We often use terms without really thinking about what they mean: “Justice,” for example, or “lost.” Human beings are able to handle a lot of ambiguity without feeling too troubled by it. However, when an entire world view (or the building of a new nation) depends upon particular terms, it is critical to achieve clarity of meaning.
The theory of evolution depends upon being able to account for the “changes” that are said to take place… changes “sweeping” and “significant enough that supposedly one “sort” of organism can (over “significant” time) morph or “branch” into another “sort.” And, as we noted regarding the Peppered Moth example, the theory is not sufficient if all it can “explain” is a change in probabilistic distribution of a trait. Even full-blown “adaptation” within a “sort” of organism still has us talking about one particular “sort” of organism, while evolutionary theory must explain how (and what demarcates) one “sort” of organism “branching” into another.
Biologists are almost universally agreed that the relevant line of demarcation occurs at the species level of taxonomy. So, we must get clear about what a species is and what differentiates one species from another. Ultimately, we are trying to define a “speciation event,” which is purported to be the branching event when one species of organism can truly be said to have produced another species.
Evolutionists believe that there have been countless speciation events during the course of biological time on Earth. The fossil record is supposedly full of the evidences of speciation events, and it is claimed that they are even now happening all around us. However, before we can evaluate such claims, we must first get clear about the terms. In this context it is certainly the case that the terminology matters!
What is a species?
The term “species” is relevant only in a taxonomic hierarchy, which is itself a more or less arbitrary schema for “sorting” and grouping living organisms. While the divide between “kingdom” and “domain” remains contentious among some scientists, and some (cladistic) scientist have abandoned the kingdom division entirely, the widely accepted taxonomic hierarchy today is as follows (with some examples), with the top positions being more general and the bottom positions being more specific.
Domain — the cell type possessed by the organism (widely divided into three types)
Kingdom — plant vs. animal is the most well-known division here
Phylum — general notion of body plan or genetic “relatedness”
Class — mammals vs. birds vs. reptiles, etc.
Order — primates vs. bears, cats, dogs, etc.
Family — hominids vs. other primates, or felidae (cats) vs. canidae (dogs)
Genus — Such as “felis” vs.”pantherinae” among cats
Species — Such as “felis catus” for domestic cat vs. “felis bengalensis“ for the Asian Leopard Cat
Some taxonomies further distinguish the very top of the hierarchy with life vs. non-life, but this division is moot in a biological taxonomy, which already presumes to distinguish among living things.
Using the above taxonomy, contemporary humans can be sorted (Domain down) as: Eukarya, Animalia, Chordata, Mammalia, Primate, Hominidae, Homo, sapien.
The first question one might ask when confronted with such a hierarchy is: How does a taxonomist clearly distinguish between, say, animals in a given level of the hierarchy? In other words, what features of an animal are relevant to each level of the hierarchy from Phylum down?
There are two broads strands of identifying such features: Phenetics and Phylogenetics.
Phenetic classification looks for morphological (form and function) similarities without regard to genetic “relatedness.”
Phylogenetic classification takes as primary genome-mapping techniques to determine genetic “relatedness.”
So, at the level of Phylum, one might look at four-legged vs two-legged body plans and detect an obvious phenetic difference. Or, one might look at genetic “relatedness” and determine that some four-legged creatures are more “closely related” to some two-legged creatures than are all two-legged creatures related to each other (e.g.: humans are more “closely related” to cows than to any bird).
There can be a lot of flux in such systems. Some, for example, place the cat vs. dog distinction at the Order level, while most place that distinction at the Family level. And many taxonomies are rife with almost countless subdivisions, such sub and super Classes, etc.
So, is there in principle anything approaching an accurate classification system? Well, what would “accurate” even mean, and what would motivate a certain sort of accuracy?
