Flour beetles A group selection example

Michael Wade set up a laboratory experiment to investigate group selection with the goal of showing that it occurs. In his experiment, Wade used flour beetles — little insects that are happy to grow and reproduce in a vial of flour. They eat flour, but they're also cannibalistic: Adult beetles eat larvae and eggs, and the larvae eat eggs.

Wade set up four experimental treatments, each consisting of 48 populations iff of beetles. For each population, he allowed 16 individual beetles to grow and reproduce for 37 days. Then he took 16 individuals from each of these populations, put them in new vials of flour, and allowed them to grow for 37 days. He continued in this way until he had the number of populations he needed. What differed between the populations in the various treatments was the factor that Wade selected for. For his experiment, he decided that population size — a group rather than an individual characteristic — would be the favored characteristic:

1 Treatment 1 (the control treatment). In this treatment, individual selection was the key. For the control group, Wade randomly picked 16 beetles from a population to start the next flask. As a result, individual beetles that were more successful at reproducing were more likely to have offspring — and, hence, their genes — in the next generation. (If you make twice as many offspring as the next beetle, you have twice the chance of getting some children into the next generation.)

Individual selection is always happening, so scientists can study group selection only by keeping track of individual selection at the same time and then comparing the results from the group selection treatments to the individual selection control treatment. The control group is the one in which selection acts at the individual level.

1 Treatment 2 (a group selection treatment selected for large population size). Wade created his new populations by using beetles from only the biggest populations.

1 Treatment 3 (a group selection treatment selected for a low population size). Wade used only individuals from the smallest populations to found new populations.

Treatments 2 and 3 represent group selection, because not all beetles in all vials contribute to the next generation — only those that meet the selection criteria (in the experiment, either large or small group size). You could be a really fit beetle in your particular vial (that is, a disproportionate number of the offspring are yours), but if you exist in a population that isn't large (or small) enough to be used, none of your genes get into the next generation.

Here's what Wade found: The original beetle stock used in this experiment generated population sizes of about 200 beetles after 37 days. But after nine generations, population size decreased in all his treatments. Population dropped in the control treatment (individual selection); it decreased more in the group selection treatment for small populations and less in the group selection treatment for large populations. The following sections explain why.

In the control treatment (Treatment 1)

In this case, selection was acting at the level of the individual. Whatever heritable traits increased the chance of a beetle's having descendents in the next generation increased in frequency. It just so happened that as a result of this selection on individuals, population size decreased from about 200 beetles in the stock population to 50 beetles after 9 generations. Although that result may seem odd, keep in mind what these beetles like to eat: flour and baby beetles (those in the larval and egg stages). Essentially, this treatment selected for beetles that were more voracious cannibals.

In Treatment 2 (group selection favoring large population size)

In this treatment, population size had decreased after nine generations, but not as much as in Treatment 1 (the individual selection control). Selection at the level of the group for larger population size had an effect in the opposite direction of selection at the level of the individual.

Selection at different levels can act in the same direction or in a different direction. If selection acts in the same direction, it compounds the effect; if it acts in a different direction, it mitigates the effect.

In Treatment 3 (selection favoring groups with small populations size)

In this treatment, population size decreased even more than in the control population. Selection at the level of the individual and selection at the level of the group each had an effect on population size, and these effects were in the same direction. After 9 generations of selection, the population had decreased from 200 beetles to 19.

0 0

Post a comment