Lesson Overview 17.1 Genes and Variation

THINK ABOUT IT Darwin developed his theory of evolution without knowing how heritable traits passed from one generation to the next or where heritable variation came from.   What would happen when genetics answered questions about how heredity works?

Genetics Joins Evolutionary Theory How is evolution defined in genetic terms?

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.

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.

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.

Genotype and Phenotype in Evolution Natural selection acts directly on phenotype, not genotype.   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.

Populations and Gene Pools 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.

Populations and Gene Pools Researchers study gene pools by examining the relative frequency of an allele. The relative frequency of an allele 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.  

For example, this diagram shows the gene pool for fur color in a population of mice.  

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.

Sources of Genetic Variation What are the sources of genetic variation?

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.

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.   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.

Genetic Recombination in Sexual Reproduction 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.   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.

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.

Single-Gene and Polygenic Traits What determines the number of phenotypes for a given trait?

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.

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.

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.

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.

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.