We had some good discussion on the thread about elephants yesterday, and by “good” I don’t mean “everyone agreed,” because they didn’t. I was pleased to see people exchanging and defending their divergent views.
The quarter starts today, teaching looms, and I don’t have a lot of time to write this morning, but I’d like to mount a brief defense of the biological species concept (BSC). Today I’ll just explain what the BSC is and state what I consider to be the “problem of speciation,” recognizing that not everyone will agree. Later today, or tomorrow, I’ll explain why I think the BSC is the best concept to address that problem.
The BSC defines (or rather conceptualizes) species as groups of interbreeding individuals that are separated from other species by reproductive isolating barriers. By “reproductive isolating barriers,” I mean genetically based traits of organisms that prevent them from exchanging genes with members of other species. (Gene exchange can occur only when members of two different groups can form fertile and viable hybrids).
These barriers can include genetically based ecological differences that limit species to different habitats, so that they never meet and thus can’t mate; “sexual isolation”, the very common tendency for individuals to court and mate preferentially with members of their own species; the inability of pollen from one species of plant to grow down the stigma of members of another; the preference, when a female is multiply inseminated by a member of her species as well as a member of another, for using the same-species sperm to fertilize eggs; and the various forms of “postzygotic isolation” that prevent gene flow after members of different species have already mated and formed fertilized eggs. These last forms include the inviability of hybrids or their sterility (the mule, a hybrid between a donkey and a horse, is a classic example of hybrid sterility).
All of these reproductive barriers keep members of different species from exchanging genes, that is, the barriers maintain the integrity of species. That doesn’t mean that they evolved to maintain the integrity of species. In most cases they didn’t. As Allen Orr and I show in our book Speciation (from which all of these ideas are taken), in nearly all cases reproductive isolating barriers (RIBs, a good Chicago acronym), are the accidental byproduct of divergent evolution between populations.
For example, two geographically isolated populations (don’t mix up geographic isolation with reproductive isolation!), may evolve by natural selection to adapt to different habitats. When that divergent evolution has proceeded sufficiently far, the genomes of the different “populations” may have diverged so much that they can’t work well when combined in a hybrid individual. The hybrid could then be inviable or sterile, a form of reproductive isolation that prevents gene exchange. As this example shows, such barriers can simply be “accidents”: the byproduct of what happens when populations evolve along different paths.
I think the existence of species, in most cases, simply reflects these accidents of evolution. These discrete groups of organisms (see below) are simply what transpires when geographically isolated populations evolve away from each other. And their discreteness then becomes evident when those newly-evolved species come back to coexist in the same area. (For truly, one can see the discreteness of species only when they’re living in the same place. That gives a clue to the connection between the species problem and the BSC.)
In a few cases, however, natural selection can directly favor the production of discrete groups. One of them is “reinforcement”, which we’ve discussed before, in connection with my student Daniel Matute’s recent paper. The other, of interest mostly to evolutionists, is sympatric speciation.
If you read Speciation—and I realize that most readers haven’t—you’ll see that we have extensive discussion about alternative species concepts, and of the problems of both those concepts and the BSC. (By the way, if you’re at all interested in speciation you should read the book. I don’t like sounding like Chris Mooney with incessant repetitions of “read my book,” but I’m quite proud of it. It took Allen and me six years to write, and involved reading hundreds and hundreds of papers.)
I know that the BSC can’t extend to asexual organisms, and in many cases is somewhat subjective. What do you do, for example, if two groups have just a limited amount of gene exchange? What about if two populations occur in different areas, like the elk and the red deer, so there’s no possibility of them encountering each other—something that is almost required to determine whether they can exchange genes? (You can force-mate them in zoos, of course, but that’s a one-way test. If they do produce fertile hybrids in a zoo, that doesn’t tell you that they’d do so in nature, for the enforced confinement of a zoo may break down reproductive barriers that would operate in the wild. But if they do produce inviable or sterile hybrids in a zoo, like the Indian and African elephants, that tells you that they’re almost certainly biological species.)
