Natural selection simulation

[Aerlinn]

We were doing a simulation about natural selection acting on a beetle population of 30 at school. We had dice and cards representing the beetles, and randomly dealt card pairs to represent pairs mating, and rolled dice to decide the offspring of each pair and which beetles would be eaten by the 'predator'. That was repeated several times to model several years.

So we ended up with a table showing changes in the population. At the start there were equal numbers of black, brown and white beetles (10 of each), by the second year black ones had completely disappeared, so they were obviously selected against. By the 7th year or something, the brown ones were gone, and all the population were white. Their chances of predation were 3 black: 2 brown: 1 white (ratio)

Now, I'm having trouble with questions related to the activity.
-Suggest how the simulation could be improved to more accurately model the scenario?
-Does the chance of survival of any of the phenotypes change from year to year?
This last question got me very confused. Through the years, does the survival chance of the white beetles increase, and does the survival chance of the brown or black beetles decrease, or do I have the wrong idea?

Any help on this would be great!!! It's really important, due tomorrow.
Thanks in advance![/u]

[Aerlinn]

Anyone???

[mith]

Genetic drift, look it up

[Aerlinn]

Genetic drift? Thought they changed as a result of the predator. Genetic drift happens slowly...

[mith]

Why would it happen slowly?

[Aerlinn]

Well, I dont know, but I think it happens over a long period, because it's due to chance...

[mith]

Isn't it chance at work here?

[AstusAleator]

Your scenario was relying entirely on random chance (stochastic events) rather than selection pressure. therefore none of your populations had a fitness advantage other than numbers. For example, if there are 10 white and only 2 black, the whites have the advantage of numbers in the case of random deletion.
To improve your model, determine a set of selection factors, to simulate variable fitness in the different phenotypes.

[Aerlinn]

The 'activity' is over anyway so all I can do now is figure out what I did right... No, I think the change in numbers is mainly due to predation. There was a predator. The blacks had most chances of being eaten by that predator, and so it makes sense that, by my results, by the third year, blacks disappeared competely. As I said, genetic drift would happen over many generations. An allele doesnt just die out over a few generations. So, I'm not sure I understand what you're saying...

[AstusAleator]

I think you'd need to describe your methods more precisely before we can comment on your activity. In your predation calculations, you factored in that black would be more likely to be killed?

Genetic drift can occur instantly, as a stochastic event.
Example: You have a population of n=500 moths. half (250) are AA and the other half are aa.
Now half of their habitat is suddenly destroyed, killing whatever moths were in that habitat. The law of averages says that the losses of both genotypes should be roughly proportional, but behavioral and environmental factors come into play. It could be that of the killed moths, 200 were AA and 67 were aa. Or it could have been more extreme, and 99% of all the moths were destroyed, leaving 5 aa moths and no AA moths.
You see how this changes the genetic composition of the population?

[mith]

Btw, stochastic is a fancy word for random lol

[Aerlinn]

Yea, I understand, what you both mean, and with your example, AstusAleator, but that seems to point more towards something like some random destructive event bringing about a bottleneck effect. The general definition of genetic drift, is, I think something like this, 'Genetic drift is the term used in population genetics to refer to the statistical drift over time of allele frequencies in a finite population due to random sampling effects in the formation of successive generations. In a narrower sense, genetic drift refers to the expected population dynamics of neutral alleles (those defined as having no positive or negative impact on fitness), which are predicted to eventually become fixed at zero or 100% frequency in the absence of other mechanisms affecting allele distributions.'

But then, I could be wrong Only in my second year of Biol after all ^^