General Ecology

Adaptation and Evolution cont: Population Genetics

Evolution in Populations

Evolution is often a process of adaptation.

Adaptation is not possible w/o genetic variation.

Organisms present a phenotype to the environment (selective regime). The phenotype is an expression of the genotype.

Evolution in Populations

The phenotypic variation we see can be a consequence of a number of things:– Genotypic or genetic variation.– Environmental variation.– Error

Basic Population Genetics

We have defined evolution as a change in allele frequencies over time.

The sum of all genes in a population is the gene pool.

We characterize the gene pool be measuring allele frequencies.

Popuation Genetics

In pea plants, there are red flowers and white flowers. Flower color is controlled by a single gene with 2 alleles.

R is dominant and r is recessive. RR and Rr individuals produce red

flowers while rr individuals produce white flowers.

Population Genetics ‘RR’ is homozygous dominant ‘Rr’ is heterozygous ‘rr’ is homozygous recessive. If there are 100 individuals in a

population, there are 200 flower color alleles.

The number of those alleles that are ‘R’ is the allele frequency of ‘R’, and 1 minus this is the allele frequency of ‘r’.

Population Genetics

If we note that the frequency of R has changed from .20 to .30 in 1 generation, then evolution has occurred.

Imagine we have 30 RR individuals, 20 Rr individuals, and 50 rr individuals.

Let p be the frequency of R and 1-p, or q, be the frequency of r. Note p + q = 1.

Population Genetics

For our example,

p 2 30 20200 0 4( ) .

Population Genetics

Also,

q 2 50 20200 0 6( ) .

Population Genetics

Note that p+q=1. Now, imagine that the individuals in this

population mate panmictically. What is the probability that a R allele

will combine with a R allele? This is simply p2. This is also the expected frequency of

RR individuals in the next generation.

Population Genetics

The probability of rr will be q2, and the probability of Rr is 2pq.

Note: there are 2 ways of getting Rr. You can get Rr or rR.

Note also that p2+2pq+q2 = 1 Finally, note that this is (p+q)2

Population Genetics

What happens? If we repeat this for many generations, we find that the system reaches equilibrium: a point at which allele frequencies no longer change. This is called Hardy-Weinberg equilibrium. In other words, allele frequencies will not change unless something happens.

Population Genetics

The system is dependent on a number of assumptions:– Panmictic breeding– Equal survival and reproduction of

individuals.– The population is closed.– No mutation.

Neither of these is likely to be true. Thus, evolution is inevitable.

Mechanisms of Evolution

What can disrupt HW equilibrium?– Forms of selection already discussed.– Genetic drift.

• Here, changes in allele frequencies can occur by chance alone. This is a small population size phenomenon. Effective population size is critical.

– Gene flow• This can wash out changes in allele

frequencies, introduce new alleles, or promote change.

Mechnisms of Evolution

Modeling natural selection Imagine the homozygous recessive

genotype does not survive as well as the heterozygote or homozygous dominant. We can denote this with a selction coefficient ‘s’.

If the fitness of the homozygous dominant and heterozygote are 1, then the fitness of the homozygous recessive is 1-s.

Mechanisms of Evolution

We can use algebra to model changes in allele frequency. Let p = change in frequency of p from original generation, to generation after selection has occurred. So, p = p – p’

We want to compute the new frequency of p, p’. We do this just as before, but know that some alleles were removed by selection.

Mechanisms of Evolution

The portion removed by selection is q2 * their selective disadvantage ‘s’. Thus, the number of alleles in the next generation is 1-sq2,

since (p2+2pq+q2)-sq2. Also, the total number of alleles is

2(1-sq2)

Mechanism of Evolution

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Mechanism of Evolution

Note that the rate of change in allele frequency is a consequence of the strength of selection (s) and the initial allele frequency (p and q).

Finally

We often characterize selection in a number of ways:

Density independent

Density dependent

Frequency independent

Hard Seln Density Depn

Frequency dependent

Freq. Depn

Soft Seln

Finally

Convergent evolution ESS: Evolutionary stable strategy. Wrights adaptive landscape.