In the process of speciation, exactly what causes an inability to breed with the "parent" species?

Question

I understand that if there is enough genetic mutation, a new species is created because it can no longer reproduce with the parent species. What are the mechanics of this? Thanks

Answer 1

There can be a variety of reasons for the failure
of interbreeding between two species. One of them is a difference in chromosome number.
When the chromosomes pair up during meiosis
to produce the sperm and ova they can not pair
up properly (at least in some cases) if they are
present in different numbers. This produces ova
and sperm that can not produce viable zygotes
because some chromosomes are missing or
there are too many of them.

There are also situations in which it might be possible for the organisms to interbreed with each
other if they ever met, but they do not for one
reason or another. Some of them become mature
and mate only in the spring and others in the fall,
for example, or some live in deserts and others in
marshes.

There is an entire spectrum of possibilities with
regard to interbreeding. Sometimes there just is
no way for them to produce a fertilized egg, even
if they do mate - the ovum does not recognize the
sperm, so it can not enter, for example. At other
times they do mate and produce offspring, but
the offspring are sterile, as with the mule. In some
groups fertile hybrids can be produced within a
group of genera, but not outside that group. In
orchids there are fertile hybrid plants whose
parentage includes as many as nine different
genera. Cactus plants also have produced a
number of intergeneric hybrids. Naturally formed
hybrids, as opposed to those produced by humans, are comparatively rare, usually, but in a
few groups of organisms they occur frequently.
This seems to be mostly in evolutionarily young
groups of organisms that have not yet diverged
enough to prevent hybridization.

Answer 2

Typically, there is some barrier to reproduction between two populations. A good example of this would be geographic isolation. Over a long period of time, the two populations would likely follow divergent evolutionary paths eventually resulting in new species.

Usually, the species that was in the original ecosystem is considered to be the parent species, but in all likelihood, has diverged as well.

Answer 3

In a broad sense- there is a finally a genetic incompatibility at some level. I would expect most speciation events to be rather unique- e.g. what causes speciation in species a&b is totally different set of type of genes from species d&e...