Download discuss-the-relative-roles-of-selection-and-drift-in

Discuss the relative roles of selection and drift in speciation.
When looking at speciation using the Biological Species concept, it is clear speciation is the
evolution of reproductive isolation. A question that scientists have still yet to agree on is
whether this isolation occurs due to natural selection or genetic drift. The isolation can be prezygotic or post-zygotic and these could occur together or at different times and so the evolution
of these different reproductive barriers could technically also be due to different processes.
The traditional view of speciation is that when populations become geographically separated
and so now live in allopatry, genetic drift will inevitably occur and lead to speciation. The
reproductive isolation is a by-product of genetic divergence. If only a small subpopulation is
separated geographically then they will undergo and bottleneck and this will reduce the genetic
variability within the population and so the effective population size will be small and genetic
drift will be strong. This will lead to genetic divergence between the subpopulation and the
original population. It is argued that this is the reason why there are many species on islands
which are endemic, one or more breeding pairs from the mainland arrive on an island and
undergo genetic divergence due to drift, the founder effect. In an experiment carried out using
Drosophila 78 populations were subjected to repeated bottleneck situations and after crossing
experiments no reproductive isolation was found to have evolved. This suggests that
bottlenecking alone is not enough to cause speciation, despite some traditional views. Perhaps
if the population undergoes a bottleneck in a new environment and then adapts to the new
environment under selection then this could lead to reproductive isolation.
The view that allopatric speciation could occur due to the populations now living in two
different habitats and so adapting to different environments leading to genetic divergence is
another view. In this case the speciation is still due to geographic separation but natural
selection plays a role. Again reproductive isolation evolves as a by-product of selection on other
traits. One experiment which tested this hypothesis evolved replicate populations in the lab.
Half were fed on maltose-based food and half on starch-based food. After fewer than eight
generations the different populations were found to preferentially mate with their own kind in
mate choice experiments. These results suggest that pre-zygotic barriers had evolved in a short
space of time due to different ecological conditions. However, a recent experiment has
complicated this interpretation. The experiment was replicated and again it was found that
mate choice evolved rapidly. However, it was shown that the choice was due to the flies’ gut
bacteria which are vertically transmitted and alter levels of cuticular sex pheromones. If
antibiotics were administered to the flies the preference disappeared. This shows that the
results of the original experiment may just be an artefact of using sterile food, highlighting how
experiments carried out in the laboratory are not representative of circumstances in the field.
If species living in allopatry are geographically separated but are not under different selection
pressures as they live in similar environments they may under uniform selection. Similar
phenotypes may evolve by fixation of advantageous mutations which are adapting to the same
environment. However, due to the chance involved in mutation they will be genetically adapting
in different ways. This is known as mutation-order speciation. Different genetic responses to the
same selective pressures have been shown to occur in microorganisms in the laboratory.
Surprisingly in many cases the most beneficial mutations are unique to one population although
parallel evolution also occurs. However, what is harder to prove is that this genetic divergence
will lead to reproductive isolation. Despite the lack of evidence is it plausible that reproductive
isolation will occur as a by-product. What is harder to imagine is that these circumstances are
representative of situations in the field. It would be very rare for two populations to be under
exactly the same selective pressures and so perhaps this theory is not applicable in most
circumstances.
Natural selection could in fact play an important part in speciation. If different subpopulations
living in sympatry begin to adapt to different niches then this could involve diversifying selection
and reproductive isolation could then occur as a by-product. This is known as ecological
speciation. In large-eared horseshoe bats three distinct size morphs echolocate at different
harmonics of the same fundamental frequency. These bats live in sympatry. By using different
harmonics the bat will eat different prey and this can lead to disruptive selection as they occupy
different ecological niches. Also, the call frequency functions as a form of communication
between the bats and so this could lead to assortative mating and therefore reproductive
isolation and speciation. This example also illustrates how sexual selection can lead to speciation
as mate choice is important in whether there is gene flow between subpopulations.
A well-known example in which sexual selection is key is the cichlids of the African Great Lakes.
In Lake Victoria many species can interbreed with no post-zygotic barriers and the offspring are
viable. However, in the field they are reproductively isolated by pre-zygotic barriers so in this
case pre-zygotic barriers evolved first. This is because mate choice has led to a reduction in
interbreeding among species. When closely related species live in sympatry they have male
colouration at opposite ends of the spectrum, usually blue and red or blue and yellow, and
females choose mates based on this. Experiments in the lab have shown that when tanks are
poorly illuminated mate choice breaks down as females cannot distinguish colour. This
reinforces that sexual selection is the mechanism that is leading to the maintenance of species
in this case. It also suggests that sexual selection could have been what caused the speciation in
the first place. If two subpopulations within a species started to develop different mate
preferences then this could lead to reproductive isolation due to a pre-zygotic mating barrier. In
turn this could lead to speciation as interbreeding would no longer occur.
In all of these theories reproductive isolation occurs ‘accidently’ due to drift or natural selection
on other traits. However, one theory suggests that prezygotic isolation occurs as reinforcement.
It is costly to mate if a postzygotic barrier already exists as the offspring will not be successful
and so it cannot lead to Darwinian success. It is therefore advantageous to avoid mating with
other species. In this theory pre-zygotic reproductive barriers evolve due to direct selection as it
advantageous to avoid mating with other species. Daniel Matute compared populations of two
species of Drosophila on different islands. He found that if the species lived in sympatry the
females show gametic isolation but this is not the same for those living in allopatry. To support
this observation he carried out an experiment in the laboratory. He evolved populations of both
species either separately or together. Prezygotic isolation only occurred in those kept together.
This is strong evidence that prezygotic isolation occurs as a direct result of direct selection and
not genetic drift or indirect selection.
It is important to consider that it is not solely a case of natural selection versus genetic drift
regarding speciation. Some traits may be under selection and some may be undergoing genetic
drift and so we cannot consider the two processes in isolation as they are acting together. What
is useful is deciding which process is having the greater effect and hence driving speciation.
Ecological differences lead to different adaptations due to natural selection and reproductive
isolation occurs as a by-product of this. However, the impact of direct selection producing prezygotic barriers has been vastly underestimated and it could be argued that in the majority of
speciation events direct selection has reinforced the isolation. It is evident that selection is the
driving force of speciation.