But let’s put this aside for the nonce. I’d recommend reading Chapter 1 and the Appendix of Speciation if you’re interested in species concepts. (You don’t have to buy the book: many libraries have it.) Many of you might not agree with our take, but at least read it before you belabor me for not considering the problems of the BSC and the “advantages” of other species concepts.
Why I like the BSC is because it has a natural connection to what I—and many early evolutionists at the beginning of the “Modern Synthesis“—consider the “big species problem.”
The problem is to explain why nature is discontinuous rather than continuous. Why, in one patch of forest, do all the birds, insects, and mammals (and yes, most of the plants, too!) fall into a limited number of discrete, objectively recognized categories? Nature is not a continuum, with blackbirds blurring into starlings blurring into sparrows and so on. If that were true, what good would Peterson’s bird guide be? Nature—at least that moiety of nature that reproduces sexually—is discrete.
This problem was stated at the very beginning of the book that is widely regarded as having launched the modern synthetic theory of evolution, Genetics and the Origin of Species (1937), written by my academic grandfather, Theodosius Dobzhansky:
Theodosius Dobzhansky (1900-1975) at the microscope. He looked at flies until the day he died.
Right at the beginning of this book (and I recommend the first edition to budding evolutionists) Dobzhansky sets out the “species problem”. On the first page he describes how many species there are on earth (he estimated 822,765 at that time!), and then, on the second page, is a section called “Discontinuity.” I quote at length, because this is perhaps the best existing statement of the problem of speciation. (Note, by the way, what a splendid writer Dobzhansky was. And his native language was not English, but Russian!)
Organic diversity is an observational fact more or less familiar to everyone. It is perceived by us as something apart from ourselves, a phenomenon given in experience but independent of the working of our minds. A more intimate acquaintance with the living world discloses another fact almost as striking as the diversity itself. This is the discontinuity of organic variation.
If we assemble as many individuals living at a given time as we can, we notice at once that the observed variation does not form a single probability distribution or any other kind of continuous distribution. Instead, a multitude of separate, discrete, distributions are found. In other words, the living world is not a single array of individuals in which any two variants are connected by unbroken series of intergrades, but an array of more or less distinctly separate arrays, intermediates between which are absent or at least rare. Each array is a cluster of individuals, usually possessing some common characteristics and gravitating to a definite modal point in their variations. . .
Dobzhansky then goes on to talk about the discreteness of cat species (bless his heart), and shows that there’s no continuum between house cats, lions, and other felid species. Cat species are discrete, and any cat from nature is easily recognizable (that’s why field guides work!). He continues:
What has been said above with respect to the species Felis domestica and Felis leo holds for innumerable pairs of species, genera, and other groups. [JAC: I’d maintain that Doby was wrong about groups above the species level: genera, families, etc. are not as discrete, or as objectively recognized, as species.] Discrete groups are encountered among animals as well as plants, in those that are structurally simple as well as in those that are very complex. Formation of discrete groups is so nearly universal that it must be regarded as a fundamental characteristic of organic diversity. An adequate solution of the problem of organic diversity must consequently include, first, a description of the extent, nature, and origin of the differences between living beings, and second, an analysis of the nature and the origin of the discrete groups into which the living world is differentiated.
(My emphasis.) Although students no longer seem to be interested, or even aware of, this fundamental problem, it was something that preoccupied the founders of the modern synthesis, including Dobzhansky and Ernst Mayr.
As we point out in Speciation, there is no “best” species concept. Different species concepts are useful for different purposes. But I contend that the biological species concept is the only one that enables you to tackle, and ultimately to solve, the “species problem” described by Dobzhansky. It’s the only one you can use to address the problem of why animals and plants in one area fall into a finite number of discrete groups.
In the next (and final) post on this, I’ll explain why.
On the other hand, if one were to collect all living organisms and all of their extinct ancestors, one would observe a continuum of variation. Thus the species problem only applies to extant organisms, right? Are there such things as fossil species, by the BSC?
Yes, obviously as you go back in history the temporal sequence of forms will blur and come together. But one can try to delineate species among fossil assemblages living in one place at one time: look for discrete, easily recognizable forms in sympatric assemblages.
