Microevolution

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