posted
Without wanting to rip up the wounds from that closed thread, Bob said something that I think is factually wrong, which I'd like to correct :
quote:First: the bred from two individuals thing...Doesn't evolution tell us exactly the same thing? At some point in every species' past there MUST have been a time where two individuals in one generation were "different enough" to have been counted as a separate species -- they bred and voila -- the separate species is extant.
Um, no. That's just not true. There is never a time when a child is a different species from its parent. What you do get is a time when one tribe/pack/group is sufficiently different that it cannot mate with the apparently very similar group just across the mountain. You also get a time when a child is not the same species as its great-to-the-1000th grandparents. But there is absolutely no requirement for any given species to descend from two individuals.
An interesting example, where the different species are spread out in space rather than time, is ring species. Consider the gulls mentioned in that article : The Herring gull and lesser Black-backed gulls cannot interbreed; they are, therefore, different species. Yet there exists a species that can breed with the Herring gull; another species can breed with that one; and so you get a ring going around the pole, until you come back to the Black-backed gull. If these species were spread in time and not space, you would have exactly the situation of humans : At any given time, there is a large population capable of interbreeding, but the endpoints are definitely different species.
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posted
Most definitely on it generally being gradual with no boundary. Species is a fuzzy concept.
However, sometimes it is abrupt, with certain types of speciation, typically due to polyploidy or inter-species offspring or certain forms of symbiosis. This is just relatively rare.
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posted
Um. Perhaps I should have added, 'assuming the child is normally fertile'. Consider a child with a mutation such that it is infertile; it'll never breed, so in principle it is not human, but you would hardly classify it as its own species, for all that. I think mules fall under a similar exception.
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posted
There are cross species offspring (becoming new species) that are interfertile, rarely. Also, see the previously mentioned polyploidy situation -- there are some species of plants that frequently speciate in ways where polyploid offspring are interfertile, but not with the original, monoploid parents.
I don't think we've ever directly observed an abrupt speciation event involving a symbiote, but we have substantial evidence they have occurred -- where the presence of a symbiote changes the behavior and/or physiology of its inhabitants so drastically that they are a separate species from the variety without the symbiote. One example would be the introduction of mitochondria.
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OK, ok, I should know better than to say 'there is never X' when it comes to evolution. Nature always finds a way to contradict you. Let me instead say, 'this was almost certainly never the case with humans, and there is absolutely no need for it.'
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quote: I think mules fall under a similar exception.
He was refencing Asimov's Foundation series, not meaning literally 'mules'. The Mule was a character who was a mutant. But I don't think he was fertile, so your point probably covers him, too.
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Oh, that Mule. Well, I really don't think it's necessary to include fictional characters in my data sample.
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But a Hinney(or hinny, I've seen both spellings) is a cross between a female donkey and a male horse, they exist, but are inferior (show more donkey than horse charecteristics).
And both female hinney's and female mules have reproduced (according to my search it's one in a million though I think that was a pulled out of the... statistic). The resulting baby is almost always a horse (when I say that i mean there have been like five in the past hundred years) No male mules or hinney's have ever been recorded of reproduction but that could be attributed to several things, including most being castrated at birth and well you just can't tell if that mare's baby belongs to the stallion you bred her to or the ungelded mule that you left her with.
posted
yeah - breyer beat me to it. Hinney are somewhat uncommon because they are consider worthless.
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Not lots of differences, but the hinny does have more donkey characteristics, it just is lacking the "good ones" extra surefootedness, strength.
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Strange that you get a different animal depending on the gender of the parents. I wonder why that is?
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I've never seen any data to explain that, but it could be something like "stronger traits come from the mother" or something.
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That's something that puzzles me as well, Penguin, though I have no intention of ever breeding mules. It suggests to me that the the genes from the female have some sort of dominance over the ones from the male, which would make zero sense given my understanding of genetics.
