow 

  • Gene flow is the movement of alleles between populations
  • If a population has lost alleles due to genetic drift, then the arrival of new alleles via gene flow should increase genetic diversity. 
  • If increased genetic diversity results in better resistance to infections by bacteria or viruses or other parasites, for example, gene flow would increase the average fitness of individuals.

23.6 Mutation 

  • ==Natural selection often favors certain alleles and leads to a decrease in overall genetic variation in a population.==
  • ==Genetic drift tends to decrease genetic diversity over time, as alleles are randomly lost or fixed in a population.== 
  • ==Gene flow increases genetic diversity in a recipient population if new alleles arrive with immigrating individuals.== But gene flow may decrease genetic variation in the source population if alleles leave with emigrating individuals. 
  • Mutations can occur in a number of ways:
    • Point mutation: A change in a single base pair in DNA. 
    • Chromosome-level mutation: A change in the number or composition of chromosomes.
    • Lateral gene transfer: The transfer of genetic information from one species to another, rather than from parent to offspring.
  • Beneficial allele is an allele that allows individuals to produce more surviving offspring, increasing fitness. 
  • Neutral allele is an allele with no effect on fitness, as occurs when a point mutation is silent
  • Key information about mutations are that 
    • Mutation is the ultimate source of genetic variation
    • Mutations are random with respect to fitness 
    • If mutation did not occur, evolution would eventually stop. 
    • Mutation alone is usually inconsequential in changing allele frequencies at a particular gene.