1. Unit 9: Evolution
9.5 Genes and Variation

2. Quick Review
Remember that heritable traits are controlled by genes that are carried on chromosomes.
These changes to genes on the chromosomes generate the variation in a population.
Typical plants and animals contain 2 sets of alleles-one from each parent
The genotype is the combination of alleles an organism carries
The phenotype is the physical trait you see; it’s an interaction between the genome and the environment.

3. Genetics and Natural Selection
Natural selection acts directly on phenotype, not genotype.
Some individuals have phenotypes that are better suited to their environment than the phenotypes of other individuals.
The better suited individuals produce more offspring than the less fit individuals do.
Therefore, the better traits get passed on to the next generation.
Natural Selection never acts directly on the genes because the entire organisms-not individual genes-either survives to reproduce or dies without reproducing.

4. Populations and Gene Pools
A population is a group of individuals of the same species that mate and produce offspring.
Because members of a population breed, they share a common group of genes called a gene pool.
A gene pool consists of all the genes, including all the different alleles for each gene, that are present in a population.
Researchers study gene pools by looking at the number of different alleles they contain.
Allele frequency is the number of times an allele occurs in the gene pool compared to the total number of alleles in that pool for the same gene.
Evolution, in genetic terms, involves a change in the frequency of alleles in a population over time.

5. Example There are 50 mice total in this population.
40% have the allele for black fur
60% have the allele for brown fur
If, over time, the brown fur allele drops to 30% and the black fur allele rises to 70%, we can conclude that the population is evolving.
Because evolution is looking at the change in gene frequency of a population over time, by definition, individuals can’t evolve!

6. Other Sources of Genetic Variation
Mutations: a change in the genetic material of the cell.
Can change the phenotype and be harmful causing the individual to die without reproducing
Can change the phenotype and be beneficial.
New research estimates that each human is born with about 300 mutations that makes us different from our parents. Only one or two may be harmful/beneficial. The rest are neutral.
Genetic Recombination in Sexual Reproduction
Each chromosome separates independently of each other during gamete formation. (In humans-this creates 8.4 million gene combinations!)
Crossing-over: paired chromosomes swap sections of DNA
Lateral Gene Transfer: the passing of genes from one individual to another that is not their offspring.
Bacteria trade genes on plasmid. This helps to create antibiotic resistance in bacteria populations.

7. Single-Gene and Polygenic Traits
The number of phenotypes produced for a trait depends on how many genes control the trait.
Single-gene traits: a trait controlled by only one gene.
Example: the dark bands on the shells of snails
Polygenic Traits: a trait controlled by two or more genes.
Examples: height in humans
Tend to produce a bell curve when graphed