Editing Natural selection (section)

===By effect on genetic diversity===

Alternatively, selection can be divided according to its effect on [[genetic diversity]]. [[Negative selection (natural selection)|Purifying or negative selection]] acts to remove genetic variation from the population (and is opposed by [[Mutation#By inheritance|''de novo'' mutation]], which introduces new variation.<ref>{{harvnb|Lemey|Salemi|Vandamme|2009}}</ref><ref>{{cite web |url=http://www.nature.com/scitable/topicpage/Negative-Selection-1136 |title=Negative Selection |last=Loewe |first=Laurence |year=2008 |work=Nature Education |publisher=[[Nature Publishing Group]] |location=Cambridge, MA |oclc=310450541}}</ref> In contrast, [[balancing selection]] acts to maintain genetic variation in a population, even in the absence of ''de novo'' mutation, by negative [[frequency-dependent selection]]. One mechanism for this is [[heterozygote advantage]], where individuals with two different alleles have a selective advantage over individuals with just one allele. The polymorphism at the human [[ABO blood group]] locus has been explained in this way.<ref>{{cite journal |last1=Villanea |first1=Fernando A. |last2=Safi |first2=Kristin N. |last3=Busch |first3=Jeremiah W. |title=A General Model of Negative Frequency Dependent Selection Explains Global Patterns of Human ABO Polymorphism |journal=PLOS ONE |date=May 2015 |volume=10 |issue=5 |pages=e0125003 |doi=10.1371/journal.pone.0125003 |pmid=25946124 |pmc=4422588|bibcode=2015PLoSO..1025003V |doi-access=free }}</ref>

[[File:Life cycle of a sexually reproducing organism.svg|thumb|upright=1.1|Different types of selection act at each [[Biological life cycle|life cycle stage]] of a sexually reproducing organism.<ref name=Christiansen1984/>]]