Understanding Biological Evolution and Population Genetics
Species: Organisms capable of interbreeding with each other naturally, but do not cross with members of another species. (If crosses occur, the offspring are infertile).
Population: A group of individuals of the same species found in the same place and time, and are interbreeding.
Gene pool: All alleles of all genes found in all individuals of a population. (The sum of the genotypes in a population).
Population genetics: The study of the frequency, distribution, and inheritance of alleles in a population.
Biological evolution: Any change in the proportions of genotypes in a population from one generation to the next. A continuous process of transformation of species over successive generations.
(This may explain the descent of modern organisms from earlier life forms).
(Evolution is a property of populations, not individuals).
Equilibrium population: A population where the allele frequency and genotype distribution do not change from one generation to another.
It is a hypothetical population that only exists under certain conditions…
Conditions for Equilibrium (Hardy-Weinberg Law)
- There must be no mutations.
- There should be no gene flow between populations.
- The population must be very large.
- Everything must be random mating (random).
- There should be no natural selection.
Causes of Evolution: Five Leading Causes
- Mutations: Changing a gene in DNA. Inherited mutations [when in the gametes] are the original or ultimate source of genetic variability, not the result of needs, and are not directed to an end, they just happen. They create new alleles and increase variability. If the change is helpful, harmful, or neutral in the future will depend on environmental conditions.
- Gene flow: The flow of genes between populations changes the frequencies of alleles, increasing the similarity of different populations.
- Genetic drift: In small populations, a fortuitous event (rain, hail, volcanic eruption, etc.) can produce a random change in allele frequency.
- Population bottleneck: For example, the northern elephant seal in the nineteenth century was hunted to near extinction. Although the number has increased, all northern elephant seals are genetically almost identical.
- Founder effect: When a small number of organisms establish isolated colonies. For example, a flock of birds that are lost during migration. Another example is the Mennonites of Lancaster County in Pennsylvania, who have a higher frequency of a genetic disease because they are descendants of a group of 200 Mennonites, among them a couple with the alleles of the disease.
- Nonrandom mating: By itself, it does not alter the frequency of alleles in a population, but certain genotypes are more common. Examples include inbreeding and assortative mating (with similar individuals).
- Natural selection: Favored or differential reproduction of organisms with genetic variations that help them cope with the environment. The fittest can reproduce and survive. It involves the elimination of individuals who do not have the necessary characteristics to survive and reproduce in an environment.