Fitness is measured on an individual level and is NOT necessarily a measure of strength or power. Rather, fitness is how evolutionary biologists quantify an individual's reproductive success based on the relative contribution of their particular phenotype to the next generation's gene pool. Fitness can be absolute or relative, direct or indirect. Absolute fitness can be quantified as the number of successful offspring an individual produces in its lifetime. Relative fitness compares this number to the number of offspring produced by individuals of other phenotypes.
For this blog, I was wondering how one would measure fitness of individuals in species that are cooperative breeders, such as meerkats. In meerkat society, there is a dominant female and male pair that does most of the breeding for the group. Other members help raise these offspring even though they are not their own. So how is this beneficial to them and how to measure fitness? This is where direct vs. indirect fitness comes in. Direct fitness measures an individual's production of direct descendants while indirect fitness measures production of offspring that are not direct descendants but are still related. Because they are related, the offspring still share some genetic material with their non-parent caregivers, therefore passing on that phenotype. So fitness of non-dominant meerkats is measured as indirect fitness - how much of their phenotype is contributed to their non-offspring kin. To compare fitness of dominant individuals, we can measure their lifetime reproductive success, which is the number of young they produce in their lifetime that survive to independence. Cooperative breeding behavior in meerkats increases the chance of survival of young since with more members of the family they can work together to guard and feed young more effectively than lone parents could.
Sources:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3856740/
https://www.pbs.org/wnet/nature/blog/meerkat-fact-sheet/
https://www.biologyonline.com/dictionary/relative-fitness
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3982663/
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