Thursday, 10 September 2009

Founder Effect

One of the neat things about genetics is it allows us to see things that have long since passed. I don't have a time machine that allows me to investigate the distribution and abundance of moose 20k years ago, but thanks to a basic understanding of how ancestry works, we can infer what they were up to.

Imagine you have a bucket marbles. And these marbles can mate, and have baby marbles of similar colour. Bear with this stupid analogy for a moment.

So you start with a bucket, and the next generation inside that bucket looks a lot like the first. Things are at a balance.
This turns into that.

Now, let's pretend a new bucket opens up. That is to say, it gets set close to the other bucket, and a few random marbles get to jump into the new one and live out their little marble lives there.
I've circled the ones that get to jump onto the new bucket.

 Now, in our new bucket, only the ones that got there get to make their little marble babies inside the bucket. There are no dark blue or black marbles because they never made it. Just Red, Green, and Light Blue. They are fruitful, multiply, and reach the brim of the second bucket.
The second bucket looks a lot like this.
Now a THIRD bucket is placed near the second bucket. Why do I have all these buckets? Well, I've got a lot of chores, and I need some for the sink, some for fishing, some for mixing stuff, and some for packing water. Again, only some marbles get to jump into the new bucket, mostly at random.
I've just circled one blue and two red. Green doesn't get to go to the new bucket.
Finally, we're down stream at our last bucket, and the founding marbles have little marble babies, and populate the bucket. This leaves us with three things we can compare.
Bucket 1
Bucket 2
... and Bucket 3 

You can see that each successive colonization results in fewer and fewer colours being represented in the population. The same is true about Genes - obviously it's true, because otherwise why would I make such a dumb analogy. There are other factors at play, which I won't go into, but this is the nut of the Founder Effect. You might realize that this is a special case of a bottleneck, and you'd be right! It's the same principle, where a few number of individuals make up what ends up on the other side. But in this case, your source population is (normally) maintained.

So what you can do is look at the areas where you see animals, and measure their genetic diversity. This'll allow you to puzzle out where a group of animals had their founders. This had actually been done for humans, and it's considered heavy support for the Out of Africa hypothesis - Africa is the centre of human genetic diversity. It's frequently said that there's more genetic diversity in one village in Africa than there is in some whole countries!

But humans are boring. And right now, I'd write about moose, except I've used quite enough words for one post, so I'll postpone it until another.


dragonfly said...

Great explanation, you're pretty good at that. Do you teach? And hey, are those my marbles? I seem to have lost a few.

gpc said...

I agree with dragonfly - you are good. Now the out-of-africa theory makes sense to me (you wouldn't believe the simplistic explanation I was given in school!)