1. Microevolution Chapter 18 contined

2. Microevolution  Generation to generation  Changes in allele frequencies within a population  Causes: Nonrandom mating Mutations Genetic drift Gene flow Natural selection

3. Nonrandom mating  Inbreeding Impacts the entire genome, not just certain alleles Individuals are more likely to mate with neighbors than distant members of the population  increases chances of genetically similar individuals mating Common in plants (self-fertilization an extreme example) In animals it often causes inbreeding depression, where offspring are less fit

4. Nonrandom mating con’t…  Assortative mating Mates are chosen based on a particular phenotype Only impacts the genes involved in the selected phenotype  Positive  select mate with the same phenotype  Negative  select mate with the opposite phenotype

5. Mutations  Changes in: Nucleotide base pairs Arrangement of genes on a chromosome Chromosome structure  Only mutations in gametes are inherited  Most mutations are silent: Only a small % of the DNA is expressed Mutations that are expressed are usually harmful Mutations do NOT cause evolution, but natural selection needs the variations mutations create

6. Genetic Drift  It is ‘easier’ to lose a rare allele in a small population due to chance  This may produce random evolutionary changes  It can decrease genetic variation within a population  It can also increase genetic variation between different populations

7. Genetic Drift con’t…  Bottleneck Can occur if the population decreases suddenly – causes a dramatic change in allele frequencies  Founder effect Decreased variation in a small population that has broken off from the parent population

8. Gene Flow  Due to migration of breeding individuals from one population to another  Isolated populations tend to be different from surrounding populations – increased gene flow changes this: Makes the population internally more varied Makes the population less varied from other populations

9. Natural Selection  Occurs over time  Increases the frequency of favorable, adaptive traits  ‘weeds out’ less adapted traits  Only operates on the phenotype, not the genotype  Phenotypes are usually due to interactions of genotypes and the environment  Most polygenic phenotypes show a normal distribution – most of the population is in the middle, with fewer at either extreme  Natural selection can only ‘work’ if there is pre- existing variation within the population

10. Types of Natural Selection:  Stabilizing Selection Population is well-adapted to its environment Selection is against phenotypic extremes

11. Types of Natural Selection…  Directional Selection Changes in the environment cause selection of a particular extreme phenotype so that one phenotype gradually replaces another

12. Types of Natural Selection…  Disruptive Selection Extreme changes in the environment may favor more than one phenotype

13. Genetic Variation in Populations  Sources: Mutation Sexual reproduction  crossing-over  independent assortment of chromosomes  random union of gametes

14. Genetic Polymorphism:  The presence of 2 or more alleles in a population  May not be evident if it does not produce distinct phenotypes  Balanced polymorphism 2 or more alleles persist due to natural selection May be due to heterozygous advantage  Sickle cell anemia is actually selected for in area where malaria exists