The Biological Species Concept: Videos & Practice Problems

The biological species concept defines species based on reproductive isolation, which prevents gene flow between populations. This isolation can be categorized into prezygotic barriers, such as habitat, temporal, behavioral, mechanical, and gametic isolation, and postzygotic barriers, including reduced viability, hybrid sterility, and hybrid breakdown. Understanding these concepts is crucial for studying speciation and biodiversity, as they illustrate how different species evolve and maintain distinct genetic identities.

Biological Species Concept

In the study of biology, defining a species can vary based on the context, leading to the development of different species concepts. Among these, the biological species concept is the most widely recognized and utilized. This concept defines species based on their reproductive isolation, which is the existence of barriers that prevent gene flow between different species. Gene flow typically makes populations more similar, so to classify organisms as distinct species, it is essential to block this flow.

The biological species concept employs a two-part test to determine if populations are separate species. The first part assesses whether the species can interbreed in nature. If two populations do not have the potential to mate, they are considered different species. For instance, humans and chimpanzees, despite being closely related, do not interbreed, demonstrating their reproductive isolation.

The second part of the test comes into play if species attempt to mate. In this case, they must not produce viable or fertile offspring. Viable offspring can survive, while fertile offspring can reproduce. A classic example is the mating of horses (Equus ferus caballus) and donkeys (Equus africanus asinus), which results in mules. Mules are viable but sterile, meaning they cannot reproduce, thus preventing gene flow between horses and donkeys and confirming their status as separate species.

Reproductive isolation is not only a criterion for defining species but also a crucial factor in the process of speciation—the evolution of new species. Speciation begins when populations become reproductively isolated, and this isolation serves as the test for distinguishing different species under the biological species concept. Understanding reproductive isolation is fundamental to grasping how species evolve and diverge over time.

The Biological Species Concept Example 1

The peppered moths, commonly found in the United Kingdom, serve as a classic example of natural selection. These moths exhibit two distinct color morphs: light and dark. Prior to the industrial revolution, the majority of these moths were light-colored, which allowed them to blend in with the lichen on trees. However, as the industrial revolution progressed and soot covered the trees, the darker moths became more prevalent due to their improved camouflage against the sooty backgrounds.

Despite the observable changes in the population, it is important to note that the dark and light moths are not considered separate species. This conclusion is based on the principle of interbreeding; both color morphs can mate and produce viable offspring, indicating that there is no reproductive isolation between them. Reproductive isolation is a key factor in defining a species according to the biological species concept, which states that a species is a group of organisms that can interbreed and produce fertile offspring. Since the peppered moths do not exhibit reproductive barriers, they remain a single species despite their color variations.

This example highlights the process of natural selection, where environmental changes can influence the frequency of certain traits within a population. The shift from light to dark moths illustrates how adaptive traits can enhance survival in changing environments, reinforcing the concept that evolution is driven by the interaction between organisms and their surroundings.

Which of the following is the primary criteria for defining species based on the biological species concept?

• A. Ability to produce viable hybrids.
• B. Geographical isolation.
• C. Reproductive isolation.
• D. Similarity in appearance.

Answer: (C)

Giraffes live throughout large regions of Africa. Historically, only one species of giraffe has been recognized, but many scientists argue that giraffe populations actually represent four distinct species. What piece of evidence would best determine whether different giraffe populations should be considered different species according to the biological species concept?

• A. Whether or not there are regular identifiable physical characteristics that make the populations distinct.
• B. Whether or not the populations have historically interbred or are capable of producing offspring.
• C. Whether or not the populations interact with the environment differently in regular and meaningful ways.
• D. Whether or not there are regular differences in reproductive traits.

Answer: (B)

Types of Reproductive Isolation

The biological species concept emphasizes reproductive isolation, which can be categorized into two primary types of barriers: prezygotic barriers and postzygotic barriers. Understanding these barriers is crucial for grasping how species remain distinct from one another.

Prezygotic barriers occur before fertilization, preventing mating or hindering the fertilization process itself. The term "prezygotic" combines "pre," meaning before, and "zygotic," which refers to the zygote, or fertilized egg. An example of a prezygotic barrier is observed in the eastern and western spotted skunks of North America. Although their ranges may overlap, one species mates in the summer while the other mates in the winter, leading to reproductive isolation due to the absence of simultaneous mating opportunities.