Let’s start with the motivation question. Taxonomy as a discipline can be traced back to at least Aristotle. The great botanist, Linnaeus, produced the most rigorous and formalized taxonomy of anybody up to his time. And this taxonomy was highly regarded until Darwin came along, at which point things were shuffled around in the discipline of taxonomy to “better fit” the Darwinian theory about what the tree of life would look like. Even now, the divide between Phenetic and Phylogenetic taxonomies results from a recognition that “similarities” of all different sorts can be used to classify in all sorts of different ways, with all of those ways having no objective relation to “relatedness.” The motivation behind Phylogenetic classification schemes is to bring some objectivity to “relatedness” and “distance” based upon genetic mapping, such that “divisions” between “sorts” of organisms derive from how similar are their genetics.
Almost certainly, the most popular form of Phylogenetic classification today is called “cladistics,” from the Greek “klados,” or “branch.” Virtually all Phylogenetic relations are structured and represented as trees, but what makes cladistics unique is its focus upon genetic similarities that exist between what are thought to be “peer” branches of a tree and that are shared by a purported common ancestor, and where the similarities in question are not possessed by presumed earlier ancestors or by non-peer branches. Cladistics, then, is presumed to be “more accurate” in a crucial way that is driven by the crucial motivation. The motivation is to discover the actual “descent” that is represented by all “trees of life.” So, “accuracy” in this context just means discovering the genuine genetic relations between organisms, from which it can be “accurately” inferred what the “lineage” of ancestors has been for any particular organism.
By focusing on the similarities between “branches” that are not shared by “earlier ancestors” than the most recent branching point, cladistics promises to accurately portray what are the real branches in taxonomy. Today, computers are able to process the vast amounts of genetic-characteristic data necessary to search for and catalog genetic similarities. And as the genomes of more and more creatures are mapped, there is more and more genetic data for the computer to chew on and thereby find more and more similarities (and dissimilarities).
Now, technically, modern cladistics employs both Phenetic and Phylogenetic data. However, there is no doubt that Phylogenetic data carries the most weight in classification, simply due to the fact that there is more perceived objectivity to saying, “This gene sequence is the same sequence in these two organisms,” than to say, “These two organisms look a lot alike.”
However, this objectivity is a chimera, as all literature on the subject will admit in one form or another. One Wikipedia page offhandedly says it this way: “Different datasets and different methods, not to mention violations of the mentioned assumptions, often result in different cladograms. Only scientific investigation can show which is more likely to be correct.” And one is quite right to immediately ask the obvious follow-up question: What would the nature of this “scientific investigation” be? Presumably it is scientists who are producing these divergent cladograms. Notice that a lot of weight is put on the phrase “more likely to be correct.” You can read this Wikipedia page for an overview of cladistics and divergent results from different “scientific investigation.”
All of this discussion has been an effort to get clear about what a species is, and you are already seeing that there is no simple answer to that question! Indeed, even certain modern taxonomies (such as cladistics) do not map neatly onto the classical “… family, genus, species” taxonomy at all. Cladistics talks in terms of larger and smaller “clades” rather than “species,” and the characteristics of differentiation are different between classical taxonomy and cladistics.
Nevertheless, the vast majority of evolutionary biologists still talk in terms of “species,” and “speciation events” are indeed the holy grail of evolutionary evidence. But, with all the talk we’ve heard about “probabilistic distributions of traits” and, now, “genetic mapping to discover clades,” and so forth, evolutionary scientists seem to be moving away from “clear, bright lines” between species. So how can “speciation events” even be relevant to evolutionary science anymore?
More rocks and hard places
Rock: evolutionary scientists need to demonstrate an unbroken continuum of life, where any “sort” of creature could in principle “branch” or morph into any other sort of creature (given enough time).
Hard Place: evolutionary scientists need to demonstrate that some sort of “hard break” between “sorts” of creatures does actually occur, because if in the short term we are not seeing significant enough changes to indicate things like: “This lizard is becoming a totally different kind of lizard,” then the inference that these “significant changes” could accumulate a LOT over time is greatly weakened!
