Ch 24.3 Hybrid Zone – Notes

Hybrid zone – region in which members of different species meet and mate, producing at least
some offspring of mixed ancestry
o Typically occurs in areas where species have undergone allopatric speciation and come
back into contact with one another.
o This is just my assumption, but I would assume this is because the 2 species have been
separated long enough to accumulate some differences, but not to build up enough
differences to have complete reproductive isolation
Patterns within a hybrid zone
1. Narrow bands: Basically, a thin line of land that exists between 2 separate populations where
hybrids exist (hybrids do not exist in either of the parent populations)
a. Obviously, there is something that is prohibiting gene flow in these regions or else you
would see alleles from both populations on both areas
b. In the example given in the book about yellow-bellied and fire-bellied toads the obstacle
is reduced hybrid viability (hybrid offspring are less likely to survive due to
morphological problems so they do not survive to spread the alleles of each population
to one another)
2. More complex scenarios
a. Not always just a clear cut band. Example from book is about 2 separate cricket species
in the Appalachian Mountains. One survives better in the warm and one in the cold. The
mountains in this area cause lots of valleys and areas that receive better or worse
sunlight. This causes patches of these different species throughout. There are some
areas where both species are found and hybrids are produced. Sometimes these hybrids
have better fitness than the parents
Hybrid zones over time
3 different possibilities:
1. Reinforcement - Reproductive barriers strengthen between the two species (limiting the
formation of hybrids). Called reinforcement because this process involves reinforcing
reproductive barriers
a. This occurs when hybrids are less fit then the parents
b. Prezygotic barriers are strengthened (because those who mate and produced hybrids
who die are less fit than those who do not due to the fact they are wasting energy
creating progeny who don’t survive)
c. The effects of these barriers are stronger in sympatric species than in allopatric
i. For example, the looks of the males of 2 different species of birds are much
more extreme in sympatric populations than in allopatric. This is because the
looks don’t need to be different in allopatric populations because the females
cannot reach the males of other species to mate. Even so, in sympatric
populations females often mate with males from both populations if they look
similar (and experienced reduced hybrid viability); therefore, the males of these
2 species began to have much more extreme phenotypes to inhibit the creation
of these hybrids
2. Fusion - Reproductive barriers weaken (causing the 2 species to fuse into a single species)
a. Basically, the reproductive barriers are weak so there is lots of hybridization. There is so
much gene flow that the reproductive barriers weaken further and the gene pools of the
2 species become increasingly alike.
b. Example in book is about cichlids and their speciation based off of mate-choice
determined by male color (example from ppt). Increased pollution has reduced visibility
and hindered female mate-choice due to the fact that the colors of the males aren’t as
visible. This has caused increased mating between species.
3. Stability - Hybrids may continue to be produced, creating a long-term stable hybrid zone
a. This occurs in the yellow-bellied and fire-bellied toads. Since there is reduced hybrid
viability, one would assume that reinforcement would occur. Even so, it does not in this
case. It is believed that this is because the hybrid zone is so narrow that the amount of
gene flow from outside this zone outweighs the decreased fitness of the hybrids (In
other words, hybrids are continually produced because there are continued flow from
each population into this area from the other 2 healthy populations allows the
continued production of hybrids.
b. This also occurs in the cricket population in Appalachian. The hybrids sometimes have
increased fitness so one would expect continued gene flow between the parent species
and hybrids in this area leading to fusion. Even so, it was stated earlier that these 2
species are warm or cold tolerable. Sometimes these populations meet in areas that are
on the edge of tolerance for one of the parent population. Slight changes in
temperature can cause extinction of that parent species in this area and thus eliminates
the possibility for hybrids in that area.