Gene Flow - Online Tutor, Practice Problems & Exam Prep

Gene Flow

Now I want to take a closer look at gene flow and think about how that can affect allele frequencies and therefore cause evolution. But we're going to define gene flow here as just the movement of individuals or gametes between populations with different allele frequencies. Alright. So you can imagine two populations. They have different allele frequencies.

Individuals start moving between them. They bring their alleles with them. That's going to change the allele frequencies in those populations, and generally, it's going to make those two populations more similar to each other. Now it also may introduce, we're going to say here, new alleles to a population. Right?

You can imagine you have two populations. One population has new alleles in it that are, you know, maybe sweeping through by natural selection or something. Gene flow, while those could introduce those alleles into that other population. It could also sort of reintroduce alleles that may have been lost in a population due to genetic drift or some other mechanism. When those individuals move, they bring those alleles that were previously lost with them.

So altogether, we're going to say up here in this purple box, the effect on allele frequency. We're going to say that this reduces genetic variation between two populations. We have our just very simple image here to illustrate this. We have these two populations of butterflies. They're, you know, separated by some barrier there or river, it looks like.

And we can see that these populations are different from each other. But, you know, if individuals are moving between these populations, then those populations are going to be more similar as they bring those alleles along with them. Alright. With that, we are going to look at all our different mechanisms of evolution. More coming up with examples and practice problems.

You definitely want to check it out. I'll see you there.

Gene Flow Example 1

Our example is about Isle Royale, and Isle Royale is this kind of famous island in ecology because people have been studying the wolf and moose populations there for a long time. Let's start out just by looking at the map here. Well, we see we're looking at Lake Superior, and here's this Isle Royale. And you can see this is going to be the Canadian coast up here, and there's this ice bridge that we're going to talk about that sometimes connects that island to the mainland. Alright.

So with that information, let's read the problem. It says Isle Royale is a small island off the coast of Michigan's Upper Peninsula in Lake Superior. The island has a small wolf population that ranges between roughly 10 to 50 wolves depending on the year. Just to be clear, that's a really small population size.

Now during cold winters, ice bridges form between the island and the Canadian coast, allowing for wolves and other animals to cross. Due to increased temperatures from global warming, ice bridges have been forming less frequently in recent years. Now with all that information, let's check out the questions. Question A says, based on the wolf population size on Isle Royale, what evolutionary mechanism would you expect to be happening relatively rapidly there?

What do you think? Well, given a really small population size, I would expect genetic drift to be happening rapidly. Genetic drift, we said, happens the most. It's most accentuated in small population sizes, where those random occurrences have much more influence. So genetic drift, small population, expect that to be happening a lot. Question B asks, due to the small population, the wolves on Isle Royale are closely related. How would you expect this to affect the fitness of the population?

I'm going to say it would decrease fitness. It doesn't specifically ask why here, but I'll put in parentheses due to inbreeding depression. When those related individuals keep mating with each other, they're more likely to create homozygotes. That increased homozygosity means that there's more likely that those rare deleterious recessive alleles, they're paired with each other, and that reduces the fitness of organisms.

Question C then asks, what evolutionary mechanism would the formation of ice bridges allow to occur that otherwise would be unlikely? And what do you think about that? Well, we said this ice bridge allows animals to cross between the mainland and the island. That to me sounds like gene flow. And that is one of our evolutionary mechanisms, and you can think genetic drift and inbreeding depression, that's going to be influenced a lot by gene flow. And sure enough, question D asks, how would your answer to C affect the mechanisms that were your answers to A and B? So, the answer to C is gene flow.

How would that affect genetic drift and well that inbreeding depression? I'm just going to say it would counteract them. I'm going to say counteract, and I'll just note, it is because it would introduce new alleles. Genetic drift, you lose alleles in your population. Gene flow will introduce alleles that were potentially lost. As for inbreeding depression, now you have a high number of homozygotes. The more alleles you have in that population, the less inbreeding there is, the less homozygosity you're going to have. So gene flow will counteract both of those.

We got one more question here we're going to scroll down for. It says, imagine that you are a conservation geneticist. Propose a simple solution to maintain the fitness of the wolf population on Isle Royale if ice bridges no longer form between the island and the mainland. So what could you do? What steps could you take to maintain this population? Well, I'm just going to say you could import wolves. If every generation or every few generations, at least, you bring some wolves from the mainland to the island, that's going to sorta renew that gene flow. It's going to introduce those new alleles, and that will slow down the genetic drift or counteract the genetic drift in that inbreeding depression, and it will maintain that wolf population. Now if there's no gene flow going forward, it's really hard to imagine a population size of only 10 to 50 really persisting for a very long time because of those two factors.

With that, we'll practice more going forward, and give it a try.

To boost wild populations of fish such as Atlantic Salmon, scientists will sometimes breed fish in captivity and then release the fish into the wild population. Below, scientists measured the fitness of the wild fish, the captive-bred fish after being released into the wild population, and the first-generation offspring of wild and captive-bred fish. The fitness values are reported as relative values between 0 and 1. What description below best explains the data?

• A. The first-generation offspring of wild and captive-bred fish are likely less fit due to inbreeding depression.
• B. Gene flow into the wild population from the captive population has increased genetic variation in the wild fish; therefore, wild-caught fish now have the highest fitness.
• C. Wild populations likely have the highest population size and, therefore, are less subject to genetic drift, meaning they will have the highest fitness.
• D. Captive-bred fish are poorly adapted to living in the wild, and gene flow from captive populations into wild populations can be expected to reduce the overall fitness of the wild population.