When Good Genes Go Bad

The National Evolutionary Synthesis Center is holding a contest - $750 towards attending SciOnline 2010 for two bloggers who write exceptional evolutionary blog posts in 2009. Living in Hawaii, it's tough to afford the round trip to North Carolina all by me onesies, so here's my attempt at getting the funds!

UPDATE: I WON! Thanks, NESCent!! See you all at SciOnline 2010!

When Good Genes Go Bad

We tend to think of evolution as a directional process, whether its from simplicity to complexity or 'less fit' to 'more fit.' Even the classic images of evolution, like the evolution of man, reveal our inherent bias. But that is not always the case; species all over exhibit a variety of traits which damage their reproductive fitness despite the best-laid plans of genes and evolution.

You see, times are always changing, and what might be good for you one day could be disastrous the next (just imagine if the flock of seagulls haircut was genetic, and you can see what I mean). The big problem is that even if a trait was once adaptive or even just neutral, it can later become maladaptive as pressures shift. And that, unfortunately, is what has happened to the Hawaii Akepa.

Akepa (Loxops coccineus) are one of native species of Hawaiian honeycreeper that lives in the upper elevations of Hawaii's forests. They're stunning birds, with the males turning bright red when they reach adulthood and females remaining a pretty green. For centuries, they've used their specialized, crossed bills to eat caterpillars and other bugs and drink nectar from the native Koa and Ohi'a trees.

Like other native species, they took a hit when people arrived. Habitats were leveled for farming, invasive plants overtook native trees creating less food and habitat, and invasive birds began to take what little was left. However, they dodged a bullet that is devastating some of their closest kin by living too high up for the introduced mosquitoes and avian malaria to attack their blood. Sure, the loss of their habitat due to farming was tough, but we've started fixing that, so they're primed to re-expand back into their native range.

That is, they would be primed to, but they aren't. Scientists were confused. Despite the relatively long breeding season they have in which to produce young, which lasts from early spring until winter, the akepa just don't have the reproductive rate they need to bounce back. So scientists looked to see if the females were producing young - and they were. The females are pumping out baby birds like it's their job, so the issue isn't that they don't have enough young akepa. Despite the female's efforts, the akepa population's growth rate is still dangerously low. Scientists wanted to know why. What's going wrong?

Well, it all starts with these invasive Japanese White-Eyes I've mentioned before. They're up in the akepa's forests, eating their food, and putting stress on breeding mothers. The competition doesn't help, and its lowering the overall fitness of the breeding birds, hampering their survival rates. But the white-eyes aren't entirely to blame: they breed later in the year, so the competition is only really bad during the later half of the breeding season, which still leaves an entire early breeding season to make babies - and the akepa do. There are plenty of baby Akepa in the earlier part of the year which survive into adulthood, but the population numbers still aren't growing. It's not just competition with the invasive species - something else, scientists realized, is going wrong.

Blame evolution. You see, the akepa have evolved a trait which is dooming their population.

Once upon a time, before the white-eyes moved in, the akepa populations were booming. There were a lot of males fighting for a good number of females, and it worked out that the bigger, healthier males managed to get laid far more often than smaller ones. It also worked out that those mothers who nested earlier got more food and had bigger, healthier babies. Somehow, a genetic switch was turned on, and the akepa started biasing their young's sex, in a process called Sex Allocation.

It's a phenomenon you've probably heard about in context with deer. Female deer, when healthy and well fed, are more likely to have males. The theory goes that because in deer society only the best males reproduce, to produce a weak male is a waste of reproductive effort. Girls, on the other hand, are always welcome in a top male's harem, so small, weaker girls still can still pass on genes.

Something similar occurs in akepa. The females who produced males earlier in the breeding season were rewarded with strong, dominant boys that had lots of kids because, in general, males are able to have more offspring than females (the whole sperm versus egg thing). Somehow, through the magic of natural selection and genetics, it became the rule. Breed early, have a boy, breed later, have a girl.

It's a battle over resources, and the white-eyes are winning. Photos c/o AAAS

Then the white-eyes made the Hawaiian forests their home. They breed later in the year, and consequently, compete far more voraciously for food and nesting sites with the akepa during the later half of the breeding season. During that time, akepa that breed have trouble getting a good nest site and enough food to feed their young, meaning that few of the later-breeding akepa chicks survive. Since competition is lower in the early season, the chicks born then do just fine. The problem is, they're almost all males.

From 1987, when white-eyes were few and far between, to 2005, after a major explosion in the white-eye populations, the sex ratio of male to female akepa shifted from close to 50-50 males-females (57% female) to only 13% female. In the early years, from 1987-1999, 32 female chicks and 24 male chicks survived to adulthood. From 2000-2006, 27 male chicks made it, but only four female chicks did. FOUR females.

What was once an adaptive strategy - producing mostly males earlier in the breeding season - has become dangerously maladaptive. And, unfortunately, there's no evidence to suggest that evolution is coming to their rescue and dismantling the seasonal variation in sex allocation. The now-endangered species appears to have little hope unless serious management practices dramatically increase the survival rates of akepa chicks in the later months of their breeding season. In the akepa, good genes have gone bad, and the species is likely to go extinct from it.

Leonard A. Freed, Rebecca L. Cann, & Karl Diller (2009). Sexual dimorphism and the evolution of seasonal variation in sex allocation in the Hawaii akepa Evolutionary Ecology Research, 11, 731-757