Biological species and related concepts

This post is part of a series on species. Read the previous parts here and here.

The Biological Species Concept (BSC) is perhaps the most famous of them all. In science, it is one of the things evolutionary biologists Theodosius Dobzhansky and (in particular) Ernst Mayr are best known for. It is basically what most non-scientists would also come up with when asked what a species is, unless they have never given the topic a thought before: species are breeding groups, i.e. groups of individuals that form a reproductive community.

Many colleagues will tell you that the BSC is very popular with zoologists but not so much with botanists, supposedly because it works well with animals but not so much with plants. And indeed I know many well accepted plant species that can interbreed, at least potentially. But that is of course circular reasoning: should they then really be separate species? Maybe botanists are simply splitting species too much.

On the other hand, the idea that some interbreeding or even potential interbreeding already makes two populations conspecific may be a caricature of the BSC as promoted, for example, by Coyne & Orr's book Speciation (which, by the way, is a fantastic resource for those interested in species); really the BSC is often understood to allow for some rare gene flow between biological species.

Botany picture #52: Spathicarpa sagittifolia

Spathicarpa sagittifolia (Araceae) from the Botanic Garden of Göttingen, 2008. The predominantly tropical Araceae are a group of monocots with strongly reduced flowers arranged in a spike. This spike is then usually subtended by a large bract called a spathe which often serves to attract pollinators as a pseudo-petal. Just like most people think that the daisy head is a flower so they also would usually call this complex Araceae-inflorescence of many flowers a flower. Many Araceae look pretty bizarre and alien, and this is one of the strangest I have seen so far. The spike is here fused to the spathe.

What is that "we" incompatibilists are talking about?

Before I continue with writing about species, I would like to address very concisely something that pops up every few weeks or so in a part of the blogosphere that I read: the controversy over determinism and free will. The occasion is a recent post by Jerry Coyne, who linked to a study showing that certain decisions are made unconsciously several seconds before we become conscious of them to argue that, and the following word is where the crux lies, "we" do not really make any decisions at all:

The implications of this research are obvious: by the time we’re conscious of having made a “choice”, that choice has already been made for us—by our genes and our environments—and the consciousness is merely reporting something determined beforehand in the brain.

To summarize the determinism vs free will issue, there are three main positions. The first is held by many religious believers, Cartesian dualists, people with an esoteric bend etc. They believe that in addition to the material world following its physical rules there are souls or spirits or whatever immaterial components to our mind (for the sake of simplicity I will subsequently simply speak of souls), and that those souls are somehow transcending the cause-and-effect of the material world.

Botany picture #51: Cardamine pratensis

Cardamine pratensis (Brassicaceae), Germany, 2008. I am not overly fond of Brassicaceae. Although the family is economically very useful - cabbage, broccoli, rapeseed, radish) - and provides with Arabidopsis thaliana the most important model organism from the plant kingdom, their flowers are pretty boring, and they are one of those families that are generally hard to identify without fruits. In addition I unfortunately hate cabbage with the exception of red kraut. The spring flowering species above I always found quite attractive though.

Tidbinbilla

We made a family trip to Tidbinbilla Nature Reserve today. It is a superb destination for a day trip from Canberra, especially if you are interested in seeing wildlife.

Part of the Nature Discovery Playground; several barbecues are available at the site, and it was quite full with families having lunch.

Hanging Rock peeking out of the Eucalyptus forest. Yes, the rock is as big as it looks next to that tree.

I have never before seen so many turtles as today at the Sanctuary wetlands of Tidbinbilla. They were constantly popping up to take air, then diving back in. Some of them looked at us curiously as if they were expecting to be fed, however.

And another cute little reptile! Presumably a red-bellied black snake?

Finally, something botanical: a frond of Pteridium esculentum, the native bracken, breaking through the bitumen of a footpath. Unfortunately, most of the plants that were in flower were introduced weeds. Autumn is really starting here.

Classifying the species concepts

This post continues a series on species that began with this introduction.

So looking over the literature and the aforementioned list of 26 species concepts, let's try to classify the concepts into different groups:

Synchronous concepts
    Reproductive community concepts
            *Compilospecies
        Biological species
        Cohesion species
        Genetic species
        Genic species (?)
        Recognition species
        Reproductive competition species
    Phenetic and cluster concepts
        Autapomorphic species
        Ecospecies
        Genotypic cluster
            *Agamospecies
        Morphospecies or typological species
        Phenospecies
    Phylogenetic concepts
        Phylogenetic species
        Genealogical concordance species
Asynchronous concepts
    Internodal species
    Composite species
        *Successional species
Incertae sedis
    Evolutionary species

The first division, indicated by bold font, is whether the concepts deal only with species in the here and now or, more generally speaking, with those existing together in one time-slice (synchronous) or whether they have a historical dimension (asynchronous). Note that the latter are rarely of interest for anything beyond theoretical discussion unless we are talking about a group with an excellent continuous fossil record. I am not sure where to place the Evolutionary Species Concept because I may not quite understand it. It sounds a bit like a reproductive community seen from an asynchronous perspective, so I have placed it apart for the moment. (Similarly, I previously understood the genic species to mean something different from how it is described in the list I linked to, so there is a question mark behind it.)

Within the synchronous concepts that deal with delimiting and circumscribing species in the present we have a second division, indicated in italics, where the concepts are grouped by their main grouping and, if defined, ranking criteria. The first group is what we might call the family of the biological species concept, different concepts that attempt to delimit species by the ability or propensity of individual organisms to interbreed. The second group defines species via some kind of character combination or overall similarity. In all these cases, species are groups of individuals that are ecologically, morphologically or genetically more similar to each other than they are to individuals belonging to any other species. In contrast to this essentially phenetic approach, the small third group employs some kind of phylogenetic approach to species circumscription.

I have left out several concepts that are silly or too similar to others. The ones that are further indented and marked with asterisks are also a bit dubious as they appear to represent merely special cases of the concepts directly above them. Even having reduced the number already, it will still be useful to lump them further and discuss, for example, all reproductive community concepts together. On the other hand, it appears useful to take a more granular approach to the phenetic concepts - but that may just reflect my personal biases.

Botany picture #50: Eryngium campestre

Eryngium campestre (Apiaceae), Germany, 2007. The Apiaceae are best known for the carrot and all the spices they provide - fennel, parsley, caraway, anise, etc. - and most of them look kind of the same: deeply divided leaves and small usually white or sometimes yellow flowers arranged in a double umbel. But there are a few more unusual groups that a beginner in botany would not even recognize as part of the family. The genus Eryngium is one of those - many of them look more like thistles, and the South American representatives look much like monocots. Unfortunately I do not have good pictures of the latter, so here is a European representative.