On the other hand, postzygotic barriers take effect after fertilization. The term "postzygotic" combines "post," meaning after, with "zygotic." In this case, fertilization occurs, resulting in a hybrid, but these hybrids often exhibit low fitness, meaning they either do not survive or are sterile. A classic example is the mule, a hybrid of a horse and a donkey. While a mule can develop from the fertilized egg, it is sterile, preventing any gene flow between the parent species. Other forms of postzygotic barriers include reduced viability, where hybrids may not survive to maturity.

In summary, the key distinction between prezygotic and postzygotic barriers lies in whether fertilization occurs. Prezygotic barriers prevent the formation of a zygote, while postzygotic barriers affect the viability or reproductive capability of the hybrid after fertilization. Understanding these concepts is essential for studying speciation and the mechanisms that maintain species boundaries.

The Biological Species Concept Example 2

In the study of reproductive barriers among species, it is essential to understand the distinction between prezygotic and postzygotic barriers. Prezygotic barriers occur before fertilization, preventing the formation of a zygote, while postzygotic barriers take effect after fertilization, affecting the viability or fertility of the resulting hybrids.

For example, consider the orchids H. davidii and H. 40i. When analyzing various statements regarding their reproductive interactions, we can categorize them based on whether they represent a reproductive barrier:

A: Both flowers are found in the same region. This does not impede reproduction, so it is classified as none.

B: When fertilized by pollen of the other species, hybrids fail to produce seeds. This indicates a postzygotic barrier, as it occurs after fertilization and results in sterile hybrids, thus it is classified as post.

C: Both H. davidii and H. 40i are pollinated by a single pollinator, the hawk moth. This scenario does not create a barrier to reproduction, so it is classified as none.

D: Both flowers bloom in July. Since they bloom at the same time, there is no temporal barrier to reproduction, thus it is classified as none.

E: The species' unique flower structures release pollen on different body parts of the pollinator, preventing pollen transfer between the two species. This situation creates a prezygotic barrier, as it prevents fertilization from occurring, so it is classified as pre.

Understanding these barriers is crucial for studying speciation and the mechanisms that maintain species integrity in ecosystems.

Which of the following describes a postzygotic barrier?

• A. Hybrids between Muscovy and Mallard ducks are often larger than either parent species but are sterile.
• B. Songbirds are only attracted to songs made by birds of the same species.
• C. Jaguars live in South America, while leopards live in Africa and Asia.
• D. Different flower species often bloom at different times of the year, preventing cross-pollination.

Answer: (A)

Prezygotic Barriers

Reproductive barriers play a crucial role in the process of speciation, and they can be classified into two main categories: prezygotic and postzygotic barriers. Prezygotic barriers are those that prevent mating or fertilization from occurring, effectively isolating populations before the formation of a zygote, or fertilized egg. There are five primary types of prezygotic barriers, each with distinct mechanisms that prevent interbreeding.

The first type is habitat isolation, which occurs when populations are geographically separated and occupy different habitats. For instance, the Kaibab and Abirt squirrels are closely related species that live in similar environments but are separated by the Grand Canyon, preventing any interaction or mating.

Next is temporal isolation, where populations breed at different times, whether seasonally or daily. An example of this is seen in two species of spotted skunks that mate in different seasons. Even if they share the same habitat, their differing mating times prevent them from interbreeding.

Behavioral isolation refers to differences in mating behaviors or rituals that prevent species from recognizing each other as potential mates. For example, the southern and northern cricket frogs have distinct mating calls; thus, even if they encounter one another, they do not respond to each other's signals, leading to reproductive isolation.

Mechanical isolation involves anatomical differences that prevent successful mating. This is often observed in insects, such as damselflies, where the reproductive structures of different species are incompatible, making mating physically impossible.

Finally, gametic isolation occurs when the sperm and egg of different species are incompatible at a biochemical level. A classic example is found in the plant Arabidopsis thaliana, where pollen from a foreign species fails to adhere and fertilize the egg due to specific biochemical interactions that are not recognized by the flower.