I would argue not, or rather – we cannot find evidence for it. Most paleontological species descriptions are based simply on morphological characters. I agree with Jerry about the importance of the BSC (partly due to my zoological prejudices, no doubt), but – like Jerry, as we will see in the next part of this post, I presume – I recognise that it is difficult (= nigh impossible) to apply it either to fossil taxa that occurred contemporaneously, or to different taxa that are temporally separated (eg humans and our hominin ancestors). I’m not worried in the slightest about the different definitions of “species” that people use – if you asked a range of biologists for a definition of “gene” you’d come up with some very different approaches. The things still exist, though.
Thanks for expanding on this!
Is there any chance that there’s some way to attach a number to the concept?
That is, is there any way to quantify that the two groups of elephants from yesterday’s discussion are N units differentiated whereas each are N × x units differentiated? Perhaps using multiple axes, multiple measurements?
Cheers,
b&
Oh, for a “preview” button!
Strike that last paragraph for this, please:
That is, is there any way to quantify that the two groups of elephants from yesterday’s discussion are N units differentiated from each other whereas each are (approximately) N × x units differentiated from giraffes? Perhaps using multiple axes, multiple measurements?
I suppose I should expand on that a bit.
We can (for the most part) identify the number of generations between two individuals, or the average between two populations. Permit me to invent a unit, the Dawkins, to represent this figure. Of course, since different species often have different temporal generational intervals, it needs to be a dual-valued number. Pulling numbers out of my ass, let’s suggest that humans and chimps are about 600 Kilodawkinses distantly related.
We can also identify the degree of genetic differences between individuals, though I don’t know how one would reduce that down to one or two numbers. But permit me to handwave a bit and suggest we measure this in Venters.
Then there’d be the odds of successful reproduction, both theoretical (in the lab) and actual (in the wild). If the Coyne number is 1, the two populations can and regularly do successfully interbreed (such as between Russians and Poles; if the Coyne number is 0, it’s even theoretically impossible (such as between tigers and tiger lilies). As with Dawkinses, this would have to be a composite measurement, for both theoretical and observed interbreeding.
Finally, one could create suitable scaling factors and plot organisms and populations in a multi-dimensional space. The distance between points in that space would unambiguously identify the degree of relatedness and of speciation.
Cheers,
b&
Random (non-biology) nit picky question: was Dobzhansky’s native language Russian or Ukrainian? I’m sure that he might have spoken fluent (native) Russian, but if his family was Ukrainian, might his first native language not have been Ukrainian?
Considering when and where he was born, I wouldn’t at all be surprised if he spoke three or more languages fluently before he was ten. Which of those would have been his “first native language”?
I also wouldn’t be surprised if he could communicate, if not fluently, in a dozen or more languages by the time he finished his formal education.
Cheers,
b&
Dobzhansky was born in Nemirov in 1900, now in the western part of the Ukraine, at the time part of Russia. Dobzhansky, in his writings, referred to himself as Russian, and historians have concurred in his self-identification. It is possible that Dobzhansky used ‘Russian’ in a more inclusive sense (including ‘Little Russia’), a not uncommon usage in his time. I don’t know if he spoke Ukrainian or Russian at home as a child. There are probably a few people still alive who know, and it is possible that Dobzhansky commented on it (in a source I haven’t seen).
@Ben–Sorry, the “first” was confusing. By first native language I meant primary native language (when someone has more than one native language but identifies primarily with one). A person can of course have more than one first/native language, making them bilingual or multilingual. I’m guessing this was the case for Dobzhansky. I’m not particularly well versed on the sociolinguistics of speaking/learning/using Russian vs. speaking/learning/using other languages in Eastern Europe during the late Russian Empire/early Soviet Empire, nor do I know Dobzhansky’s background well enough to make any educated guesses, but it’s certainly possible that he grew up with Ukrainian as a primary language (even if he was a fluent native speaker of Russian since birth).
