Chapter 22 How Genetic Variation is Maintained within Populations Evolution Chapter 22 How Genetic Variation is Maintained within Populations

Genetic Variation Neutral allele Some mutations do not affect the function of the proteins Does not affect the fitness of organism Alleles that are unaffected by natural selection Tend to accumulate in a population Provides genetic variation Allele is no better or worse than the alternative alleles Phenotypic variation that we observe is not neutral.

Genetic Variation Sexual recombination amplifies the number of possible genotypes Crossing over Independent assortment Combination of genetic material during fertilization Generates endless variety of genotypic combinations Disadvantages of Sexual Recombination Recombination breaks up adaptive combination of genes Sex reduces the rate at which females pass genes on Dividing offspring into separate genders reduces the overall reproductive rate

Genetic Variation Frequency-dependent selection Polymorphism two or more different phenotypes exist in the same population of a species Fitness of a genotype depends on its frequency in a population When a genotype is rare, it is relatively favored by selection and it will increase in frequency as it becomes more common, its fitness decreases and there may come a point at which it is no longer favored.

The frequency of the scale-eating cichlid, Perissodus microlepis, oscillates around 50%. Each year, the prey species learn to look over a particular "shoulder" to guard against predation by the scale-eaters. The rarer morph (e.g., Left-jawed in 1984) has an advantage and increases in frequency. The following season the other morph becomes rare (e.g., right-jawed in 85) in the following season

Genetic Variation Environmental variation favors genetic variation No single genotype can perform well under all conditions

Constraints on Evolution Lack of genetic variation If the allele for a trait does not exist in the population, the trait cannot evolve.

Constraints on Evolution Developmental processes All evolutionary innovations are modifications of previous existing structures lineage’s development may limit the sorts of phenotype that it can evolve

Constraints on Evolution Trade-offs For an adaptation to evolve, fitness benefits must exceed fitness cost organisms have an energy budget that they can “spend” in different ways. for example, can invest in growth early on or can invest in developing large gonads egg size can vary too (next slide) But energy is limiting – it’s a constraint. So tradeoffs occur.

Constraints on Evolution Trade offs: Number of eggs vs. size of eggs Allocation: many eggs or few? Degree of protection of the young parental care or not Effort: one time or many?

Constraints on Evolution Short-term & Long-term evolutionary outcomes sometimes differ Patterns of change influenced by: Events that occur infrequently Meteorite impact Events that occur slowly Continental drift Different lineages may evolve in different directions.

Human Influence on Evolution Humans have changed environment Selective forces have also changed Examples: few are killed by predators, bad weather Differences in survival & reproductive success related to health conditions Hypertension, diabetes, etc. Attempt to control species considered pests & increase species considered desirable Undesirable consequences Pathogens resistant to antibiotics & pests to pesticides

Human Influence on Evolution Some changes are unintentional Hunters are pushing their prey to evolve faster than they would naturally Results in smaller and younger individuals over time Moved thousands of species around the globe Modified organisms using biotechnology Activities changed the climate Increased the rate of extinction