Overview of Speciation Types & Barriers

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Have you ever wondered how a new species arises? This lesson will examine speciation, or when a new species forms, as well as barriers that prevent separate species from mating.

Species and Speciation

Picture a group of imaginary critters. These critters are the same species, or group of organisms that can mate and produce fertile offspring. The critters inhabit a forest until a flood shakes up the landscape, thus dividing them into two populations: Critter 1 and Critter 2. Over thousands of years of separation, the two groups evolve differently. For example, the members of Critter 1 live in a swampy environment so the ones that are better at swimming survive and reproduce. Eventually members of Critter 1 are streamlined and hydrodynamic. Meanwhile, the Critter 2 group inhabits an environment that is slightly more mountainous. This group eventually becomes muscular and stocky. Over time, the two groups become more and more dissimilar, until they could no longer reproduce if they were to come into contact with one another. In this case, speciation has occurred, meaning one (or more) new species has developed. In other words, the critters are now two separate species. Crazy! You just witnessed speciation!

Types of Speciation

The critters in our example underwent a type of speciation called allopatric speciation. This means that the species were separated by a geological barrier and over time each population evolved differently, which eventually resulted in their inability to mate with one another. Allopatric speciation usually occurs when some sort of slow geologic process, like glaciation or the formation of a water body, occurs.

A real word example of allopatric speciation can be seen in the snapping shrimp. Approximately 3 million years ago the Isthmus of Panama closed, thus separating the snapping shrimp. Scientists found snapping shrimp on both sides of the isthmus that appeared identical, but when they put them together they found they snapped at each other and wouldn't mate. Over the 3 million years since they separated, they evolved differently and have become separate species.

The land separating the Pacific and Atlantic Ocean, or Isthmus of Panama, closed approximately 3 million years ago
panama

It doesn't necessarily take an event like a flood or a glaciation to cause speciation. In fact, sympatric speciation happens when speciation occurs without a barrier. The prefix 'sym' means 'same,' so sympatric is referring to species living in the same area that undergo speciation. So if they aren't separated, what prevents the same species from interbreeding? Oftentimes sympatric speciation is the result of a mutation in the offspring. For example, sometimes the number of chromosomes is not copied correctly and offspring end up with more or fewer chromosomes than their parents. While this is often fatal in animals, it is quite common in plants. When the offspring of plants have extra chromosomes, it makes mating within the species impossible. The plants with extra chromosomes can, however, mate with other plants that have the extra chromosomes. This immediately results in a new species.

An example of sympatric speciation that does not have to with mutations is that of resident and transient orcas. Resident orcas remain in the same region, whereas transient orcas travel. Scientists believe that even though the orca ranges overlap, the transient orcas do not mate with the resident orcas. Both groups also have different diets, different vocalizations and different social behaviors. Although the orcas are still considered the same species, they may be undergoing sympatric speciation.

Barriers

So now that you know what speciation means and what types of speciation occur, you may be wondering what prevents separate species from producing offspring? There are two main barriers: prezygotic barriers and postzygotic barriers.

Let's start with prezygotic barriers, which are barriers that prevent mating from ever occurring. I know, another vocabulary word, but if you examine the word, it will be easier to remember. A zygote is the result of a sperm and egg uniting, so a prezygotic barrier is a barrier before the zygote forms, hence the name 'prezygotic.' Prezygotic barriers can be different courtship behaviors, different mating seasons or different niches within a habitat, just to name a few.

Let's take a moment to explore a few different types, starting with behavioral isolation. This occurs when two species do not mate because they have different courtship behaviors. For example, the blue-footed booby is a type of bird that has a complex courtship process. Even though there are other closely related species inhabiting the same region, the female blue-footed booby will only mate with males that undergo a specific courtship ritual.

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