Reproductive barriers are biological features that reduce interbreeding between populations. By limiting gene flow, these barriers help populations diverge genetically and maintain distinct gene pools even when they live near one another.

Core idea: reproductive isolation blocks gene flow

This diagram illustrates how an original population splits into two populations and, as reproductive isolation increases, gene exchange (gene flow) between them decreases. It provides a visual timeline for how barriers to interbreeding allow populations to diverge genetically and become distinct lineages. Source

When reproductive isolation is strong, alleles are less likely to move between populations, so differences caused by selection, drift, or mutation are not “blended away.”

Prezygotic barriers (before fertilisation)

Prezygotic barriers stop a zygote from forming, often making them energetically “efficient” barriers because no resources are invested in producing low-fitness offspring.

Habitat (ecological) isolation

• Populations occupy different microhabitats in the same region, so potential mates rarely encounter each other.

• Example patterns: one population breeds in fast-flowing water while another uses still pools; different host plants for herbivorous insects.

Temporal isolation

• Populations reproduce at different times.

• Differences may be:

  • Seasonal (spring vs fall flowering)

  • Daily (nocturnal vs diurnal mating activity)

• Even small timing shifts can sharply reduce mating opportunities.

Behavioral isolation

• Differences in courtship signals prevent mate recognition.

• Common cues include:

  • Birdsong patterns

  • Pheromones in insects

  • Visual displays (color patches, flashing patterns)

• If signals are not recognized, mating does not proceed, even if individuals meet.

Mechanical isolation

This figure compares reproductive structures in different damselfly species, highlighting shape differences that prevent successful copulation. It concretely demonstrates mechanical isolation as a prezygotic barrier: mating attempts may occur, but fertilization is prevented because the structures are incompatible. Source

• Structural differences prevent successful copulation or pollination.

• Examples of mismatches:

  • Incompatible genital structures in some animals

  • Flower shape not fitting a particular pollinator’s body plan

• Mechanical isolation can evolve rapidly when traits involved in mating are under strong selection.

Gametic isolation

• Mating (or gamete release) occurs, but sperm and egg (or pollen and ovule) do not fuse.

• Causes include:

  • Sperm cannot survive in the female reproductive tract

  • Molecular incompatibilities between sperm surface proteins and egg receptors

  • Pollen tube fails to grow to the ovule

• Gametic isolation is especially important in externally fertilizing species (many aquatic organisms), where species-specific gamete recognition reduces hybridization.

Postzygotic barriers (after fertilisation)

Postzygotic barriers allow a zygote to form but reduce the fitness of hybrid offspring, limiting allele flow across populations despite occasional mating.

Reduced hybrid viability (hybrid inviability)

• Hybrid embryos may fail to develop properly, or hybrids may be frail and die before reproducing.

• Typical biological causes:

  • Incompatible interactions among genes from different populations (disrupted development)

  • Improper expression of key regulatory genes

• The result is fewer or no hybrid individuals contributing genes to future generations.

Reduced hybrid fertility (hybrid sterility)

• Hybrids survive but cannot produce functional gametes.

• A common cause is failure of homologous chromosomes to pair correctly during meiosis due to differences in chromosome structure or number.

• Sterile hybrids block gene flow completely at the reproductive stage.

Hybrid breakdown

• First-generation hybrids (F1) may be viable and fertile, but later generations (F2 or backcrosses) show reduced viability or fertility.

• This can occur when gene combinations are “reshuffled” in later generations, revealing incompatible allele interactions.

How these barriers maintain isolation

• Prezygotic barriers reduce the probability of fertilization and thus strongly reduce gene flow at the earliest stage.

• Postzygotic barriers reduce the genetic contribution of hybrids to future generations, limiting introgression even when mating occurs.

• Multiple barriers can act together; a single pair of populations may show temporal isolation plus gametic incompatibility, creating layered protection against gene flow.