The Three Patterns of Natural Selection

Hey Nerds,

In biology lecture this week, my class is covering the mechanisms of evolution. I needed some extra practice, so what better practice than to explain the concepts to someone else. LUCKY YOU! :)Here goes…

It should be pretty obvious that there’s a serious amount of diversity on Earth – even among individuals of the same species… in the same population! Even then, life just happens; sometimes things happen that are beyond an individual’s control, and when the situation changes, sometimes individuals in a population can’t deal with those changes. You already know that when this happens, we call it natural selection. Those individuals that can still deal with the environment after the situation has changed are the ones that we say have been “selected for.”

There are three ways that this can occur:

1. Stabilizing Selection

Remember what we said about there being a lot of diversity? Think about this: there are three strains to a species of butterfly (white, gray, and black). The white and black butterflies are what we would refer to as the extremes, and the gray is what we would call an intermediate. Makes sense, yes? Most populations have divisions like this (with extremes and intermediates).

In the case of stabilizing selection, the strains “selected for” are usually the intermediates. It STABILIZES the species. In terms of our example, this would mean that something is keeping white and black butterflies from being able to survive and reproduce as very well. The species is stabilized as mainly gray.

NOTE: this doesn’t mean that there’s any loss of ability to produce offspring that land in the extreme category. Those white and black butterflies can still be produced. However, they don’t have many children.

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2. Directional Selection

Can you guess how this mechanism of selection works? Here’s a hint: it leans toward one direction. ;) To be specific, this means that selection favors ONE extreme over the other extreme AND the intermediates.

So, if there was some sort of factor that kept black AND gray butterflies from being able to survive and reproduce as well as the white butterflies, then it could be said that selection favors the white butterfly. Because the white butterfly is an extreme, this would be a form of directional selection. It would also be directional selection if the favored butterfly was black.

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Disruptive Selection

So now that you have a pretty good idea of how this kinda works you probably know the only other way that this can work.

In the case of disruptive selection, both of the extremes are selected over the intermediates. NO GRAY BUTTERFLIES. It’s kinda like double-directional selection. I can’t think of anything creative at the moment, but you’re a smart person. You can think of something. ;)

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I hope this helps.

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Best of Luck,

Grey