Courtship Song and Speciation Events...
Thursday, February 22, 2007
With the world developing as fast as it is, there are many things going on around us without our knowledge. Most things in nature occur at a slow pace. Speciation is one of these events that occurs relatively quickly, but usually not at a rate that you can sit back and watch each step in your lifetime.
On the science blog, I wrote about how hard it is to define a species and speciation. There are many points of view and criteria that can define a given species. But all that aside, all definitions understand the importance of reproductive isolation. This is the absence of gene flow between two or more populations. This is not necessary to define a species concept, but it does define species boundaries.
What is reproductive isolation? This is when two populations (of the same species) no longer reproduce with each other (no genes flowing back and forth between the populations). This will lead to changes in allele frequencies dependent on the population. There are many types of barriers/obstacles that can block reproduction. We can further categorize these obstacles into pre-zygotic (before actual mating) and post-zygotic (after mating) barriers. Some pre-mating are behavioral isolation (not recoginzing as potential mates) and spatial isolation (not near enough to mate). Post-mating are mechanisms that block copulation all the way to the egg being fertilized but aborted. There can be mechanisms after birth such as sterility of all offspring.
As we all know (i.e., nature shows, class, etc.) animals behave differently across species. One courtship may be different than another courtship even in closely related species. Think of courtship as a test. When the male (most often) performs the courtship correctly, he may win the prize (passing on genes) with the other partner administering the test. This even more tricky, because the questions being asked by each sex may be different based upon fundamental sex differences concerning current and future investment in the offspring. It is through the courtship ritual that the questions are answered by communicating through visual, chemical, auditory and tactile cues.
So what of species that only differ in their courtship signals? It can be assumed that greater differentiation in signals significantly act as species recognition factors. This is especially true in sympatric species (reside in same environment). Closely related (but not reproducing with each other) would have very different courtships only recognized by their species. On the other hand, it can also be assumed, closely related species (only differening in their courtship signals) raised allopatrically (different environments) could have very similar, if not almost identical, courtship displays. In fact, this is what is seen very often. There are the Laupala crickets of Hawaii that have undergone rapid, intra-island species radiations on different islands of the archipelago. Thus, species confined to different islands do not have the problem of confusing their signals and can have nearly identical courtship songs without the consequences of wasted time and energy on females that will not mate with them. You might wonder what the females would do in a situation where both species of males are present. It has been shown that the females will choose a conspecific male (her species) over a heterospecific male (other species) most often.
Why is there divergence of mating signals? Actually, this is not yet clear. What we do know is that species recognition of mating signals will reach stabilizing selection (most population favors the majority displayed pattern). However, divergence of signals can only come about when their is a shift in the preference of mating signals in the direction on the extremes (species specific). So, changes in the one receiving the signal will in turn cause changes in the one sending the signal, causing the chain reaction of directional selection.
To understand this point, imagine a stand bell-shaped curve. On the x-axis is courtship behavior. On the y-axis is the number of individuals in the population. On the x-axis at 50% most of the population prefers the same thing. But at 10% and 90%, these individuals prefer something else that the majority does not. So, a directional shift would favor one of the two extremes, outside of what the majority prefers. Thus, causing a directional shift (left or right) away from the majority of the population. By doing this, you get two populations (one smaller than the other) that prefer different mating signals, therefore, you start the divergence of mating signals in a single population (same species). This is how a speciation event is thought to occur when they all live in the same physical environment and no physical boundaries separate the population. Over time, the females in population one will stop recognizing the mating calls from population two (and vice versa) leading to two separate species that do not interbreed based only on courtship/mating signals. They can physically still mate, but they do not recognize each other as potential mates (Behavioral Isolation). This if very different from two species that do try to mate but either conception doesn't happen, they are born but soon die or the offspring are born sterile as in the case of the horse and donkey making a mule, which is sterile (Reproductive Isolation; pre & post-zygotic isolation examples).
There are other hypotheses proposed that discuss that divergence is a by-product of genetic divergence of populations in allopatry (different evironments). The mating signals will gradually change due to mutations in the population, genetic-drifts, and environmental factors. When the populations separated later reside in the same environment, their phenotypic characteristics (mating signals to general behavior) will be very different compared to the other population. So a new stabilizing selection will occur, one for each population, independent of the other population.
In summary, defining a population is difficult. However, when discussing reproductive isolation, the boundaries are clear. There are a multitude of factors that can lead to reproductive isolation. I have discussed how courtship can lead to divergence of populations and possibly to different species in the future.
posted by Sonya @ 2/22/2007 11:09:00 AM,
