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

That’s actually written in my notes. Looking through some of my old notes to write this article, I found that quote and it was too good to pass up. Thankfully, I’ve grown a bit since then, and I can better explain this stuff to you (but of course, with the same teenaged angst).

As far as vocab terms go, “genetic variation” is one of the most straightforward ones that you’ll run across. For those of you who are a little slower than others, it means “VARIATION in a population based on GENETIC differences.” Are we all together here again?

Here’s something to keep in mind: you can’t tell differences in genetic variation by simply looking at an individual. Sure, you can look at hair color or eye color, and you’ll probably already know that these differences are based on genetics. BUT not every trait is genetic. Extreme “buff-ness,” for example is a trait that almost all members of the bodybuilder population have. Unfortunately, that’s not based on genetics. THEREFORE, we have to make a difference between GENOTYPE and PHENOTYPE. If you don’t see how that ties in right at this moment, that’s ok… it’s just something to keep in mind.

Discrete Characters vs. Quantitative Characters

Let’s talk turkey. Let’s say, for instance, that there exist only three genes in turkeys: one for feather color, one for gender, and one for everything else. For now, we’re gonna ignore the “everything else” gene and hone in on the other two.

The result of the gender gene is what we’d call a DISCRETE CHARACTER. It’s only got a certain number of results: male or female. There’s not really much of any middle ground there, so it’s a discrete characteristic.

The result of the feather color gene would be understood as a QUANTITATIVE CHARACTER. Although it’s possible that in some species color is either black or white (in which case, it would be discrete), but in most species, colors are the result of a few pigments blending together to give a certain final result. Like the range of browns (from almost white to almost black) in human skin, quantitative characteristics are usually part of a gradient.

If you’re ever having trouble assessing whether a characteristic is discrete or quantitative, ask yourself “are there only a few possibilities for this trait or are there only a few?”

Here’s an example: Is foot size a discrete or quantitative characteristic? (technically this is a trick question, but just follow me here) Because human adults show foot sizes that range from super small to super large, it’s pretty obvious that there are more possibilities than black vs. white. So foot size would be a quantitative character.

Average Heterozygosity

This is going to be the shortest paragraph I’ve written thus far. Average heterozygosity is the percentage of genes in a genome that are heterozygous. Boom. That’s all. If a population of Granny Smith apples had 500 genes, with heterozygosity in around 150 genes, then it could be said that that population has an average heterozygosity of 30%.

It’s actually that simple. But here’s the catch: it can’t measure variation any smaller than whole genes. If there was a mutation in a gene that changed the sequence but NOT the final result, using average heterozygosity would completely overlook that.

Nucleotide Variability

That’s why they invented nucleotide variability. Here’s how it works. Pick one individual from a population; compare his/her genome sequence to that of another individual from the same population. Do this again… A WHOLE LOT. Record results. What this does is tell scientists how much variation there is in the genomes of the population.

For instance, in a population of iPhones, it was found that there are approximately 122 million base pairs in the genome. After comparing the sequences of numerous iPhones, it was observed that any two iPhones differ on average by about 61 million pairs. Therefore, the average nucleotide variability among iPhones in that population was around 50%.

Other Sources of Variation

Those are the big boys that you should really be familiar with, however there are a few others that you should at least recognize.

Geographic Variation – brought about by changes in location that force populations to change in isolation from one another.

Cline – form of geographic variation. However, this is characterized by a gradient between numerous locations. If the percentage of people with a gene for black hair were to increase gradually as you move from Los Angeles to New York, that would be referred to as a cline.

Mutation – The ultimate source of all new alleles. The introduction of new characteristics due to random chance.

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I really hope this helps someone. Be sure to comment.

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Best of Luck,

Grey