So how do we look at the variation among the individuals in a biological population? Well, one way is that we can look at the protein level. So looking at the phenotype, how does one fruit fly differ from another in what it looks like?
How does one person, one human differ from another? We can look at the protein level. One way that we can do that, aside from sequencing the amino acids in the protein, the primary structure of the proteins, really kind of a quick and dirty way of looking at the proteins, is starch gel electrophoresis. And this was kind of a state-of-the-art molecular study of 30 or 40 years ago, and people still do this sometimes, but it's not too common. And what we do is we take a gel made of starch and a homogenate of tissue from the organism, and we run an electric current through the gel with the tissue in it.
And so what we have here is samples from a number of individuals. We have 18 different individuals, and we start the sample down here where the arrow is. where we started the sample and we put an electrical charge across the gel and the proteins are electrically charged, either positive or negative. We have an anode at one end of the gel and a cathode at the other end of the gel.
And so these proteins, based on their electric charge, will migrate through the gel slowly. We might run this for an hour or two. And these proteins will migrate.
towards the electrode based on their electric charge. Because proteins are different in their electric charge, some of them will migrate faster than others. There are different forms of proteins and these different forms of proteins that can be distinguished by electrophoresis are called allozyme. Now some of these will go faster than others.
see here it says some are fast and some are slow and so after a certain period of time you know we stop the gel and then we stain it with a particular stain that allows us to identify this particular protein and this is probably an enzyme this protein and we actually put in the substrate of the enzyme and whatever it needs to visualize it to make it show up on the gel I used to do this this kind of thing actually Three years ago, I guess. And so we can see how far each of these proteins went through the gel. And so we can see individual number one here, each protein went this far, and so did individual number two, and so on. Here's a heterozygote, though. Number four, here's a heterozygote.
Here's another heterozygote. And, you know, some of these you can see that had different forms of the protein. So you can identify the heterozygotes and you can see which individuals have different allozymes. This however shows variation that may not show up at the phenotypic level. However, this does underestimate the whole level of the variation.
Could be variation in the proteins that doesn't show up in the starch gel electrophoresis. It could be variation in even in the amino acid sequences of the protein. And even further than that, there could be, of course, variation in the DNA that doesn't show up in the protein for reasons that I'm sure that you understand that mutations in the DNA codon doesn't necessarily translate into a difference in the primary structure of the protein that it codes for.
And so if you look at this figure, and this is a figure from your textbook, it shows here, for example, right there is a single nucleotide difference that makes the difference between alcohol dehydrogenase slow allele and the alcohol dehydrogenase fast allele. And that one mutation is what makes the difference in the gel, on the electrophoresis. Here's a variable site that we can pick up by... Restriction enzyme analysis, and I'm sure you know about restriction enzymes, taken genetics.
And in addition, we can see in this whole sequence of 2600 base pairs of DNA of this allele of alcohol dehydrogenase, there are variable bases that we can pick up that if we sequence the whole gene and look at the population of all of these individuals of the same species in this one gene, this one locus, all the individuals are different. They all have variation that we can see. So remember population thinking even at this one locus everything's different.
Thinking about variation, we have geographic variation of various sorts. This is a flicker population. This is this is a woodpecker and there's what ornithologists recognize as the red shafted flicker in the west and the yellow shafted flicker in the east. They hybridize where they meet genetically and phenotypically you can see differences between the populations but the alleles can get between those two populations. So we generally consider this to be a single species of woodpecker.
Now this is another case of the different type of variation. This is a plant, Potentilla glandulosa, and there are three distinct subspecies, Potentilla glandulosa typica, Hansenii, and Nevada ensis. And these are found at different elevations, at coastal, mid-altitude, and alpine, going in from the west coast. This is found near Stanford, Mather, and Kimberline, and you can see the elevations here.
And the important phenotypic... variation that we're looking for is the height of the plant. And the tallest plant is this mid-altitude one.
Once you get up into the really high elevations, they grow very short. And the take-home message here is how much variation is due to environmental effects, how much is due to genetic effects. So if you take these, all three of these plants, these three subspecies, species and grow them in different conditions, you can see by looking at the three plants grown at Stanford at 30 meters of elevation, how different are they in their height?
And this height difference is due to genetics. They're all grown in the same location. And this height difference across the different... elevations is due to environmental effect.
Now another aspect of geographic variation that I want to talk about is what's called a cline and you should be familiar with that term. A cline refers to a gradual change in phenotype or allele frequencies. If you look at the geographic distribution in allele frequencies or any kind of phenotypic character, sometimes you can see a distinct break in the character.
But sometimes it's a very gradual change. And this is, again, alcohol dehydrogenase. There's a fast allele and a slow allele based on starch gel electrophoresis.
And if you look at this, there's a gradual change. The pie chart represents the amount of the fast allele. present in various locations in Australia and again in North America. These are fruit flies, Drosophila populations, and you can see a clinal variation in the allele frequencies.
Here's another figure that shows the same thing. This is human skin color. The lighter skin and darker skin graphed against the distance from the equator in degrees latitude showing a gradual clinal variation.
The farther north you get going in the northern hemisphere, higher latitudes, lighter skin color based on percentage of skin reflectance. It's a clinal variation. There's no point along that continuum where there's a distinct break. It's just a gradual change.
If you map out these data across there, it's a gradual change. And so we refer to that kind of Gradual change has declined.