Speciation - Online Tutor, Practice Problems & Exam Prep

Speciation is the formation of new species through reproductive isolation, which can be prezygotic or postzygotic. Prezygotic mechanisms prevent mating before a zygote forms, while postzygotic mechanisms affect offspring viability or fertility, such as mules being sterile. Allopatric speciation occurs due to geographic barriers, while sympatric speciation arises without such barriers, often through changes in behavior or resource use. Understanding these processes is crucial for grasping evolutionary biology and the biological species concept, which defines species based on reproductive capabilities.

Speciation

So, speciation is the formation of a new species. This occurs through a process called reproductive isolation. And, what is reproductive isolation? Well, if you've seen the Hardy Weinberg videos, we talk about it a little then, but I just want to talk about it again here. Reproductive isolation is various mechanisms that block reproduction between two individuals or two organisms. There are two types: there's prezygotic and postzygotic, named based on their reference to the zygote, the first cell that's formed. Prezygotic mechanisms happen to block reproduction before the zygote's formed. Postzygotic mechanisms occur after the zygote is formed. So, for prezygotic mechanisms, what are some examples? It could be different behaviors. If one animal is nocturnal and one is awake during the daytime, they're most likely not going to mate, because they are not awake at the same times even though they live in the same environment and could otherwise mate. This is known as temporal isolation. Other examples include different mating dances, or physical separations, such as animals that live in trees or fly and some that live below ground. All of these factors can block reproduction. There are also mechanical blocks where the body parts do not fit together, preventing successful mating. Different types of gamete isolation, where the gametes are only produced at certain times that may not coincide with the mating periods of other organisms, also play a role.

Postzygotic mechanisms occur after the offspring has been formed, often affecting the reproductive success of the offspring in terms of viability or sterility. An example of this is mules, which are produced by mating between a horse and a donkey. These two species can create a mule, so it's postzygotic since the mules are sterile. They can't reproduce, thus establishing reproductive isolation where mules cannot produce more mules, nor can they produce horses or donkeys. There is also what's known as the biological species concept, which describes the differences necessary to consider two organisms different species. This might seem straightforward, considering whether they can reproduce with each other. However, it can be more complex since technically, horses and donkeys can reproduce together, but their offspring, mules, are sterile. Determining what features make two species separate can be challenging, but this concept seeks to define those differences.

An example of this process starts with an initial sample of flies. Then, in a laboratory setting to facilitate evolution, the flies are separated into two groups given different types of media or food sources; one with a lot of starch and the other with a lot of maltose sugar. Over several generations, these two different environments lead to the formation of two distinct fly populations, represented as yellow and brown flies. When these flies are reintroduced, they may not mate despite originally being the same species, showcasing reproductive isolation driven by dietary preferences.

There are two main types of speciation mentioned in this class: allopatric and sympatric. Allopatric speciation occurs when a geographic barrier is present, splitting a population into two groups. These groups cannot exchange genetic information due to the presence of a barrier like distance, a mountain, or a river. Sympatric speciation, however, happens without a geographic barrier. It involves alternative types of reproductive isolation such as environmental or behavioral changes that discourage interbreeding, allowing two populations to evolve into separate species without a physical barrier. An example of allopatric speciation starts with a homogeneous group of blue-colored organisms; a barrier forms that isolates them, resulting in one group developing a green variation. Over time, these two groups develop into separate species, green and blue, that even once the barrier is removed and they can coexist, they remain distinct and cannot produce interbreeding offspring. Thus, they remain two separate species.

So that is speciation and how these new species are created through evolution. So with that, let's now move on.

Which of the following is an example of a postzygotic reproductive isolation mechanism?

• A. Two organisms do not produce offspring because one is sleeping while the other is awake
• B. Two organisms do not produce offspring because one does not contain the appropriate body parts to mate with the other organisms
• C. Two organisms do not produce offspring because the fetus is always lost through miscarriage
• D. Two organisms do not produce offspring because one lives in water and the other lives on land

Which type of speciation occurs when a geographic barrier splits the population into two or more groups?

• A. Allopatric
• B. Sympatric
• C. Metapatric
• D. Geopatric

Here’s what students ask on this topic:

What is speciation and how does it occur?

Speciation is the formation of new species through the process of reproductive isolation. This can occur via prezygotic mechanisms, which prevent mating before a zygote forms, or postzygotic mechanisms, which affect the viability or fertility of the offspring. For example, prezygotic mechanisms include differences in mating behaviors or physical incompatibilities, while postzygotic mechanisms can result in sterile offspring, like mules. Speciation can be allopatric, involving geographic barriers that separate populations, or sympatric, where reproductive isolation occurs without physical barriers, often due to changes in behavior or resource use. Understanding these processes is crucial for grasping evolutionary biology and the biological species concept, which defines species based on reproductive capabilities.

What are the differences between allopatric and sympatric speciation?

Allopatric speciation occurs when a geographic barrier, such as a mountain or river, physically separates a population into two groups, preventing gene flow between them. Over time, these isolated populations accumulate genetic differences, eventually leading to the formation of distinct species. In contrast, sympatric speciation happens without geographic barriers. Instead, reproductive isolation arises from factors like changes in behavior, mating preferences, or resource use. For example, one group might adapt to feeding on a different food source, leading to reproductive isolation even though they live in the same area. Both processes result in the formation of new species, but the mechanisms driving them differ.

What are prezygotic and postzygotic mechanisms in reproductive isolation?

Prezygotic mechanisms are barriers that prevent mating or fertilization before a zygote forms. Examples include temporal isolation (different mating times), behavioral isolation (different mating rituals), mechanical isolation (incompatible reproductive structures), and gametic isolation (incompatibility of sperm and egg). Postzygotic mechanisms occur after fertilization and affect the viability or fertility of the offspring. For instance, hybrid inviability results in offspring that do not develop properly, while hybrid sterility, like in mules, results in offspring that cannot reproduce. Both types of mechanisms contribute to reproductive isolation, which is essential for speciation.

How does the biological species concept define a species?

The biological species concept defines a species as a group of organisms that can interbreed and produce viable, fertile offspring. This concept emphasizes reproductive isolation as the key criterion for distinguishing species. If two populations cannot mate successfully or produce fertile offspring, they are considered separate species. However, this concept can be challenging to apply in cases where hybridization occurs, such as with horses and donkeys producing sterile mules. Despite these complexities, the biological species concept remains a fundamental framework for understanding speciation and the diversity of life.

Can you provide an example of speciation in a laboratory setting?

An example of speciation in a laboratory setting involves fruit flies. Researchers can start with a homogeneous population of flies and separate them into two groups, each given a different food source, such as starch and maltose. Over several generations, these flies adapt to their specific food sources, leading to genetic and behavioral differences. Eventually, when reintroduced, the two groups may no longer mate with each other, demonstrating reproductive isolation. This experiment illustrates how environmental factors and selective pressures can drive speciation, even in controlled settings.