Microevolution and its processes

1. 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 • Mutation • Nonrandom mating • Natural selection • Genetic drift • Gene flow

2. Microevolution • 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

3. Microevolution • 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

4. Microevolution • Nonrandom Mating • 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 • 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

5. Microevolution • 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

6. Microevolution • 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

7. Fig. 23.9

8. Microevolution • 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

9. Microevolution • Genetic Drift • 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

10. Fig. 23.10

11. Fig. 23.11

12. Microevolution • Genetic Drift • Bottleneck effect • Ex: Elevated frequency of Tay-Sachs Disease in Ashkenazi Jews • Ex: Genetic homogeneity in populations of African cheetahs • 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