Nearly everything about evolution by natural selection involves some sort of cost-benefit analysis. Is the cost of the mutation offset by its benefit? Does having an elaborate tail help you more than hurt you? Natural selection favors the trait that confers the most benefit from a fitness perspective, meaning that traits that enhance your ability to get your genes into the next generation increase in frequency in subsequent generations.
The situation is no different for reproduction. If, for some reason, you were unable to produce any offspring of your own, helping produce even one extra niece or nephew would still enable you to get some of your genes into the next generation. Or if by forgoing producing a single offspring of your own, you were able to help produce three nieces or nephews who would not otherwise have existed, you come out ahead, evolutionarily speaking.
Because the cost (the single offspring that you did not produce who would have shared half of your genes) is less than the benefit (the three nieces or nephews who each share one quarter of your genes), natural selection will favor helping your sibling reproduce. For this reason, genes that are responsible for behaviors that help relatives reproduce can increase in frequency even if they decrease individual fitness as long as they increase inclusive fitness. This type of selective force is called kin selection.
The concept of kin selection explains many of the altruistic behaviors that Charles Darwin found confusing. For Darwin, who lacked modern knowledge of genetics, it wasn't as obvious how helping a related individual could be , advantageous to the helper.
It's a salamander-eat-salamander world
David Pfennig raised tiger salamander larvae in lated individuals. The results: Full siblings were groups with and without siblings. Tiger sala- least likely to cannibalize their neighbors, and manders are cannibalistic. For a tiger salaman- unrelated individuals were most likely. Half-sib-
der, eating one of your own kind is good if it ling groups were in between.
increases your chance of getting your genes into the next generation — but not as good if the one you eat has your genes.
The result makes sense from the standpoint of inclusive fitness and kin selection. Genes that allow a salamander to tell kin from stranger (and In the study, Pfennig raised his salamanders in to avoid eating the kin) appear to have been groups of full siblings, half siblings, and unre- selectively favored.
170 Part lll: What Evolution Does_
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