Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg theorem
Microevolution is deviation from Hardy- Weinberg equilibrium
Genetic variation must exist for natural selection to occur .
• 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
. Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg theorem
Microevolution is deviation from Hardy- Weinberg equilibrium
Genetic variation must exist for natural selection to occur . Microevolution is a change in allele frequencies or genotype frequencies in a population over time
population – a localized group of individuals capable of interbreeding and producing fertile offspring, and that are more or less isolated from other such groups
gene pool – all alleles present in a population at a given time
phenotype frequency – proportion of a population with a given phenotype
genotype frequency – proportion of a population with a given genotype
allele frequency – proportion of a specific allele in a population
. Microevolution is a change in allele frequencies or genotype frequencies in a population over time
allele frequency – proportion of a specific allele in a population
diploid individuals have two alleles for each gene
if you know genotype frequencies, it is easy to calculate allele frequencies
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 (sum rule of probability)
. Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg theorem
Microevolution is deviation from Hardy- Weinberg equilibrium
Genetic variation must exist for natural selection to occur .
• 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
. Make up and do some pop gen problems.
Suggestions: Start with a population of either 100 or 10,000 individuals. Have the number of individuals with the recessive phenotype be the square of a whole number (so that q2 is easy to solve). Then answer the frequency questions below.
What is the frequency in the population of: • the recessive phenotype? • the dominant phenotype? • the dominant allele? • the recessive allele? • homozygous recessive individuals? • homozygous dominant individuals? • heterozygous individuals?
. The Hardy-Weinberg Theorem
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)
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
Then p + q = 1 (following the sum rule for probability)
. The Hardy-Weinberg Theorem
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
. Hardy-Weinberg Equilibrium
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
in such cases you can use data from one generation to predict the allele, genotype, and phenotype frequencies for the next generation
such populations are not evolving, but are static instead . Make up and do some pop gen problems.
Suggestions: Start with a population of either 100 or 10,000 individuals. Have the number of individuals with the recessive phenotype be the square of a whole number (so that q2 is easy to solve). Then answer the frequency questions below.
What is the frequency in the population of: • the recessive phenotype? • the dominant phenotype? • the dominant allele? • the recessive allele? • homozygous recessive individuals? • homozygous dominant individuals? • heterozygous individuals?
.
• Describe the assumptions of the Hardy- Weinberg equilibrium model.
. Hardy-Weinberg Equilibrium
the assumptions of this model are:
large population size (due to statistical constraints, to minimize genetic drift)
no migration – no exchange of alleles with other populations (no gene flow)
no mutations of the alleles under study occur
random mating of all genotypes
no natural selection
.
• Describe the assumptions of the Hardy- Weinberg equilibrium model.
. Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg theorem
Microevolution is deviation from Hardy- Weinberg equilibrium
Genetic variation must exist for natural selection to occur .
• Describe conditions that can keep populations from establishing or maintaining genetic equilibrium.
. Microevolution is a deviation from Hardy-Weinberg equilibrium
if allele and/or genotype frequencies in a population change over time, then it is by definition evolving (evolution means changing over time), undergoing microevolution
things that can cause microevolution
small population size: genetic drift
migration – gene flow; individuals leave and/or join a population
mutations
nonrandom mating
natural selection
. Microevolution is a deviation from Hardy-Weinberg equilibrium
consequences of small population size: genetic drift
Consider taking a small sample of individuals from a larger population
If only two individuals were picked they almost certainly won’t reflect the allele frequency in the larger population (in many cases, they can’t even possibly do so).
The same holds true for 3, 4, or 5 individuals.
As the selected sample gets larger it becomes more likely that the sample reflects the allele frequency in the larger population.
Mating to produce the next generation is effectively sampling the population
It takes a very large sampling size (thousands) to have a good chance of the sample essentially matching the allele frequencies and genotype frequencies of the population.