How can you tell the difference between a mule and a hinny?
quote: Really, there is no reliable way to do so. The mule has a jack father and mare mother. (Mules can be either male or female). Hinnys (also either male or female) have a stallion father and jennet mother. Hinnies are more rare than mules, but for all purposed are grouped together as MULES (for example - horses, donkeys, mules, zebras, zebra hybrids). Most known hinnies - where both parents are known- look just like mules. Some so seem to have slightly more horse-like or donkey-like features, but so do some mules. Not all hinnies are small - the size can be influenced by the parents. However, Draft Hinnies are Extremely rare, while Miniature and Pony hinnies are more common. It is more difficult to breed for hinnies - the fertility rates drop when the chromosome number is lower in the female. The male donkey has 62 chromosomes, the female horse 64, and this is a viable, easy cross. In the reciprocal cross, the horse (64) to jennet (62) the female's count is lower and the conception rate, as well as the live birth ratio, drops drastically. Old wives tales say the hinny lacks hybrid vigor, but no eveidence has been found to prove this. However, modern hinnies are larger than the "Common"; hinny of times past - which would probably have been from a standard jennet (44-48") and a local stallion - not necessarily a good one either!!! One way that some breeders say is reliable to tell a hinny from a mule is to put them in with a mixed pasture - some donkeys and some mules. The hinny will pair with the donkeys, the species that raised it, the mule with the horses. But other than actually knowing the parents for certain, even this is just a "Best-guess" method.
I had always heard (often from mule people) that hinny's were more donkey like, while horses were more horse like, and hinny's were less sturdy and useful.
Ni, I agree with you, it is weird, this is probably one of the weirdest areas of genetics, equine or otherwise.
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posted
It's true with tiger lion crosses as well, that you get a totally different animal depending on whether it's a male tiger + female lion or male lion + female tiger. One is bigger than either parent and the other is smaller, but I forget which is which.
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posted
KoM, while I agree that gradualism is the main method of transition from one species to the next, my point was about small local inbreeding populations being one source of speciation. Geographic isolation is, I believe, as powerful a concept in evolution as the genetic isolation you are focusing on. The other factor is behavioral isolation.
And really, all it takes is one "just right" mutation in some species for this to happen. Or, far more likely, the sudden appearance of a geographic boundary.
Let's look at behavioral isolation just for grins. A mutation causes an animal that's usually nocturnal to produce an offspring that is awake during the day. That critter is behaviorally isolated. It grows up. Can't find a mate. Dies. Evoluationary dead end.
That same critter has a twin with the same mutation. The twins are male and female. A speciation event has occurred.
It doesn't mean they wouldn't be cross-fertile with the more normal members of their "species" but they never meet because they aren't awake at the same times. So, eventually they pile up genetic differences, but the moment of speciation occurred with the mutation back in the first generation where the twins were born.
The "speciation event" can be quite sudden, and random.
In more modern parlance, we might call these two critters a sub-species because some poor grad student would be assigned the task of finding out if they can interbreed to produce fertile offspring. But really, in the wild, they are behaviorally isolated and that's all that matters. At least to them...
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quote: For animals where male and female matter, being a twin vs. a regular sibling wouldn't matter.
I think the reason Bob assumed twin was that the probability of such a mutation is so low that the probability that it happen in two separate fertilizations is incredibly small. If we allow for twins we allow the mutation to occur once and result in two individuals with the same mutation.
I think... (don't want to speak for Bob...)
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posted
Bob: I'm pretty sure most speciation events are not sudden. Most theories we have about evolutionary paths involve small, incremental changes (though a whole bunch of these might happen in a geologically "short" period), not dramatic changes such as between diurnal and nocturnal. The differences between two varieties of salamander, for instance, are usually individually small but collectively constitute species differentiation.
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Plants do whatever they want to do whenever they want. And they often are self fertile so they can survive. I always found plants interesting because they did things genetically that were very unusual (like weird ploidy rules, multiple genomes from strange matings, etc).
