Thanks for following up!
It looks like you are interested in comparing traits under natural selection compared to artificial selection, which is a fascinating topic. The problem is that I do not think you will be able to demonstrate natural selection (for comparison) in a short time. In order to observe natural selection, there must be a selective pressure
on that trait. This means that certain versions of the trait lead to better survival than others. I am not sure that height is under any selective pressure in natural populations of radish plants.
However, do not lose hope! I have suggestions that will still allow you to do a project similar to what you have proposed. If you are truly interested in the power of artificial selection, you can still perform experiments that will select for taller radish plants, and compare the height of your selected offspring to the average height of the original plants (and to plants in the wild, if you are so inclined). This will require two important things: (1) that you perform multiple crosses - maybe three generations - with multiple plants (say, 5 plants per cross), and (2) that height is a quantitative character
, that is, a trait controlled by multiple genes.
If height is only controlled by a single gene with two alleles
(versions), then you will only see two phenotypes
: tall and short , and you should not expect the tall plants to get any taller. If height is controlled by multiple genes in radishes (as it is in humans), then you can expect to get taller and taller plants when you select for height.
Before I address your hypothesis and experimental design (including your control), I would like to ensure that you are on the right track with your project. Here are some suggestions for directions in which you can take the project. Please post again with the choice that most interests you, and I can help you design a solid experiment.
1. Determine (experimentally) whether height is controlled by only one gene or by multiple genes in radishes.
2. Perform artificial selection to grow radishes that are taller than would be expected in the wild. (** This will only work if height is controlled by multiple genes. **)
3. Perform artificial selection on a different trait in radishes that researchers know to be quantitative.
To help you with finding alternative traits, I found a paper that looked at different quantitative traits in wild radishes: http://www.genetics.org/content/180/2/945.full.pdf
This is a scientific paper, so it may have some unfamiliar terms. But you can use it to see some of the different radish traits and how they were measured.
I am looking forward to seeing where you'd like to take this project!