An experimental way of seeing how selection works is using the fruit fly (Drosophila melanogaster). You can design a laboratory protocol to see how selection eliminates progressively a mutation. Mutants are relatively easy to obtain in this specie. For example, you can use mutation such as sepia (dark brown eyes), ebony (black body), scarlet (bright red eyes), vestigial (rudimentary wings), etc.

Preparing the population base

The first step is to prepare an initial population by crossing in a flask with culture medium three females from a normal pure strain with three males from a pure mutant strain (the reciprocal cross can be also used). Females should be virgins, so it is impossible that they have already crossed with males from their same pure strain. The female genotype will be AA, and the male’s one will be aa (remember that A dominates over a).

About 5 days later and once we have observed larvae in the culture medium, you must give dead to the progenitors so any of them could cross with the descendents females. In the first filial generation all descendents will be normal, as they are heterozygous (Aa). Then, it is necessary to select by random three females and three males to obtain next generation (G0) in a new flask. In this new generation (G0), we will count how many normal individuals (about 3/4) and how many mutants (about 3/4) appear. Mutant phenotype flies must be rejected, and we will use three females and three males with a normal phenotype as progenitors for next generation (G1), and therefore we will put them in a new culture flask. It is desirable that females are virgin.

It is possible to repeat the process with same conditions as many generations you want (G2, G3,…). If in a specific generation mutant flies do not appear, it doesn’t mean that the mutant allele is lost, it can be maintained in heterozygote (Aa). If the process is repeated with enough generations, eventually you will reach the total elimination of the mutant alleles.

Why do mutants disappear?

 The interesting thing of the experiment is that you can verify that in each generation less mutant flies will appear, and that is due to selection. We are eliminating they in each generation. In fact, this also happens in nature because mutant individuals are less viable than normal ones. Remember that while some mutations are viable, most are less useful than normal phenotypes.

Other evolutionary forces

You must keep in mind that natural selection is not the only evolutionary force that is acting in our experiment. The genetic drift is also acting, as we are talking about a population with very few progenitors. Furthermore, consanguinity is a important thing because after few generations individuals are related. Finally, we can mention mutation, that can generate the appearance of new mutations (a), but in our case, because its rate is very small (about 10-5) its effect will be insignificant.

How to do this experiment?

How we can cultivate D. melanogaster in the laboratory? This specie is very easy to maintain. We need culture flasks, either plastic or glass, with a volume equivalent to a small jar of mayonnaise or jam. The top must not be hermetic, and we will make one with a very compact cotton ball. Inside the clean flasks we will introduce the culture medium. Normally you can use a mash made of cornstarch with sugar and agar (to get gelled), and once cold you must add baker’s yeast (Saccharomyces cerevisiae). But you can also use other “domestic” cultures. For example, a ripe banana paste with a bit of sugar and baker’s yeast can be used. But something important is that yeast (adult flies food) must be “authentic” (Saccharomyces cerevisiae), and not bicarbonates that can be also used for making cakes and that sold in stores as yeasts.

It doesn’t matter the culture medium you use, but it is important to introduce a folded paper like an accordion to provide a dry physical support to larvae so they can develop their pupation. The temperature requirements are not particular strict. The optimum temperature is 25ºC , but in spring or autumn a temperature between 20 and 25 degrees is enough. At temperatures below 25 degrees the biologic cycle slows, while with/at higher temperatures the cycle accelerates (but there is the risk that flies develop sterile). At the optimum temperature the biologic cycle last about 10 days.

Where drosophilas can be obtained?

Schools can contact the Centres de Recursos Pedagògics to order Drosophila melanogaster individuals.