@Greg–My family refers to itself as Russian, even though we technically came from what is now Ukraine. It’s not clear to me if my ancestors culturally identified as Russian or Ukrainian (or some mix of both). It seems to me that much of Eastern European slavic culture and identity was often glossed as simply being Russian. As an adult, Dobzhansky truly might have seen himself as being specifically Russian (and not, at least primarily, Ukrainian) in identity. It also seems possible that he might have identified as culturally Russian while still speaking Ukrainian as a primary language (even if he was also a native speaker of Russian). I was just curious if we had any data to point one way or the other.
From the Royal Society obituary by E.B. Ford:
Ford, who knew Dobzhansky personally, also notes that he spoke Russian, German, English, French, Spanish, and Portuguese (the first three fluently).
“It took Allen and I six years to write …”
Am I the only one left nowadays who winces when reading such a phrase? You wouldn’t say “It took I six years to write”, and adding “Allen and …” doesn’t change things. It should be “It took Allen and me six years to write” just as it is “It took me six years to write”. Sorry for the pedantry!
Yes, you’re right. I fixed it, thx.
As the years pass and genome sequencing becomes faster and cheaper I expect the situation regarding species to undergo some degree of change. Separate species being amalgamated into one, a single species being split into two or more. Systematics will become a matter of not just the genes alone, or morphology alone, but a blend of the two, with other elements playing a role. For example, combining dogs, wolves, and coyotes into one species, with dogs and coyotes sub-species of wolf. On the other hand, we may wind up with as many as five species of chimpanzee, with the common chimp having three sub-species.
Note that this is all speculation on my part, with the final answer awaiting the sequencing of the genome of each of the current sub-species of common chimp plus the bili ape population because of their unique features.
The field of systematics is going to be going through quite a few changes.
I don’t think so. Your post implies that the degree of sequence divergence, or morphological divergence, will accurately predict the degree of reproductive isolation and therefore species boundaries. We know enough to say that it won’t. The key is not overall genomic differences but difference in key characters that mediate pre- or post-zygotic isolation. Finally, given how speciation works, we would expect to find, and do find, some sets of populations for which the question, “Do they represent one species or two?” HAS NO ANSWER – that is, their degree of isolation is partial and quirky (population A interbreeds with B, B with C, but not A with C, etc.).
I find eukaryotic speciation a fascinating topic. I’m a virologist and sadly have limited knowledge of the topic.
There are obviously many speciation mechanisms of which I suspect I can imagine only a few. I’m surprised that no one has mentioned that in theory a point mutation is sufficient to create a new biological species. On the other hand it’s been mentioned that there are cases where widely divergent populations readily interbreed.
I’m curious about prezygotic isolation and why it’s common in some species and not in others. Dogs for example are promiscuous wheras some drosopholids are highly discriminating. Is there an example of related taxa where one branch is promiscuous and the other fastidious?
Doesn’t Darwin’s finches mitigate this somewhat? Is it a matter of degree? Different species with different beaks to eat different foods? Granted, it is not continuous but baby steps?
So, if organisms can mate, and produce fertile offspring in the lab but don’t do it in nature, you count them as separate species?
This is a suprise to me. In “the Ancestors Tale”, Richard Dawkins gives some such examples among insects and identifies them as different “races”, not species, and then he detours to a discussion on what a “race” is. Of course this wouldn’t hold true for human ethnic groups, because their interbreeding is not limited to any “experimental” situation.
their interbreeding is not limited to any “experimental” situation.
Perhaps you just lack imagination?
Insight: So, if organisms can mate, and produce fertile offspring in the lab but don’t do it in nature, you count them as separate species?
I would, without question. Lots of species that are not even very closely related can be crossed under controlled conditions where their normal isolating mechanisms are bypassed.
In orchids, trigeneric hybrids have been made by artificial pollination. Elaborate and highly specific pollination systems are what keep them apart in nature, but those can be bypassed by a person with a fine brush.
A male donkey crossed with a female horse produces a fertile female hybrid called a jenny. But this cross cannot happen in nature because male donkeys are just too short. They need some help.
I know that the BSC can’t extend to asexual organisms, and in many cases is somewhat subjective. What do you do, for example, if two groups have just a limited amount of gene exchange? …
But let’s put this aside for the nonce.
