Balancing selection refers to environments that vary over long periods of evolutionary time. For example as original hunter gather groups began to migrate to different climates. The idea behind this theory is that no single adaptation is optimal consistently The balance changes, not allowing any one particular adaptation to become fixated and universal. An excellent metaphor is 'the early bird catches the worm, but the second mouse gets the cheese'. In some environments those risk taking individuals would benefit, and in other environments those who are more cautious would benefit, balancing out the process of natural selection.
Recent Selective Sweeps as an Explanation for Individual Differences
2. Recent selective sweeps implies that logically natural selection has not just stopped. However the hunter gather lifestyle prevailed for over 90% of our history, and so natural selection to the modern environment simply has not yet had chance to fixate and force other strains to extinction. So individual differences in this theory are simply adaptations in still in the process of beingnaturally selected.
Mutation-Selection Balance as an Explanation for Individual Differences
3. The final theory mutation-selection balance in a nutshell implies that each replication of genes in an offspring is not perfect and individual differences are the result of these mutations. An average person carries around 500 mutations that are potentially mildly harmful to optimal fitness. Eventually natural selection will force these mutations to extinction if they impact negatively on fitness. However the theory implies that there are so many small mutations that whilst one is becoming extinct, there will always be another present.
Collective Conclusion to Individual Differences
Each theory can successfully explain individual differences in evolutionary terms. These theories can also often be used in conjunction, or differing theories for different types of individual difference. For example mutation selection balance is based around genetics, whilst balancing selection refers to the environment and in the middle is recent selective sweeps which can be a combination of both.
Sources:
Price, M. E. (In press). (2010). Cooperation as a classic problem in behavioural biology. In Evolutionary Psychology: A Critical Introduction, V. Swami, Ed.
Penke, L., Denissen, J. J. A., & Miller, G. F. (2007). The evolutionary genetics of personality. European Journal of Personality, 21, 549-587.
Buss , D. M . ( 2009 ). How can evolutionary psychology successfully explain personality and individual differences? Perspectives on Psychological Science , 4 , 359 - 366 .
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