Editing Extinction (section)

=== Genetics and demographic phenomena ===
{{See also|Extinction vortex|Genetic erosion|Mutational meltdown}}
If [[adaptation]] increasing population [[fitness (biology)|fitness]] is slower than [[environmental degradation]] plus the accumulation of slightly deleterious [[mutation]]s, then a population will go extinct.<ref>{{cite journal |last1=Bertram |first1=J |last2=Gomez |first2=K |last3=Masel |first3=J |title=Predicting patterns of long-term adaptation and extinction with population genetics |journal=Evolution |date=February 2017 |volume=71 |issue=2 |pages=204–214 |doi=10.1111/evo.13116 |pmid=27868195 |arxiv=1605.08717 |s2cid=4705439}}</ref> Smaller populations have fewer beneficial mutations entering the population each generation, slowing adaptation. It is also easier for slightly deleterious mutations to [[fixation (population genetics)|fix]] in small populations; the resulting positive feedback loop between small population size and low fitness can cause [[mutational meltdown]].

Limited geographic range is the most important determinant of [[genus]] extinction at background rates but becomes increasingly irrelevant as [[#Mass extinctions|mass extinction]] arises.<ref>{{cite journal |author1=Payne, J.L. |author2=S. Finnegan |year=2007 |title=The effect of geographic range on extinction risk during background and mass extinction |journal=[[PNAS|Proc. Natl. Acad. Sci.]] |volume=104 |issue=25 |pages=10506–10511 |doi=10.1073/pnas.0701257104 |pmid=17563357 |pmc=1890565 |bibcode=2007PNAS..10410506P |doi-access=free}}</ref> Limited geographic range is a cause both of small population size and of greater vulnerability to local environmental catastrophes.

Extinction rates can be affected not just by population size, but by any factor that affects [[evolvability]], including [[balancing selection]], [[cryptic genetic variation]], [[phenotypic plasticity]], and [[robustness (evolution)|robustness]]. A diverse or deep [[gene pool]] gives a population a higher chance in the short term of surviving an adverse change in conditions. Effects that cause or reward a loss in [[genetic diversity]] can increase the chances of extinction of a species. [[Population bottleneck]]s can dramatically reduce genetic diversity by severely limiting the number of reproducing individuals and make [[inbreeding]] more frequent.