Population Genetics Notes 1

 There are an excess of progeny

 Types of Variation

      Discontinuous

      Continuous

      The hereditary code carried by an individual is the genotype. 

      The observable expression of the genotype is the phenotype.

      Phenotypic plasticity

 

Genes=informational units of the DNA molecule

Alleles=genes occupying the same locus on a pair of chromosomes.

Homozygous=each member of a pair of alleles affects a trait in the same manner

Heterozygous=each affects a trait in a different manner

Major source of genetic variation is recombination.

Mutations

Macromutations

Polyploidy = duplication of sets of chromosomes (e.g., 4n)

Deletions tend to be lethal (e.g.cri-du-chats syndrome

Duplications are frequently harmless

Inversions prevent recombination

Translocation is exchange of sequences between two nonhomologous chromosomes

Micromutations=alteration of one or a few nucleotides

Homozygous=individuals in which alleles are identical (AA or aa)

Heterozygous=individuals in which the alleles are not identcal (Aa)

If one mates randomly mates equal populations of homozygous dominants and homozygous recessives the expected genotypic frequencies of the offspring are 0.25AA,0.25aa, and 0.5Aa.The expected allele frequencies are 0.5A and 0.5a.

 

Hardy-Weinberg Equilibrium

Proportions will be maintained under the following conditions:

1.mating is random;

2.mutations do not occur or are balanced;

3.the population is closed;

4.population size is infinite;

5.no natural selection occurs

 

p2 + 2pq + q2 = 1

p + q = 1

For 3 alleles: p2 + 2pq + q2 + 2pr + 2qr + r2 = (p + q + r)2

 

Natural Selection

Nonrandom reproduction

 

Fitness = W

Selection coefficient, s = 1-W

where the most productive genotype = 1

If AA produces 250 and BB produces 200 progeny

then W for AA = 1 and W for BB = 0.80

 

 

Stabilizing Selection

Disruptive Selection

Directional Selection

Biston betularia

Group Selection

Interdemic selection: Given an altruistic allele that occurs with a frequency of p=0.5 where demes have frequencies of p ranging from 0.3-0.5 the global frequency of p may increase if altruism increases the overall reproductive success of the deme.

 

Group

1

2

3

4

5

P

.3

.4

.5

.6

.7

P1

.29

.39

.49

.59

.69

N

110

10

10

10

10

N1

16

17

19

21

22

 

(adapted from Wilson 1983)

 

Genes for altruism can be selected for when:

1.k>1/r, where k may be considered the ratio of fitness to loss of fitness

2.where r is the proportion of genes that are identical by common descent, and

3.k = b/c where b = recipient’s benefits, and c = altruist costs

 

r for identical twins = 1

r for parent offspring = 0.5

r for full sib = 0.5

r for half sib aunt,uncle, niece or nephew = 0.25

r for first cousin = 0.125.

(adapted from Smith 1996)

 

If one sacrifices oneself for 6 full sibs, then 6/1>1/.5, and the altruistic

behavior is to one's selective advantage.

How many cousins would you have to save?

 

An individual's direct reproductive success + that of of its relatives possesing genes that are identical by common descent is called the inclusive fitness.

 

In Hymenoptera, females have all the genes from their father (r = 1) and

one-half their mother’s genes (r = 0.5)

 

So for full sisters, r = 0.75.

 

Therefore, females increase their own fitness most by helping their mother

produce more sisters rather than producing offspring.

 

*Cooperative Breeding (birds: e.g., Red-cockaded woodpecker)