Microevolution

1

• Microevolution
• Evolution occurs when populations dont meet all
  the H-W assumptions
• Process by which a populations 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
  doesnt 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 loss of rare alleles

7
Fig. 23.8
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. 23.9
11
Fig. 23.10
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

13

• Microevolution
• Gene Flow
• Movement of fertile individuals or gametes among
  populations
• Tends to
• Increase diversity within populations
• Decrease diversity among populations
• Elevated gene flow can amalgamate separate
  populations into a single population

14
Fig. 23.12