I don’t disagree with this. It is quite likely that genes from other species introduced via rare hybridization can contribute to adaptation in the receiving species.
What I would mostly disagree with is your earlier remark that the hybridization is “deliberate, not accidental.” Any advantage gained is fortuitous. In general, species can’t be “programmed” to be receptive to hybridization with other species, or else the integrity of the two species would break down and they would merge. Hybridization can’t be selected for over the long term; if it is, then the two species will eventually become one. And there are plenty of examples of that process apparently at work, for example in the Red-shafted and Yellow-shafted subspecies of Northern Flicker, which freely hybridize where their ranges meet.
Colibri, you’re correct, my use of the words “deliberate, not accidental” was poor. Evolution is not end directed but, as you say, fortuitous.
But I think you may be making a related error stating that “hybridization can’t be selected for over the long term” without definition of “long term”. In these species (or species groups, be they diverging or converging at this moment) hybridization seems to be an important means whereby species integrity is maintained (for the at-this-moment clearly identifiable species) while at the very same time some gene flow continues between them. How long this may continue before these species either merge or develop more complete barriers is an open question. So, for this indeterminate time at least, hybridization is clearly being reinforced by overall selection, and thereby is being maintained.
In fact, given the numbers of species that commonly hybridize (see chart in former cite) perhaps this mechanism is more important in evolution than is commonly recognized. Maybe evolutionary pressures don’t have to drive an incipient species all the way into that final, genetically isolated condition that we once upon a time thought was going to allow us to define a species. Maybe this mechanism of dynamically separated yet still joined species functions to allow speciation to take advantage of local conditions through specialization, perhaps several times over in slightly different ways, while still maintaining a means whereby additional variation can be rapidly recovered should there be dramatic change in those local conditions.
Ah, haven’t had a good discussion of the mechanisms of evolution since I left the hallowed halls of academia!
Here, and in the rest of your post, you seem to be thinking about this in terms of selection of the species. While this may possibly occur, it is generally assumed that most selection is at the level of the individual, with kin-selection sometimes being important. In general, the tendency to hybridize is going to be selected for or against at the level of the individual. If those that hybridize leave more offspring than those that don’t, or they are equal, then the species will merge. (This presumes that hybrids have an advantage over the parental forms, or at least no disadvantage.) If those that hybridize leave fewer offspring, species isolating mechanisms will be reinforced.
While it is possible that the tendency to hybridize might confer some long term advantage on a species, this is tenuous at best. This kind of selection is much less likely to be important than that at the level of the individual.
It was once argued ( but I am at least a decade out of date in terms of recent literature ) that hybrid parthenogensis in whiptail lizards in the American Southwest might be a “base-covering” strategy suited to changeable, marginal habitats. At least one ( old ) paper talked about how asexual lineages don’t seem to exhibit much in the way of an individual heterosis advantage over the bisexual lineages. But the superior reproductive potential of unisexual “species” probably swamp that out, up until the point Muller’s ratchet starts working against them - at which point fresher unisexual lineages produced by bisexual hybridizations could replace them.
'course I always enjoy mentioning unisexual Cnemidophorus and related critters in these discussions, insomuch as they tend to make all species concepts twist and strain at the seams ;).
Bill Bryson relates a story similar to this in A Walk in the Woods about Bachman’s Warbler. He claims that two different men shot specimens within a few days of each other in 1939 and the bird was never seen again. Like much else in Bryson (whom I generally enjoy) this isn’t accurate, the last confirmed sightings were in 1988, but this wasn’t a newly discovered species in 1939 anyway so maybe you heard a different story.
My (least) favorite story of this type concerns the Stephen Island Wren. A lighthousekeeper’s cat brought in to its human carcasses of what turned out to be a previously unknown species of bird. After a while the cat stopped bringing the birds in. Why? Because they were extinct!
Can you clarify what you mean here? I’m having trouble understanding what “hybrid parthenogenisis” and “bisexual hybridization[s]” is. The first term seems like an oxymoron, and the second seems to imply a hybridization within a species. Maybe it’s the difficulty of applying the BSC to a population which reproduces asexually…?
