In an aside to his contribution to our recent discussion of same-sex marriage (my contribution is here), Big Think’s Peter Lawler wrote that Darwinists agree with many religiously observant people in “thinking that the main point of any social animal is to generate replacements and raise them right.” The idea that the purpose of our lives should be to pass on our genes is often attributed to Darwinists, but it’s not something you will learn in an evolutionary biology class. That’s because it’s not at all what the theory of evolution says.
Evolutionary theory offers a naturalistic explanation for the diversity of life. The theory of evolution is essentially that organisms adapt to their environment as new, heritable traits that help them survive and reproduce are passed on to their offspring. The theory accords beautifully with the observed evidence and is completely uncontroversial among biologists, although any sort of naturalistic explanation of life may run counter to the basic (and ultimately non-falsifiable) metaphysical beliefs of many people.
Evolutionary theory doesn’t say anything about what is right or wrong. It offers an explanation for why human beings developed they way they did. But on its own it doesn’t say anything about what what we should do now that we are here. We can say that passing on our genes is "the point" of human biology only in the sense that specific human characteristics developed largely because they enabled us to pass on our genes. But this is different from saying, as religious people sometimes do, that it is "the point" of human life in the sense that we have a moral imperative to go forth and multiply.
The fact that humans developed the way we did because it contributed to our ability to reproduce certainly doesn’t mean that we should devote our lives solely to passing on our genes. Consider that natural selection rewards passing on our genes at all costs. It might help your children’s chance of survival under certain circumstances, for example, to murder your step-children. People who murdered their step-children in times of scarce resources might in fact be more likely to pass their genes on to future generations. But even if that were the case, murdering your step-children would still be a terrible thing to do. Even religions that say we have some duty to procreate would agree that passing on our genes doesn't justify any crime.
The idea that evolutionary theory tells us that the goal of human life should be to reproduce a version of what the philosopher G.E. Moore called “the naturalistic fallacy.” As Moore pointed out, the fact that something is something happens naturally does not make it good. Gravity may explain why things roll downhill, but tells us nothing about whether it’s better for things to be at the bottom of hills than at the top of them. Likewise, the fact that there may be a natural explanation for why human beings murder and rape certainly does not justify murder or rape, even if it may incline us to be more understanding of our baser impulses.
The naturalistic fallacy is related to David Hume’s famous claim that we cannot derive what ought to be from what actually is. Science can and should inform our choices, but there are no empirical observations we can make to answer fundamental moral questions. We cannot prove that murder is wrong by looking at the fossil record any more than we could prove it's wrong by shooting a beam of particles at gold foil. That's why moral philosophers and theologians do not work in the field or perform experiments in laboratories.
It’s also why you won’t hear prominent biologists testifying in court that relationships that don’t involve biological reproduction are immoral. Because that’s just not what the science says.