Ch 24 (3)
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What happens if allopatric populations come back into contact with one another?
- One possible outcome is formation of a __, a region in which members of different species meet and mate, producing at least some offspring of mixed ancestry.
__ exhibit a variety of structures. Some __ form as narrow bands.
hybrid zones x2
What causes uch a pattern of allele frequencies across a hybrid zone?
We can infer that there is an obstacle to __- otherwise alleles from one parent species would also be found in the __ of the other parent species.
True or False:
Hybrid toads have increased rates of embryonic mortality and a variety of morphological abnormalities, including ribs fused to the spine and malformed tadpole mouthparts.
As a result of malfunctions or abnormalities, __ rarely serve as a stepping-stone from which allleles are passed from one species to the other.
Other _- zones have been more complicated spatial patterns.
Studying a __ is like observing a natural experiement on __.
-Will the result be the rapid formation of a new species, as occurred by polyploidy in the goatsbeard plants of the Pacific Northwest?
o If not, there are three possible outcomes for the hybrid zone over time.
What are they?
- hybrid zone
- o REINFORCEMENT: reproductive barriers btw species may be strengthened over time (limiting the formation of hybrids0
- o FUSION: weakened over time (causing the two species to fuse into a single species)
- o STABILITY: hybrids may continue to be produced, creating a long term stable hyrbid zone
When hybrids are less fit than members of their parent species, we might expect natural selection to strengthen __ to reproduction, thus reducing the formation of unift hybrids. Because this process involvies __ reproductive barriers, it is called __. If __ is occurring, we would predict that barriers to reproduction between species should be stronger for __ species than __ species.
- prezygotic barriers
- reinforcement x2
When two species contact one another in a hybrid zone, but the barriers to reproduction are not strong, so much __ may occur that reproductive barriers weaken further and the gene pools of the two species become increasingly alike. In fact, the __ process reverses, evenutally causing the two hybridizing species to fuse into a single species.
True or False:
Many hybrid zones are stale in the sense that hybrids continue to be produced.
recall that hybrids are at a strong disadvantage in the Bombina hybrid zone. As a result, the offspring of individuals that prefer to mate with members of their own species should survive or reproduce better than the unfit hybrid offspring of individuals that mate indiscriminately with members of other species. This suggests that __ should occur, strengthening reproductive barriers and thereby limiting the production of hybrid toads.
True or False
In short, sometimes the outcomes in hybrid zones match our predictions and sometimes they dont. But whether our predictions are upheld or not, events in hybrid zones can shed light on how barriers to reproduction between closely related species change over time.
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