Lecture 9: Natural Selection on the Phenotype
- What is Natural Selection?
- Natural selection is differential survival or reproduction of individuals with different phenotypes
- High mortality or low reproduction within a population is not natural selection unless it is differential across individuals with different phenotypes [when food is in low supply everyone will suffer; there may not be selection]
- Evolution occurs when natural selection causes changes in relative frequencies of alleles in the gene pool
Natural selection acts directly on the phenotype, not the genotype
If a single locus has a clear influence on the phenotype, then we can track the effects of differential survival or reproduction on gene frequencies
- Industrial melanism a case of rapid evolution (dramatic change in allele frequencies in 50 years); example of selection for or against a particular allele
Sometimes natural selection acts to maintain genetic variation
- Sickle cell anemia: heterozygote advantage; balanced polymorphism
3. Modes of selection: Natural selection acts in 3 ways on the distribution of phenotypes (Fig. 23.11)
- Stabilizing selection: an intermediate phenotype is favored → reduced phenotypic variation
- Directional selection: one end of a phenotypic distribution is favored → shifts the mean phenotype
- Diversifying selection: favors phenotypes of opposite extremes; disfavors intermediate phenotypes → expands the phenotypic distribution and can lead to 2 peaks in the frequency distribution
- No trait is adaptive in all environments
eg. brain size
- The way that natural selection acts on any trait depends on the environment and the particular organism
- Differential environmental situations favor different phenotypes
5. Kin selection: selection for traits which increase the survival of close relatives (survival and reproduction of close relatives increases ones inclusive fitness)