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

2. 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.

3. 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

4. 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.

5. 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

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

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

8. 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

9. 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.

10. 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?

11. 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.

12. 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

13. 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