Biology 106: Lecture Topic # 9 Microevolution: Genetics of Populations Cont.

I. Conditions of H-W equilibrium (No Evolution)

A. Random mating

B. No Mutation

C. No Migration or gene flow

D. Large Population

E. No Selection

II. Non-Random Mating **

A. No change in allele freq., only genotype freq., i.e., AA, Aa, aa

B. Assortative - like with like

Assortative mating reduces genetic variation, eg:

1. Inbreeding (relatives mate)

a. selfing most extreme form of inbreeding.

2. Consequences of Inbreeding

a. increased freq. of homozygotes

Example: phenylketonuria; Weak mindedness (King George III)

freq. of (a) = 0.001 or 1/1,000

only aa individuals express disease

In random mating population:

if q = 0.001, then freq. of aa = qxq = 1/million

In selfing population:

no heterozygotes

freq. of aa = q = 1/1000

Inbreeding is deleterious, most organisms avoid inbreeding

C. Disassortative- dissimilar types mate

1. Maintains genetic variation

III. Mutation

A -----------> a

mutation

- Mutations occur at low rate. Little effect on allele freq.

IV. Migration or Gene Flow

AA AA AA AA aa aa aa

AA AA AA aa aa aa aa

- How many migrants?

- Differences between populations?

V. Large population size

IF population consisted of 200 indiv. versus a popln consisting of 20 individuals

In popln of n individuals, 2n copies of locus; gametes are a sample of this gene pool

Sampling variance increases with decreasing sample size: obs. in lab this week

Direction of change is RANDOM=DRIFT

Why are populations small?

Population Bottleneck = population periodically crashes in size

Founder Effect = one or a few individuals start new population

VI. NO Selection

Agents of selection:(1) environment, abiotic and biotic component, (2) random events

VII. Take Home Message: Natural selection is not the only force to cause allele frequencies at a particular locus to change.