1. Genes, Variations & Evolution

2. Gene Pools
Population – collection of individuals of the same species in a given area and is the smallest biological unit that can evolve (individuals can’t evolve)
Ex. All the students in SHS is an example of a population
All members of a population can interbreed and they share a common group of genes
Gene Pool – combined genetic information of all the members of a particular population / total collection of alleles
Gene pool contains two or more forms of a certain gene for each trait
Ex. Brown or black fur in mice

3. Relative Frequency
Relative Frequency – the number of times an allele occurs in a gene pool compared with the number of times the other allele(s) occur
When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale.
Sample Population
Frequency of Alleles
allele for brown fur
allele for black fur
48% heterozygous black
16% homozygous black
36% homozygous brown

4. Sources of Genetic Variation
Darwin couldn’t explain how variations occurred – he didn’t know about genetics
We know now that mutations and genetic shuffling that results from sexual reproduction are sources of variations

5. Mutations
Change in a sequence of DNA that occurs due to mistakes in DNA replication or from radiation/chemicals in the environment
Produce changes in an organism’s phenotype (physical characteristics) – can affect fitness (ability to survive and reproduce in the environment)

6. Gene Shuffling
Chromosomes “move around” the cell during meiosis (formation of sperm and egg cells)
Crossing over of the chromosomes occurs during meiosis creating different genetic combinations
Sexual reproduction is a major source of variation within a population (even more so than mutations)

7. Single-Gene and Polygenic Traits
Number of phenotypes produced for a given trait depends on how many genes control that trait
Variable traits in a population may be
polygenic, resulting from the combined effects of several genes, or
determined by a single gene.
Polygenic traits tend to produce phenotypes that vary more or less continuously.
Single-gene traits tend to produce only a few distinct phenotypes

8. Single-Gene Trait
Trait controlled by one gene with two alleles (Ww) – variation leads to only TWO phenotypes
Ex. Widow’s Peak or no Widow’s Peak
100 80 60 40 20
Frequency of Phenotype
(%)
Widow’s peak
No widow’s peak
Phenotype

9. Frequency of Phenotype
Polygenic Traits
Traits controlled by two or more genes. Each gene of a polygenic trait has two or more alleles = many phenotypes possible.
Ex. Height in humans – short, tall, and everything inbetween. When the average is graphed, a “bell-curve” is created for that trait
Frequency of Phenotype
Phenotype (height)

10. Natural Selection & Genetics
****Natural Selection acts directly on phenotypes**** - affects which individuals having different phenotypes will survive and which will not
Natural selection does change relative frequencies of alleles in a population over time
Any factor that causes alleles to be added or removed from a population will change relative frequencies of alleles. If an individual dies without reproducing, its genes are removed from the population. If an individual reproduces a lot, that individual’s genes increase in the gene pool

11. Natural Selection & Genetics (continued)
Evolution is any change in the relative frequencies of alleles in a population’s gene pool
****Evolution acts on populations, NOT individuals

12. Natural Selection on Single-Gene Traits
Can lead to changes in allele frequencies and thus to evolution
Ex. Lizard color – brown color is normal, whereas red and black are mutations. These lizards live on dark soil. The brown and black lizards are hard for birds to see, but the red is easy for birds to see. Predict what will happen to the 3 variations of color…

13. Analyzing Gene Pools A gene pool
consists of all the alleles in a population at any one time and
is a reservoir from which the next generation draws its alleles.
Alleles in a gene pool occur in certain frequencies.

14. Analyzing Gene Pools (continued)
Alleles can be symbolized by
p for the relative frequency of the dominant allele in the population (ex. R),
q for the frequency of the recessive allele in the population (ex. r), and
p + q = 1.
Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele.

15. Analyzing Gene Pools (continued)
Genotype frequencies can be calculated from allele frequencies (if the gene pool is stable = not evolving).
The Hardy-Weinberg formula
p2 + 2pq + q2 = 1
can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles.
p2 = frequency of homozygote for one allele (ex. RR)
2pq = frequency of heterozygotes (ex. Rr)
q2 = frequency of other homozygote (ex. rr)

16. Analyzing Gene Pools (continued)
Alleles can be symbolized by
p for the relative frequency of the dominant allele in the population (ex. R),
q for the frequency of the recessive allele in the population (ex. r), and
p + q = 1.
Note that if we know the frequency of either allele in the gene pool, we can subtract it from 1 to calculate the frequency of the other allele.