BIOL359 - Evolution Winter 2013
Chapter 6: Mendelian Genetics in Populations I: Selection and Mutation as Mechanisms of Evolution
- Population Genetics: Integrates Darwin’s Theory of Evolution by Natural Selection with Mendelian genetics
o Changes in the relative abundance of traits in a population is tied to changes in the relative
abundance of the alleles that influences them
Change across generations in the frequencies of alleles
6.1. Mendelian Genetics in Populations: The Hardy-Weinberg Equilibrium Principle
- Population: Group of interbreeding individuals and their offspring
- Adults Gametes Zygotes Juveniles Adults (Track the frequency of alleles across generations)
- Gene pool: The total of all the eggs & sperms of a generation of one population
- If the ending frequencies are different from starting frequencies, the population has evolved
o Evolution resulting from blind luck “genetic drift”: Allele frequencies change somewhat across
generations.
o G. Udny Yule (1902): First biologist to trace the frequencies of Mendelian alleles across generations,
but he concluded only 0.5 & 0.5 frequencies will result in equilibrium
o Reginald Punnett: A population with allele frequencies that sums to 1 will remain unchanged
p + 2pq + q = 1
Evolution is a change in allele frequencies
- The Hardy-Weinberg equilibrium principle:
o The allele frequencies in a population will not change, generation after generation
o If the allele frequencies in a population are given by p and q, the genotype frequencies will be given
2 2
by p , 2pq, and q
If both conclusions are true, the population is in Hardy-Weinberg equilibrium
- Assumptions that cannot be violated for the Hardy-Weinberg equilibrium principle to be true (If violated, the
allele frequencies of the population will change, the set of events that can cause evolution)
o There is no selection, where all members survived at equal rates & contributed equal numbers of
gametes to the gene pool
o There is no mutation, alleles are not converted “mutated” into copies of other existing alleles, & no
new alleles are created
o There is no migration, no individuals moved into or out of the population
o There are no chance events, or the model population is infinitely large, so individuals with different
genotypes will pass on their alleles to the next generation equally
o Individuals choose their mates at random, so there’s no mating preference for certain genotypes
6.2. Selection
- Selection: Violation of the first assumption, individuals with particular phenotypes survive better or
reproduce more than individuals with other phenotypes “reproductive success higher”
o Leads to evolution if the phenotypes are heritable “associated with certain genotypes”
o In nature, selection results in small, cumulative changes in allele frequencies
- Drosophila melanogaster:
o Experimental group with ethanol spiked in their food
o Control group with normal, non-sFiked food
o Over generations, the allele (Adh ) that contains the gene encoding the enzyme ADH (alcohol
dehydrogenase) increased in the experimental group, with no changes in the control group
o Adh homozygous encoded ADH breaks down ethanol 2x the rate compares to Adh homozygous
- Genetic variation for resistance to kuru “fatal neurological disorder of the Fore people”
o Kuru “uncontrollable shivering & trembling” (E.g. Bovine spongiform encephalopathy)
Belongs to a group of maladies known as the spongiform encephalopathies “brain looks like a
sponge degenerating tissues through the host’s production of mis-folded PrP proteins,
unknown vector as of today” BIOL359 - Evolution Winter 2013
The genotype for PrP gene on chromosome 20 influences an individuals’ susceptibility to the
disease 2 alleles @ position 129, 1 encodes for valine & other encodes for methionine
All victims of Kuru had Met/Met genotype (Homozygous diseased) not Met/Val or Val/Val
Statistically significant excess of heterozygotes & deficit of homozygotes Homozygotes are
susceptible to kuru and heterozygotes are resistant
6.3. Patterns of Selection: Testing Predictions of Population Genetic Theory
- Selection on Recessive and Dominant Alleles
o Flour beetles, heterozygote fonder, initial allele frequency of the 2 alleles are 0.5 & 0.5, homozygous
recessive individuals have zero fitness
Over 12 generation, dramatic increase of the non-lethal dominant allele, rate of evolution is
rapid at first, but slows down as the experiment proceeds
o When a recessive allele is common, evolution by natural selection is rapid
o When a recessive allele is rare & a dominant allele is common, evolution by natural selection is slow
Most are phenotypically hidden inside heterozygous individuals, immune from selection
- Selection on Heterozygotes & Homozygotes
o D. melanogaster, heterozygotes founders, initial allele frequency of the 2 alleles are 0.5 & 0.5,
homozygous recessive individuals have zero fitness
Frequency of the viable allele increased rapidly over the first few generations, but rate of
evolution slowed down & reached equilibrium at a frequency of 0.79 instead of 0.94 like the
flour beetles example shown above
o Heterozygote superiority/Overdominance: Heterozygotes have higher fitness than either
homozygotes, at equilibrium, the selective advantage enjoyed by the lethal allele in heterozygotes
balances the obvious disadvantage it suffers when it is in homozygotes Maintenance of genetic
diversity among populations
o
More
Less
