chapter 23: population genetics (microevolution) · chapter 23: population genetics...

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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

http://www.auburn.edu/academic/classes/biol/1020/bowling/

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• 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


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(Microevolution)






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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


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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)


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(Microevolution)






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• 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


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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?


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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)


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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


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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?


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• Describe the assumptions of the Hardy-

Weinberg equilibrium model.


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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


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• Describe the assumptions of the Hardy-

Weinberg equilibrium model.


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(Microevolution)






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• Describe conditions that can keep

populations from establishing or

maintaining genetic equilibrium.


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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


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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.


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genetic drift is a change in gene frequencies of populations because of small population size


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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


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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


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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


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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


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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


http://cnews.canoe.ca/CNEWS/Science/2004/06/24/512617.html

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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


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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


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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


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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


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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.


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there are three types of natural selection

stabilizing selection

directional selection

disruptive selection


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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


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Hardy-Weinberg equilibrium directional selection – permits species to adapt

to environmental change by favoring selection of one extreme over the other

example – peppered moth


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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


http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TCY-49MX3DJ-7&_user=409620&_coverDate=11%2F30%2F2003&_rdoc=2&_fmt=high&_orig=browse&_srch=doc-info%28%23toc%235183%232003%23999809988%23466594%23FLA%23display%23Volume%29&_cdi=5183&_sort=d&_docanchor=&view=c&_ct=15&_acct=C000019518&_version=1&_urlVersion=0&_userid=409620&md5=8f7589fcb6f6bbf11dcd4d753b614f3e

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TCY-49MX3DJ-7&_user=409620&_coverDate=11%2F30%2F2003&_rdoc=2&_fmt=high&_orig=browse&_srch=doc-info%28%23toc%235183%232003%23999809988%23466594%23FLA%23display%23Volume%29&_cdi=5183&_sort=d&_docanchor=&view=c&_ct=15&_acct=C000019518&_version=1&_urlVersion=0&_userid=409620&md5=8f7589fcb6f6bbf11dcd4d753b614f3e

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• Explain three main types of natural

selection.


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(Microevolution)






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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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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


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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


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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




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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




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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.


chapter 23: population genetics (microevolution) · chapter 23: population genetics...

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