Evolution of Populations

Genes and Variations A gene pool is all the different alleles present in a particular population. A population is a particular species that lives within a designated area. Evolution is a change in the relative frequency of alleles in a population. Why do these frequencies change? Mutations Crossing over occurs during prophase I of meiosis among homologous chromosomes. Nondisjunction results in cells with wrong number of chromosomes. Usually a big problem in meiosis…..Down Syndrome. Duplication, Translocation, Deletion, Insertion, Inversion All of the above cause changes in the chromosomal pattern. They can be random, or caused by chemical or environmental exposure. Natural selection of a single gene trait can cause changes in alleles frequency in a population and thus evolution. Natural selection is more complicated when polygenic traits are involved. Directional selection occurs over time when members of the population on one end are more fit. Stabilizing selection occurs when members of the population in the middle are more fit. Disruptive selection occurs when the members at the high and low ends of the scale are more fit. Evolutionary change is sometimes just by chance. In genetic drift, a particular allele might become more prevalent because the individuals that had it produced more babies that survived. This is known as the founder effect. Hardy-Weinberg Principle Allele frequencies will remain the same in a population, unless one or more factors cause a change. In this kind of population the following conditions would need to exist: 1. Random mating 2. Population must be very large 3. No movement of members in or out of the population 4. No mutations 5. No natural selection The population would be in genetic equilibrium and therefore would not evolve. How do different species evolve? As populations evolve, they can become reproductively isolated from one another in 3 ways: 1. Behavioral isolation- they are capable of breeding, but courtship is different 2. Geographical isolation 3. Temporal isolation-breed at different times Over time, any one of these factors can lead to new species. All of the factors previously discussed were reasons for the famous speciation of Galapagos Islands Finches discovered by Darwin.