Genes are packets of DNA that are inherited by children from their parents in distinct chunks. Genes are the smallest discrete units that are inherited by offspring intact, without being broken up. According to modern evolutionary biologists, evolution operates by the process of differential gene r eproduction, defined by reproductiv success relative to others. The genes of or ganisms that reproduce more than others get passed down to future generations at a relatively greater frequency than do the genes of those that reproduce less. Since survival is usually critical for reproductive success, characteristics that lead to greater survival get passed along. Since success in mating is also critical for reproductive success, the qualities that lead to success in same-sex competition or to success at being chosen as a mate get passed along. Successful survival and successful mate competition, therefore, are both part of dif feren-tial gene reproduction.
The characteristics that lead to the greater reproduction of genes that code for them are selected and, hence, evolve over time. In this sense, survival is important only inasmuch as it is necessary for reproduction. Nonetheless, many biologists maintain the distinction between natural, or survival, selection and sexual selection because it helps clarify two important types of adaptations—those that help or ganisms survive (e.g., fear of snakes) and those that help or ganisms reproduce (e.g., lar ge antlers for same-sex combat).
The modern evolutionary theory based on dif ferential gene reproduction is called inclusive fitness theor (Hamilton, 1964). The "inclusive" part is the fact that the characteristics that facilitate reproduction need not af fect the personal production of offspring. They can af fect the survival and reproduction of genetic relatives as well. For example, if you take a personal risk to defend or protect your sister or another relative, then this might enable her to better survive and reproduce. Since you share genes with your sister—50 percent on average in the case of siblings—
then helping her survive and reproduce will also lead to the spread of your genes (successful gene reproduction).
A critical condition for such helping to evolve is that the cost to your reproduction as a result of the helping must be less than the benefits to the reproduc tion of your genes that reside in your relative. If helping your sister survive—for example, by jumping into rushing rapids to save her from drowning—puts your own life at risk, the odds of saving her must exceed twice the odds of your dying in order for evolution to select for mechanisms underlying this helping behavior. Thus, inclusive fitness can be defined as on s personal reproductive success (roughly , the number of children you produce) plus the effects you have on the reproduction of your genetic relatives, weighted by the degree of genetic relatedness. Inclusive fitness lead you to take some risks for the welfare of your genetic relatives, but not too great a risk. Inclusive fitness theor , as an expansion and elaboration of Darwin's theory, represented a major advance in understanding human traits, such as altruism.
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