Great post, but once again: hardly anybody I know doubts that the BSC is an ideal, very satisfying concept. It is just hard to set aside what I encounter in every genus I have studied in my admittedly young career. Limited gene flow instead of a none / free interbreeding binary system is what seems to happen a lot (in plants). And I also do not see why using a concept that allows limited gene flow as long as it is not enough to muddle up the ecological and morphological distinctness of the two species is a problem for addressing the species problem. It is nearly the same, just allowing more fuzziness. My point is simply that there are too many grey zones here for what the BSC envisions – both over time on the way through incipient speciation and in the number of cases throughout the diversity of life.
I am looking forward to the next post, but will be away on a field trip, so I will unfortunately only see the discussion after it is over.
Alex: Great post, but once again: hardly anybody I know doubts that the BSC is an ideal, very satisfying concept. It is just hard to set aside what I encounter in every genus I have studied in my admittedly young career. Limited gene flow instead of a none / free interbreeding binary system is what seems to happen a lot (in plants). And I also do not see why using a concept that allows limited gene flow as long as it is not enough to muddle up the ecological and morphological distinctness of the two species is a problem for addressing the species problem. It is nearly the same, just allowing more fuzziness. My point is simply that there are too many grey zones here for what the BSC envisions – both over time on the way through incipient speciation and in the number of cases throughout the diversity of life.
I agree with that 100%. I work on plants too and as a result have become very eclectic in my approach to species concepts. Annuals seem to be more like animals in that they seem to develop isolating mechanisms rapidly and the BSC works pretty well many times. Trees and shrubs just don’t seem to lose much by mixing it up with differently adapted species, so they’re seemingly very slow about building walls. VERY different plants can be completely interfertile and might be kept separate only by the limited amount of “hybrid habitat” where the numerous hybrid seeds produced every year can grow. Edgar Anderson published a lot on this back around 1949.
I’m always happy when I find sympatric taxa that don’t/can’t cross for some reason. But if two species are just kept apart by ecological factors such as soil type, or simple physical isolation, I’ll take it. If they’ve diverged and are operating differently, they’re species in my view. BSC is great, gold standard, when you can use it — but often no help at all, IMO.
Dr. Coyne;
I have read ‘Speciation’. An excellent book. The amount of research that it sparked is in itself amazing.
The only review I could find was by James Mallet http://www.ucl.ac.uk/taxome/jim/pap/mallet_coyne_rev05.pdf called ‘Speciation in the 21st Century’ which makes a continuum case. This concept of discrete and continuous goes all through biology, the concept of modularity, whether you want to look at how species evolve (how does gene flow shut off) or what happens while species evolve (while gene flow is still posssible). BSC seems fine for me for most diploid cases. Isolation seems, at least, a necessary condition. A sufficient condition, that’s open. The time for complete isolation of species can be greater than climatic time and in some cases geological time so there may be a period of introgression and maybe repeated periods of introgression and isolation before full speciation. Whether some gene flow is also necessary for speciation? An open question and my take on Mallet’s critique.
I’m enjoying this thread & the elephant one. Interesting. So…
Questions from a beginner Jerry:
Does the notion of species have value in the microbe world ?
How does one use the species concept in any life form that isn’t always reproducing sexually ?
Peace’n’Love
Michael Fisher
MrLoki: How does one use the species concept in any life form that isn’t always reproducing sexually ?
BSC doesn’t work in those cases and one of the lineage concepts would be better, or even just a plain old morphological concept. Rubus (blackberries)m Hieracium (hawkweeds) and Taraxacum (dandelions)are notorious for having numerous “taxa” composed of asexual lines. Some have treated each of the lines as separate species. Madness lies down that path, IMO. Personally, I’d prefer to treat each asexual (clonal) line as an individual, since the separate plants of each line are all genetically identical. I’d unite the lines under one species based on shared aspects of morphology, genetics, and ecology, plus common ancestry (maybe). Sometimes there are 6 reasonable ways you could do something and so you just have to make a decision and tell people why you decided as you did.
Dan Janzen wrote a wonderful paper back in the 70s on dandelions and how we might think about them as individuals. I don’t have the ref. before me but will dig it out if anyone cares. That paper influenced my thinking about this a lot.
