The Evolution of Populations

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In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible.

Slide 9

Hardy-Weinberg equation tests whether a sexually reproducing population is evolving

A population is a localized group of individuals (a species in an area) capable of interbreeding and producing fertile offspring.

A gene pool consists of all the alleles for all loci in a population.

A locus is fixed if all individuals in a population are homozygous for the same allele.

Slide 10

Hardy-Weinberg equations

The frequency of an allele in a population can be calculated.

If there are 2 alleles at a locus, p and q are used to represent their frequencies.

The frequency of all alleles in a population will add up to 1:

p + q = 1

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The Hardy-Weinberg Principle: a Population

The Hardy-Weinberg principle describes an ideal population that is not evolving.

The closer a population is to the criteria of the Hardy-Weinberg principle, the more stable the population is likely to be.

Calculating Genotype Frequencies

p2 + 2pq + q2 = 1

where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype.

Slide 12

The five conditions for nonevolving populations are rarely met in nature:

No mutations

Random mating

No natural selection

Extremely large population

No gene flow

Hardy-Weinberg Ideal Conditions

Slide 13

Applying the Hardy-Weinberg Principle

We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:

The PKU gene mutation rate is low

Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions

The population is large

Migration has no effect as many other populations have similar allele frequencies

Slide 14

The occurrence of PKU is 1 per 10,000 births

q2 = 0.0001

q = 0.01

The frequency of normal alleles is

p = 1 – q = 1 – 0.01 = 0.99

The frequency of heterozygotes / carriers is

2pq = 2 x 0.99 x 0.01 = 0.0198

or approximately 2% of the U.S. population.

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