Oh, sorry - bisexual here refers to a species having two sexes ( i.e. male and female ), unisexual to a species consisting only of parthenogentic females. In the genus Cnemidophorus ( or Aspidoscelis, but I learned the older name and I haven’t read the paper apportioning the new one ), the whiptail lizards, two individuals from two different closely related bisexual species can hybridize to produce a new parthenogentic hybrid “species”. This also occurs in several other genera as well.
We see this happening multiple times with different crosses of two species of Cnemidophorus to produce a partheogentic third. So through karyotyping we can discover that the parthengentic whiptail species C. neomexicanus was created by a hybridization between the bisexual species C. tigris and C. inortatus. Further the parthenogenetic females are also apparently capable of backcrossing with males from bisexual species to produce further, triploid, parthenogentic lineages. I find the damn things fascinating :).
But it brings up thorny issues in species concepts. Say twenty different hybridization events have occurred to produce twenty different lineages of “C. neomexicanus” - are those twenty seperate species? Traditionally they are lumped into one based primarily on morphology. But they don’t breed with each other and they are de facto on “seperate evolutionary trajectories.” Critters like this are the sort of curveball nature throws to disrupt philosophically consistent human musings.
It appears that the evolutionary (or phylogenetic) species concept is the one most frequently used in these cases (see abstracts here and here, for example). In that second example, reference is made to an “evolutionarily significant unit”, which appears to be a conservation term (defined here). The ESU concept appears to be a variation of the BSC, but includes the additional criterion of “represent[ing] an important component in the evolutionary legacy of the species”. Thus, if parthenogenetic hybrids are geologically transient, or similar enough in habit (and habitat) or overall adaptations to the parent stocks, then I would suspect they would not be considered ESUs, and would therefore not be classed as separate species (under that definition, anyway).
It would also seem that asexual species would be regarded as separate species by the ESU definition, but since I’m not sure how many of those are under scrutiny for conservation under the Endangered Species Act, I don’t know if it’s ever actually been applied to them.
It seems to me that the first part of the definition is actually an oxymoron with respect to the BSC:
If an ESU is “substantially” reproductively isolated from other populations, then by the BSC they are ipso facto not conspecific with them.
However, it would seem that the ESU provides an “out” when there are populations of conservation concern that have not yet received formal designation as species. As such it could be a valuable tool for conserving such species.
It is a fact of life that described and formally recognized species get the most attention when it comes to conservation. Isolated populations that may be critically endangered are often ignored simply because they are not recognized as species, even though they may be morphologically and genetically distinct.
We have exactly this situation in one case here in Panama. The Azuero Parakeet, found only in a tiny range in the southern Azuero Peninsula, was originally described as a subspecies of the Painted Parakeet, which is widespread in South America. Several recent studies have shown that it is morphologically and genetically distinct and should be considered separate from Painted Parakeet (whose South American populations should also be split up into several species). Although partly protected in a national park, its range is being actively deforested and it should be considered globally endangered.
However, information is lacking on several small populations in Colombia, and it is uncertain whether these belong with the Azuero Parakeet or with one of the South American groups. Because of this the American Ornithologists’ Union has refrained from designating the Azuero as a full species, awaiting further information on the Colombian populations.
Considering the security situation in Colombia, new studies on those populations are unlikely. They are also threatened by deforestation. The Azuero Parakeet and the Colombian populations could well become extinct before they acquire formal recognition as full species.
The kind of situation I am talking about is shown by several species of North American orioles.
Baltimore Oriole Icterus galbula breeds in the eastern US; Bullock’s I. bullocki in the western US and northern Mexico; Black-backed I. abeilli in Central Mexico; and Streak-backed I. pustulosus from western Mexico southward.
Baltimore and Bullock’s hybridize to a limited extent on the Great Plains, and Bullock’s and Black-backed hybridize to a limited extent where their ranges come into contact. Traditionally Baltimore and Bullock’s were considered different species, while Black-backed was usually considered a subspecies of Bullock’s. In 1974, based on this hybridization, all three were merged by the American Ornithologists’ Union as Northern Oriole I. galbula.