Understanding these prezygotic barriers is essential for grasping how species maintain their distinct identities and how speciation occurs in nature. Each barrier plays a significant role in preventing gene flow between populations, thereby contributing to the diversity of life on Earth.

The Biological Species Concept Example 3

In the study of closely related species, such as Iris domestica and Iris dichotima, understanding reproductive isolation is crucial for comprehending how these species coexist despite overlapping ranges and similar pollinators. Both species exhibit distinct flowering times, which plays a significant role in their reproductive strategies.

The graph illustrating honeybee visits reveals key insights into their temporal isolation. On the y-axis, the number of honeybee visits ranges from 0 to 18, while the x-axis represents the time of day from 7 AM to 11 PM. The flowering times of Iris domestica (shown in orange) and Iris dichotima (shown in purple) indicate that although there is a period when both flowers are open, their peak visitation times by honeybees differ significantly.

Specifically, honeybees tend to visit Iris domestica in the morning, while Iris dichotima attracts bees in the evening. This temporal separation effectively prevents cross-pollination between the two species, demonstrating a clear example of temporal isolation. Temporal isolation occurs when species breed at different times, which in this case is evidenced by the distinct visitation patterns of honeybees.

Understanding these dynamics is essential for studying biodiversity and the mechanisms that maintain species boundaries in ecosystems. The concept of temporal isolation highlights the importance of time in reproductive strategies, ensuring that even closely related species can thrive without interbreeding.

Abalone, a type of sea snail, are broadcast spawners, meaning they release their gametes into open water, where the sperm and egg drift until they meet and fertilization occurs. The sperm of the red abalone does not recognize the eggs of the pink abalone and vice versa due to biochemical incompatibilities. This is an example of what type of reproductive barrier?

• A. Mechanical isolation.
• B. Habitual isolation.
• C. Gametic isolation.
• D. Behavioral isolation.

Answer: (C)

Postzygotic Barriers

Reproductive isolation is a crucial concept in understanding how species remain distinct from one another. While prezygotic barriers prevent fertilization from occurring, postzygotic barriers come into play after fertilization has taken place. These barriers lead to incompatibilities that reduce the fitness of hybrid offspring, ultimately hindering gene flow between species.

There are three main types of postzygotic barriers: reduced viability, hybrid sterility, and hybrid breakdown. Reduced viability refers to the decreased likelihood of hybrid offspring surviving compared to their non-hybrid counterparts. For instance, in the genus Ficedula, certain species may mate and produce fertilized eggs, but the resulting hybrids often fail to hatch, leading to reproductive isolation.

The second type, hybrid sterility, occurs when hybrids are viable and healthy but cannot reproduce. A classic example is the mule, which is a hybrid of a horse and a donkey. Mules are strong and healthy animals, yet they are sterile, preventing any gene flow back to either parent species.

Lastly, hybrid breakdown involves the first generation of hybrids being healthy and fertile, but their offspring in subsequent generations exhibit reduced fitness. An example of this is seen in certain cotton species, where the first generation of hybrids is robust, but the second generation produces weak and sterile offspring. This decline in fitness further reinforces reproductive isolation.

It is important to note that species can exhibit multiple forms of reproductive isolation, including both prezygotic and postzygotic barriers. For a group to be classified as distinct species, at least one form of these barriers must be present, ensuring that gene flow is effectively blocked.

The Biological Species Concept Example 4

In the context of animal breeding, particularly with domestic cattle (Bos taurus) and yaks (Bos grunniens), it is evident that these two species exhibit significant reproductive isolation. This isolation suggests that yaks and domestic cattle should indeed be classified as different species. The breeding of these animals results in hybrids known as Zho, which inherit desirable traits from both parent species. However, while male Zho are typically sterile, female Zho are fertile but are rarely bred due to the inferior traits of their second-generation offspring, which tend to be smaller and weaker than both parent species.

When considering the barriers to reproduction, it is important to distinguish between prezygotic and postzygotic barriers. In this case, the primary barriers observed are postzygotic. After mating occurs, the resulting hybrids exhibit two specific forms of postzygotic barriers: hybrid sterility and hybrid breakdown. Hybrid sterility is evident in the male Zho, which are unable to reproduce, while hybrid breakdown is observed in the second-generation hybrids, which are less viable and exhibit reduced fitness compared to their parents.

