Chapter 18: Genetics and Evolution

Genetics and Variation

• A genetic definition of a species is a population of physically similar, interbreeding organisms that do not interbreed with other such groups.
• A population shares a common gene pool, which consists of all genes present in a population, including all alleles for each gene.
• Researchers describe gene pools by the numbers of different alleles they contain.
• Allele frequency is the number of times an allele occurs in a gene pool, as a percentage of the total occurrence of all alleles for that gene.

Genotype, Phenotype, and Evolution

• Evolution involves any change in the frequency of alleles in a population over time.
• Some individuals have phenotypes that are better suited to their environment than phenotypes of other individuals.

Sources of Genetic Variation

• Genetic variation is produced in three main ways: mutation, genetic recombination during sexual reproduction, and lateral gene transfer.
• Mutation is a heritable change in genetic information.
• Genetic recombination during sexual reproduction produces genetic variation through crossing over and shuffling of chromosomes during meiosis.
• Lateral gene transfer occurs when genes pass from one individual to another individual that is not its offspring.

Single Gene and Polygenic Traits

• The number of phenotypes produced for a trait depends on how many genes control the trait.
• Single gene traits are controlled by only one gene and produce only two or three distinct phenotypes.
• Polygenic traits are controlled by two or more genes and produce a range of phenotypes that form a bell curve.

Evolution as Genetic Change

• Natural selection works on single gene traits and polygenic traits.
• Directional selection occurs when individuals at one end of the curve have higher fitness than individuals elsewhere in the curve.
• Stabilizing selection occurs when individuals near the center of the curve have higher fitness than individuals at either end.
• Disruptive selection occurs when phenotypes at both upper and lower ends of the curve have higher fitness than individuals near the curve.

Genetic Drift

• Genetic drift occurs when individuals that carry a particular allele may leave more descendants than other individuals, just by chance.
• Genetic bottlenecks occur when a population experiences a dramatic reduction in size, leading to a change in allele frequency.
• The founder effect occurs when a few individuals colonize a new habitat, resulting in a new gene pool with allele frequencies that differ from those of the parent gene pool.

Evolution vs. Genetic Equilibrium

• If a population is not evolving, allele frequencies in its gene pool are not changing, and that population is in genetic equilibrium.
• The Hardy-Weinberg principle states that allele frequencies in a population should remain constant unless one or more factors causes those frequencies to change.

Process of Speciation

• Isolating mechanisms, such as behavioral, geographic, and temporal isolation, can lead to reproductive isolation and eventually speciation.
• Reproductive isolation can develop in several ways, including behavioral isolation, geographic isolation, and temporal isolation.
• Speciation in Darwin's finches occurred through the founding of a new population, geographic isolation, changes in the new population's gene pool, behavioral isolation, and ecological competition.### Speciation and Evolution
• The combination of founder effect, geographic isolation, and natural selection led to the evolution of a new species (Species A) from an initial population.
• This new species experienced differential reproductive success, adaptation, and evolution, resulting in distinct characteristics.

Gene Pools and Isolation

• When a few birds from Species A crossed to another island, they became geographically isolated, and their gene pools diverged from the original population.
• Over time, natural selection caused the isolated population to adapt and evolve, forming new populations with distinct phenotypes.

Behavioral Isolation and Competition

• Finches prefer to mate with birds that have the same beak size, leading to reproductive isolation.
• As two species live together on the same island, they compete for resources (seeds), and those with different beak sizes have higher fitness during dry seasons.

Molecular Evolution

• New genes can evolve through gene duplication, followed by modification of existing genes.
• Gene duplication can occur during crossing-over, resulting in an "extra" copy of a gene that can gain new functions over time.

Genetic Rearrangement

• Chromosomes are dynamic and can break, reconnect, lose/gain DNA sequences, and acquire DNA from viruses or other chromosomes.
• These changes can produce new genes that code for different proteins.

Developmental Genes and Body Plans

• Hox genes determine which part of an embryo develops into arms, legs, or wings, and control the sizes and shapes of structures.
• Small changes in Hox gene activity during development can produce large changes in adult animals.

Molecular Clocks

• Molecular clocks use mutation rates in DNA to estimate the time since two species diverged.
• Neutral mutations accumulate at a steady rate, allowing researchers to estimate the time elapsed since two species shared a common ancestor.
• Researchers calibrate molecular clocks by comparing DNA sequences and estimating mutation rates in species whose ages are known from other methods.