1. Evolution of Populations
Chapter 16

2. The study of genetics helps scientists understand the relationship between inheritance and evolution
Scientists know that…
genes control traits and that many genes have at least two forms, or alleles
-members of all species are heterozygous for many genes

3. In genetic terms, evolution is any change in the relative frequency of alleles in a population

4. A population is a group of individuals of the same species that can interbreed
Members of a population share a gene pool
- all the genes, and their different alleles, in the population

5. The number of times the alleles occur in a gene pool compared to the number of times that other alleles for the same gene occur is the relative frequency of the allele

7. The two main sources of genetic variation are mutations and gene shuffling
A mutation is any change in a sequence of DNA
Gene shuffling occurs during gamete formation (crossing-over)

8. It can produce millions of different gene combinations, but doesn’t change the allele frequency in the population
Both mutations and gene shuffling increase genetic variation by increasing the number of different genotypes

9. The number of phenotypes for a trait depends on how many genes control the trait
A single-gene trait is a trait controlled by only one gene
If there are two alleles for the gene, two phenotypes are possible

10. In humans, a single-gene trait is the presence of a widow’s peak
Allele for a widow’s peak is dominant over the allele for hairline with no peak.
As a result, there are only two phenotypes – having a widow’s peak or not

11. A polygenic trait is controlled by two or more genes
Each gene of a polygenic trait may have more than one allele
Polygenic traits form many phenotypes
Variation in a polygenic trait in a population often forms a bell-shaped curve with most members near the middle
example: height in humans

12. 16.2 Genetic Change
Natural selection on single-gene traits can lead to changes in allele frequencies… microevolution

13. Normal Distribution
Frequency is highest near the mean value and decreases steadily toward extreme end of range
Population follows a normal distribution when it’s not under natural selection for the trait

14. Natural Selection on Polygenic Traits
Directional Selection takes place when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

15. Stabilizing Selection
Occurs when individuals near the center of the curve have higher fitness than individuals at either end of the curve.

16. Disruptive Selection
Occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle
Favors phenotypes at both extremes

17. Genetic Drift/Founder Effect
Random change in allele frequency
Looking at small populations
* Think about the way a natural disaster would lead to the founder effect.

18. Hardy-Weinberg Principle
Allele frequencies in a population will remain constant unless one or more factors cause them to change
Genetic Equilibrium

19. 5 Conditions to Maintain G.E.
Must be random mating
Population must be very large
No movement into or out of the population
No mutations
No natural selection