= Solution
\b[<reinforcement> is the strengthening of reproductive barriers by selection against costly hybridization. Its importance must be demonstrated causally, rather than inferred solely from greater mating discrimination where <species> meet.] Typically, partially isolated lineages encounter one another, produce low-fitness hybrids, and experience selection favouring discrimination that reduces such matings. <secondary contact> after divergence is a common setting, but the definition concerns the selective mechanism rather than one obligatory geographical history.
Consider a rare inherited preference that reduces the probability of producing a hybrid by $\delta$, when hybrid offspring lose a proportion $d$ of <reproductive success> relative to within-population offspring. In a simplified accounting with a common offspring-equivalent denominator, the benefit is $\delta d$. If stronger discrimination costs $c$ through delayed mating or rejection of suitable mates, it is favoured when:
$$
\delta d>c.
$$
This expresses the logic of <reinforcement>, not a complete <population genetics> model. Encounter rates determine $\delta$, sex-specific costs can differ, and <gene flow>, <recombination> and associations between preference and ancestry determine whether discrimination can evolve and persist. The mere existence of low-fitness hybrids does not guarantee that an effective preference variant is available or that its costs are small.
The first empirical requirement is evidence for contact and opportunity to hybridize. Maps should establish coexistence at the scale of mating, not merely occurrence within the same region. Field observations and parentage or genomic ancestry estimates can establish heterospecific mating and recent <gene flow>. Historical contact may also be relevant if successful <reinforcement> has already made contemporary hybrids rare. Complete present-day separation therefore does not disprove a past role, but it makes that historical role harder to identify. Models fitted to genomic data should compare alternative demographic histories rather than equating shared <alleles> automatically with recent <introgression>.
The second requirement is a reproductive cost. Reciprocal crosses should measure fertilization, survival, fertility and subsequent <reproductive success>, including later-generation and backcross offspring where feasible. Reduced performance may be intrinsic, such as <hybrid sterility>, or ecological, such as failure to use either parental habitat effectively. Laboratory survival alone can miss the latter. Reciprocal environmental comparisons with parental controls test whether hybrid disadvantages persist under natural conditions. Handling effects, parental condition and rearing density should be standardized. Selection against hybrids must be distinguished from lower mating rates caused solely by ecological separation.
The third requirement is variation in barriers consistent with selection against those costs. A classic prediction is <reproductive character displacement>: stronger discrimination in <sympatry> than in populations outside the contact zone. Female <Drosophila pseudoobscura> from regions shared with <Drosophila persimilis> showed greater sexual isolation than females from outside that overlap, providing an influential test of this prediction. <Speciation driven by natural selection in Drosophila by Mohamed A. Noor>[Nature 375, pp. 674–675; geographic variation in female discrimination against D. persimilis]{ref} Comparisons should use several independent populations, matched environments and standardized male stimuli. Both choice and no-choice assays are useful: the former measures relative preference among available partners, whereas the latter asks whether rejection persists when an alternative partner is absent. Neither alone estimates realized <gene flow> in the field.
The geographic pattern has important alternatives. Different environments can independently favour different signals or sensory responses; <sexual selection> within each lineage can generate divergence; existing discrimination can permit coexistence while poorly discriminating populations disappear. This last process can sort populations without increasing discrimination within them. Costly heterospecific courtship can also favour signal divergence without selection against hybrid offspring. Such reproductive interference is relevant to isolation, but should not automatically be called <reinforcement> in its strict hybrid-cost sense. <Common garden experiments> help distinguish inherited differences from immediate effects of environment or experience, but do not on their own identify the historical selective cause.
Crosses and mapping provide a further line of evidence. In the <Drosophila pseudoobscura> system, crosses between populations with stronger and weaker discrimination showed inherited variation in enhanced discrimination, with high discrimination expressed in first-generation crosses. Chromosome substitutions and finer mapping implicated multiple regions, and their contributions differed among sampled populations. Controls measured male mating attempts, helping separate female rejection from altered male effort. Importantly, candidate regions enriched for olfactory functions were suggestive rather than proof of a particular sensory mechanism; mapping an interval is not identifying every causal <gene>. <The Genetics of Speciation by Reinforcement by Daniel Ortiz-Barrientos and others>[PLoS Biology 2, e416; crosses, mate discrimination and the genetic basis of enhanced isolation]{ref} This evidence makes an evolutionary response plausible and helps distinguish basal isolation from its subsequent enhancement.
The most direct prospective test would use replicated <experimental evolution>. Start with the same partially isolated populations, allow contact in treatment lines, and vary the <biological fitness> consequences of hybrid offspring while retaining appropriately matched controls. Across generations, test discrimination in a common environment, measure actual hybrid production and follow parental and hybrid <reproductive success>. Lines must be independent <experimental units>. Artificially removing hybrids can demonstrate a possible response to selection, but complete elimination of all hybrid-mediated <gene flow> simplifies the problem that the natural hypothesis seeks to explain. Changes due to crowding, <learning> or laboratory <adaptation> must also be separated from inherited changes in mate discrimination. A field intervention changing encounter rates could be informative, but ethical and ecological constraints often make longitudinal natural contact zones preferable.
Finally, importance in <speciation> is a broader question than existence in one pair of <species>. Comparative data should estimate how often hybrid costs, geographical overlap and enhanced barriers coincide, controlling for divergence time, ecology and <phylogenetic nonindependence>. Ascertainment matters: conspicuous surviving hybrid zones may omit cases where contact caused fusion, exclusion or near-complete isolation. A stronger preference is evolutionarily consequential only if it reduces effective exchange or helps maintain distinct lineages. Its effect should be measured alongside habitat separation, seasonal timing and <postzygotic isolation>, because sequential barriers act on the individuals that reach each stage.
\b[Convincing evidence combines hybrid costs, opportunity for contact, inherited enhancement of isolation and exclusion of competing selective explanations.] <reinforcement> can complete or stabilize a separation already under way, but geographic differences in signals alone establish neither its mechanism nor its frequency as a cause of <speciation>.
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