BIOL 1020 – CHAPTER 23 LECTURE NOTES
Chapter 23: Population Genetics (Microevolution)
1. Explain what terms in the Hardy-Weinberg equation give:
· allele frequencies (dominant allele, recessive allele, etc.)
· each genotype frequency (homozygous dominant, heterozygous, etc.)
· each phenotype frequency
2. [do some pop gen problems from slides, and make some more up if there is time]
3. Describe the assumptions of the Hardy-Weinberg equilibrium model.
4. Describe conditions that can keep populations from establishing or maintaining genetic equilibrium.
5. Explain three main types of natural selection.
6. Discuss the importance of genetic variation for evolution, and the concept of neutral variation.
7. Give a hypothetical example of how genetic variation that was once neutral may no longer be neutral.
I. Microevolution is a change in allele frequencies or genotype frequencies in a population over time
A. population – a localized group of individuals capable of interbreeding and producing fertile offspring, and that are more or less isolated from other such groups
B. gene pool – all alleles present in a population at a given time
C. phenotype frequency – proportion of a population with a given phenotype
D. genotype frequency – proportion of a population with a given genotype
E. allele frequency – proportion of a specific allele in a population
1. diploid individuals have two alleles for each gene
2. if you know genotype frequencies, it is easy to calculate allele frequencies
3. example:
· population (1000) = genotypes AA (490) + Aa (420) + aa (90)
· allele number (2000) = A (490x2 + 420) + a (420 + 90x2) = A (1400) + a (600)
· freq[A] = 1400/2000 = 0.70
· freq[a] = 600/2000 = 0.30
· note that the sum of all allele frequencies is 1.0
II. Genetic equilibrium in populations: the Hardy-Weinberg theorem
A. the Hardy-Weinberg theorem describes the frequencies of genotypes in a population based on the frequency of occurrence of alleles in the population that is in a state of genetic equilibrium (that is, not evolving)
1. the usual case for calculations: if allele “A” is dominant to “a”, and they are the only two alleles possible at the A-locus, then
· p = freq[A] = the frequency of occurrence of the A-allele in the population
· q = freq[a] = the frequency of occurrence of the a-allele in the population
2. Then p + q = 1 (following the sum rule for probability)
3. Allele associations follow the product rule for probability, so you multiply to predict the genotype frequencies:
· ( p + q ) x ( p + q ) = p2 + 2 pq + q2
· p2 = frequency of homozygous dominant genotypes
· 2 pq = frequency of heterozygous genotypes
· q2 = frequency of homozygous recessive genotypes
· note that ( p + q ) x ( p + q ) = 1 x 1 = 1, so p2 + 2 pq + q2 = 1
B. Hardy-Weinberg equilibrium
1. if the Hardy-Weinberg theorem can be used to accurately predict genotype frequencies from allele frequencies for a population then the population is in Hardy-Weinberg equilibrium or genetic equilibrium
2. in such cases you can use data from one generation to predict the allele, genotype, and phenotype frequencies for the next generation
3. such populations are not evolving, but are static instead
C. the assumptions of this model are:
1. large population size (due to statistical constraints, to minimize genetic drift)
2. no migration – no exchange of alleles with other populations (no gene flow)
3. no mutations of the alleles under study occur
4. random mating of all genotypes
5. no natural selection
III. Microevolution thus can be described as deviation from Hardy-Weinberg equilibrium, where the allele or genotype frequencies in a population change over time
A. consequences of small population size: genetic drift
1. Consider taking a small sample of individuals from a larger population. If only two individuals were picked they cannot reflect the allele frequency in the larger population. Neither can 3, 4, or 5 individuals, but as the selected sample gets larger it becomes more likely that the sample reflects the allele frequency in the larger population.
