An important thing to note about a species is that it is fundamentally distinct from other terms that are used in classification, such as family, genus, order, class, etc. The key difference is that the latter terms are arbitrary - a genus is not a defined unit. If you are comparing two genera, for example, and one has more species than another, this does not tell you that the one with more species has evolved or diversified more quickly - there is no strict definition as to what constitutes a genus or any other term besides species - as long as the higher ranks are monophyletic (see Sci-Day #1 for more details on what monophyletic means), it doesn't matter where they are placed on a cladogram or phylogeny. A species, however, is a meaningful and distinct unit - it is a unique population of organisms that is not arbitrary. One of the biggest problems in biology, however, is figuring out how to identify species. This can also be referred to as "the species problem."
This question has no clear or 'right' answer, because the issue is so complex. One scientist named Edward Wiley stated in 1978 that "a species is a single lineage of ancestor-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate" [quote retrieved from Evolutionary Biology lecture]. This essentially means that a species is in some way distinct from other organisms, and its population evolves on its own trajectory - ie, the population may be influenced by changes in environment or other organisms that it interacts with (like in coevolution), but the species does not evolve the exact same changes as the other or evolves in its own unique way. This definition of a species is called the "Evolutionary Species Concept".
There are many different species concepts, all of which have benefits and drawbacks in their application. The most relevant one in paleontology is called the Phenetic [or Morphological] Species Concept. This concept identifies species based on morphological characters such as length of certain bones or other structures, or any observable morphological character. To distinguish species, one can create a chart in "phenotype space", as shown in this image:
However, there are several drawbacks to this concept. One issue is choosing characters to analyze. How do we choose what characters to plot? Which characters are important to distinguish species, and which ones are simply representations of variation between localities or individuals? Another drawback is that some species may actually look very similar, but are actually different species (known due to a lack of gene flow between them) - these are called cryptic species. Unfortunately, it is the only species concept that we can use when examining fossil taxa - this could mean that specimens which we all lump into a single species may actually represent several different species - differences between them might have been limited to soft tissue or genetics, which would mean if this were the case we might never even know it. This issue with applying species concepts to modern taxa is also displayed by the fact that the species status of several dinosaurs (including Dracorex, Stygimoloch, Nanotyrannus, and Torosaurus latus) is disputed as being due to ontogenetic variation. If we had fully intact specimens like we have of modern animals, we would be able to tell whether or not these are species in their own right or are just juvenile/adult forms of other species.
Another concept that I will briefly address is one of the most commonly used ones in biology of extant organisms - the Biological Species Concept. the Biological Species Concept says that "species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups" [Quote retrieved from Evolutionary Biology lecture]. Observations that led to the formulation of this concept include variation within populations, sexual dimorphisms, life cycle (some organisms display extreme morphological changes in development), geographic variation, and cryptic species [as described above]. While this is beneficial due to addressing some of the limitations of the Phenetic Species Concept, it too has its drawbacks.
Drawbacks of the BSC are mostly related to the issue of 'reproductive isolation'. How can we determine the species boundaries of animals if we have no data on their reproductive habits or capabilities? Another issue is that occasionally, two species might interbreed and hybridize, which would mean they are the same species if one takes the BSC at face value. However, if this is only occasional and the populations seem to represent distinct species in all practical purposes, should we really lump them together? Additionally, many times the offspring of these hybridizations are nonviable or infertile - meaning that they are an evolutionary dead end. What do we do in these cases? This is a conundrum that the BSC has trouble answering.
I have only talked about two commonly used Species Concepts, but there are many more, each with their own pros and cons. I do not have time to talk about all of them in detail, but if you are interested you can find plenty of information about them on the good ol' internet! I hope that this post has educated you a bit more on a very hotly debated topic that we may never truly have an answer for!
A special thanks to my professors in Evolutionary Biology, as their lectures are very helpful for providing good quotes and other useful information!
No comments:
Post a Comment