Reproductive Isolation: Pre-zygotic, Post-zygotic, Genetics

The definition of reproductive isolation is “the inability of a species to successfully breed with related species due to geographical, behavioral, physiological, or genetic barriers or differences.”

A crucial component of speciation is reproductive isolation. It refers to the controls that keep populations that are closely related from reproducing with one another, which might eventually result in the emergence of new species. Reproductive isolation can occur before or after conception, and it is typically the consequence of genetic, ecological, or behavioural causes. We shall examine prezygotic isolation, postzygotic isolation, and the role of genetics in reproductive isolation as well as other mechanisms of reproductive isolation in this article.

Prezygotic Isolation

This category of isolation occurs before the zygote formation can take place, where mating of the organisms does not occur. There are different forms of pre-zygotic isolation which include temporal, behavioural, mechanical, and gametic isolation.

• Temporal or habitat isolation: This happens when two species that are closely related reproduce at various times or in multiple environments, prohibiting interbreeding. For instance, two bird species may reside in different locations and never meet, or two frog species may marry at different times of the year.
• Behavioural isolation: When two species have dissimilar courtship customs, mating habits, or other behaviours that hinder effective mating, this happens. For example, male fireflies may have distinct flashing patterns, and females will only mate with males of the same species who exhibit the right pattern.
• Mechanical isolation: When the genitalia of two species are incompatible, successful mating is prevented. For instance, two species of insects may not be able to mate because the morphology of their genitalia differs.
• Gametic isolation: This happens if the ensuing zygote cannot develop normally or if the sperm of one species cannot fertilize the eggs of another species. For instance, it’s possible that the eggs of one species of sea urchin can’t be penetrated by the sperm of a different species.

Post-zygotic Isolation

This type of isolation occurs after mating between members of two different species, forming a zygote which leads to the formation of a hybrid. These hybrids are often infertile, hence unable to reproduce. This type of production of hybrid isn’t considered a successful mating and are sexually immature.   

• Zygote mortality and non-viability of hybrids: When two species combine to form a zygote, this happens when the zygote perishes before becoming a viable embryo. As an alternative, the hybrid embryo might develop but not live to adulthood. For instance, a mule- a cross between a horse and a donkey—is sterile and incapable of procreation.
• Hybrid sterility: When a hybrid is born, but is unable to produce live gametes, it is unable to procreate. For instance, a hybrid mouse between two species could be produced, but it might not be able to produce healthy sperm or eggs.
• Multiple mechanisms: To stop interbreeding, several reproductive isolation mechanisms may occasionally cooperate. For instance, two plant species may have incompatible chromosomes, distinct pollinators, and different flowering dates, all of which hinder interbreeding.
• Hybrid sex: According to Haldane’s rule, the sex that is absent, infrequent, or sterile in a hybrid between two species is more likely to be the heterogametic sex (the sex with two different sex chromosomes). Male offspring from a hybrid between two species with distinct sex chromosomes may be infertile because, for instance, in mammals, men are XY and females are XX.

Genetics and Reproductive Isolation

• Animal’s Pre-copulatory Mechanisms: Genes that regulate courting, mating behaviour, and other behaviours that affect reproductive success are a part of these processes. For instance, in some bird species, the song of the male is genetically determined, and females will only mate with males that sing the proper song.
• Animal’s Post-copulation or Fertilisation Mechanisms: Genes that regulate zygote and embryo development are a part of these systems. For example, in some species of fruit flies, genes control the development of the embryo, and if the genes are not compatible, the embryo will not develop properly.
• In Plants: Genes that regulate flower morphology, flowering time, and other aspects of reproduction can also be responsible for reproductive isolation in plants. For example, in some species of plants, the shape and size of the flowers may be controlled by genes, and if the genes are not compatible, interbreeding will not occur.
• Pre-fertilization Mechanisms Examples: These include the genes in charge of pollen formation, pollination, and other pre-fertilization processes of reproduction. For instance, in some plant species, genes regulate pollen production, and if the stigma of one species does not recognise the pollen of another species, fertilisation will not take place.
• Post-Fertilization Mechanisms Examples: These include the genes that regulate the zygote’s and the embryo’s post-fertilization growth. For instance, in some species of plants, the endosperm, the tissue that feeds the growing embryo, is developed under the direction of genes; if the genes are incompatible, the embryo will not develop properly.
• Effects of Hybrid Necrosis: The phenomenon known as hybrid necrosis occurs when the offspring of two species exhibit abnormal growth or development and may even pass away too soon. Genes that are incompatible between the two species may be to blame for this. For instance, in certain plant species, hybrid necrosis can happen if the genes that drive disease resistance are incompatible.
• Chromosomal Rearrangements in Yeast: In some species, chromosomal rearrangements can cause reproductive isolation. For example, in yeast, chromosomal rearrangements can result in hybrid sterility or inviability.
• Incompatibility Caused by Microorganisms: Some bacteria can create reproductive isolation by interfering with reproduction in their hosts. For example, Wolbachia bacteria can cause sterility or male death in insects, which can hinder interbreeding between closely related species.

One of the most important processes in the evolution of new species is reproductive isolation. Without it, closely related species may reproduce with one another and eventually unite into one species, eliminating the diversity of life that we see today. The various mechanisms of reproductive isolation, including pre-zygotic and post-zygotic isolation, genetics, and incompatibility caused by microorganisms, ensure that closely related species remain distinct and can continue to evolve separately. It is crucial to comprehend these mechanisms in order to comprehend the variety of life and how it has changed over time.

Reproductive isolation has substantial practical repercussions in addition to its involvement in the development of new species. For example, understanding the mechanisms of reproductive isolation is essential for controlling the spread of invasive species. If closely related species can interbreed, the introduction of an invasive species into a new ecosystem could result in hybridization and the creation of new, potentially harmful hybrid species.

Conclusion

Reproductive isolation plays a critical role in the evolution of new species and is essential for maintaining the diversity of life on Earth. The various mechanisms of reproductive isolation, including pre-zygotic and post-zygotic isolation, genetics, and incompatibility caused by microorganisms, ensure that closely related species remain distinct and can continue to evolve separately.

Prezygotic isolation mechanisms such as temporal or habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation act to prevent mating or fertilization between different species.

Post-zygotic isolation mechanisms such as zygote mortality, hybrid sterility, and genetic incompatibility act to prevent the development or survival of hybrid offspring. Understanding the various mechanisms of reproductive isolation and their interactions is essential not only for understanding the diversity of life but also for practical applications such as controlling the spread of invasive species.

The complexity and diversity of reproductive isolation mechanisms highlight the incredible intricacies of the natural world and the crucial role that evolution plays in shaping it. A better understanding of reproductive isolation will continue to be essential in preserving the diversity of life on Earth for generations to come.

References

1. Reproduction Isolation: Introduction – https://www.sparknotes.com/biology/evolution/reproductiveisolation/summary/
2. Reproductive isolation – https://evolution.berkeley.edu/evolution-101/speciation/reproductive-isolation/
3. Reproductive Isolation Types & Examples – https://study.com/learn/lesson/reproductive-isolation-types-examples.html
4. What is reproductive isolation? – https://onlinelibrary.wiley.com/doi/full/10.1111/jeb.14005
5. Speciation – https://global.oup.com/academic/product/speciation-9780878930890?cc=in&lang=en&#
6. Coyne, Jerry A., and H. Allen Orr. “Patterns of speciation in Drosophila” revisited.” Evolution (1997): 295-303.