Which Best Describes Biogeographic Isolation? Answer, Examples, and Exam Tips

Quick Answer

Biogeographic isolation is best described as the separation of populations of a species by physical or geographic barriers. It also prevents gene flow, leading to independent evolutionary paths and speciation.

Which best describes biogeographic isolation?

• A. It always leads to the formation of new species.
• B. It cannot lead to evolution.
• C. It is a mechanism for evolution.
• D. It occurs only through geographic forces.

Correct answer: C, "It is a mechanism for evolution."

Questions on biology during exams are often asked in such a way that they seem to be easy, but they come with a smart trap, and the biogeographic isolation definition is one of those objects of confusion. In one sentence, the definition of biogeographic isolation can be defined as an evolutionary process or mechanism by which a population is divided.

Any type of barrier, such as a mountain, ocean, or land bridge, can produce divergence, and that is where errors happen, and all the reasons you are using the divergence model become confusing. Moreover, when you work on a biology assignment or exam question, it is possible that you will get confused by something like simple reproductive isolation.

It seems that this topic is likely to be a complex one, but let me assure you that this topic is far from complicated. Everything will become clear within ten minutes, with examples and in an effective way. Continue reading to find out the exact characteristics, examples, and distractors of the topic.

What Biogeographic Isolation Means

Simply put, a population is divided, and the two halves no longer exchange genes. Gene flow is when alleles are passed between groups via migration and reproduction. It has a tendency to standardize populations. 

In its absence mutation, genetic drift and natural selection may make the two populations to develop in different directions. Since all these mechanisms depend on modifications at cell level and gene expression, understanding basics of cell biology will make clear how a mutation at the gene level affects whole traits. 

Three points are worth fixing in your head:

• It is relative to the species: A river may pose a barrier to a mouse, but would be insignificant to a bird. A dry valley may be a barrier to a frog, but not a hawk.
• A barrier can be anything: Poor habitat is equally as effective. Tree squirrels will not cross a treeless desert, because the desert is a barrier for them.
• Geographic and Geological are not the same: Geographic is accomplished by geographical isolation of the populations by natural earth processes such as uplift, volcanic island formation, continental drift, and similar events. But it is not just geographic isolation, as climate change, river changes, and man-made objects such as roads and dams also contribute.

There are alternate terms for biogeographic, and you will find geographic and geographical in some textbooks. On the test, use the term that your class uses.

Geographic barriers are of various types. There are a number of common types of geographic barriers, and the same landscape may be impermeable to one species and yet open to another.

• Land animals and many plants are separated by water: Oceans, wide rivers, and lakes. Islands, typically, are the classic case.
• Lowland and forest species are divided by rising ranges of mountains and deep gorges: Like the Grand Canyon did for squirrels.
• Deserts and glaciers: Dry or frozen regions that keep moist, shady, or warm conditions species apart.
• Warming and cooling caused by climate change: Can cause small populations to be separated on the top of mountains or in cool areas.
• Human structures: Highways, dams, canals, and farmland can fragment a population within a few decades. That is why a geographic barrier is not necessarily natural, and therefore option D is incorrect.

Why the Other Three Options Are Wrong

How a Barrier Becomes a New Species

Typically, it is allopatric speciation (speciation in different places). It is a process that can be divided into four steps:

1. A barrier is formed: A river changes course, a glacier advances, a land bridge closes off, or a highway splits a habitat.
2. There is no crossing any longer: Individuals are no longer able to cross to find their partners.
3. There is a divergence between the groups: In several generations, mutations, genetic drift, and natural selection can occur independently on each group of individuals.
4. Reproductive isolation occurs: If the groups are sufficiently different so that interbreeding is impossible, then they are regarded as different species.

The word may comes into play in step four. These species typically take thousands of generations, and many splits never even complete. Groups can become one population if the barrier goes out of existence first.

What Decides the Outcome

This does not always mean that two separate populations must go the same route. There are five factors that determine the extent of the split:

• Time: The more time passes, the more change will have occurred. A few thousand years might yield a distinct subspecies, and full speciation can take a longer period of time.
• Complete barrier: No gene flow occurs at a complete barrier. A leaky one, such as a narrow strait, lets a few migrants through and slows divergence.
• Small population size: Small isolated populations change rapidly as a result of genetic drift. They also are more likely to be wiped out.
• Selection pressure: The more different groups are in terms of food, climate, and predators, the more selection pressure drives them apart. They might remain the same if the conditions are the same.
• Dispersal ability: Some species can definitely get over barriers more easily than other species, because they can fly like birds or bats, whereas salamanders and snails might not be able. Some species are hit harder by the barrier than others.

Real Examples of Geographic Isolation

Remember the examples when discussing geographic isolation. There are four examples: a canyon, an ocean archipelago, a continent, and an island chain, each providing a lesson in the impact of separation on living things.

• Canyon: Isolation may result in a new form, but not a new species.
• Archipelago: Local food and open water drive local adaptation.
• Continent: Long isolation leads to a wide diversity.
• Island chains: Like habitats mean like features.