So, cladists seem content to sit on their “rock,” smiling knowingly at what they perceive as a grand continuum of life. However, most evolutionary scientists recognize that the “rock” cannot be all there is to taxonomy, because it is obvious that elephants are a very, very, VERY different “kind” of creature than are chimpanzees (or human beings, for that matter). So, even as cladistics proposes “branching characteristics” that could in principle have produced a very different tree from the one(s) that are now highly regarded as “accurate,” nevertheless, everybody knows that some of those branches simply had to be “hard breaks” of some sort that ultimately produced a “lineage” of creatures that were “evolutionarily isolated” from their “peers” and from their earlier ancestors (i.e.: the “hard place”).
The point we are reaching is that taxonomy is often presented to the public as “objective evidence” of evolution, and cladistics presents to the public all sorts of claims about “evolutionary distance” and other such notions. But all of taxonomy is arbitrary. All of it is people focusing upon this or that “similarity” between diverse things and thus employing this or that “similarity” to “group” and “classify” things. The point is that all classification is a matter of psychological projection! Cladistics is the newest classification fad, but it is in reality no more objective than have been all other biological classification “techniques.”
And, regardless of all classification techniques, scientists still recognize and acknowledge that we should observe “hard breaks” between (for lack of a better term, and because scientists still use it) species! We should observe genuine speciation events, where we observe one “sort” of organism literally branching (a la cladistics or otherwise) into another “sort” of organism. And that notion of “sorting” trumps all forms of classification, because it alone indicates when genuine (of the sort that matters) evolution is happening!
So, the really relevant question of sorting and classification is: When can we say that one organism really has branched into another organism? There is no point in talking about “evolutionary distance” between organisms as a classification modality until it can be demonstrated that any organism can in principle have any “evolutionary distance” from any other organism! And to demonstrate that, you must demonstrate speciation events. Thus, the question of what a species is really has far more weight than just “where does ‘species’ fit into this or that classification scheme?”
Four concepts of “species”
There are four widely-accepted approaches that scientists have used to distinguish between species (in the evolutionarily relevant sense). Thus, these are the four concepts that might underlie perceiving a speciation event.
Taxonomic or Phenetic (Folk) morphological resemblance — Crudely, the notion that you can tell species apart by looking at them. This includes intuitions from Biological Speciation (dogs beget dogs, etc.). The taxonomic approach can take more into consideration that strictly morphology (as we saw in our discussion of taxonomy above). But morphology plays heavily into the quite arbitrary divisions between the “levels” and what creatures exemplify the “levels” of all taxonomies.
Ecological — Based on ecological niches or “adaptive zones.” Darwin’s Finches are a classic example of “speciation” based upon bill/beak size and shape, in turn supposed to result from “the same” finch species branching out into different ecological niches containing different sorts of nuts and seeds.
Phylogenetic — Based on the smallest evolutionary unit that can be discerned, the smallest cluster of organisms that possesses at least one “diagnostic characteristic.” This characteristic may be morphological, biochemical, or molecular, and must be fixed in reproductively cohesive units. (This would constitute a subset of reproductively compatible units under the Biological Species concept, since not all interbreeding individuals need to have the Phylogenetic character of the Phylogenetic Species. In other words, the Phylogenetic Species Concept would recognize as “different species” many organisms within one Biological species, just as we see different “breeds” of cats within what we believe to be one Biological species.)
The individuals of a Phylogenetic Species need to have either: A) descended from a common ancestor; or: B) the property of being more closely related to each other than to any other organisms. (See: Baum, D. 1992. “Phylogenetic Species Concepts,” Trends in Ecology and Evolution, 7:1-3. From this TalkOrigins article.)
Biological — Based on natural interbreeding and producing fertile offspring. This is the most theoretically prominent notion of “species” today. Thus, a “species” is a “reproductive community.” The notion of interbreeding within an ecological niche is often included. However, the Biological Species Concept (BSC) overarches the Ecological Species Concept (ESC) in any case that it might contain the ESC. Thus, the ESC might only be employed within the BSC to provide some account for how a particular species became “reproductively isolated” from its parent species. Examples like so-called “ring species” demonstrate the ESC working within the BSC, as we will further discuss.
