Ronald Zee said:
Hi Rasmus
I am amazed to read that the Andean Coot (Fulica ardesiaca) has three differnt morphs, why aren't they considered different species?
As far as I know this species of Coots must be the only one with varying beak/shield colours.
Alejandro, sorry for this slight twist compared to the original question – hope you don’t mind…
Ronald, as far as I know you have a deeper interest in Coots, so I took the time to elaborate my answer (had quite a bit of it already from things I’ve written earlier). Furthermore, I checked my memory against my notes (to make sure I got it right). While the following is specifically aimed at this species, much of the info is relevant for other polymorphic species.
The different morphs (phenotypes) involved are regarded as a single species because they interbreed to a large extent (morphs don’t equal species. Otherwise we would have to say humans with blue eyes and humans with brown eyes were different species, too!). Actually, different phenotypes in terms of bill & shield are also known from another species in this genus; the Hawaiian Coot. To my knowledge, there have not been any thorough studies of the possible reasons for this in the case of the Andean, but other polymorphic species have been studied to great extent, leading to several different possible explanations (obviously, it is somewhat simplified and usually morphs are the result of a combination of the following rather than just one):
1) The genes contributing to the colour of the bill & shield could be placed on neutral alleles. This results in phenotypes that are equal, and thus often approach 50/50. As far as I know such morphs are relatively infrequent.
2) The phenotype has various advantages/problems compared to the other phenotype(s). It could be that one morph is more desirable as a partner or perhaps better at hiding. However, the advantage isn’t directly visible in all cases, e.g. it could be that one morph also has a better immune system (usually due to connected alleles). This usually leads to one phenotype being commoner than another. Of course such examples require a careful balance. If the advantage one morph has is too large, the other will go extinct.
3) Few genes are co-dominant. Thus, phenotypes expressed by dominant genes are likely to be more common unless that form has a serious disadvantage over other phenotypes in life (see above #2). In reality dominance among genes is a bit more complex, but there’s really no reason to go into that in this context.
Back to the Andean Coot: In large parts of the range the morphs with white or yellow shield and white/yellow bill are the commonest by far, especially in the northern part of its range (I’ve never seen the red-fronted morph in Ecuador, though I know it occur infrequently – perhaps I just didn’t spend enough time looking at every single individual I have encountered). However, it seems the roles are switched in the southern part of the range where the red-shielded morph is commoner. Indeed, there are areas in Bolivia where I’m not sure I ever saw a white-shielded or a yellow-shielded individual! This indicate several things: First, in my experience, the morph with white/yellow bill and yellow shield appear to be roughly equally common as the morph with white bill and shield. Hence, these two are likely not to have an advantage over the other, and thus the two are likely to be examples of #1 described above. However, in northern regions the red-shielded morph is rare. This could lead us to believe that the red-shielded morph has a disadvantage over the other two (as described above in #2), or it could be that the genes leading to the red-shielded morph are recessive (as described above in #3). However, as mentioned previously, the situation is a bit more complex, as it seems the red-shielded morph is commoner than the other in the southern part of the range. Hence, it is unlikely that one form is genetically dominant over the other.
In this case I think the likelihood is that a specific phenotype is the preferred partner. This is supported by a number of observations. First, in regions where several species of coots are sympatric, the different species have different colours of the bills and/or shields. While this could be a coincidence, the bills/frontal shields are known to be important in breeding where pairs have displays involving face-to-face “head shaking”. Thus, the difference in colour of bill and/or shield is likely to be an important factor in maintaining pure lines in areas where several species occur. As you may note, in most areas where the Andean Coot occur, it is the only species of Coot. The only exceptions are in the high barren Andean regions where the very different Giant Coot occur and Laguna de Mejía in SW Peru where a relatively small population of Red-fronted exist. The population of Red-fronted is at Laguna de Mejía is some distance from other populations, so it is likely to be a relatively recent arrival or perhaps a relict population. Furthermore, Andean Coot is generally restricted to the highlands in this region, meaning that it only will come face-to-face with a Red-fronted infrequently. The situation is comparable in Hawaii, where the Hawaiian Coot is to only species, except for accidental visits by the American Coot. Being the only one result in some liberty in terms of morphs, as there is no risk of ending up looking too much like closely related species (which could lead to frequent hybridization). While above may be an indication of the reason for one morph to be commoner than the other in certain areas, there actually is another thing supporting this view. Apparently, there are observations (not mine, haven’t spend enough time watching pairs of this species to notice) that indicate that red-shielded individual seem to prefer red-shielded as partners if they have the possibility to choose, and likewise with pale-shielded preferring other pale-shielded as partners (apparently, white or yellow shield isn’t important in this context). There are also indications of a level of habitat segregation between the two forms with red-shielded generally being commoner at lakes with much vegetation and pale-shielded in more barren regions (this could lead to speculations over possible benefits one morph may have in a specific habitat). However, if no partner with the same shield-colour is available, it appears they happily will take a partner belonging to the population with the other shield-colour. Thus, interbreeding between the forms is extensive and they are regarded as one species. Historically, the taxonomical treatment of this species has been problematic. For a long period the pale-shielded (including both the yellow-shielded and the white-shielded) were considered a subspecies of the American Coot and known under the Scientific name of Fulica americana peruviana. The red-shielded morph was considered a different species under the name F. ardesiaca. Then it became apparent that the two happily would – and did – interbreed to a significant degree, and they were lumped together as a subspecies of the American Coot, this time known as F. americana ardesiaca. Finally, studies in Colombia were able to show that F. americana columbiana occurred together with F. ardesiaca without any signs of interbreeding. This has later been supported by genetics. So, finally this resulted in F. ardesiaca being recognized as a species distinct from the F. americana, but including the various morphs. While it hasn’t been mentioned here, there are actually a number of other (generally very rare) variants. Among others, there are more than one yellow shield colour…
BTW: Morphs exist in quite a few species, though often the difference is rather limited. One of the most striking examples of polymorphy in a species is the black versus the red morph in the Papuan Lorikeet - check this links - exact same species, just two different morphs:
http://api.corbis.com/wscorbisone/f...3HDq6lpZWgqgKcGTKb8+tngWh97kYxzmqt475H8zM9LA=
Several species of herons also have very different morphs such as the Reddish Egret (there's also a white morph), the Great Blue Heron (there's also a white morph) and the various Reef Herons (where there usually is a white and a very dark morph).
