In my career, I have found that the most exciting research is when the results are exactly the opposite of what I had expected. Certainly, it’s nice to show that what you thought was correct, but you really learn something when the opposite occurs–it makes you look at questions in a new way and often leads to new insights. This has happened to me several times, most recently in our experimental study of founder effects in Bahamian anoles (paper downloadable here).
Here’s the story: we have been conducting studies on anoles in the Bahamas for quite some time, using tiny islands as experimental test tubes. We had seen island populations wiped out by hurricanes, and we had documented anoles colonizing these islands, so we knew that populations often must be founded by overwater dispersal, probably by one or very few individuals. Given the long-running controversy over the evolutionary significance of founder effects, we had long discussed whether we could create an experimental founder effect, in replicate, to see what would happen. But we never started such an experiment, for a simple reason: suitable islands for our various ecological and evolutionary experiments were in short supply, and this experiment wasn’t a high priority.
Enter Hurricanes Frances and Jeanne in short succession in the late summer of 2004. These twin maelstroms, Category IV and III respectively, walloped the Bahamas in a three-week span and cleared many low-lying islands of their lizard inhabitats. Suddenly, we had no shortage of test tubes, so in the spring of 2005, we collected 14 individuals (7 male, 7 female) from a nearby larger island and introduced a pair to each of seven islands. The recipient islands, though quite small and scraggly, had maintained lizard populations in the past, so we knew they were suitable.
Why, you might ask, did we put two lizards on an island? Isn’t colonization usually by a single lizard? This was a subject of great debate among our team. On one hand, I thought that putting one female on an island would more closely mimic what occurs most commonly. This was not a trivial point. The key to the founder effect is the skewing of the gene pool–relative to the ancestral population–due to the fact that the new population is created by relatively few individuals. As a result, some alleles (gene variants) are lost entirely from the new population because the founders by chance don’t carry that variant; other alleles might experience a great increase in frequency if they were rare in the source population but happened to be carried by one of the founders. In this context, the number of founding individuals is critically important–the fewer the founders, the greater the likelihood of a founder effect (which is defined specifically as the genetic difference between the source and founding population due solely to the random statistical consequences of founding a population by a few individuals). More specifically, we imagined that if we put a single, gravid female on an island (and almost all females are gravid in May), the population would have two founders: the female, and whichever male had fertilized her eggs. By contrast, if we added a male, that would make three founding individuals–the female and male on the island, and the male who had impregnated the female–a 50% increase in genetic diversity, and thus a substantial watering down of the founder effect. So, I said, one gravid female is the way to go to maximize the founder effect.
My colleagues were not swayed. Their counterargument was simple: suppose the female actually hadn’t mated and was not gravid. Then the introduction would fail and what would we learn? Nothing. Better to play it safe and increase the odds of a successful population establishment. And, they pointed out, occasionally we had seen colonization events involving two lizards–not that surprising after all, because one means of colonization is by floating on vegetation, which could carry more than one lizard.
We voted. I lost. Two colonists it was, a male and a female. I regretted the increased genetic diversity we’d be introducing, thus decreasing our hopes of observing a founder effect, but so be it.
Sidebar 1: It’s been known for decades that anoles can store sperm, sometimes for 7 or 10 months, or even more. That’s how a single female arriving on an island can pump out egg after egg, all fertile, without help from a manfriend (remember, anoles only lay egg at a time, at approximately weekly intervals when times are good).
Sidebar 2: For many years, it was believed that what are technically called “extra pair copulations” and what most people know as “sneaking around” didn’t occur in anoles. Females of most species have small territories nested within the larger territories of males, and it was assumed that females mated only with their guy, the territory holder. This belief persisted long after similar ideas about birds had been debunked with the advent of genetic paternity testing, showing that in fact the females in many species often were quite promiscuous, mating with and having offspring fathered by multiple males other than their territory holder. We all assumed that anoles were more virtuous, and the surprising dearth of paternity studies until recently allowed that idea to persist. However, the data are now in and–guess what!–it turns out that female anoles get around just like birds. Several studies have shown a relatively high level of multiple paternity–in one study, 80% of females brought into captivity produced eggs fathered by more than one male, with the eggs of some females having as many as four fathers!
Sidebar #3: In some animals, like fruitflies, there is a phenomenon known as last male sperm precedence, in which the eggs are fertilized by the sperm of the last male with whom the female has mated.
Now, back to our story. When we started this experiment, the results on multiple paternity were unknown. Had we known of them, we would not have expected such a dramatic founder effect from the introduction of a single female, because her offspring might contain the the genes of a number of different males–still a founder effect, but with a founder size greater than two.
In any case, we didn’t put just a female on the island–we put a male there, too. So, what happened? When we returned a year later, we captured every lizard on the islands (we may have missed one), a number ranging from 7-17–not a bad growth rate from two founding individuals!. From each animal, we pinched off a tiny bit of the tail to use for DNA sampling. We’d done the same for the founders, so we knew the genetic composition of the founding population. Thus, we could ask, what percentage of the offspring had been fathered by the male we put on the island? The answer: almost all of them: the males had been busy, and they had fathered 94% of the population. No matter how much sperm the females were storing from who-knows-how-many males, they weren’t using it. And that led to the perverse result: introducing a male alongside the female actually decreased the genetic diversity of the introduction, perhaps enhancing the founder effect, because the number of founding animals truly was two, rather than some greater number that might have resulted from the use of stored sperm in the absence of a male on the island. I was wrong again, but we learned something interesting as a result.
And the results of the experiment? Indeed, a founder effect did occur. Compared to the source population, the introduced populations varied greatly in both genotype and phenotype. One year after the introduction, some of the populations were quite different in genetic constitution from the source, and these differences have persisted over the four year course of the experiment. Similarly, the populations also were immediately different in mean hindlimb length, with some populations having longer legs than the source, and others having shorter legs. Most surprisingly, however, as the introduced populations evolved shorter limbs over the course of the experiment–the expected adaptive response to living in an environment of narrow-branched bushes instead of a forest–the relative differences among these populations–the founder effect–persisted. The populations that initially had longer legs continued to have the relatively longest legs, and those with the shortest legs continued to have the shortest legs, even as all populations experienced a decline in limb length. Of course, the experiment has only been running for four years, and we’ll see whether these founder effects persist through time. Assuming, of course, that the populations themselves persist, something we’re not sure about after last fall’s Hurricane Irene. We’ll be heading out in May to see if the populations are still around–stay tuned!