Introduction to evolution: Difference between revisions

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== Genetic drift ==

{{Further|Genetic drift}}

Genetic drift is a cause of allelic frequency change within populations of a species. [[Allele]]s are different variations of specific genes. They determine things like [[Human hair color|hair colour]], [[Human skin color|skin tone]], [[eye colour]] and [[blood type]]; in other words, all the genetic traits that vary between individuals. Genetic drift does not introduce new alleles to a population, but it can reduce variation within a population by removing an allele from the gene pool. Genetic drift is caused by random sampling of alleles. A truly random sample is a sample in which no outside forces affect what is selected. It is like pulling marbles of the same size and weight but of different colours from a brown paper bag. In any offspring, the alleles present are samples of the previous generationsgeneration's alleles, and chance plays a role in whether an individual survives to reproduce and to pass a sample of their generation onward to the next. The allelic frequency of a population is the ratio of the copies of one specific allele that share the same form compared to the number of all forms of the allele present in the population.<ref>{{harvnb|Futuyma|1998|p=Glossary}}</ref>

Genetic drift affects smaller populations more than it affects larger populations.<ref name="Ellstrand1993">{{cite journal |last1=Ellstrand |first1=Norman C. |last2=Elam |first2=Diane R. |date=November 1993 |title=Population Genetic Consequences of Small Population Size: Implications for Plant Conservation |journal=[[Annual Review of Ecology, Evolution, and Systematics|Annual Review of Ecology and Systematics]] |volume=24 |issue=1 |pages=217–242 |doi=10.1146/annurev.es.24.110193.001245|bibcode=1993AnRES..24..217E }}</ref>