Barriers to Hybridization And Sterility of Interspecific Hybrids as Mechanisms Preventing the Mixing of Alleles Between Species
- Barriers to hybridization are mechanisms that prevent different species from mating and producing viable, fertile offspring.
- These barriers maintain the genetic distinctness of species by reducing or eliminating gene flow between them.
Gene flow
The movement of genes from one population to another
What Are Barriers to Hybridization?
- Barriers can be classified into two types:
- Prezygotic Barriers: Prevent mating or fertilization.
- Postzygotic Barriers: Prevent hybrid offspring from surviving or reproducing.
- Each play a vital role in preserving species boundaries and biodiversity.
Prezygotic Barriers: Preventing Fertilization
Prezygotic barriers
Prezygotic barriers act before fertilization, stopping different species from mating or ensuring that fertilization is unsuccessful.
1. Temporal Isolation
Species reproduce at different times of the day, season, or year.
ExampleTwo populations of pine processionary moths in Portugal breed in different seasons, preventing interbreeding.
2. Behavioral Isolation
Differences in courtship behaviors or mating rituals prevent interbreeding.
ExampleBirds of paradise perform species-specific courtship displays that ensure individuals recognize suitable mates of their own species.
3. Mechanical Isolation
Differences in reproductive anatomy make mating physically impossible.
ExampleInsects often have species-specific genital structures that ensure reproductive compatibility only with their own species.
4. Gametic Isolation
Even if mating occurs, sperm and egg cells of different species may be incompatible, preventing fertilization.
ExampleIn sea urchins, surface proteins on eggs and sperm must match for successful fertilization.
Postzygotic Barriers: Preventing Viable or Fertile Offspring
Postzygotic barriers
Postzygotic barriers occur after fertilization, ensuring that hybrid offspring are inviable or sterile.
1. Hybrid Inviability
Hybrid embryos fail to develop properly, often due to genetic incompatibilities.
Example- The mule, a sterile hybrid of a horse (Equus caballus) and a donkey (Equus asinus), combines the strength of a horse with the hardiness of a donkey.
- However, chromosomal mismatches during meiosis prevent mules from reproducing, ensuring no further mixing of horse and donkey alleles.
2. Hybrid Sterility
Hybrids are unable to reproduce because their chromosomes cannot align correctly during meiosis.
ExampleMules (horse × donkey hybrids) are sterile due to having 63 chromosomes, an uneven number that prevents proper pairing during gamete formation.
