All species are transitional

Excellent post! You might want to retitle it to all individuals (who reproduce) are transitionals.

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What we need to realise is that we are trying to impose an arbitrary system of discrete divisions on a continuous set of elements. With species it is no different. If you translate the example of the numbers to species, you can imagine a continuous sequence of intermediates from any ancestor you'd care to start with, right up to yourself. You are the same species as your parents, and they the same as theirs. At the other end of the line, the ancestor you started with is the same species as its offspring, and they are the same species as their offspring.

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The discrete unit is the individual and in fact there is a bit more difference than one would assume. You are a jumble of the genes of your parents. But in addition, each indivdual carries mutations not present in either parent and thus, the genome contains novel variation both in content and in recombined inherited variation. This also overcomes some of the problems with dealing with asexual organisms as they also accumulate mutations though many individuals may be clonal. I think the problem is people confuse the discrete character which is the individual with the non-discrete character which is the population or species. If you add the number of unique individuals composing the effective population size, you have a huge genetic and morphologic pool of variation for most species. Even a single population has a huge amount of variation. Which one individual represents the species? Which variant is non-representative? Which individual represents a transition from one state to another? If speciation is occuring, how would you know? It is often defined as reproductive isolation i.e. accumulation of mutations to the point that members of a previously single population cannot interbreed. But ring species demonstrate that this need not be a discrete characteristic and there are "species" that can potentially interbreed but in the wild do not..or thier offspring quickly disappear from the gene pool i.e. forest African and savannah African elephant hybrids. As you point out, when you are comparing a parent to the offspring, you would not notice that it is a "transitional" as it may represent only a relatively small incremental difference morphologically and genetically. But if you compare an individual to some other species, or even a population that is now extinct (i.e. neandertal genes for example) you suddenly see a much greater difference...this progresses the farther you go back in generations. So the difference between me and my father will be smaller than that between me and my grandfather which will be much smaller than that between me and any H. erectus individual and so on. It is sort of like growing taller...each day you may look in the mirror and not seem to be any different. But if you look at yourself when you were 5 and then 10 you would see a huge difference.

Not to make this more complicated, you cannot really tell from current trends or trajectories if they will continue to head towards isolation. One might posit that had human populations never developed agriculture and expanded in size, the different groups would have remained isolated hunter gatherers which might have eventually speciated. However, agriculture and expansion lead to the intermixing of all human populations after they had begun to diverge genetically and morphologically and has homogenized our gene pool to a large extent i.e. we are a single population in effect.Similarly, in a study of Darwin's finches,
Grant PR, Grant BR.
Unpredictable evolution in a 30-year study of Darwin's finches.
Science. 2002 Apr 26;296(5568):707-11.they tracked environmental factors and morphological changes and demonstrated that you could not predict the evolutionary trajectory of the traits even though for extended periods there was selection and a shift in the allele frequencies that was easily observable. Like humans, hybridization of populations worked against the isolating mechanisms that would increase the likelihood that the two populations would diverge enough to become reproductively isolated.This long winded and poorly written post is only to ad to what you are saying that in addition to not being able to look at small steps and recognize discrete transitionals..the "transition" has not even necessarily been unidirectional with tendencies towards isolation being reversed and then reversed again until much later you look at two clearly distinct species that may have had several false starts in getting to a point where they are recognizeabley members of distinct gene pools.

I don't think that simple models that may be off are not useful. Hardy Weinberg equilibrium in population genetics is a useful concept although almost no population is in HW equilibrium. It is the same with evolutionary concepts of human origins like the out of Africa hypothesis. Genetic data suggests out and in and around Africa since populations don't just move in a straight line out of one region into the next but often expand (and contract) chaotically.It might be a useful idea to consider a row of fossils showing a change in form over time as a progression. But if you had access to the finer details (say at the level of the Darwin's finches study) you might find that beaks got shorter, then longer then shorter and it would not look like a progression at all. Rather lurching back and forth as the selective forces vary as one would expect from a non-static system like the environment as a selective force.Laboratory studies (in bacteria for example) tend to overlook this because they use artificial selection such as temperature or antibiotics as a constant selective force and then monitor the changes in the bacterial genomes over time in response. It demonstrates the principle for strong selection. But much like mendelian genetics, it only applies to a subset of extreme characters but is not as informative when faced with more complex situations.

