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Topics for February 9

Hardy-Weinberg: the assumptions that must be met for the Hardy-Weinberg principle to be true.

Genetic diversity in natural populations: how much diversity is there?

Natural selection:

variation, inheritance, fitness differences --->selection

Hardy-Weinberg: an example.

Consider a population in which there are 500 diploid individuals, and that the frequency of A is: p=0.60, and the frequency of a is: q=0.40, then we expect:

Frequency of AA=p2=0.36 (180individuals)

Frequency of Aa=2pq=0.48 (240 individ.)

Frequency of aa=q2=0.16 (80 individuals)

Summary of what the Hardy-Weinberg principle predicts.

For a particular gene locus, there may be 2 different alleles. If so, the frequency of one allele (A) is p and the frequency of the second (a) is q. The allele frequencies are p and q. (p+q=1)

The expected genotype frequencies are: frequency of AA=p2, frequency of Aa=2pq, and the frequency of aa=q2 .

An example with 500 diploid individuals.

The following data agree with the prediction of the Hardy-Weinberg principle:

500 diploid individuals, of which 180 are AA, 240 are Aa, and 80 are aa. For this case, p=0.60 and q=0.40 (as seen above).

The observed frequencies of the genotypes are p2=180/500=0.36, 2pq=240/500=0.48, and q2=80/500=0.16.

In this example, the data agree with Hardy-Weinberg predictions

Are the numbers always so precisely "right"?

No. There is usually statistical variation.

Hardy-Weinberg predicts that given gene frequencies of p and q, the frequencies of the genotypes will be p2, 2pq, and q2. We would use a particular statistical test to see if the data are really good enough to say that the expected equilibrium is actually being met.

A summary of Hardy-Weinberg (so far...)

Gene (or allele) frequency: the relative abundance of an allele at its locus

Genotype frequency: the relative abundance of a genotype in the population

Hardy-Weinberg predicts the expected relationship between allele and genotype frequencies, if...

5 conditions are met. (migration, mutation, selection, size, mating)

The conditions which must be met for Hardy-Weinberg

Condition 1: Large population (no small scale effects due to chance allowed.)

Condition 2: An isolated population (no alleles coming from neighbors.)

Condition 3: No mutations (which would introduce new alleles.)

Condition 4: Random mating.

Condition 5: No natural selection.

If these conditions are met, then what?

1. There will be a predictable relationship between gene frequencies (p and q) and genotype frequencies (p2, 2pq, q2).

2. These frequencies will remain unchanged from one generation to the next.

Why are these conditions necessary?

No migration and no mutation means the frequencies of the alleles in a population won’t change by addition. There is no new source of alleles.

No selection means the frequencies of the alleles in a population won’t change by subtraction. No alleles are removed because they have a negative effect.

Why are these conditions necessary? (cont.)

Large population means there will be no change in allele frequencies because of small scale sampling effects. No change in frequency due to chance.

Random mating means that each genotype has an "equal" chance to occur. That is, we expect AA to be pxp, Aa (and aA) to be 2xpxq, and aa to be qxq.

Are the conditions of Hardy-Weinberg always met?

NO. Hardy-Weinberg provides the benchmark, or null-hypothesis against which real population genetics may be compared.

In real populations, the necessary conditions for Hardy-Weinberg to be met are often not true. Nevertheless, the H/W principle is useful to evaluate the departures from the standard model.

An example of a departure from Hardy-Weinberg

The allele which causes PKU originates by mutation (rare) and produces a carrier.

When two carriers happen to marry and have a child with PKU (homozygous recessive), the child will die before reproducing (no longer the case now).

The result is a balance: the PKU allele is introduced into the human gene pool by mutation, and removed by selection.

How much genetic diversity is there in typical populations?

LOTS! Excepting unusual cases like cheetahs, all natural populations of plants and animals have a lot of genetic diversity.

Methods to identify the presence of genetic diversity include: breeding experiments, karyotype comparisons, DNA testing.

The results: typically, 5% to 20% of all gene loci are have 2 or more alleles.

The genetic diversity is important

Genetic diversity in populations means that each individual is genetically unique.

If there is a change in environmental conditions, some of the individuals may have genotypes which give them a reproductive advantage.

Reproductive advantage (based on genotype) is the basis of natural selection.

How do we know that there is genetic diversity?

Direct observation: Peppered moth (p229), Cepaea snails (p224), and horses (p240).

Artificial breeding experiments: Cole vegetables (cabbage, broccoli, brussels sprouts, etc.) and domestic dogs.

Molecular biology: gel electrophoresis of soluble proteins; DNA sequencing (p208)

Where does genetic diversity come from?