. Microevolution is a deviation from Hardy-Weinberg equilibrium
genetic drift is a change in gene frequencies of populations because of small population size
. Microevolution is a deviation from Hardy-Weinberg equilibrium
genetic drift tends to decrease genetic variation within a population
genetic drift tends to increase genetic variation between populations
NOTE: genetic drift places a major factor in evolution, especially when populations are split, but does NOT involve natural selection
. Microevolution is a deviation from Hardy-Weinberg equilibrium
consequences of small population size: genetic drift
two general mechanisms lead to small population sizes
genetic bottlenecks are created by dramatic reduction in the population size – endangered species face a genetic bottleneck on a species-wide scale, and suffer lasting effects even if population size later recovers
. Microevolution is a deviation from Hardy-Weinberg equilibrium
consequences of small population size: genetic drift
two general mechanisms lead to small population sizes
founder effect – when a new population is established, typically only a few individuals (founders) are involved in colonizing the new area, essentially an “isolation bottleneck” for the new population; this is common for islands
. Microevolution is a deviation from Hardy-Weinberg equilibrium
migration – when individuals leave or join a population
migrating individuals carry their alleles with them (gene flow), usually resulting in changes in allele frequencies
gene flow tends to decrease genetic variation between populations
. Microevolution is a deviation from Hardy-Weinberg equilibrium
mutations increase variation in the gene pool of a species
remember that mutations may be neutral, harmful, or beneficial
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
. Microevolution is a deviation from Hardy-Weinberg equilibrium nonrandom mating
if individuals do not mate at random, then Hardy-Weinberg equilibrium is not achieved
the most common cases of nonrandom mating involve inbreeding – mating between individuals of similar genotypes, either by choice or due to environmental factors such as location
. Microevolution is a deviation from Hardy-Weinberg equilibrium
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
. Microevolution is a deviation from Hardy-Weinberg equilibrium
assortive mating – a type of nonrandom mating where mates are (sexually) selected based on phenotypes – really is an aspect of natural selection
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
. Microevolution is a deviation from Hardy-Weinberg equilibrium
natural selection changes allele frequencies in a way that leads to adaptation to the environment
fitness is the ability of an organism to compete successfully in passing its alleles on to the next generation (in a vessel that can continue that process)
populations undergoing natural selection are evolving, with alleles that contribute to better fitness increasing in frequency over successive generations
natural selection only operates based on the current environment – as conditions change, different alleles will be selected for
. Microevolution is a deviation from Hardy-Weinberg equilibrium sexual selection (mate choices based on inherited characteristics) is an aspect of natural selection
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• Describe conditions that can keep populations from establishing or maintaining genetic equilibrium.
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• Explain three main types of natural selection.
. Microevolution is a deviation from Hardy-Weinberg equilibrium
there are three types of natural selection
. Microevolution is a deviation from Hardy-Weinberg equilibrium
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
. Microevolution is a deviation from Hardy-Weinberg equilibrium directional selection – permits species to adapt to environmental change by favoring selection of one extreme over the other
example – peppered moth
. Microevolution is a deviation from Hardy-Weinberg equilibrium
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 the Tularosa Basin of New Mexico
Michael E.N Majerus, Nicholas I Mundy. Mammalian melanism: natural selection in black and white. Trends in Genetics Volume 19, Issue 11, November 2003, Pages 585-588
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• Explain three main types of natural selection.
. Chapter 23: Population Genetics (Microevolution) Microevolution is a change in allele frequencies or genotype frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg theorem
Microevolution is deviation from Hardy- Weinberg equilibrium
Genetic variation must exist for natural selection to occur .
• Discuss the importance of genetic variation for evolution, and the concept of neutral variation.
• Give a hypothetical example of how genetic variation that was once neutral may no longer be neutral.
. Genetic variation must exist for natural selection to occur
the ultimate source of genetic variation is mutations
once variation exists, it can be affected by independent assortment and genetic recombination during gamete formation
consider the cross AaBb x AaBb – 9 different genotypes arise
this involves only 2 alleles at 2 loci; if there were 6 alleles possible at just 5 loci, over 4 million genotypes are possible
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
. Genetic variation must exist for natural selection to occur
the demonstrated presence of two or more alleles at a given locus is genetic polymorphism
biologists have produced tools for studying the genetic polymorphism of populations at the molecular level (RFLP, DNA sequencing, etc.)
these tools can be used to demonstrate and study polymorphism in populations without necessarily knowing the specific genes involved
. Genetic variation must exist for natural selection to occur
genetic variation can be maintained by heterozygote advantage or hybrid vigor
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
…but when heterozygous genotypes have advantage over either of the homozygous genotypes, variation tends to increase in the population
example - sickle cell anemia and malaria resistance
. Genetic variation must exist for natural selection to occur
genetic polymorphisms can be maintained due to frequency- dependent selection
there are cases where the frequency of a given genotype affects the degree to which it is or isn't selected in the population
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
. Genetic variation must exist for natural selection to occur
much of the genetic variation in a population will produce no selective advantage – called neutral variation
the role of neutral variation in evolution is debated today
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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• Discuss the importance of genetic variation for evolution, and the concept of neutral variation.
• Give a hypothetical example of how genetic variation that was once neutral may no longer be neutral.
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