Found ref. to that old Janzen paper:
Janzen, D. H. (1977): What are dandelions and aphids? – The American Naturalist 111, 586-589.
@Achrachno
Thank you !
regards ~ Michael
But – if BSC alone gives discreteness (“the connection between the species problem and the BSC”), it follows that most populations or the exceedingly dominant part of “the living world” is “a single array of individuals” et cetera!?
Perhaps Dobzhansky overstated?
The very embodiment of “time flies”!
No, not overstated, the array of distinctly separate arrays still exists. Perhaps better characterized as skewed the perspective?
Jerry writes:
“This problem [the fact of discontinuous species] was stated at the very beginning of the book that is widely regarded as having launched the modern synthetic theory of evolution, ‘Genetics and the Origin of Species’ (1937), written by my academic grandfather, Theodosius Dobzhansky.”
Jerry’s passage might suggest (I’m not sure that he is actually suggesting this) that the problem of discrete species arose for evolutionary theory only in the 1930s. As with many other aspects of evolution–its problems and solutions–the master had recognized it originally. Darwin writes in his “Autobiography”:
“At the time [in the mid-1840s], I overlooked one problem of great importance . . . This problem is the tendency in organic beings descended from the same stock to diverge in character as they become modified. That they have diverged greatly is obvious from the manner in which species of all kinds can be classed under genera, genera under families, families under suborders, and so forth; and I can remember the very spot in the road, whilst in my carriage, when to my joy the solution occurred to me; and this was long after I had come to Down. The solution, as I believe, is that modified offspring of all dominant and increasing forms tend to become adapted to many and highly diversified places in the economy of nature.”
Darwin spent some 40 pages of the manuscript for the “Origin of Species” dealing with the problem. His solution involved sympatric speciation, though earlier he had assumed allopatric–one of the few “mistakes” Ernst Mayr was willing to ascribe to Darwin. Darwin’s treatment is much more complex than his statement in the “Autobiography” implies. His change of mind about speciation reveals deeper features of his theory of natural selection.
Bob,
Thanks for this, but I’m not sure that the passage quoted really shows that Darwin conceived of DISCRETENESS as the big problem, rather than simply hierarchical distributions of character states produced by branching speciation. Maybe that’s in the other manuscript pages, but I don’t see it here. Do elaborate if you can.
jac
The biological species concept is SO ZOO-CENTRIC! Many of the species in Populus (Salicaceae) hybridise with fertile offspring, both in the wild and in cultivation. For example.
When I read Speciation several years ago I tried to explain this to my neighbor, a creationist and discovered what may be a core problem in the creation/evolution challenge: creationists–at least my neighbor and a few others–think science sees the theory of evolution as absolute fact without uncertainty.
“The BSC defines (or rather conceptualizes) species…” is an excellent clarification of the challenge. while my education includes enough science to understand the difficulties, Speciation left me shaking my head, thinking, “I wish I understood all I know about this.”
It seems we are tremendous amount of energy arguing with these people.On one level,it is probably necessary.But then consider this thought experiment-suppose we could view the unfolding of earths history in its entirety(like those speeded up movies they showed me in the auditorium in grade school)Now,of course its IMPOSSIBLE to ever see this entire “movie”.Or is it?Could we someday,somehow-like go out in space say,recapture the light earth reflected and somehow from that light reconstruct the entire play.Just a thought experiment.The point of this bit o off the wall speculation is this-we have some many fascinating mysteries to solve-why waste our precious time fighting these morons.Isnt that their strategy-to distract us?
Back when I was thinking seriously about species and speciation (10+ years ago now), I was a fan of Templeton’s (1989) cohesion concept. I prefer concepts that don’t require the knowledge of other species to work; we often don’t know the phylogeny of a group or even all of it members. The same thing applies to species descriptions – they should be able to stand alone without any need for comparison to other members of the genus.
MF: The same thing applies to species descriptions – they should be able to stand alone without any need for comparison to other members of the genus.
I don’t know. Comparison to similar species is pretty much mandatory in new species descriptions, and I’d hate to have to try and use a bunch of “free standing” descriptions that didn’t explain how X differs from Y.
