As the world warms, fish can swim north and butterflies can head to higher ground, but what is a 2-metre-long, 200-kilogram Antarctic fur seal stranded on a remote island to do? More to the point, what are several million of them to do?

The common refrain for species facing climate change is that they must migrate or adapt — or perish. Some, such as trees, will find it difficult to move their ranges quickly enough. And others, including polar bears, already live at the limit of their habitable range, so have nowhere to go.

Adaptation is nature’s best response to a crisis: after all, evolution and natural selection have been turning niche problems into opportunities for billions of years. It is no coincidence that people who are opposed to action on curbing carbon emissions talk up humanity’s ability to adapt as if it were a deliberate choice of action; some species just make it look so easy.

But adaptation is not as simple as it looks, especially if future generations are to benefit from the changes. As a special issue of the journal Evolutionary Applications made clear in January (see go.nature.com/8pruey), researchers too often infer that a species is responding to threats such as global warming through genetic evolution, just because the change seems to be the right thing for that species to do in the circumstances. Of course it would seem intuitive that a particular sea bird has evolved to be smaller with climate change: a warmer world saps less of the bird’s internal warmth, so it can be lighter — right?

Not always. As a study on page 462 of this issue makes clear, the situation is often more involved than that. Back to the Antarctic fur seals (Arctocephalus gazella) on that remote island, the polar outpost of South Georgia. On the front line of exposure to rising temperatures and with little scope for migration, the fur seals seem a prime example of a species that must adapt to survive, and quickly. Sure enough, as a genetic analysis of the population over the past few decades indicates, breeding female seals have become more heterozygous — a standard measure of Darwinian fitness and of an individual’s resilience in the face of environmental adversity.

Again, this simple narrative is intuitive: of course the seals would respond to stressful conditions with greater genetic capacity to deal with them.

Why, then, has the study found that the fur-seal population is shrinking? Heterozygosity is valuable, but not heritable. The average heterozygosity of the seal pups being born has not changed. But the number of less-heterozygous pups that survive to breed has declined — the less fit are being weeded out as expected. The survivors are older when they breed and do so less often than in previous decades, which contributes to the observed 25% reduction in the population.

What is driving this effect? Climate change, or its proxy of locally changed weather patterns, seems to have an indirect role. Models suggest that weather changes reduce the availability of the seal’s preferred food, Antarctic krill. So too, however, could altered fishing practices and the recovery of whale numbers.

What is a seal to do? The answer is more complicated than it seems. But so is the question.