1. Lesson Overview
17.1 Genes and Variation

2. Genetics Joins Evolutionary Theory
How is evolution defined in genetic terms?
In genetic terms, evolution is any change in the relative frequency of alleles in the gene pool of a population over time.

3. Genetics Joins Evolutionary Theory
Researchers discovered that heritable traits are controlled by genes.
Changes in genes and chromosomes generate variation.
For example, all of these children received their genes from the same parents, but they all look different.

4. Genotype and Phenotype in Evolution
Typical plants and animals contain 2 sets of genes, one contributed by each parent. These sets are homologous.

5. Genotype and Phenotype in Evolution
Specific forms of a gene, called alleles, may vary from individual to individual.

6. Genotype and Phenotype in Evolution
An organism’s genotype is the particular combination of alleles it carries.
An individual’s genotype, together with environmental conditions, produces its phenotype.
Phenotype includes all physical, physiological, and behavioral characteristics of an organism.

7. Genotype and Phenotype in Evolution
Some individuals have phenotypes that are better suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next generation.
Therefore, natural selection acts directly on phenotype, not genotype. This is due to the fact that it is an entire organism, not a single gene, that either survives and reproduces or dies without reproducing.

8. Populations and Gene Pools
Genetic variation and evolution are studied in populations, not individuals.
A population is a group of individuals of the same species that mate and produce offspring.
A gene pool consists of all the genes, including all the different alleles for each gene that are present in a population.

9. Populations and Gene Pools
Researchers study gene pools by examining the relative frequency of an allele.
The allele frequency is the number of times a particular allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

10. For example, this diagram shows the gene pool for fur color in a population of mice.
What is the allele frequency of the dominant B allele (black fur)? 40%
What is the allele frequency of the recessive b allele (brown fur)? 60%
The frequency of an allele has nothing to do with whether the allele is dominant or recessive.

11. Populations and Gene Pools
Evolution is any change in the relative frequency of alleles in the gene pool of a population over time.
Natural selection operates on individuals, but resulting changes in allele frequencies show up in populations.
Populations, rather than individuals, evolve.

12. Sources of Genetic Variation
What are the sources of genetic variation?
Three sources of genetic variation are mutation, genetic recombination during sexual reproduction, and lateral gene transfer.

13. Mutations
A mutation is any change in the genetic material of a cell. Mutations can involve changes within an individual gene or parts of a chromosome.
Neutral mutations are mutations that don’t change an organisms phenotype.

14. Mutations
Mutations that produce changes in phenotype may or may not affect fitness. Some mutations may be lethal or may lower fitness; others may be beneficial by improving an individual’s ability to survive and reproduce.
Mutations matter in evolution only if they can be passed from generation to generation. The mutation must occur in the germ line cells that produce either eggs or sperm.

15. Genetic Recombination in Sexual Reproduction
Mutations are not the only source of heritable variation.
You don’t look exactly like your parents and you look even less like your siblings (if you have them).
Most heritable differences are due to genetic recombination during sexual reproduction.
In humans, who have 23 pairs of chromosomes, meiosis can produce 8.4 million gene combinations!
What processes result in genetic recombination?

16. Genetic Recombination in Sexual Reproduction
Remember crossing over and independent assortment of chromosomes in meiosis?

17. Lateral Gene Transfer
Lateral gene transfer occurs when organisms pass genes from one individual to another that is not its offspring.
It can occur between organisms of the same species or organisms of different species.
Lateral gene transfer can increase genetic variation in a species that picks up the “new” genes.

18. Single-Gene and Polygenic Traits
What determines the number of phenotypes for a given trait?
The number of phenotypes produced for a trait depends on how many genes control the trait.

19. Single-Gene Traits
A single-gene trait is a trait controlled by only one gene. Single-gene traits may have just two or three distinct phenotypes.
The most common form of the allele can be dominant or recessive.

20. Dominance of an allele for a single-gene trait does not necessarily mean that the dominant phenotype will always appear with greater frequency in a given population.
An example of a single-gene trait is the presence of dark bands that appear on the shells of a certain species of snails. Even though the allele for shells without bands is dominant, a population may show a greater frequency of the “with bands” phenotype.

21. Polygenic Traits
Polygenic traits are traits controlled by two or more genes.
Each gene of a polygenic trait often has two or more alleles.
A single polygenic trait often has
many possible genotypes and even
more different phenotypes.
Often these phenotypes are not clearly
distinct from one another, like in skin color.

22. Polygenic Traits
Human height, which varies from very short to very tall, is an example of a polygenic trait.
The bell-shaped curve in the graph is typical of polygenic traits.