However, subsequent genetic work showed that Baltimore Orioles were more closely related to Black-backed than they were to Bullock’s, while Bullock’s were more closely related to Streak-backed, which they do not hybridize with. Also, it was determined that the hybridization on the Great Plains was limited, and the two forms did not appear to be in the process of merging. Based on this, in 1984 the AOU reversed position and considered Baltimore, Bullock’s, and Black-backed as separate species.
Now, the PSC regards the ability to hybridize as a shared ancestral condition, and thus of no value in identifying clades, which must be based on unique derived conditions. Also, since Bullock’s is more closely related to Streak-backed than either of the other forms with which it hybridizes, there would be no justification for considering it as part of the same species as the latter, while excluding Streak-backed. According to the PSC, the four forms would automatically be considered separate species, regardless of the hybridization.
What is currently happening in birds is that, although the BSC is still the most prevalent species definition, it is being modified in interpretation by the influence of the PSC. Limited hybridization is no longer considered grounds for merging two forms, and there is an increased tendency to recognize isolated populations as distinct species instead of subspecies. The end result has been a wave of species-splitting over recent years.
The PSC, though, has some problems of its own. For one thing, there is no criterion for a “minimum” difference on which species can be recognized - species could be defined on the basis of small genetic differences impossible to recognize in the field. Second, speciation is not simply a cladistic process: clades originate, but they also merge; evolution at this level is reticulate. Therefore, in my view, clades are not the only criterion that should be taken into account in classification at the species level.
Personally, I think the BSC is likely to be retained for the foreseeable future with regards to sexually reproducing species. However, it will be modified due to the influence of the PSC, which I think is a good thing.
Got it. And that was exactly the the question that was going thru my mind as I was reading the earlier stuff in your post. Seems like that would have to worked out in order for that concept to be widely accepted, and even then wouldn’t it still be difficult to nail down a specific quantification protocol?
For those who don’t know what “reticulate” means (like me), the definition is:
netted; covered with a network.
netlike.
Botany. having the veins or nerves disposed like the threads of a net.*
Yes. The proponents of the PSC, who are often museum based and work mainly with specimens, tout it as being more “objective” than the BSC. In practice, however, the PSC also requires a lot of judgment calls, since we often lack data from much of a species’ range and thus don’t know the extent of variation. Field biologists, I think, tend to prefer the BSC, which is more likely to recognize entities that can be identified in nature.
Sorry. By that I meant that at its lowest levels the speciation process shows a netlike pattern. Clades form when populations become isolated, then often merge when those populations come into secondary contact. The PSC is apt to recognize as species entities that may be ephemeral. The PSC emphasizes the very beginning of the speciation process, the BSC the point when speciation has essentially been completed.
I don’t know - I’ve always thought of it this way: Theorhetically speaking, I would like to use the PSC in virtually all cases, practically speaking, I wind up using a Morphological Species Concept simply by virtue of the manner in which I identify specimens, but I hope that my MSC is backed up by research connecting it to the BSC - and am overjoyed if someone has connected it to the PSC.
I’ve always been frustrated by the impracticality of both the PSC and BSC.
Well, the problems are basically that 1) we rarely have sufficient information to make a fully informed decision; and 2) Mother Nature is a slob (as well as being a bitch).
I would say that if that were the case, there wouldn’t be 6 billion+ of us, the vast majority of which (if not the whole damn lot) are of mixed ancestry.
As Darwin’s Finch indicates, in terms of breeding behavior there is absolutely no evidence that that is the case. While in some times and places there have been some social or cultural barriers to interbreeding, these have usually been honored in the breach. Left to their own devices, humans will screw like minks with whoever happens to be available, whether or not they look the same. There is essentially zero evidence for any biological or innate reproductive isolation between any populations of modern humans.
By that I meant that at its lowest levels the speciation process shows a netlike pattern. Clades form when populations become isolated, then often merge when those populations come into secondary contact. The PSC is apt to recognize as species entities that may be ephemeral. The PSC emphasizes the very beginning of the speciation process, the BSC the point when speciation has essentially been completed.