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 Changes in allele frequencies result from five evolutionary processes 1) 1)Mutation 2) 2)Nonrandom mating 3) 3)Natural selection 4) 4)Genetic drift 5) 5)Gene flow

I. I.Microevolution A. 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. I.Microevolution B. 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. I.Microevolution B. B.Nonrandom Mating 1. 1.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. 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. I.Microevolution C. 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. I.Microevolution D. 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. I.Microevolution D. 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. I.Microevolution D. D.Genetic Drift 1. 1.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

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I. I.Microevolution D. D.Genetic Drift 1. 1.Bottleneck effect Ex: Elevated frequency of Tay-Sachs Disease in Ashkenazi Jews Ex: Genetic homogeneity in populations of African cheetahs 2. 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