Critiques of these species concepts
Taxonomic — This is obviously too crude, “folk,” and ambiguous to count as scientifically rigorous.
Ecological — This tells us only about population distributions; it does not distinguish between individuals in fine-grained fashion to tell us what, exactly, about individuals differentiates them into genuine “kinds.” Worse, the ESC can be neatly subsumed into the BSC and is best employed as a device to explain how ecological niches can eventually produce reproductive isolation, which just is the BSC.
Phylogenetic — This is a practically useless delineation, undermined by the service of the Biological Speciation Concept (BSC). Not only is it so fine-grained that we can see “speciation events” everywhere, even in what are clearly not true speciation events, but it offers nothing to explain the true “hard break” between “kinds” that evolutionary theory must presume happens frequently. As a taxonomic modality, it has sustained the “comfy rock” some evolutionists (such as cladists) occupy. But it provides nothing needed by those evolutionists that rightly recognize that they are in a “hard place” unless they can demonstrate genuine speciation events widely recognized as the sorts of “hard breaks” that could truly lead to such diversity as elephants and human beings.
Biological — The others are so troubled that the BSC has become the leading species concept. However, it is not without troubles of its own.
It doesn’t apply to asexual species of animals.
It doesn’t apply to self-pollinating plants.
It seems to be “inapplicable in practice”–The BSC does not provide any practical way to actually delimit species in the field. Imagine 1,000 lakes containing bluegill. Are all these bluegill of the same species? It would take 499,500 separate crosses to determine interbreedability. But the crosses should be reciprocal (male to female, and female to male), needing a total of 999,000 crosses. There should be replicates, say three, bringing the total up to 2,997,000 crosses. But not just any two bluegill in a bucket will mate; in nature, bluegill create and defend nests, and females chose mates. So, we must simulate a colony for each test; this would require, say, 20 fish. So, we would need 60,000,000 fish for our tests! But, there are actually many more than 1000 lakes containing bluegill…. So, in practice, it is impossible to determine if “bluegill” constitute “a species” under the BSC.
Breeding in the wild is influenced by many environmental factors. Thus, just because we can’t get the critters to breed under experimental conditions doesn’t indicate that they can’t interbreed. Difficulties in breeding captive Pandas indicate the extent of this issue. So, the BSC seems at best inconclusive.
There must be “persistence” of reproductive isolation, and this is virtually impossible to demonstrate!
In practice, if there is apparent Phenetic Speciation, it is assumed that there is Biological Speciation, unless a reproductive barrier is demonstrated. Some, notably cladists, argue that sexual compatibility is a primitive trait, so that organisms that are no longer genetically closely related may retain the capacity to interbreed. So, since interbreeding is not a derived characteristic, it is an invalid characteristic upon which to distinguish between species.
Finally, groups not occurring together in time cannot be evaluated. The BSC cannot distinguish between species when any one of the species to be compared is extinct!
The only game in town
With all of its problems, the BSC still rises far above the other contenders. For one thing, it remains the most rigorous and broadly-applicable (even with its limitations regarding supposed evolution among organisms that do not “breed” to reproduce). Most importantly is that the BSC is the one speciation concept that can in principle demonstrate “hard breaks” occurring among species, such that reproductively isolated species can indeed be said to be “different kinds” of organisms.
Indeed, the BSC is so widely accepted as “the only game in town” among species concepts, that you will find that every published example of a supposed speciation event appeals to reproductive isolation as the primary evidence that a speciation event has taken place.
So, going forward into next week’s topic, we will presume exactly what the vast majority of evolutionists presume: the BSC is the best way to distinguish between species and detect speciation events (whenever it is in-principle appropriate to do so). Thus, we will proceed by examining a variety of famous speciation events among animals that reproduce sexually, and we will do so in light of the BSC. We will see if the claims of true speciation events can be sustained.