I think you need to do both, but you don’t always know all the species in the group – as many will remain undescribed. If you can make a stand alone description, then you are screwed when additional species are found. Taxonomic keys are useful for playing off other species, but species descriptions should not require you to go find every other species in the group for them to make sense.
Jerry,
Here’s just one of the ways Darwin discusses the problem of divergence or discreteness, which he believes is solved by his “principle of divergence.” It is not so much about hierarchy of Linnaean categories and their justification, but about discreteness of species. Prior to the passage in the manuscript quoted below, Darwin has tried to provide a mathematical demonstration of these relationships: 1) large genera (i.e., having many species) have large species (i.e., many varieties); 2) dominant species (i.e., widespread over a region) tend to be large species and to be in large genera (his calculations were suppressed in the “Origin”). So here’s the passage:
“from the species of larger genera tending to vary most & so to give rise to more species, & from their being somewhat less liable to extinction, I believe that the genera now large in any area, are now generally tending to become still larger. . . Here in one way comes in the importance of our so-called principle of divergence: as in the long run, more descendants from a common parent will survive, the more widely they become diversified in habits, constitution & structure so as to fill as many places as possible in the polity of nature, the extreme varieties & the extreme species will have a better chance of surviving or escaping extinction, than the intermediate & less modified varieties or species. . . the principle of divergence always favoring the most extreme forms & consequently leading to the extinction of the intermediate and less extreme, will taken together give rise to that broken yet connected series of living & extinct organisms, whose affinities we attempt to represent in our natural classifications.”
The problematic consideration is that natural selection (or is it the principle of divergence?) favors the extremes. It’s not clear, at least to me, how that can occur, if selection normally works on just small advantageous traits. In time, the consequence may be the groups are greatly separated (i.e., become extremely different from one another), but that would seem to be a consequence of selection against different and changing environments. The problem I’ve been working on is the relationship Darwin sees between his principle of natural selection and his principle of divergence: are they independent or is divergence simply a consequence of natural selection or is it a kind of selection, perhaps like sexual selection. Darwin thought it quite important, as he wrote to Joseph Hooker in 1858: “the ‘Principle of Divergence,’ . . . along with ‘Natural Selection,’ is the keystone of my book; and I have very great confidence it is sound.” Darwin was worried precisely about the problem that Dobzhansky suggests–why isn’t there smooth continuity among varieties, species, etc. The creationists could explain discreteness, but his theory, at least on the surface, seemed to imply continuity (which, of course, it does if one takes ancestors into account).
I think it might justify the whole “diavlogging” concept if you and John
Wilkins could get together on the Internet and discuss species concepts, while we all get to watch and ask questions.
I’d vote for that!
Where else shall I try to get answers to my post # 11 above ?
I don’t know that anyone has made a count, but I surmise that the vast majority of animal species are genetically isolated, bounded entities. We study hybridization because it is rare and unusual, and therefore interesting. The exception which tests the rule.
Why is hybridization so rare among animal species which are syntopic and have the opportunity to hybridize? Why are there isolating mechanisms? I think it is because hybrids are usually less fit than individuals of the parent species, and that members of parent species which produce hybrids are less fit than those individuals who do not hybridize. There are studies in the hybrid literature which support both statements. Unfortunately, there are also studies in the hybrid literature which can be cited to support just about any hypothesis one can think up.
So far as continuity back to the origin of life. If allopatric speciation is as common in animals as some think it is, each speciation event is a little blip or constriction of the continuity, I suppose.
The definition of the majority of modern day species is. “An entity which a competent taxonomist says is a species.” Most recognized species are recognized on strictly morphological grounds. Remember that use of DNA in species determinations is maybe 20-25 years old, and species have been described ever since 1858 (and even before in a few instances.) So paleospecies and most recent species of animals are recognized and defined on morphological characters. It is just that we neontologists have better and more complete data than most paleontologists encounter.
A diagnosis, in which the author tells how to distinguish the new species from other similar species’ is a required element of the species description, at least according to me.
Typo, since 1758, not 1858.