Question

List the barriers that prevent interbreeding, and give an example of each.

Short answer

Answer: The main types of interbreeding barriers are prezygotic and postzygotic barriers. Prezygotic barriers can be classified into habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation. Examples include salamanders living in different habitats, flowers blooming at different times, fireflies using distinct light patterns, incompatible pollination between bees, and selective fertilization in sea urchins. Postzygotic barriers include hybrid inviability, hybrid sterility, and hybrid breakdown. Examples are health complications in ligers and tigons, sterility in mules, and reduced fertility in hybrid rice plants.

Step-by-step solution

Prezygotic barriers

Prezygotic barriers occur before the formation of a zygote, preventing fertilization between different species. They can be further classified into five categories: habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation.

Postzygotic barriers

Postzygotic barriers occur after the fertilization between different species. These barriers prevent the successful development of viable, fertile hybrid offspring. Postzygotic barriers can be classified into three categories: hybrid inviability, hybrid sterility, and hybrid breakdown. Now that we have defined the main types of interbreeding barriers, let's provide examples for each.

Examples of prezygotic barriers

1. Habitat isolation: Two species of salamanders live in the same geographic region but occupy different habitats (one prefers forested areas while the other prefers grasslands), making it unlikely for them to encounter each other and reproduce. 2. Temporal isolation: One species of flower blooms in the morning, whereas another related species blooms in the afternoon. Pollinators are attracted during their respective blooming periods, preventing pollen exchange between the two species. 3. Behavioral isolation: Male fireflies of different species emit different light patterns to attract females. Females of each species are only attracted to the light pattern of their own species, resulting in mating between conspecifics. 4. Mechanical isolation: Orchard bees and bumblebees may visit the same species of flower, but their different body sizes and flower-handling behaviors prevent them from transferring pollen between one another, preventing potential interbreeding. 5. Gametic isolation: Sea urchins release their gametes into the water, but proteins on the surface of their eggs only bind to sperm of their own species, preventing fertilization by sperm from other species.

Examples of postzygotic barriers

1. Hybrid inviability: In the case of tiger and lion hybrid offspring (ligers and tigons), they often suffer from various health complications due to the incompatible genetic information from their parents, leading to their early death. 2. Hybrid sterility: Mules are a result of mating between a female horse and a male donkey. However, mules are typically sterile, lacking the ability to create viable offspring, due to differing chromosome numbers between their parent species. 3. Hybrid breakdown: Rice plants can be crossbred to produce hybrid offspring, but when these hybrids are further bred, their offspring often displays reduced fertility or survival rates, preventing the continued reproduction of these hybrid offspring in subsequent generations.

Key concepts

Prezygotic Barriers

Prezygotic barriers are mechanisms that prevent different species from mating and creating fertilized eggs. These barriers occur before the formation of a zygote, effectively stopping fertilization. Prezygotic barriers help maintain the identity of species and include several types:

• Habitat Isolation: Species live in different habitats within the same area, thus rarely encountering one another. For example, some salamanders prefer forest areas while others are adapted to grasslands.
• Temporal Isolation: Species breed at different times. For instance, flowers that bloom during different times of the day or year prevent cross-pollination.
• Behavioral Isolation: Differences in mating behaviors or rituals, like specific light patterns in fireflies, ensure mating occurs only within the same species.
• Mechanical Isolation: Differences in physical characteristics make mating impossible, like bees of different sizes being unable to handle the same flowers in ways that allow pollen transfer.
• Gametic Isolation: Even if gametes meet, they may fail to fuse due to incompatibilities, such as seen in some sea urchins.

Postzygotic Barriers

Postzygotic barriers come into play after fertilization has occurred. These obstacles prevent the development of robust, fertile hybrid offspring. They ensure species remain distinct even if prezygotic barriers are breached. Key forms include:

• Hybrid Inviability: Some hybrids, like ligers and tigons, suffer from health issues due to genetic mismatches from their parents and often die early.
• Hybrid Sterility: Hybrids may survive but are unable to reproduce. Mules, offspring of horses and donkeys, are typical examples due to chromosome mismatches.
• Hybrid Breakdown: Though the first-generation hybrids may be viable and fertile, their descendants often experience reduced fertility or survival rates, seen in some hybridized rice plants.

Hybrid Sterility

Hybrid sterility is a common postzygotic barrier where hybrid offspring cannot reproduce. This phenomenon often results from differences in parental species' chromosome numbers or structures. For example, mules are sterile hybrids produced by mating a horse with a donkey. Horses have 64 chromosomes, while donkeys have 62, leading their offspring, the mule, to possess 63 chromosomes, which complicates meiosis, hence rendering them infertile. This barrier helps ensure that species with different evolutionary backgrounds remain separate, even if they occasionally interbreed and produce offspring.

Habitat Isolation

Habitat isolation is a type of prezygotic barrier. It occurs when two species occupy different habitats within the same geographical area, preventing them from meeting and mating. This separation can happen even if the habitats are close together because the species have adapted to different conditions. A classic example is when one salamander species prefers moist forest habitats, while another thrives in open grasslands. Despite living in the same larger region, their choice of habitat keeps them isolated and maintains species integrity by reducing opportunities for interbreeding.