1. Chapter 11 Evolution of Populations

2. Genes and Variation Darwin’s theory had 2 big gaps:
1. He had no idea how heritable traits passed from generation to generation
2. He had no idea how variation appeared
It wasn’t until the 1930’s that Darwin’s theories were connected to Gregor Mendel’s genetics principles
- focus then turned to genes as mechanism for passing heritable traits
- Watson and Crick’s contribution of DNA also helped explain the origins of mutation and variation

3. How Common is Variation?
Every gene has two alleles
Each gene that is heterozygous for a trait has a chance of showing variation
-Certain insects may have 15% of genome with heterozygous genes
-Most mammals are between 4 & 8%

4. Variation and Gene Pools
Population is a group of individuals of the same species that can successfully interbreed
- these members share a common group of genes
- Gene pool – accumulation of all the different genes that are present in a population
- Relative frequency – refers to the number of times an allele occurs in the gene pool
- has nothing to do with dominant or recessive
- Higher % means it occurs more often vs. lower % being less often
Evolution is any change in relative frequency of alleles in a population

5. Sources of Genetic Variation
Where does variation come from?
2 Main sources of genetic variation:
1. Mutation – any change in the sequence of DNA either internally or externally ( radiation or chemicals in environment)
2. Gene shuffling from sexual reproduction – crossing over during meiosis is major source of variation among eukaryotes
- 8.4 million combinations possible in humans
- even though many combinations are possible, does not alter the relative frequency of genes

6. Single vs. Polygenic
The number of phenotypes produced for a given trait depends on how many genes control it
Single –gene trait – has only two alleles that control the single gene
Polygenic traits – two or more genes control the expressing of a trait – more than 2 alleles
Ex: height of students in class

7. Evolution as Genetic Change
Evolutionary fitness is measured as an individuals success at passing genes to the next generation
Natural selection acts only on the organism not the genes
Organisms that produce many offspring keep their alleles in the gene pool and may increase the frequency of those alleles
Populations are the smallest entity, not the individuals that can evolve over time

8. Natural Selection on Traits
Natural selection on single gene traits can lead to changes in allele frequencies, or evolution
Ex: red or brown vs. black lizards

9. Natural Selection on Traits
Polygenic traits can be affected 3 ways by natural selection:
1. Directional selection – those at one end of curve have higher fitness vs. middle or other end
2. Stabilizing selection – favors those at the center of curve in terms of fitness
3. Disruptive Selection – those at the extreme ends have higher fitness than the middle group

10. Genetic Drift
Genetic drift occurs when allele frequencies change unpredictably or by chance
- can make an allele more common in the population
- separation to a new habitat can be a source of change – founder effect
*population will be genetically different from parent population

11. Gene Flow
Gene flow – movement of alleles from one population to another
- can increase genetic variation of the receiving population
- lack of gene flow from neighboring populations can increase the separation of species
Ex: animals migrate between populations and reproduce
for plants and fungi wind blown dispersal of spores and seeds

12. Are there conditions where evolution will not occur?
Hardy- Weinberg principle – states that allele frequencies in a population will remain constant unless one or more factors cause a change
-creates genetic equilibrium
-established in 1908 as the counter principle to evolutionary change

13. Hardy- Weinberg Principle
5 conditions must be met to maintain genetic equilibrium:
1. Random mating must occur
2. Population must be very large
3. No migration
4. No mutations
5. No natural selection
Rarely are all conditions met in nature – thus change is imminent or evolution will occur

14. Speciation Speciation – origin of a species
Species – group that can breed and produce fertile offspring
- common genes in the gene pool
- any changes that increase fitness will as change allele frequency as it passes from generation to generation

15. Isolating Mechanisms
Reproductive isolation – members of two populations cannot interbreed and produce fertile offspring
- leads to establishment of new species
Two types of reproductive isolation:
1. Behavioral – two populations in close proximity will not interbreed because of different courtship rituals, or reproductive strategies
2. Temporal – species reproduce at different times (either day, season, or year)

16. Isolating Mechanisms
Geographic (allopatric) isolation – populations are separated by a geographic barrier
Ex: rivers, mountains, or larger bodies of water
- isolation depends on organisms mobility
- can create new species with genetic changes not present in other group
- does not always guarantee formation of new species; if two population can still interbreed then still one species

17. Natural Selection in Nature

18. Speciation of Darwin’s Finches
Speciation of Galapagos finches occurred by:
- Founding a new population
- Geographic isolation
- changes in new gene pool
- reproductive isolation
- ecological competition
Collectively known as Adaptive radiation

19. Adaptive Radiation
evolution of many diversely adapted species from a common ancestor after being introduced into new environmental opportunities and challenges
Explains differences between each islands inhabitants and differences from having organisms in close proximity