Hi Percy

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And the whole reason for this is that species change occurs in tiny immeasurable steps. It simply isn't possible to point to a step and say, "Before this step it was X, after this step it was Y."

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Perhaps a bit picky but this is not necessarily true. One can very clearly measure changes within species over time. Each base change, epigenetic modification etc. in every individual could (in theory) be characterized for each generation. Richard Lenski's 20,000 generation bacteria studies sort of do this and one can both demonstrate change over time and make predictions about what changes and how they will change under experimental conditions. The issue is that when one sees change over time, one cannot be sure it will lead to speciation. Often it is because we do not know what the selective forces are that are operating as opposed to not knowing what types of changes are occurring.I agree that defining species is an arbitrary concept. There are some simplistic idealized concepts in use of separation of populations that probably do not not represent any real world situations...much like other useful idealized conditions such as Hardy-Weinberg equilibrium which may never occur in a natural population. But it serves as a baseline. Trying to rigidly define a continuous process into discrete steps is always going to be a challenge.

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RAZD mentioned the possibility that later on the members of the 2 groups might start interbreeding again, due to hard times, like some finches he mentioned did. If that happens, then the earlier designation that we had 2 separate species would be seen to be mistaken, according to the definition I'm using

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I don't think this works very well. You may have heard of hybrid zones where the ranges of two species overlap and they interbreed. Often, though not always, the interspecies hybrids are sterile. The species interbreed when given a chance and in some cases, like African elephants, there can be introgression of genes from one gene pool to another. But it is not widespread and in all other characteristics the two groups continue to diverge in morphology and behavior. By your definition of separate gene pools, they are one gene pool. But the genetic differentiation between the two groups overall suggests that they are species and introgression is range restricted, occurring only where the two groups overlap..and still, not often. Even gene pools separated by millions of years can potential mix like the single hybrid produced by a female Asian elephant and male African elephant..they are different genera! A similar type of hybrid zone exists for baboons. I think the best studies are in crickets however. All sub-generic designations will be arbitrary because the process of speciation is continuous.

Hi Percy,
I had avoided plants because I can barely keep track of the definitions of xylem and floem In any case, here is an example from the UC Berkley site on rapid speciation by ploidy changes in plants. Plants are much better and rapidly becoming reproductively isolated by changing ploidy than most other groups.Causes of speciation - Understanding Evolution

Just another point, given that plants can often self fertilize, one would not necessarily need the sudden appearance of a reproductively isolated population. The hybridization of two species generating an individual (or several) with novel ploidy that cannot breed with either parent species could propagate by self fertilization. During the founder event when all individuals of the new species are genetically very similar to near identical, it would be very difficult to distinguish progeny resulting from self fertilization or sexual reproduction...either way, it would not require the instantaneous generation of a new population. This however is not a viable method of speciation among mammals...even if John Salty Davidson thinks it is...

Which mice? I am not aware of any mammals that can form hybrids with novel ploidy that go on to reproduce asexually and then at some point, sexually. Mice on an island could speciate sympatrically but it would have nothing to do with sudden changes in ploidy like in plants or frogs. So mammals would never face that situation.Are you thinking of ring species?

Hi ned,
Thanks for the clarification. But I see a difference in this case compared to the plant example I gave. Let me see if I can clarify.

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The different populations are allopatric; isolated in different valleys leading down to the sea.
* The distinct and uniform karyotype found in each population probably arose from genetic drift rather than natural selection.

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The mouse populations with different karyotypes are separated. It is quite possible their karyotypes changed after they speciated i.e. not the direct cause. In the plant example, we are talking about the generation of an individual or group of individuals (from the same meotic event) that have a new ploidy and cannot mate with the parent stock within a single population. They then mate either with each other (if more than one are present) or reproduce asexually until they can mate sexually. All within one population and within a single generation.Second, the mouse example is also reduction of chromosome numbers as opposed to complete changes in ploidy as can be seen in plants with complete duplication of the genome i.e. from diploid to triploid, tetraploid etc. Sometimes, even karyotypically different animals can still produce fertile offspring. They would have to test this assumption "these would probably be infertile as proper synapsis and segregation of such different chromosomes would be difficult when the hybrids attempted to form gametes by meiosis".