Chapter 25 Evolutionary Processes
There are four mechanisms that shift allele frequencies in populations:
1. Natural selection increases the frequency of certain alleles- the ones that contribute to success in
survival and reproduction.
2. Genetic drift causes allele frequencies to change randomly. In some cases, drift may even cause
alleles that decrease fitness to increase in frequency.
3. Gene flow occurs when individuals leave one population, join another, and breed.Allele
frequencies may change when gene flow occurs, because arriving individuals introduce alleles to
their new population and departing individuals remove alleles from their old population.
4. Mutation modifies allele frequencies by continually introducing new alleles. The alleles created
by mutation may be beneficial or detrimental or have no effect on fitness.
25.1Analyzing Change inAllele Frequencies: the Hardy-Weinberg Principle
-Hardy and Weinberg imagined all of the gametes produced in each generation go into a single group
called gene pool and then combine at random to form offspring.
-To determine which genotypes would be present in the next generation and in what frequency, Hardy and
Weinberg simply had to calculate what happened when two gametes were plucked at random out of the
gene pool, many time, and each of these gamete pair was then combined to form offspring. These
calculations would predict the genotypes of the offspring that would be produced as well as the frequency
of each genotype.
-Hardy Weinberg rule:
1.A =1pA = q2p+q=1
2.A A =p A A = 2pq A A = q 2
1 1 1 2 2 2
3. p +2pq+q = 1
Two fundamental claims:
1. If the frequencies of allelesA1 andA2 in a population are given by p and q, then the frequencies
of genotypes A1A1, A1A2 andA2A2 will be given by p2, 2pq, q2 and generation after generation
2. When alleles are transmitted according to the rules of Mendelian inheritance, their frequencies do
not change over time. For evolution to occur, some other factor(s) must come into play.
The Hardy-Weinberg Model Makes Important Assumptions
1. No natural selection at the gene in question
2. No genetic drift, or random allele frequency changes 3. No gene flow. No alleles added by immigration or lost through emigration
4. No mutation
5. Random mating with respect to the gene in question
How Does the Hardy-Weinberg Principle Serve as a Null Hypothesis?
Biologists often want to test whether natural selection is acting on a particular gene, non-random mating
is occurring, or one of the other evolutionary mechanisms is at work. In addressing questions like these,
the Hardy-Weinberg Principle functions as a null hypothesis. Given a set of allele frequencies, it predicts
what genotype frequencies will be when natural selection, mutation, genetic drift, and gene flow are not
affecting the gene; and when mating is random with respect to that gene. If biologists observe genotype
frequencies that do not conform to the Hardy-Weinberg prediction, it means that something interesting is
going on: Either non-random mating is occurring, or allele frequencies are changing for some reason.
Further research is needed to determine which of the five Hardy-Weinberg conditions is being violated.
25.2 Types of Natural Selection
Balancing selection/heterozygote advantage: Heterozygous individuals have higher fitness than
homozygous individuals do. The consequence of this pattern is that genetic variation is maintained in
Genetic variation: The number and relative frequency of alleles that are present in a particular population.
Directional Selection: Average phenotype of the populations change in one direction
Most often, directional selection tend to reduce the genetic diversity of populations. If directional
selection continues over time, the favoured alleles will eventually reach a frequency of 1.0 while
disadvantageous alleles will reach a frequency of 0.0.Alleles that reach a frequency of 1.0 are said to be
fixed; those that reach a frequency of 0.0 are said to be lost.
Purifying selection: When disadvantageous alleles decline in frequency.
Commonly, one cause of directional selection on a trait is counterbalanced by a different factor that
causes selection in the opposite direction. Known as fitness trade-off.
Stabilizing Selection: Selection can reduce both extremes in a population. Two important consequences:
- There is no change in the average value of a trait over time
- Genetic variation in the population is reduced
Disruptive Selection: Opposite effect of stabilizing selection. Instead of favoring phenotypes near the
average value and eliminating extreme phenotypes, it eliminates phenotypes near the average value and
favors extreme phenotypes. Overall amount of genetic variation in population is maintained.
-Important because sometimes plays a part in speciation, or formation of new species.
25.3 Genetic Drift -Undirected
-Any change in allele frequencies in a population that is due to chance.
-sampling error: allele frequencies change due to blind luck.
-Random with respect to fitness. The allele frequency changes it produces are not adaptive.
-Most pronounced in special populations. The smaller the sample the larger the sampling error.
-Over time, genetic drift can lead to the random loss or fixation of alleles.
-genetic marker-a specific allele that causes a distinctive phenotype.
-founder effect: c