Geographic Isolation — Definition & Examples

How Does Geographic Isolation Cause Speciation?

When a population is separated because of a geographic feature, like distance, a canyon, a river, or a mountain range, those two subgroups of the population are no longer able to reproduce together.

Since the two groups are in their own unique ecosystems and each experience unique pressures, they will adapt to their environment over time and can eventually become very different from each other. This has the end result of speciation.

For example, if snails slowly travel beyond a big canyon, the snails on one side will adapt to that environment and the snails on the other side will do the same with their environment. On one side, dark shells might hide them from predators. On the other side, bright coloring might let them stay cool in strong sunlight.

Over time, the two populations become more genetically differentiated and might become two distinct species with independent geographic ranges.

Physical Separation

When two populations of the same species are prevented from mating by a barrier or distance, they are experiencing geographic isolation.

Barriers can be created by changing environments, like shifting mountains and rivers. They can also be produced by natural disasters, like forest fires, earthquakes, and floods.

Isolation stops the exchange of genetic material between the two populations. They start to evolve separately from each other because their environments are different. Differences in climate and food sources can create selective pressures unique to each population. Eventually, the two populations might become different species, but not always.

A classic example of geographic isolation involves the Galápagos finches. Populations of finches found on the different islands of Galápagos evolved separately and therefore split into nearly 15 different species. They all look different and utilize different food sources.

As strange as it seems, this type of isolation can still happen in human populations. Particularly, this has happened with isolated tribes around the world. For example, there is still a tribe in the Amazon that has not made contact with the rest of Brazil, much less the rest of the world. Although we know nothing about these people, we can only imagine that their allele frequencies and acquired mutations are very different from ours.

How Isolation Leads to Speciation

Geographic isolation is an isolating mechanism in nature. Over long periods of time, it can lead to speciation. As the name suggests, it happens when populations of a species become separated, well, geographically. This can be a physical barrier that prevents the organisms on either side from breeding (like a mountain range or river), or it can even be a large distance between the two ends of the population's range.

Here's an example of geographic isolation:

We'll call this cartoon face species "little dudes". The rectangle represents the range of their habitat. As you can see, there is some phenotypic variation in the little dudes' coloration. Although the majority of the little dudes are purple, some are red and some are blue. In this hypothetical example, the different colors have no effect on an individual's fitness, so there's no particular selective pressure favoring any one phenotype.

But wait. What's this? Suddenly, the little dudes' habitat is split into two.

Immediately after the populations are split, the little dudes are still all the same species. Because they are one species, individual dudes from both sides could technically mate, if they were able to reach each other. But because of the physical separation, they can't cross over, allowing the two populations' gene pools to drift apart.

Now, imagine some time has passed. As you can see, over generations of being separated the two populations of little dudes have started to diverge, or drift apart, genetically.

At this point, the two populations are in the process of speciation. They can technically still be considered one species as long as the two populations are able to reproduce and create fertile offspring (that is, if they weren't separated).

Here, even more time has passed. The two groups of little dudes have diverged even more. Now, even if the two populations were reunited they would no longer be able to interbreed, making them two separate species. The speciation process is complete.

What's Geographic Isolation?

It's a mechanism of speciation that happens when populations of a species are divided by a geographic barrier.

Populations can become separated by rivers, mountains, or bodies of water. A pretty common example of geographic isolation is a population migrating to an island and becoming separated from the mainland population. This stops the gene flow between the groups of organisms. Over time, the two populations become reproductively isolated and they evolve separately.

A neat example is a type of squirrel that actually became two separate species because the Grand Canyon divided the populations.

Another famous example of this type of isolation is Darwin's Galápagos finches.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Speciation}}$ : One species splitting into two distinct species

$\color{#8925AE}{\text{Isolating Mechanisms}}$ : Types of isolation that lead to the formation of a new species

$\color{#8925AE}{\text{Behavioral Isolation}}$ : A portion of a species undergoes a behavior change and no longer mates with the other part of the species

$\color{#8925AE}{\text{Geographic Isolation}}$ : A portion of a species becomes separated by geography and can no longer mate with the other part of the species

$\color{#8925AE}{\text{Temporal Isolation}}$ : A portion of a species changes the time they are awake or time they breed and they can no longer mate with the other part of the species

(Video) Speciation: Of Ligers and Men

by CrashCourse

Geographic isolation is the simplest mechanism of speciation. It can occur due to any natural divide such as a canyon, river, or an island. However, it does not always guarantee that a new species will form.

When a species becomes geographically separated for long enough, the groups will follow different evolutionary paths. After enough time separated with no genetic mixing, they will no longer be able to mate and will become different species. In this video, Crash Course discusses speciation. Understanding speciation is key to understanding evolution.

Example of Geographic Isolation

Geographic isolation occurs when two populations of the same species are divided by some type of geographic event or object. This event or object physically prevents two populations from being in the same area, preventing them from mating.

As an example, let's look at a species of fish. These fish are all members of the same species, though some fish happen to have green scales and others happen to have pink scales. All the fish live in the east of the river.

One day, strong rain caused the river to flood, and the currents in the river became overpowering, sweeping some fish to the west of the river. Once in the west, the fish were unable to get back to the east.

As the picture describes, Population A happens to have more green fish than Population B. However, both groups are still members of the same species. Since the two populations will never be reunited, eventually the ratio of green fish will increase in Population A, and the ratio of pink fish will increase in Population B. Over time, each population will only have one color.

The two populations will continue to evolve separately. Eventually, they will become so different that they can no longer mate and reproduce successfully. At that point, they will be considered separate species. While the change in color alone would not lead to speciation, it demonstrates how differences in the populations can accumulate over time.