Mechanics of Evolution
Natural Selection is the mechanism by which evolution occurs. The general idea is that in order for a species to survive and proliferate, it must adapt itself to the environment. Individuals do not evolve, populations evolve.
The main ideas of Natural Selection:
1. Differential ability of individuals to survive and reproduce (survival without reproduction is a dead-end evolutionarily)
2. Occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population.
3. The product of Natural Selection is the adaptation of populations of organisms to their environment.
All populations undergo Natural Selection. In order to visualize this, consider the Hardy-Weinburg Equilibrium Hypothesis which sets up a null situation for Natural Selection. The hypothesis consists of conditions which would be necessary in a population for Natural Selection NOT to occur, therefore if one of these conditions is broken, allele frequencies will change. As these conditions are broken in every known population, this is further evidence that evolution is occurring.
1. Infinitely large population
2. Random Mating
3. No migration
4. No mutation
Types of Selection in a Gene Pool
1. Stabilizing: The average phenotype is selected for and the extremes are selected against
2. Directional: An extreme phenotype is selected for, and the average against, will cause the average to move toward the previously extreme phenotype.
3. Disruptional: The extreme phenotypes are selected for and the average against .
Mutations: Alterations in the genetic code which can either be deleterious or silent. Can be induced by mutagens such as X-rays, radiation, UV light etc, or an error in the reading or writing of DNA. There is a relatively constant rate of mutations which differ between species depending highly on the RNA polymerase. RNA polymerase is what makes RNA from DNA, which then travels into the ribosome to make proteins. RNA polymerase has a capacity to "proofread" itself for error. Some species have more accurate polymerase than others, thus the mutation rate is different.
Types of Mutations
Partial Mutations: One nucleotide is altered. In the examples below, the string of letters represents DNA, each letter represents one nucleotide.
Deletion : ABCDEF --> ABCEF (caused by non-reciprocal "crossing-over" during meiosis)
Translocation: ABCDEF --> ABXYEF (non homologous chromosomes cross over)
Duplication: ABCDEF --> ABCDCDEF
Inversion: ABCDEF --> ABDCEF
Whole Chromosome Mutations
Aneuploidy: +/- 1 chromosome
Monosomy: Minus one chromosome
Trisomy: Gain one chromosome
Polyploidy: All chromosome pairs double, triple etc. (common in plants)
Definition of a species: A group of potentially interbreeding organisms that produce viable (healthy), fertile offspring.
Speciation: The formation of a new species. Two basic types Allopatric, and Sympatric. In Allopatric speciation, there is a geographic barrier which divides the gene pool. Sympatric evolution is more difficult, can be the result of a change in behavior, but essentially there is no geographic barrier.
Neutralists vs. Selectionists: There is a debate about how much Natural Selection plays a role in evolution. Neutralists believe there is not much difference between one type of allele or another, or in other words mutations do not cause enough of a change for the environment to necessarily choose, and therefore put more of an emphasis on genetic drift. Selectionists believe that Natural Selection is the mechanism by which frequencies change.
Genetic Drift: Traits which do not affect reproductive fitness or survival change over time in a population. Fairly random process.
This page was made for Honors 210, a course offered at Monmouth College