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\b[<dispersal> determines potential encounters and genetic exchange; <mate preference> helps determine which encounters become reproduction. <speciation> depends on the resulting <gene flow> and the evolution of barriers to it.] None of the three can be understood simply from the distance animals move or from a laboratory preference score. <Dispersal> is movement, while <gene flow> requires genetic contribution to later generations. <Mate preference> is responsiveness to possible partners; realized mating also depends on encounter rate, competition, availability and acceptance by both sexes.
Reduced movement can allow populations to diverge through <natural selection> and <genetic drift>. During <allopatric speciation>, different environments, signal systems or histories can generate reproductive differences without continual exchange of <alleles>. At <secondary contact>, these differences may maintain isolation, permit extensive merging, or produce partial barriers. Current coexistence does not establish <sympatric speciation>: the populations may have diverged elsewhere. Conversely, appreciable movement need not prevent divergence if migrants seldom reproduce or their descendants perform poorly.
A small illustrative calculation shows why migrant numbers and effective contribution differ. Suppose migrants constitute a fraction $m$ of potential breeders, residents have reproductive weight one, and migrants have weight $a$ after mate acceptance, with equal survival and other fecundity. Their fraction of the successful breeding pool is then:
$$
m_{\mathrm{breed}}=\frac{ma}{1-m+ma}.
$$
For $m=0.2$ and $a=0.1$, this is $0.02/0.82=1/41\approx0.0244$. These are invented values, not a migration estimate. The normalization matters, and the expression describes weighted breeders, not automatically ancestry in every offspring or a complete model of sex-specific mating. It nevertheless demonstrates why substantial <dispersal> can coexist with much smaller genetic exchange.
Movement is often nonrandom with respect to habitat. If animals select the resource on which they also meet mates, <habitat preference> produces <assortative mating> without an explicit preference for a partner's identity. In <Rhagoletis pomonella>, apple- and hawthorn-associated populations mate in relation to their hosts. Mark-and-recapture studies found that <host fidelity> substantially restricted exchange, while host preference and differences in emergence timing contributed to the barrier. <Host fidelity is an effective premating barrier between sympatric races of the apple maggot fly by Jeffrey L. Feder and others>[PNAS 91, pp. 7990–7994; three mark-and-recapture studies and host-associated gene flow]{ref} The estimated exchange was not zero: the example concerns partial, measurable <premating isolation>, rather than complete reproductive closure. It also warns against equating geographic overlap of host plants with random encounters among insects.
Ecological specialization and mating can remain coupled when the same characteristic affects both, a <magic trait>, or when separate relevant loci are genetically linked. Pea-aphid mapping linked host-related adaptation and reproductive isolation, making <Acyrthosiphon pisum> an example in which host association has a genetic basis rather than being inferred solely from field distribution. <Genetic linkage of ecological specialization and reproductive isolation in pea aphids by David J. Hawthorne and others>[Nature 412, pp. 904–907; mapping host adaptation and habitat preference]{ref} A mapped association does not establish one causal mutation with several effects. <Pleiotropy>, tight <genetic linkage> and correlated environmental responses can produce similar phenotypic patterns and require different tests.
Direct preference for a signal creates another route. In <Heliconius>, wing patterns are involved in ecological interactions and mate recognition. Genetic crosses between <Heliconius melpomene> and <Heliconius cydno> mapped major effects on male courtship preference, including a region tightly linked to the colour-pattern locus <Optix>. <Genetic dissection of assortative mating behavior by Richard M. Merrill and others>[PLoS Biology 17, e2005902; preference QTLs, linkage to optix, and limitations of courtship assays]{ref} Such coupling helps preserve associations between cues and preferences against <genetic recombination>. Yet the mapping interval is not an identified “<speciation> <gene>”, and a male's courtship duration can also depend on a female's response. Manipulated or artificial models, standardized females and reciprocal behavioural assays help separate visual preference from other interacting traits. Both sexes can exert choice; a female-choice-only account would miss this example.
The sensory environment can itself connect habitat and <mate preference>. Lake Victoria <cichlids> show associations among depth-related light conditions, visual-pigment variation, male colours and female preferences consistent with <sensory drive>. <Speciation through sensory drive in cichlid fish by Ole Seehausen and others>[Nature 455, pp. 620–626; light gradients, visual pigments, colours and preferences]{ref} Preference need not begin as a direct mechanism to avoid hybrids: sensory adaptation for detecting resources can change the attractiveness of a mating signal. Changing the light environment can then weaken a previously effective colour barrier. Sensory sensitivity, signal appearance and preference are related measurements, however, and none should be substituted for an actual assay of mating or genetic exchange.
<Dispersal> can also create rather than dissolve reproductive novelty. Founders bring new combinations of signals and developmental backgrounds into contact, and <hybridization> can generate a distinct combination rather than merely homogenize populations. Alternatively, repeated immigration can swamp locally favoured <alleles> or continually break associations between signal and preference. The outcome depends on ecological selection, the <genetic architecture> of the barriers, recombination, density and mating costs, not on a universal migration threshold.
After contact, <reinforcement> can strengthen <premating isolation> if <hybridization> imposes a fitness cost and heritable mate discrimination reduces that cost. It requires selection against the consequences of interpopulation mating, not just a finding that sympatric individuals prefer their own type. Differences between contact and non-contact populations could instead reflect habitat, population history or differential survival. Common rearing, reciprocal crosses, controlled cue presentation and direct estimates of hybrid performance make the alternatives distinguishable. Migration can supply variation needed for such evolution while also eroding the resulting barrier.
The most informative research therefore measures the entire sequence: departure, movement, settlement, encounter, mating and descendant contribution. Tracking and mark–recapture reveal movement; choice and no-choice tests reveal different aspects of preference and acceptance; parentage and genomic ancestry reveal realized reproduction. A preference assay needs controls for condition, familiarity, developmental experience and stimulus intensity. Barriers operating at successive stages must be assessed conditionally, rather than adding percentage reductions that refer to different denominators. \b[<speciation> becomes possible when ecological and mating processes maintain sufficiently persistent reproductive separation; <dispersal> and <mate preference> jointly determine that separation and can either strengthen or weaken it.]
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