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Evolution in Action
Normal Distribution
Normal Distribution
Following the peccaries down one of their paths or trails in the desert areas of the southern United States, we come to a patch of desert on which cacti grow in abundance.

In this area of desert, a scientist lays out a 1 km2 grid and starts to look for a research subject. It does not take long. Spiny cacti are everywhere and the adaptations for survival seen on these plants make it an ideal research subject.

Spines on these cacti are important defensive adaptations. Cacti need such structures to defend themselves against predation and water loss. Spines can be considered vital adaptations to the survival of the cactus plant, yet, allelic variation in the gene pool of the study population results in some individual cacti having more spines than others.

The scientist begins to count the number of spines present on each cactus in the study area.

A hypothetical set of data might be:

# cacti # spines
4 70
12 80
24 90
10 100
7 110
2 120
1 1300

When this data is taken back to the laboratory and plotted on a graph, it gives a lot of information about the cactus population under study.

Figure legend: A Normal Distribution.

The width of the graph is an indication of the amount of variation in spine number within the population. The wider the better. More variation means that the population is better capable of withstanding stress.

The shape of the graph is symetrical; it is the same shape on both sides. When the points on the graph are connected, they form a bell-shaped or normal distribution curve. This also means that the population is not under any kind of stress.

More cacti have 90 spines than any other number. This is the mean, or average value for the number of spines in this cactus population.

Genetic equilibrium The number of spines is a clear example of Darwinian variation in the phenotype of these cacti, and this study population gives us a chance to see the forces of evolution at work.

If all five conditions for stability are met, it might be reasonably expected that this population would be in genetic equilibrium. Year after year, the investigating scientist can return to this patch of desert, repeat the counts, and obtain exactly the same distribution curve despite the fact that every year some cacti die and others germinate to take their place.

A consistent and persistent distribution curve for a particular trait implies genetic equilibrium.

© 2001, Professor John Blamire