The ultimate source of new alleles is from mutation. New alleles with small or large differences occur by accidents during DNA replication or during mitosis/meiosis. Most such mutations result in defective genes, but a few may confer an advantage to the individual that inherits it.

Is this the only source? No.....

A second source of genetic diversity: sex.

Because of sexual reproduction, existing alleles can be recombined in new ways. It is often the combination of existing alleles in new ways that produces possible advantage.

Given a large number of different alleles at many different loci, sexual reproduction can produce an astonishing number of new combinations.

Are new alleles beneficial?

Usually not: new mutations are commonly deleterious and removed by selection.

However, if a change in the environment produces a new ecological context, an existing allele which had been deleterious can become advantageous.

It depends on: (1) what an allele does and (2) the current environmental context

Exactly what is natural selection?

Natural selection occurs whenever some individuals have more than their share of offspring because they possess certain genetically determined traits.

In this context, fitness is measured by successful reproduction. More offspring means better fitness. Fewer offspring means poorer fitness.

The ingredients for natural selection.

Natural selection occurs if:

(1) Variation: If there are phenotypic differences among individuals in a population, and

(2) Inheritance: If the phenotypic differences are based on genetics, and

(3) Fitness differences: If some individuals have more offspring because of (1) and (2).

Does natural selection cause evolution?

Evolution is change in the genetics of a population.

Natural selection can cause genetic change if the population is not in equilibrium with environmental conditions (cause evolution).

Natural selection can maintain current conditions if a population is in equilibrium with its environment (maintain status quo).

An example of natural selection and no change in the gene pool.

An example of a balance maintained by natural selection. The case of PKU.

mutation--->gene pool--->selection

Mutation repeatedly adds a deleterious allele to the gene pool and selection repeatedly subtracts it. The result for the population as a whole is a balance: gene frequency doesn’t change.

An example of natural selection and no change: phenylketonuria.

The allele which causes PKU in humans originates by mutation. During DNA replication (perhaps during meiosis) the gene which codes for a particular enzyme for processing phenylalanine may be copied incorrectly.

Although rare, the mutation occurs. As a result, mutation adds to the number of carriers for PKU in the population.

PKU: a balance between mutation and selection

PKU alleles accumulate in the gene pool because of mutation.

When two carriers have children, about 1/4 of the children will be afflicted with PKU. Because of the PKU, these children do not live long enough to reproduce. Therefore, compared to other parents, carriers for PKU will, on average, leave fewer offspring.

But is this natural selection?

Yes. All 3 ingredients are present.

(1) Variation: Some individuals suffer from PKU, others do not.

(2) Fitness differences: Individuals with PKU do not reproduce.

(3) Inheritance: The PKU is based on the defective allele (in this case, recessive).

The result is: balance, not change. Nevertheless, this is natural selection.

Example of evolution (change) caused by natural selection (p228).

Populations of peppered moth (Biston betularia) have changed in historical time. This change (i.e. evolution) has been produced by natural selection.

The ingredients: variation (some moths are peppered, some black); fitness differences (moths are likely to survive if they have good camouflage); inheritance (the variation has a basis in genetics).

Why does natural selection cause change in this case?

The genetics of this population have changed over time. Evolution has occurred because of a change in the environment: good camouflage depends on fitting in with the background, and the background changed because of air pollution. (see photos, p229). (In other respects, this case is very similar to the PKU example: the unusual allele occurred by mutation, etc.)

History of the peppered moth

Originally (before 19th century), nearly all moths were peppered and were well camouflaged against the lichen covered tree trunks.

Because of occasional mutation, a few black moths appeared in the population, but remained rare. We presume they were selected against at that time.

The effect of air pollution

Coal burning during the industrial revolution in England killed the lichens and stained the tree trunks black.

Subsequently, the relative abundance of black moths increased in the population. In areas with heavy air pollution, the black variety became predominant.

Studies to interpret this event (p229)

Scientists (G. Kettlewell and students) have conducted experiments to test the interpretation that camouflage is important to the moths.

Experiments: capture-recapture studies with both color types in areas with or without air pollution; genetic studies; direct observation of bird predators and moths.

Results of studies

More of the peppered variety were recaptured in areas free of air pollution; more of the black variety were captured in areas with air pollution.

Direct observation from blinds: birds found the more poorly camouflaged moths faster.

Genetic studies identified two genes that influence the color pattern of the moths.

Table of Kettlewell’s results

Current experience

Because of air quality laws, air pollution has declined.

The shift from peppered to black variety of moth has been reversed: the peppered variety is now becoming more common again.

Is this natural selection? Is this evolution?

YES and YES

Natural selection: all 3 necessary ingredients are present: variation, fitness differences, and inheritance.

Evolution: the population genetics have changed in response to environmental change.

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