Chapter 17 Evolution of Populations 17.1 Genes and Variation

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

Genetics Joins Evolutionary Theory By the 1930’s, researchers discovered that heritable traits are controlled by genes. They realized that changes in genes and chromosomes generated variation. By the 1950’s, Watson & Crick’s model of DNA helped to demonstrate the molecular nature of variations and mutations.

Genotype and Phenotype in Evolution Recall that 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 It is important for us to remember that 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 Genetic variation is studied in populations. Recall that a population is a group of individuals of the same species that mate and produce offspring. Since they interbreed, they share 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.

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. It is often expressed as a %.  

Variation and Gene Pools For example, the relative frequency of the dominant B allele is 40%. We get that by adding one half of the heterozygous population or 24% to all of the homozygous black population or 16%. What is the relative frequency of the recessive b allele? Copyright Pearson Prentice Hall

Populations and Gene Pools In genetic terms, evolution is defined as any change in the relative frequency of alleles in the gene pool of a population over time. For example, if the frequency of the B allele changed from 40% to 30% in the mouse population, we would say the population is evolving.   Natural selection operates on individuals, but resulting changes in allele frequencies show up in populations. Populations, rather than individuals, evolve!

Sources of Genetic Variation: 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 cells that produce either eggs or sperm.

Sources of Genetic Variation Another source of genetic variation is genetic recombination during sexual reproduction. The 23 pairs of chromosomes in the gametes can produce millions of different gene combinations upon fertilization. Crossing-over, which occurs during meiosis and the production of gametes, increases the number of genotypes (gene combinations) that can appear in offspring.

Sexual reproduction can produce many different phenotypes Sexual reproduction can produce many different phenotypes. However, it does not change the relative frequency of alleles in a population. Since it does not change the frequencies of alleles, It does not cause evolution. (It merely increases variation.) Natural selection causes evolution.

Sources of Genetic Variation: 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 occurs between bacteria during transformation. Lateral gene transfer can increase genetic variation in a species that picks up the “new” genes.

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 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.