Subject: Re: NANFA-L-- two interesting articles
From: Todd D. Crail (tcrail at UTNet.UToledo.Edu)
Date: Wed Oct 20 2004 - 15:03:47 CDT
----- Original Message -----
From: Moontanman at aol.com
Dave, I have often wondered how a population of fish can start from just one
or two pairs of fish while breeding large numbers of fish over time causes
them to become sickly. North American populations of carp started from just
a few individuals as do populations of some exotics. Yet when I breed fish
just a few generations can cause significant problems.
I know this isn't true among carp. Their introduction, and more importantly, consequent reinforcement of those introductions were fairly large scale than someone putting one or two fish in a pond and they end up covering a continent. From Fishes of Ohio:
"Ohio Distribution - The first carps apparently were introduced into Ohio waters in the fall of 1879, when the U.S. Fish Commission sent shipments of 16 to 39 yearlings to each of six applicants in the Cincinnati and Fremont areas. In 1880 more than 100 applicants, scattered throughout Ohio, received carps from the Federal Government, and yearly introductions on a large scale continued until at least 1886."
It goes on to discuss the reinforcement of the populations that were done on a yearly basis from within Ohio. Essentially, they didn't need the Fed's stock any longer. While the introduction was swift, it wasn't just a couple carps.
So in this case of the carp, enough of the necessary heterozygous traits of the genome were present with that population (do a websearch on "Hardy Weinberg") to make it through the bottleneck of stocked ponds, work on the bottleneck of the wild and begin competing with natural selective forces. >From there... The margins of the habitat of that particular genome are the boundaries, and expansion happens as fast as their "strategy" and on-board "tools" allows them to move to the edge of those margins.
In the case of the carp... They grow really fast, so their predator free space becomes essentially _everywhere_ in what seems is the blink of an eye, and their only limits then (since they don't have all of their home disease and parasites either) are merely abiotic factors like climate. This is what has allowed them to become invasive. They had the initial bottleneck to overcome, and viola... We have the carp plague we see today.
As another example... I can't tell you how many Botanical gardens have Purple Loosetrife have records of having a HECK of a time establishing it in their collections from Europe. It makes me cringe. There was serious effort that went into establishing that species in just professional public gardens, and now look at what it's up to. We can barely stop it.
On the other side of this... In the case of gambusia, for example... I would suspect that the genome of the species is _incredibly_ heterozygous (have the genetic briefcase to cover a wide array of factors and be successful) to deal with what may be a very ephemeral habitat and very limited individuals as a initial population. Like they carry the "super genome" with all sorts of survival tricks in a single individual instead of having them distributed across a whole population. I'd be interested to know if anyone can shed factual light on this... I could be completely wrong, but I think this is a valid argument to explore if it hasn't been (I'm pretty sure it has though ;)
One thing that happens in aquaria, when breeding, is that the recruitment into the next generation is _not_ as selective as nature. It's not even close. And it certainly can't be what the selection would be in the wild. I mean who would cull all the way down to 1%? And what is it nature would select on? Can you determine that?
Or even a fraction of a percent? It may stand that species maintain themselves by multiple individuals contributing to the next generation.
And it works the other direction too, which end up in extinctions. The critical mass of genetic variance among a population falls below the line, and <poof>... They can't "reestablish" themselves one year to the next.
Man I wish I would have thought of using invasive exotics when we did population genetics. We did a bottleneck with beads, but they were well, just beads. I guess there's next year lol :)
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: Fri Dec 31 2004 - 11:27:51 CST