Artificial Selection: Biology Lab

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  • 0:01 Introduction
  • 0:45 Experimental Procedure
  • 3:25 Explaining the Data
  • 5:11 Lesson Summary
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Lesson Transcript
Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

Natural selection is a major way that evolution works, but what happens when humans get involved? In this lesson, we'll see how humans influence the characteristics of a population through artificial selection.

Natural vs. Artificial Selection

What do golden retrievers, thoroughbred racehorses, and juicy tomatoes have in common? The way they look, act, and, in the case of the tomato, taste, have all been brought to us by artificial selection, which is the breeding of desired traits into a species. Artificial selection is much different than natural selection.

Natural selection arises from differential reproduction; that is, the environment plays a role in which organisms survive and reproduce most successfully. Those that do are more likely to pass their genes on to a new generation. In artificial selection, though, humans decide which organisms reproduce and, therefore, control what kind of offspring they have.

Experimental Procedure

So, how can humans change the genetic makeup of a population? Let's see. For this simulation, we're going to use a kind of plant called Brassica rapa. This little plant has an interesting backstory, but all you need to know is that it's a great model organism for artificial selection because some of its traits are inherited by simple genetic laws and it grows fast.

Say we had a setup in which we grew some of the fast plants. For our simulation, ours have trichomes, or hairs, on their petioles, the stem-like structure that holds the leaf on to the stem. Each plant has a different number of trichomes - some have many trichomes, some have few, and some in between. This is called a continuous trait, a trait that varies along a range. Trichome number is inherited consistently between generations; that is, the number of trichomes is controlled by a specific set of genes.

Let's start by looking at 50 plants. Assume that we've counted trichomes on the first leaf's petiole of all 50 of these plants and created this simple graph that shows the number of trichomes on each petiole:

Histogram of petiole trichome data

What kind of statistics might you use on this type of data? Because the data are observational, we need to stick to descriptive statistics like mean, median, range, and standard deviation:

Mean Median Range Standard Deviation
41 36 80 23

These statistics show that there's a fairly large spread in the data; that is, there aren't too many values close to the average.

Now it's time to do the artificial selection. For this example, we'll do directional selection - selection of one extreme. Let's select the hairiest plants, the ten hairiest plants, and isolate them from the rest of the population. Then we'll make a new graph of the number of trichomes on just those plants. Our statistics for just these plants are:

Mean Median Range Standard Deviation
73 74 16 4

These statistics of a very small sample size indicate a much narrower spread in the data. It's important to note that this is a very small sample size.

Our next step is to isolate these plants and let them breed only with other members of the isolated population. What prediction would you make about the number of trichomes in the next generation of plants in both the isolated population and the general population? What might happen if we repeated the selection process? If you think that the offspring of these isolated plants would have more trichomes than those in the remaining population's next generation, you would be correct. But why?

Explaining the Data

To answer that question, let's dig into why trichomes might be useful in the wild. Plants, being producers, are consumed by insects, animals, and even humans. Unfortunately for the plant, their consumption either means their death or their weakening to the point where they can't reproduce.

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