Behavior evolves through natural selection, just as anatomy does. Hence if sex is enjoyable, natural selection must have been responsible for that ou… - Jared Diamond

History, as well as life itself, is complicated; neither life nor history is an enterprise for those who seek simplicity and consistency.

White blood cells and other cells of ours actively seek out and kill foreign microbes. The specific antibodies that we gradually build up against a particular microbe infecting us make us less likely to get reinfected once we become cured. As we all know from experience, there are some illnesses, such as flu and the common cold, to which our resistance is only temporary; we can eventually contract the illness again. Against other illnesses, though—including measles, mumps, rubella, pertussis, and the now defeated smallpox — our antibodies stimulated by one infection confer lifelong immunity. That's the principle of vaccination: to stimulate our antibody production without our having to go through the actual experience of the disease, by inoculating us with a dead or weakened strain of microbe.

How can we explain simultaneously this general pattern of female parental care and also its numerous exceptions?
The answer comes from the realization that genes for behavior, as well as for malaria resistance and teeth, are subject to natural selection.