2. genetic drift is a change in gene frequencies of populations because of small population size
3. genetic drift tends to decrease genetic variation within a population
4. genetic drift tends to increase genetic variation between populations
5. two general mechanisms lead to small population sizes
· genetic bottlenecks are created by dramatic reduction in population size – endangered species face a genetic bottleneck on a species-wide scale, and suffer lasting effects even if population size later recovers
· founder effect – when a new population is established, typically only a few individuals (founders) are involved in colonizing the new area; this is common for islands
B. migration – when individuals leave or join a population
1. migrating individuals carry their alleles with them (gene flow), usually resulting in changes in allele frequencies
2. gene flow tends to decrease genetic variation between populations
C. mutations increase variation in the gene pool of a species
1. remember that mutations may be neutral, harmful, or beneficial
2. even at the risk of harmful effects, mutations are necessary to increase variation in the population so that natural selection can produce organisms more suited to their environment
D. nonrandom mating
1. if individuals do not mate at random, then Hardy-Weinberg equilibrium is not achieved
2. the most common cases of nonrandom mating involve mating between individuals of similar genotypes, either by choice or location
· such inbreeding does not change allele frequencies, but increases the frequency of homozygous genotypes
· inbreeding depression is seen in some cases, where inbred individuals have lower fitness that non-inbred individuals
§ fitness – relative ability of a genotype to contribute to future generations
§ fertility declines and high juvenile mortality associated with “unmasking” harmful recessive alleles can reduce fitness for inbred individuals
§ hybrid vigor also leads to higher relative fitness for hybrids
· self-fertilization is the most extreme case of inbreeding
3. assortive mating – mates are selected by phenotypes
· positive assortive mating – selection for the same phenotype; works like inbreeding for the genes governing that phenotype, and for loci closely linked to those genes
· negative assortive mating – selection for the opposite phenotype
§ less common than positive assortive mating
§ leads to a decrease in homozygous genotypes for the genes governing the selected phenotype, and for loci closely linked to those genes
E. natural selection changes allele frequencies in a way that leads to adaptation to the environment
1. fitness is the ability of an organism to compete successfully and pass its alleles on to the next generation
2. populations undergoing natural selection are evolving, with alleles that contribute to better fitness increasing in frequency over successive generations
3. natural selection only operates based on the current environment – as environmental conditions change, different alleles will be selected for
4. sexual selection (mate choices based on inherited characteristics) is an aspect of natural selection
5. there are 3 types of natural selection
· stabilizing selection – occurs in populations well adapted to their environments, selecting against phenotypic extremes
§ this is probably the type of selection most commonly faced by populations
§ example - human birth weight
· directional selection – permits species to adapt to environmental change by favoring selection of one extreme over the other; example – peppered moth
· disruptive selection – when more than one extreme phenotype is favored over intermediate phenotypes
§ really a special case of direction selection, where there are trends in more than one direction
§ can produce a genetic “split” in a population and thus serve as a mechanism for speciation
§ example – pocket mice in New Mexico
IV. Genetic variation must exist for natural selection to occur
A. the ultimate source of genetic variation is mutations
B. once variation exists, it can be affected by independent assortment and genetic recombination during gamete formation
1. consider the cross AaBb x AaBb – 9 different genotypes arise
2. this involves only 2 alleles at 2 loci; if there were 6 alleles possible at just 5 loci, over 4 million genotypes are possible
3. thus, given that there are thousands of genes in an organism, and that many alleles are possible at most of these loci, it becomes clear that in nature there is great genetic variability
C. the demonstrated presence of two or more alleles at a given locus is genetic polymorphism
1. biologists have produced tools for studying the genetic polymorphism of populations at the molecular level (RFLP, DNA sequencing, etc.)
2. these tools can be used to demonstrate and study polymorphism in populations without necessarily knowing the specific genes involved
D. genetic polymorphisms can be maintained by heterozygote advantage or hybrid vigor
1. when either the homozygous dominant or recessive is more suited to an environment than the heterozygote, the homozygous genotype will be more likely to be fixed in the population
2. but when heterozygous genotypes have advantage over either of the homozygous genotypes, variation tends to increase in the population
3. example - sickle cell anemia and malaria resistance
E. genetic polymorphisms can be maintained due to frequency-dependent selection
1. there are cases where the frequency of a given genotype affects the degree to which it is or isn't selected in the population
2. example - predator/prey relationships, where individuals with a rare phenotype may be ignored by a predator, but as they become more abundant the selective advantage decreases because the predator is more likely to notice them
F. much of the genetic variation in a population will produce no selective advantage, i.e. it is neutral
1. the role of neutral variation in evolution is debated today
2. remember that what is neutral in one context may not be neutral in another context, so as environments change some previously neutral variation may be acted on by natural selection
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