Lec 38

Evolution of Reproductive Isolation

Introduction to Reproductive Isolation

Reproductive isolation is a key mechanism in the process of speciation, preventing gene flow between populations and leading to the formation of new species. In Drosophila (fruit flies), reproductive isolation can evolve rapidly and is often studied to understand the genetic and evolutionary basis of speciation.

Rapid Evolution of Assortative Mating

Experimental Evidence in Drosophila

• Assortative mating refers to the tendency of individuals to mate with others that are similar to themselves, often based on ecological or behavioral traits.

• In a classic experiment, Drosophila pseudoobscura populations were reared on different substrates (starch or maltose) for ~25 generations. Afterward, strong evidence for assortative mating based strictly on feeding substrate was observed, indicating rapid evolution of prezygotic isolation.

• Prezygotic isolation prevents mating or fertilization between species, while postzygotic isolation affects hybrid viability or fertility.

Prezygotic Reproductive Isolation

Measuring Prezygotic Isolation

• Prezygotic isolation can be quantified using indices that compare the probability of conspecific (same species) versus interspecific (different species) matings.

• Studies by Coyne & Orr (1997) collected data from sister pairs of Drosophila species to analyze the evolution of prezygotic isolation.

Reinforcement and Sympatry

• Reinforcement is the process by which natural selection increases reproductive isolation between populations in sympatry (overlapping ranges) to avoid the production of unfit hybrids.

• Females from sympatric populations often show stronger discrimination against heterospecific males compared to allopatric populations.

• Example: Drosophila pseudoobscura females from sympatric populations avoid mating with D. persimilis males, while allopatric females do not.

Types of Premating Barriers

• Ecological isolation: Species breed at different times (temporal) or in different habitats.

• Mating isolation: Potential mates come into contact but do not mate due to behavioral differences or specialized pollinators.

Postzygotic Reproductive Isolation

Haldane’s Rule and Genetic Basis

• Haldane’s Rule: When hybrid sterility or inviability is limited to one sex, it is usually the heterogametic sex (e.g., XY males in Drosophila).

• This suggests that sex chromosomes play a major role in the evolution of postzygotic isolation.

• Pleiotropic effects of recessive alleles may contribute to hybrid sterility or inviability, especially in geographically isolated populations.

Intrinsic Reproductive Isolation in Drosophila

• Intrinsic barriers are genetic incompatibilities that reduce hybrid fitness, such as hybrid sterility or inviability.

• Example: D. yakuba and D. santomea are sister species with distinct elevational ranges. Hybrids between these species show reduced viability and fertility.

Hybrid Incompatibility and Genomic Conflict

• Hybrid zygotes may form but often have reduced fitness due to genomic incompatibilities.

• Genes involved in essential cell functions, such as nucleoporins (e.g., Nup96), can cause hybrid inviability when divergent alleles are combined in hybrids.

• Presgraves et al. (2003) mapped hybrid inviability between D. melanogaster and D. simulans to the Nup96 gene.

Genetic Mapping of Hybrid Incompatibility

Experimental Approaches

• Systematic genetic screens can identify chromosomal regions responsible for hybrid inviability.

• In D. melanogaster × D. simulans crosses, hybrid males typically die at the pupal/larval transition, but can be rescued by specific chromosomal deletions or rescue alleles.

• Genetic mapping techniques can localize the responsible gene to a specific chromosomal region (e.g., 95AB on chromosome 3R for Nup96).

Accumulation of Reproductive Isolation

Snowball Effect Hypothesis

• Genetic incompatibilities are hypothesized to accumulate at an exponential rate over time, a concept known as the Snowball Effect.

• As more incompatibilities accumulate, reproductive isolation becomes more complete, reinforcing species boundaries.

Summary Table: Mean Development Time in Drosophila Crosses

The following table summarizes mean development time (in hours) from egg to first-instar larva in pure-species and F1 hybrids of D. yakuba and D. santomea:

Cross/Line	Mean development time (ISO)	Mean development time (SYN)
yak × yak	16.84 (0.62) A	18.34 (0.77) A
san × san	17.19 (0.55) A	19.15 (0.75) A
yak × san	22.63 (0.54) B	22.07 (0.95) B
san × yak	30.05 (1.06) C	28.64 (1.35) C

ISO refers to standard lines; SYN refers to synthetic strains. Letters indicate statistical groupings (Tukey-Kramer HSD).

Key Terms and Concepts

• Prezygotic isolation: Barriers that prevent mating or fertilization between species.

• Postzygotic isolation: Barriers that reduce the fitness of hybrids after fertilization.

• Reinforcement: Natural selection increases reproductive isolation in sympatry to avoid unfit hybrids.

• Haldane’s Rule: Hybrid sterility/inviability is more likely in the heterogametic sex.

• Snowball Effect: The exponential accumulation of genetic incompatibilities over time.

Relevant Equations

• Prezygotic Isolation Index:

$\text{Prezygotic isolation index} = 1 - \frac{\text{Interspecific mating probability}}{\text{Conspecific mating probability}}$

• Genetic Distance (D): A measure of genetic divergence between populations or species, often used to correlate with reproductive isolation.

Conclusion

Studies in Drosophila provide critical insights into the mechanisms and genetic basis of reproductive isolation and speciation. Both prezygotic and postzygotic barriers contribute to the formation of new species, and their evolution can be rapid and driven by ecological, behavioral, and genetic factors.