By Basel Kamoua
Genes and Variation
Darwin did not know how heredity worked.
This lack of knowledge led to: No idea how heritable traits pass from one generation to the next, and had no idea how that variation appeared.
Mendel’s work on heredity was connected Darwin’s during 1930s.
Biologists understood that genes control heritable traits.
Soon realized that changes in genes produces heritable variation on which natural selection can operate.
Watson and crick’s studies in DNA helped evolutionary biologists because it demonstrated the molecular nature of mutation and genetic variation.
Molecular techniques are used to test hypotheses about how heritable variation appears and how natural selection operates on that variation.
Fitness, adaptation, species, and evolutionary change are now defined in evolutionary terms.
How Common is Genetic Variation?
Many genes have at least two forms, or alleles.
Animals such as dogs, horses, and mice often have several alleles for traits such as body size or coat color.
Plants, such as peas, often have several alleles for flower color.
Involves small differences in biochemical processes.
Individual organisms is heterozygous for many genes.
Insect may be heterozygous for as many as 15 percent of its genes.
Fish, reptile, and mammals are typically heterozygous for 4-8 percent of their genes.
Variation and Gene Pools
A population is a group of individuals of the same species that interbreed,
Because they interbreed they share a common group of genes called a gene pool.
Gene Pool = Consists of all genes, including all the different alleles, that are present in a population.
Relative frequency of an allele is the number of times that the allele occurs in a gene pool compared with the number of times other alleles for the same gene occur.
Often expressed as a percentage.
Gene pools are important to evolutionary theory because evolution involves changes in populations over time.
In genetic terms, evolution is any change in the relative frequency of alleles in a population.
Sources of Genetic Variation
The two main sources of genetic variation are mutations and genetic shuffling that results from sexual reproduction.
Any change in a sequence of DNA.
Can occur because of mistakes in the replication of DNA or as a result of radiation or chemicals in an environment.
Can affect an organism’s fitness, or its ability to survive and reproduce in its environment.
Can have no effect on fitness.
Most heritable differences are due to gene shuffling that occurs during the production of gametes.
Each chromosome of a homologous pair moves independently during meiosis.
Crossing-over occurs during meiosis.
Crossing-over further increases the number of different genotypes that can appear in offspring.
Genotype is an organism’s genetic makeup.
When alleles are recombined during sexual reproduction, they can produce dramatically different phenotypes.
Sexual reproduction does not change the relative frequency of alleles in a population.
Single-Gene and Polygenic Traits
The number of phenotypes produced for a given trait depends on how many genes control the trait.
A single-gene trait is a single gene that has two alleles.
Widow’s peak is an example of a single-gene trait.
Allele for a widow’s peak is dominant over the allele for a hairline with no peak.
Many traits are controlled by two or more genes and are called polygenic traits.
Each gene of a polygenic trait often has two or more alleles.
Height in humans is one example of a polygenic trait.
16-2 Evolution as Genetic Change
Each time an organism reproduces, it passes copies of its genes to its offspring.
Natural selection never acts directly on gens.
It can only affect which individuals survive and reproduce and which do not.
If an individual dies without reproducing, the individual does not contribute its alleles to the population’s gene pool.
Natural Selection on Single - Gene Traits
Natural selection on single-gene