God clearly does not love all fetuses. Ones particularly expendable are fishy ones. The male three-spined stickleback is the fish in question right now. He goes in for cannibalism and recent research confirms an explanation:
The researchers manipulated the clutches that individual males cared for, replacing up to 100 percent of the eggs fertilized by the male with eggs fertilized by another male. They found that the higher the percentage of “alien” eggs, the more likely the male was to completely cannibalize the clutch. The researchers suggest that the fish probably take their cues from the odor of the developing eggs as the paternal genes start to kick in.
Cannibalism of a clutch with a high proportion of alien eggs makes sense for the fish, because it avoids putting a lot of energy into producing offspring that carry another fish’s genes, and because by eating the eggs it gets energy to rear future clutches of its own eggs.
But what I like about the example for teaching is that it’s a clear case where the explanation appeals to the good of the agent without giving any reason the agent actually could entertain. As we say in evo-speak, it would be much too expensive and impractical to build a fish brain that could grasp facts about passing on one’s genes. So evolution has linked his actions with odor or taste. Others’ eggs are just so yummy.
It’s also a really good way into to the apparently mythical idea that step fathers tend to kill their step children, though if I’m out of date on the many twists and turns of that debate, please someone let me know.