GENE_FREQUENCIES
Note: I'm providing this video and the notes below as a resource. The movie will be in the learning resource room (second floor of the Houff Student Center, next to the library). I recommend watching the movie. I strongly encourage you to print out this page of notes.
Program Synopsis
In this program, the presenters:
Prerequisite Knowledge
To achieve the objectives in this program, you should already understand:
Learner Expectations
After you view this program and use the suggested resources, you should be able to:
Natural Selection
Natural selection favors the reproductive success of fit individuals over those that are less fit, thereby increasing the favorable genotypes in the gene pool. The result is that the population becomes adapted to its environment.
Contingency
Stephen Jay Gould suggests that contingency explains the modern order (the number and kind of species that exist on earth today). Contingency is the view that the species populations now present on earth resulted from the occurrence of chance events. In other words, if any one of a number of chance events did not occur, then a totally different pattern of existent species would result. Therefore, the present set of species is a result of many interconnected random events. This idea goes against the view that basic laws. such as natural selection and superiority in anatomical design, guaranteed the modern order.
For Gould, the Burgess Shale represents the opportunities of contingency because so many different life forms were present almost 550 million years ago. He suggests that an evolutionary tree for the species present would be "bottom heavy." Certain body plans would survive over others at that time.
Mass Extinctions
Paleontology shows that mass extinctions have happened in the past. These episodes mark the major boundaries of the geological time scale. Darwin believed mass extinctions were examples of an incomplete fossil record. But, as Gould points out, mass extinctions seem to be genuine disruptions in the geological flow. They are not just the high points of a continuity. "They may result from environmental change at such a rate, and with so drastic a result, that organisms cannot adjust by the usual forces of natural selection" (Gould, S. J., Wonderful Life. New York, NY: W. W W. Norton & Company, Inc., 1989).
Genetic Variation
Genetic variation refers to heritable variation, an idea that is central to Darwin's theory of evolution. Variation provides the raw material for natural selection. Only the genetic component of a variation can affect adaptation through natural selection because it is the only component that passes from generation to generation.
Most heritable variation consists of polygenic (an additive effect of two or more gene loci on a single phenotypic character) characters that vary quantitatively within a population. When two or more forms of a character are present in a population, the contrasting forms are called morphs. A population is said to be polymorphic for a character if two or more morphs are each represented in high enough frequencies to be readily noticeable. Polymorphism is extensive in human populations. One example is the human ABO blood groups. (Biology, Benjamin/Cummings, 1993)
Variation can be detected by biochemical means. Electrophoresis and other methods can analyze proteins that have different molecular charges. Work on Drosophila population gene pools shows that genetic variation in the genotypes of any two flies in the population occurs at about 25% of their loci. Human populations show similar results.
Most species have geographical variation in the allele frequencies of their gene pools. Environmental factors change from one location to another, and natural selection will favor traits for each environment. Natural selection is the main cause of geographical variation.
Genetic drift (changes in the gene pool of a small population due to chance) can also cause variations among different populations.
Speciation
Speciation, or the origin of a new species, has two patterns: anagenesis and cladogenesis. Anagenesis (also called phyletic evolution) is the change of a species in an unbranched line. If the change makes the new population significantly different from the original population, the new group is called a new species. Cladogenesis (also called branching evolution) occurs where one or more new species forms from a parent species. The parent species continues to exist.
Gene
A gene is a single unit of hereditary information. It is a segment of DNA located on a chromosome. In diploid organisms each gene is found at two loci (one on each homologous chromosome). They are called alleles, and may be identical to one another or different from one another. Each allele of a specific gene may occur within a population. Its frequency of occurrence is called the allelic frequency. The terms gene frequency and allelic frequency are often used interchangeable. The more accurate term is allelic frequency.
Species
A species is a group of organisms which have similar structural and biochemical features. They can interbreed and produce fertile offspring.