Kaibab and Abert Squirrels at the Grand Canyon

Abert's squirrel is found in nine recognized subspecies. The Kaibab squirrel is found only on the Kaibab Plateau on the North Rim.

The barrier is the Grand Canyon itself plus hot, dry land below the pine forest, thus forming a separate subspecies but not a separate species.

The Mechanism: Abert's squirrels spread into the Grand Canyon area following the last Ice Age when ponderosa pine grew at lower elevations. During climate change and pine's retreat upslope, forests became fragmented, and the population in the North Rim was isolated. Other Abert's subspecies were later reintroduced to the South Rim, making it now a different subspecies there.

The Differences: The Kaibab squirrel has a dark belly, and the South Rim Abert's squirrel is white-bellied. They both have tufted ears and rely on seeds of ponderosa pine.

Exam Point: Isolation resulted in a localized type and has not led to a separate species. This is the general counterexample to option A.

Darwin's Finches in the Galápagos

General Information: It lies six hundred miles west of Ecuador and was explored by Darwin in 1835. They developed from ancestors who migrated from the mainland of South America. Their habitat, surrounded by the open sea, acts as a barrier to genetic flow but does not stop it completely.

Importance of isolation in this area: Food available on various islands may consist of hard seeds, soft fruits, and insects. Even with the ability to fly between islands, finches will not interbreed much because open water limits their mixing, enabling each group to evolve to utilize locally available food resources.

Medium ground finches have been studied on this island by the Grants since the nineteen seventies. In 1977, a bad drought reduced the seed supply. The deeper the finches' beak, the more likely they were to crack the tough seeds that were left, so more survived. The mean beak depth of their offspring was greater, around 9.5 mm prior to the drought and around 10 mm following the drought.

Exam Point: The beak shift of 1977 is an example of natural selection over an isolated population. Selection makes, and isolation sets up. Graphics, images, and text are modified from sources including HHMI BioInteractive, Open University, and LibreTexts.

Marsupials of Australia

Some one hundred million years ago, the Australian continent separated from the other continents in the south. This left a lot of time for kangaroos, koalas, and wombats to diverge without placental mammals. Marsupials are most diverse in Australia and New Guinea, although there are species in the Americas.

Anolis Lizards on Caribbean Islands

When anolis lizards colonize islands, they develop analogous body types in similar habitats: long legs are good for running along the ground, and short limbs are good for balancing on a narrow twig. This is textbook convergent evolution in these isolated populations.

At a Glance

Island Biogeography

Islands with boundaries are clearly demarcated as natural laboratories. In 1967, Robert MacArthur and Edward O. Wilson presented their equilibrium theory of island biogeography. It states that eventually the number of species on the island will reach an equilibrium between immigration of new species and the extinction of old species.

The model leads to two rules:

• Distance matters: The closer the island is to the mainland, the more species arrive compared to more distant islands.
• Population size is correlated with habitat size: Small islands lose more species. The rule of thumb is that ten times more species are found in ten times more area, which is approximately tenfold, though there are exceptions depending on region and group.

In the late nineteen sixties, a team of researchers, Simberloff and Wilson, tested this idea on small mangrove islets in the Florida Keys. First they eradicated the bugs from some islets and then observed how they re-colonised the plots. If you're doing an assignment for a biology class on a controlled experiment like this you'll want to know the best way to write a biology lab report so that your results and ideas are clearly expressed. The same is true of patches of forest, parks, or mountain tops that are isolated by farmland or human habitation. And that is the very reason for wildlife corridors: link up disconnected areas for the advantage of wildlife. 

Key Terms at a Glance

Biogeographic Isolation versus Other Isolating Mechanisms

Students struggle because the word is in all of them. What really distinguishes these is whether the groups are kept apart geographically or kept from breeding behaviorally.

Key Tip: different texts will group geographic isolation in different ways. It is sometimes grouped plus reproductive barriers, and in some instances it is the first condition listed under allopatric speciation. Use the label that your text uses.

Why This Question Trips Students Up

This is why exam writers have multiple choice questions. The trap is this is a topic about a process that cannot always lead to a result, with the distractors containing absolute words making them sound more certain.

On test day, practice these habits:

Circle absolute words: Always, only, never, cannot, and all should be looked at again.

Now go back and make each of the incorrect responses softer and make each of them a test: True statement: sometimes, isolation causes speciation. Here's the issue with the original absolute wording.

Write an example for each: If an option says there is always isolation between species does this include the Kaibab squirrel? 

Recall the pattern: The process starts with the obstacle but not the conclusion.

Quick Practice Questions

Try these first and then view the answers below.

1. A population of salamanders is separated into two groups by a highway. What kind of isolation is this? Is the barrier natural?
2. True or False: A barrier means populations must stay as two different species.
3. Why did species loss happen faster on small islands compared to larger islands?

Answers

1. Geographic isolation: The barrier is man-made, demonstrating that barriers can be artificial rather than natural.
2. False: If they have not evolved sufficiently, the groups can interbreed and be merged back into a single population.
3. Smaller populations and reduced habitats on small islands mean that one storm or disease can destroy a species.