Question

Which of the following is considered an example of a postzygotic isolating mechanism? a incompatible genitalia b different mating seasons c incompatible gametes d mountain range separating two populations e failure of fertilized eggs to develop normally

Short answer

The example of a postzygotic isolating mechanism among the options given is (e) failure of fertilized eggs to develop normally.

Step-by-step solution

Analyze the given options

Analysing the given options to understand the differences, especially considering whether they represent mechanisms that occur before or after zygote formation.

Identifying postzygotic isolating mechanisms

Identify which of the options corresponds to a mechanism that works after the formation of zygote.a) Incompatible genitalia is a prezygotic mechanism (prevents mating and, subsequently, fertilization).b) Different mating seasons is also a prezygotic mechanism, preventing different species from mating.c) Incompatible gametes, it is also a prezygotic mechanism, where sperm from one species is unable to fertilize eggs of another species.d) A mountain range separating two populations falls under geographic isolation, which is also a prezygotic isolating mechanism. It prevents different species from mating due to physical separation.

Choosing the correct option

The only option left is (e) failure of fertilized eggs to develop normally. This is a postzygotic mechanism because it occurs after fertilization. The zygote is formed but abnormal development of fertilized eggs prevents it from growing into a healthy, viable and fertile offspring. Thus, it is the correct answer.

Key concepts

prezygotic isolating mechanism

Prezygotic isolating mechanisms are fascinating barriers that prevent different species from mating and producing offspring. These mechanisms occur before the formation of a zygote, essentially blocking the fertilization process altogether.
There are various forms of prezygotic barriers, which include:

• **Mechanical Isolation**: This involves anatomical differences between species. An example, like incompatible genitalia, makes mating physically impossible.
• **Temporal Isolation**: When species have different mating seasons or times of day, they are prevented from mating, as seen with organisms that might mate in the spring compared to those in the autumn.
• **Behavioral Isolation**: Unique mating rituals or behaviors that are species-specific can prevent different species from recognizing each other as potential mates.
• **Gametic Isolation**: Even if mating occurs, the gametes (sperm and egg) do not fuse to form a zygote. This is often seen in cases where chemical markers on egg and sperm are incompatible, such as incompatible gametes.

These mechanisms ensure that only those belonging to the same species can reproduce, helping maintain species integrity over time.

fertilization

Fertilization is a critical stage in sexual reproduction, marking the union of sperm and egg. This process involves several crucial steps:

• The sperm travels towards the egg, navigating various barriers presented by the female's body.
• Upon reaching the egg, the sperm must penetrate its protective layer. This process can vary significantly across species, with specialized enzymes or physical adaptations to help in penetration.
• Once inside, the genetic material from both sperm and egg combine, forming a zygote—the first cell of a potential new organism.

Fertilization requires compatibility between gametes. If prezygotic mechanisms do not impede the process, and conditions are favorable, fertilization can successfully take place. Fertilization sets the stage for all subsequent development and is therefore one of the central focuses of both reproductive biology and the study of species isolation.

species isolation

Species isolation refers to the various mechanisms and barriers that maintain species as distinct entities. These isolation mechanisms can be crucial in evolutionary biology as they prevent different species from exchanging genetic material, which could otherwise disrupt species' adaptations. Species isolation is categorized into prezygotic and postzygotic mechanisms:

• **Prezygotic Isolation**: As mentioned earlier, these barriers stop different species from mating and include mechanical, temporal, behavioral, and gametic isolation.
• **Postzygotic Isolation**: These barriers come into effect after fertilization of the egg by sperm from a different species. For example, the hybrid offspring might be sterile or could have developmental issues, as seen in the case where fertilized eggs fail to develop normally.

Through these barriers, each species maintains its distinct characteristics and evolutionary path, preventing them from merging into one another.

reproductive barriers

Reproductive barriers are essential for maintaining species differences and involve both prezygotic and postzygotic factors.
They effectively prevent species from interbreeding and help maintain biodiversity by preserving the uniqueness of each species. ### Types of Reproductive Barriers 1. **Prezygotic Barriers**: These block fertilization from occurring through: - **Habitat Isolation**: Different species might inhabit distinct environments preventing them from encountering each other. - **Behavioral Isolation**: Differences in mating rituals or calls keep species from mating. 2. **Postzygotic Barriers**: Even after the formation of a zygote, these barriers prevent viable or fertile offspring from growing: - **Hybrid Inviability**: Hybrid offspring often fail to develop properly. - **Hybrid Sterility**: Offspring may develop but are sterile as adults, such as the mule, a hybrid of a horse and a donkey. By understanding these reproductive barriers, scientists can better grasp the intricacies of evolution and the ways in which species are adapted to their niches in the environment.