 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 3 4 5 6 7 ] Next Last |
Main article: Mutation
A mutation occurs when there is a so-called error during duplication or translation of genetic material. Mutation provides natural selection with variation; and is the sole source of new genetic material. Mutations are usually neutral, often deleterious, but occasionally adaptive.
Main article: Genetic drift
Genetic drift describes changes in gene frequency that cannot be ascribed to selective pressures, but are due instead to events that are unrelated to inherited traits. This is especially important in small mating populations, which simply cannot have enough offspring to maintain the same gene distribution as the parental generation. Such fluctuations in gene frequency between successive generations may result in some genes disappearing from the population. Two separate populations that begin with the same gene frequency might, therefore, "drift" by random fluctuation into two divergent populations with different gene sets (for example, genes that are present in one have been lost in the other). Rare sporadic events (volcanic explosion, meteor impact, etc.) might contribute to genetic drift by altering the gene frequency outside of "normal" selective pressures.
Unlike natural selection, genetic drift is the random fluctuation of gene frequencies from generation to generation in a small, relatively isolated population. Its chief mechanism of operation is chance within small populations. The term small population is relative, however. Thus, genetic drift occurs when N <= 0.5s, N <= 0.5µ, N <= 0.5m where N is the population numbered in the hundreds, s is the selective value of the allele s, µ is mutation pressure, and m is gene flow.
Main article: Gene flow
Gene flow or gene admixture is the only one of the agents that makes populations closer genetically while building larger gene pools. Migration of one population into another area occupied by a second population can result in genetic admixture. Gene flow operates when geography and culture are not obstacles.
Main article: Natural selection
Natural selection, the last of the four forces, is based on three principles: (a) there is variation within a species and this variation is heritable; (b) parents have more offspring than can survive; and (c) surviving offspring have favorable traits. The mechanism by which it operates is termed survival of the fitter meaning differential mortality and fertility. Differential mortality is the survival rate of individuals before their reproductive age. If they survive, they are then selected further by differential fertility – that is, their total genetic contribution to the next generation.
Natural selection can be categorised into ecological selection – due to differential survival – and sexual selection – due to selection of mates with desirable characteristics.
Selection by humans of organisms for desirable characteristics, e.g. for agriculture or as pets, is called artificial selection.
Main article: Microevolution
Microevolution refers to small-scale changes in gene frequencies in a population over the course of a few generations. These changes may be due to a number of processes: mutation, gene flow, genetic drift, as well as natural selection. Population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution.| Politics | Business | Finance |
 |
 |
 |
|