Wednesday, August 01, 2007

If you build it, they will come

At a game last night of the local minor league nine we got a chance to see an amazing bit of nature -- though I suspect I was in the minority marveling at it rather than being annoyed (or exhibiting gleeful sadistic destruction). The amazing site was easily millions, perhaps tens of millions, of mayflies swarming the field. Many compared the sight to a snowstorm, with observers present the previous night comparing those conditions to a blizzard. Later, when our bleachers section had largely cleared out, I could actually hear a buzzing noise from thousands of gossamer wings hitting the aluminum bleachers.

Kevin Costner needed to build his diamond in a cornfield & start playing the game, but these mayflies were simply confused by the high intensity lights being so close to their home -- home run balls splash in one of the rivers that powered the U.S.'s Industrial Revolution.

I never learned to fly fish, and so don't really know my hatches. Indeed, if I knew the right tied fly to use it would probably make identifying the critter via Google quicker. But thanks to I can specify it as a white mayfly, though I remember the wings being less translucent than in the image.

Hatches like these are probably largely synchronized by environmental cues occurring after the appropriate larval development is complete. What I've found particularly striking are the insects whose development is on a long multi-year clock. Seventeen-year 'locusts' (actually cicadas) being the classic example, and a memorable one for me -- I worked at a summer camp during the largest cohort's year and the constant hum in the woods was unforgettable. You went to sleep with it, woke up with it, ate with it, worked with it -- nowhere there could it be escaped, except by swimming underwater in the pool. The creatures were thick -- and often flew into you.

The thing I've wondered for a number of years now: how accurate are their clocks? If I took one million 17-year larvae and could somehow tag them, what would be the pattern of their emergence? What fraction would emerge 17 years later, and how many would show up 1 or 2 years early or 1 or 2 years late? Obviously, the graduate thesis project from hell. But the question is interesting. For example, if the clocks were sufficiently accurate, then each of the 17 cohorts would be effectively reproductively isolated from the other 17, meaning they would be approaching a state of being 17 different species!

A more practical experiment, which I am unaware of being executed (though I am hardly a strong watcher of the cicada literature), would be to ask how genetically isolated are each cohort from each other. By isolating a lot of members of each cohort and typing a large number of polymorphic markers, one could estimate the amount of gene flow between years. This could be done on stored samples, making it a practical project.

Or, to imagine another context, consider the standard story on Pacific salmon: when the coho's thoughts turn to love, they swim back to the exact place of their birth. Presumably this tale is supported by tag-and-release studies, but at what sample size? What error rate could be detected? How often does a chinook become confused and go up the wrong stream? Again, if the simple model of near perfect birthplace location is correct, then each salmon stream's population is reproductively isolated.

In either case, perfection is dubious. Biological systems are amazing, but noise happens & mutations occur. Keeping a biologic oscillator going for 17 years straight is truly incredible, but some of these metronomes must occasionally skip a beat. The existence of 17 different populations of 17 year cicadas suggests that alone: one original population bled over into the others. The other evolutionary alternative is that the 17-year period was selected multiple times from the proto-cicada population due to its useful properties -- a long, prime number period minimizes the chance of synchronizing with the population of a predator with a periodic population.

The 'snowstorm' we witnessed was really quite harmless to the hominids, but clearly a disaster for the white mayflies. Even without the sadistic kids pounding them into the floor, the vast majority of female flies who entered the stadium the other night would die without having any opportunity to lay their eggs back in the river. So a new threat with a periodic occurrence has entered the insect world: the schedule of night games in Single A ball.


RPM said...

I'm pretty sure a lot of the salmon reproduce every other year. That means there are two somewhat reproductively isolated lineages, those that mate in even years and those that mate in odd years. This, along with leakage between populations from different rivers, could contribute to the evolutionary dynamics of the species.

Also, aren't there cicadas with generation times of different lengths? If these species/populations can hybridize, then there will be some years in which a 10yr and 17yr species will eclose in the same season, and other years that the 10yr and a 7 year year species will sync-up. That would allow for interesting patterns of gene flow between the different types.

Silas said...

By isolating a lot of members of each cohort and typing a large number of polymorphic markers, one could estimate the amount of gene flow between years. This could be done on stored samples, making it a practical project.

I'd strongly advise grad students to stay the hell away from that project until year 16 at the earliest.

Keith Robison said...

The 'grad student project from hell' angle is exactly what that design tries to avoid -- there must be lots of extant stored collections of cicadas from each year hiding in somebody's insect collection.