The term sex can mean several things. Ask a seventh-grader straight out of health class, and you're likely to get one definition (with a few sniggers thrown in). Ask a parent of a brood of children, and you're likely to get another. Often, these definitions equate sex with sexual reproduction. But ask an evolutionary biologist to define sex and sexual reproduction, and you get definitions that are beautifully precise, even if they do suck all the fun and innuendo out of the terms.
For evolutionary biologists, sex is not mating but the combining of genetic material from different individuals, and reproduction simply means making offspring; reproduction, in other words, doesn't have to include sex:
i Sexual reproduction: Sexual reproduction is the system in which two individuals mate and produce offspring whose genes are a combination of some of the genes from each parent. Sexually reproducing organisms produce offspring that have only half of each parent's genes.
i Asexual reproduction: Asexual organisms produce offspring without mating, and these offspring contain only the single parent's genes.
For some species (like humans), reproduction always relies on sex. Other species (like rotifers and a few kinds of lizards) never have sex at all and still manage to reproduce. Still other species can reproduce with or without sex, proving that sexual reproduction isn't necessarily an all-or-nothing proposition.
¿jfjABEft As mentioned in numerous places throughout this book, fitness is all about passing on your genes to the next generation. The more successful you are at that task, the more fit you are. All other things being equal, an organism that makes twice as many offspring is twice as fit. (Go to Chapter 2 for a more in-depth discussion of fitness from an evolutionary perspective.) But in reproduction, the quality of the offspring's reproductive success also impacts an individual's fitness. If two individuals each produce four offspring, but the four offspring of one individual die before getting a chance to reproduce, the two individuals aren't equally fit.
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