| Summary: | A pair of sympatric three-spined stickleback species (Gasterosteus aculeatus complex) at
Paxton Lake, Texada Island, British Columbia, Canada hybridize regularly, with 1% of the total
adult population consisting of hybrids (McPhail 1992). Previous work has established the
presence of significant, but imperfect, premating isolation mechanisms (Nagel 1994). I
investigated two potential mechanisms of postulating isolation. The first was sexual selection
against adult F₁ hybrids and the second was natural selection against juvenile F₁ hybrids.
To test for sexual selection against adult F₁ hybrids, I examined the mating success of F₁
hybrid males and the habitat use of benthic and limnetic females. I conducted mate choice tests in
which F₁ hybrid and limnetic males competed for wild limnetic females. I found significantly
reduced mating success among F₁ hybrid males. Mean mating success of F₁ hybrid males was
33% that of limnetic males. Hatfield (1995) found that F₁ hybrid males nest in the open regions of
the littoral zone, which is the same habitat used by nesting limnetic males. I investigated the
habitat use of wild gravid females by placing minnow traps in two habitats (open and vegetated).
A significantly higher number of limnetic females were trapped at the open site relative to benthic
females and significantly more benthic females were trapped at the vegetated site. This
observation confirms that F₁ hybrid males, by nesting in open areas, are predominantly sampling
limnetic females. These results suggest that reduced mating success of F₁ hybrids may be an
effective postmating isolation barrier between benthics and limnetics.
I measured the survival and growth over two months of F₁ hybrid juveniles in an enclosure
in the littoral zone in the presence of juveniles of both parental species. I did not detect any significant reduction in relative numbers of hybrids in two separate years and growth rates were
similar to the average of the two parental species. Prey items consumed by juvenile F₁ hybrids
showed substantial overlap with juveniles of the parental species, suggesting that juvenile F₁
hybrids exploit the same resources as the parental species. Natural selection against juvenile
hybrids, at the stage in their life history at which I conducted my experiment, may not be a strong
postmating isolation barrier reducing gene flow between benthics and limnetics. === Science, Faculty of === Zoology, Department of === Graduate
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