Kin recognition and inclusive fitness

Plants and animals can recognize their relatives. Why would that ability be useful? One thought is that helping a relative could help preserve genes you share, even if you don't have offspring yourself. Keep that as a possible explanation, not something we've already proved. The opening distinguishes two questions: how recognition works and why it developed. Improved understanding of the first has prompted interest in the second.

Inclusive fitness theory develops the shared-genes idea. The traditional account described here focuses on individuals that produce the most offspring. But an organism's own offspring aren't the only route by which its genetic attributes can be represented in the next generation. Close relatives share some of those attributes, too. Think of a sibling having children: helping that sibling survive can help preserve shared genes. The theory adds that route to the explanation rather than abandoning natural selection.

Now the honeybee example makes the difference concrete. Most honeybees don't produce offspring; they nurture relatives. If we only count an individual's own offspring, that behavior is puzzling. Including the relatives' contribution helps explain why natural selection could favor it. We don't need to imagine a bee consciously calculating its family's future to understand the proposed advantage.

The next application is less pleasant: cannibalism. If relatives carry shared genes, eating them has a genetic cost. I'd therefore expect a cannibal that can tell relatives apart to avoid eating them. Do the tadpoles behave that way?

Spadefoot toad tadpoles begin as omnivores. Eating another tadpole or a freshwater shrimp can trigger a change: they become larger and exclusively carnivorous. But the key issue for this theory isn't just that they eat meat. Do they spare siblings? They nip at other tadpoles, eat nonsiblings, and release siblings unharmed. That distinction fits the prediction we were waiting to test.

Then comes a limit. A very hungry cannibal is less likely to spare its kin, apparently giving its own genetic makeup priority. So sibling protection isn't unconditional. A useful explanation has to accommodate both the usual avoidance and the change when food becomes urgent. This adds something to explain; it doesn't make the behavior inexplicable.

The final paragraph introduces another possible reason for recognition. Tiger salamanders can contract a deadly bacterium by eating diseased members of their species. At first, that explains why eating another salamander is risky, but not why recognizing a relative would matter. The next fact supplies the missing connection: the bacterium is more deadly when it comes from a close relative with a similar immune system.

Now avoiding kin could protect the eater itself. The relative also escapes being eaten, but the explanation offered here doesn't need that benefit to account for the behavior. Avoiding a particularly dangerous infection can provide a direct survival advantage.

That leaves us with a useful theory and a qualification. Inclusive fitness explains how recognizing and sparing relatives can preserve shared genes, and the tadpoles illustrate its value. The salamanders show why we shouldn't automatically give every instance the same explanation: protecting one's own life may also account for kin recognition.