In summary, the reproductive dynamics between yaks and domestic cattle highlight the complexities of hybridization and the mechanisms of reproductive isolation, emphasizing the importance of understanding both hybrid sterility and hybrid breakdown in the study of species classification and evolution.

Which of the following pairs correctly matches the postzygotic barrier with its definition?

• A. Hybrid sterility: reduced fitness of hybrids after the first generation.
• B. Hybrid breakdown: hybrids are unable to reproduce.
• C. Gametic isolation: hybrid offspring are sterile.
• D. Reduced viability: decreased fitness of hybrid offspring.

Answer: (D)

“Ring species” are species that are distributed across a habitat in a ring-like distribution. Every population is able to breed with neighboring populations, but where the ring closes, the populations at the two “ends” do not interbreed. One example of a ring species is Ensatina escholtzii, a species
of salamander in California. The species began as a single population in the North, and spread South on either side of the California Central Valley. Each color on the map shows the distribution of different named population of E. escholtzii. Hybrids created by interbreeding of the populations marked
with the circle are sterile. If hybrid sterility is present, why do you think these populations are still considered the same species?

• A. To be considered separate species there must be pre- and postzygotic barriers.
• B. Because each population interbreeds with its neighbor, gene flow is possible throughout the entire species.
• C. This is a postzygotic barrier; prezygotic barriers are necessary for speciation.
• D. To become distinct species the populations would need to be geographically isolated.

Answer: (B)

Here’s what students ask on this topic:

What is the biological species concept?

The biological species concept defines species based on reproductive isolation. This means that different species are groups of organisms that do not interbreed in nature or, if they do, they produce offspring that are not viable or fertile. Reproductive isolation prevents gene flow between populations, maintaining distinct genetic identities. This concept is widely used because it emphasizes the importance of reproductive barriers in the process of speciation, helping scientists understand how new species evolve and maintain their uniqueness.

What are prezygotic barriers in the biological species concept?

Prezygotic barriers are mechanisms that prevent mating or fertilization between different species. These barriers occur before the formation of a zygote and include habitat isolation (species live in different environments), temporal isolation (species mate at different times), behavioral isolation (differences in mating behaviors), mechanical isolation (anatomical differences prevent mating), and gametic isolation (sperm and egg are incompatible). These barriers ensure that gene flow between species is blocked, maintaining reproductive isolation and distinct species identities.

What are postzygotic barriers in the biological species concept?

Postzygotic barriers occur after fertilization and result in reduced fitness of hybrid offspring. These barriers include reduced viability (hybrids are less likely to survive), hybrid sterility (hybrids are unable to reproduce), and hybrid breakdown (first-generation hybrids are viable and fertile, but their offspring are weak or sterile). These barriers prevent gene flow between species by ensuring that hybrids do not contribute to the gene pool, maintaining reproductive isolation and distinct species identities.

How does reproductive isolation contribute to speciation?

Reproductive isolation is crucial for speciation, the process by which new species arise. It prevents gene flow between populations, allowing them to evolve independently. Over time, genetic differences accumulate, leading to the development of distinct species. Reproductive isolation can occur through prezygotic barriers (preventing mating or fertilization) or postzygotic barriers (reducing hybrid fitness). By maintaining genetic separation, reproductive isolation ensures that populations diverge and form new species, contributing to biodiversity.

Can you provide examples of prezygotic and postzygotic barriers?

Examples of prezygotic barriers include habitat isolation (Kaibob and Abirt squirrels separated by the Grand Canyon), temporal isolation (eastern and western spotted skunks mating in different seasons), behavioral isolation (southern and northern cricket frogs with different mating calls), mechanical isolation (damselflies with differently shaped reproductive structures), and gametic isolation (Arabidopsis thaliana pollen not adhering to foreign species). Examples of postzygotic barriers include reduced viability (hybrid birds of the genus Physidula with lower hatching rates), hybrid sterility (mules from horse and donkey mating), and hybrid breakdown (second-generation hybrids of cotton being weak and sterile).