I. I.Microevolution Evolution occurs when populations don’t meet all the H-W assumptions Process by which a population’s genetic structure changes = microevolution.

I. Microevolution

• Evolution occurs when populations don’t meet all the H-W assumptions

• Process by which a population’s genetic structure changes = microevolution

• Changes in allele frequencies result from five evolutionary processes

1) Mutation

2) Nonrandom mating

3) Natural selection

4) Genetic drift

5) Gene flow

I. Microevolution

A. Mutation• Unpredictable change in nucleotide sequence of DNA• Spontaneous, unpredictable, permanent• Somatic mutations seldom passed to next generation• Most mutations harmless

• Neutral variation• Not reflected in phenotype• May be passed to next generation

• Expressed mutations tend to be harmful• May increase genetic variability and influence alleles

• Mutation rates low (1 in every 100,000 genes per generation)

• Minor impact compared to recombination

I. Microevolution

B. Nonrandom Mating• Occurs when individuals select mates on the

basis of phenotype• Choice• Proximity

• Individuals living closer tend to be more closely related (genetically similar) than individuals farther away

I. Microevolution

B. Nonrandom Mating1. Inbreeding

• Increases homozygosity• Ultimate: Self-fertilization (e.g. in plants)• May lead to inbreeding depression and reduced fitness

• Declines in fertility, increased juvenile mortality• Ex: White-footed mice brought into captivity and inbred had

significantly lower survivorship when released vs. non-inbred mice

2. Assortative Mating• Mates selected based on phenotype• Ex: Fruit flies with more bristles prefer other bristly flies and

vice-versa• Increases homozygosity• May lead to shifts in genotype frequencies but doesn’t add

variation

I. Microevolution

C. Natural Selection• Alters allele frequencies to increase adaptation

to environmental conditions• Allele frequencies tend to shift toward most favorable

alleles

• Individuals that survive and produce fertile offspring have a selective advantage

I. Microevolution

D. Genetic Drift• Results from random events that change allele

frequencies within a population• Small populations more prone to substantial

changes, including reduced variation and loss of rare alleles

• May lead to increased frequency or fixing of harmful alleles

Fig. 23.9

I. Microevolution

D. Genetic Drift• Random process; alleles may be lost or

preserved independently of benefit• Typically leads to loss of alleles decrease of

genetic diversity in population• If population decreases in size and loses

diversity, then increases in size, resulting large population may display influence of genetic drift when population was small

I. Microevolution

D. Genetic Drift1. Bottleneck effect

• Usually due to rapid, severe decline in population size followed by increase in population

• May produce allele frequencies very different from pre-bottleneck conditions

Fig. 23.10

Fig. 23.11

I. Microevolution

D. Genetic Drift1. Bottleneck effect

• Ex: Elevated frequency of Tay-Sachs Disease in Ashkenazi Jews

• Ex: Genetic homogeneity in populations of African cheetahs

2. Founder effect• Allele frequencies in small populations may reflect

genotypes of founding individuals• Common in isolated populations• Ex: Finns descended from small group of people

~4000 years ago; genetically distinct from other Europeans