Speciation cont.
Postzygotic isolating mechanisms

Despite pre-zygotic mech’s, sperm may still fertilise an egg and create a
hybrid.

Hybrid inviability:
-
The zygote, embryo or offspring is inviable (unable to survive/develop
normally) so dies early in development.
-
Eg – Hybrid frogs often don’t survive in the tadpole stage for more than a
day.
Postzygotic isolating mechanisms

Hybrid sterility:
-
Hybrid reaches maturity but can’t breed as it is sterile.
-
Eg – A mule (male donkey – 2n = 62 and female horse – 2n = 64). Mules
have 63 chromosomes so can’t pair during meiosis.

Hybrid breakdown:
-
Occurs when hybrid reaches maturity and is able to breed but the next or
subsequent generations are infertile or have reduced reproductive
capacity.
-
Eg – first gen of hybrid cotton are fertile but the next gen’s have reduced
fertility.
Postzygotic isolating mechanisms

Isolating mechanisms may be individually insufficient to maintain genetic
isolation, however in nature several mech’s work together.

Sometimes iso mechs are weak: Eg – native grey duck and mallards
hybridise freely.

The degree of genetic isolation that a mech provides may reflect the
evolutionary stage from a common ancestor.
Evolution of postzygotic iso mechs
during a period of allopatry

Allopatric populations are unlikely to have the same environment, so selection
can take a different form and two pop’s will diverge genetically.

The more different the conditions, the faster the two will diverge.

Depending on how different they become, they could be subspecies or full
new species.

Genetic differences that evolve will be a result of mutation, and the
subsequent creation of new alleles.

However, this is only proven when the species become sympatric again and
they can interbreed – if they remain allopatric, we may never know them as
separate species.

Eg – morepork and boo-brook (Aus) are so similar and considered subspecies,
however we can’t interbreed them so the matter is unresolved.
Evol of prezygotic mechs during
subsequent sympatry – 3 things
1.
Some interbreeding – hybrids are less fertile so are less fit than parents =
disadvantageous. Selection would favour those with different mating
signals, breeding times etc so they mate with their own kind. *The
evolution of postzygtoic isolating mechanisms thus provides the main
impetus for the evolution of prezygotic isolating mechanisms*. It is also
likely that there is overlap in niches = competition for resources. Strong
selection pressure in favouring different food etc. This may result in a burst
of divergence, with new species forming.
2.
Populations don’t hybridise at all because during allopatry, prezygotic
mechs evolve. Two species may be closely similar in their niches =
competition = rapid rise in divergence until competition is minimised.
Evol of prezygotic mechs during
subsequent sympatry – 3 things
3. Hybridise freely – merging to reform the original species. Can be an
indication that the species are of recent origin with imperfectly evolved
isolating mechanisms.
**I have scanned pages of examples of geographical isolation you can read.
There are also questions at the end**