Professor Dave again, let’s make some new species.
Earlier in the series we talked about Darwin’s descent with modification, and elucidated
how evolution occurs by natural selection.
Inevitably, as organisms exhibit slow incremental change, members of a species will eventually
find themselves in different, totally new species, as sufficient alteration has occurred
due to being subjected to completely different sets of stimuli, such that they are no longer
able to interbreed.
When this happens, we call it speciation.
Speciation occurs in a number of different ways, and for different reasons, some of which
have been physically observed, so let’s go in for a closer look.
As we already know, a species is a group of populations whose members have the potential
to interbreed and produce viable, fertile offspring.
In addition, they do not produce viable, fertile offspring with members of other such groups.
These groups represent different species, and it is the only taxonomic branch that is
not arbitrarily determined, but rather is determined by nature.
A population of organisms will tend to remain the same species as one another as long as
there is adequate gene flow, or adequate transfer of alleles amongst them.
This ensures that any genetic changes that proliferate in a statistically significant
way will be largely shared amongst the population.
When gene flow is disrupted, this can potentially result in speciation, as different alterations
can accumulate in the two groups, according to different sets of stimuli, and after enough
time, their genomes will be sufficiently different that they can no longer interbreed, making
them by definition, two different species.
This disruption can occur due to a variety of prezygotic or postzygotic barriers, meaning
reproductive barriers that either impede mating or fertilization, if prezygotic, or impede
the viability and fertility of a hybrid, should one form.
Prezygotic barriers include habitat isolation, temporal isolation, behavioral isolation,
mechanical isolation, and gametic isolation.
Postzygotic barriers include reduced hybrid viability or fertility, and hybrid breakdown.
There are many common hybrid animals, such as the mule, which is the offpsring of a male
donkey and female horse, so hybrids can be viable.
However, mules are not fertile, but rather sterile, so this type of gene flow can not occur.
As we said, when one or more of these barriers arise and gene flow is restricted, speciation
can occur, and there are different types of speciation that we should be aware of.
These are allopatric, sympatric, peripatric, and parapatric.
Allopatric speciation is the easiest to understand.
This occurs when a geographical barrier divides a population into two smaller populations.
This could occur if the water level in a lake decreases such that some land mass divides
it into smaller bodies of water.
A river may change course and divide a population.
A mountain range or a canyon may slowly form.
Seeds from a plant may float across a body of water and land on an island.
Once the barrier is in place, speciation is only a matter of time, as mutations continue
to occur, and selective pressures on either side of the barrier must necessarily slowly diverge.
With sympatric speciation, there is no geographical barrier, but rather gene flow is restricted
in some other way, causing reproductive isolation.
Perhaps a subset of the population develops polyploidy, and thus can only reproduce amongst
one another.
Discrepancies in sexual selection can be a factor.
Habitat isolation can also be responsible.
So although this is more rare, geographical barriers are not a prerequisite for speciation.
The other two definitions are more subtle.
Peripatric speciation is when a small group of individuals breaks off from the larger
group to form a new species, due to physical barriers.
This is just like allopatric speciation, except that one group is much smaller than the other.
This is an important distinction, because the frequency of specific traits present in
this subpopulation may be different from the larger group, due to its small size, and this
will be reflected in new generations of that group, just as we discussed when we learned
about the founder effect and the bottleneck effect in a previous tutorial.
And in parapatric speciation, a species is spread out over a very large geographical
area, which results in individuals only mating with those in their immediate vicinity.
Although in this instance there are no rigid physical barriers, slight differences in habitat
within the larger area can result in speciation over time, as subpopulations explore the various
niches within the habitat.
So those are the types of speciation.
How long does speciation take?
To answer this we look to fossils.
There are many examples in the fossil record of punctuated evolution.
This involves long periods of very little change, followed by sudden rapid change.
Other times, species appear to have changed very gradually over long periods of time.
But whichever the case my be, speciation requires anywhere from a few thousand years to a few million.
This does explain why observation of speciation events is quite rare, but nevertheless they
do occur, and successive speciation over long periods of time is what results in the macroevolution
that has brought about everything that is alive today.
As to what kinds of organisms may exist on this planet in another thousand years, or
even a million, only time will tell.