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posted
I'm actually assuming that one parent carries the mutation and that among multiple offspring are two (fraternal, not identical, or they'd be the same sex) "twins" who share the particular mutation.
I personally don't know how complicated the genetic coding is for the difference between diurnal and nocturnal behavior. I'm supposing it could be fairly small from a genetic point of view.
Even if the genes for being nocturnal are many and varied (in fact, especially if they are part of a complex "thing" that has evolved over time...), one mutation could in fact render a critter incapble of living nocturnally, but it still might do well enough during daylight or twilight.
Suppose it lost the reflective nature of the retina (useful in night vision, no?) because one protein didn't fold properly. That doesn't take much genetic change at all.
But yes, sure creation of a completely new species (incapable of breeding to produce fertile offspring with the parent species) is a much longer process, involving a whole host of (not even necessarily related) genetic changes.
It's entirely possible, however, for the behavioral isolation to be due to just one genetic hiccup.
Think of sexual selection in birds of paradise. One genetic flaw that changes even a small bit of plumage would render a bird an unfit mate. Unless...there's another bird around who happens to "go" for the new look.
I want to distinguish between a "speciating event" and the point at which the research community would agree that a new species had been formed. Recall, however, that for most of the history of life on this planet, and even for the majority of extant species, we don't really have a good handle on whether or not closely related morphs are truly different species.
When you throw things like behavioral isolation into the mix, we can't even begin to answer these questions for the vast majority of living species, let alone the extinct ones.
At this level, much of evolutionary thought is really just a bunch of "just so" stories anyway.
It's good to think about, though, because it prompts us to look beyond morphology and even beyond gene maps, to things like behavior and microenvironment.
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posted
OK, that's a rather more nuanced picture, which is always good when dealing with evolution. I still think, however, that it will be very rare for a species (of mammals, anyway) to be descended from two individuals. Reticulum's point about inbreeding is well taken, after all. And besides, such events as you speak of require considerable random factors to operate, while the evolution of a larger group just requires, well, evolution.
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Bob, you still seem to be using a hard line definition of species, which is somewhat of a flaw. Species is a human-ascribed concept that only vaguely correlates with potential to interbreed.
Regarding the notion of a speciating event, sure, things like that happen on occasion. But modern evolutionary theory mostly doesn't proceed in such terms.
To borrow from Darwin, take finches. There might be some finches on two neighboring islands, and they might even interbreed on occasion. One island might have a low number of terrestrial predators and a lot of tasty fruits that drop to the ground when ripe, while another might have few arborial predators and a lot of delicious nuts with hard shells. They both have fruits, nuts, and all sorts of predators, the distributions are just different (for whatever reason).
The finch population on the fruit island will gradually become larger and fly less, using a beak appropriate for fruits. The finch population on the nut island will remain small, efficient flyers, and their beaks will become increasingly suited for cracking nuts. Even if they still have the potential to interbreed (which may also disappear with size or other changes), those differences could easily progress far enough to be called different species. In this case, there is no speciation event. The means of the two populations merely wandered due to selective pressure.
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quote:Bob, you still seem to be using a hard line definition of species, which is somewhat of a flaw. Species is a human-ascribed concept that only vaguely correlates with potential to interbreed.
No, fugu, I'm doing the exact opposite. I'm arguing that the term species means one thing to a scientist and something else entirely (and more meaningful) to the actual critters.
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That's the thing, the term species doesn't have that characteristic. Species is a fairly vague classificatory tool having no bright line standard and only approximate heuristics for assignation. It doesn't mean any one thing to scientists, its just a convenient way to talk about things, and, importantly, the concept is pretty much completely meaningless as far as the "critters" themselves are concerned. It makes far more sense from an ecological context to talk about populations instead of species, not bothering to attempt to qualify different enough and instead dealing only with degree of intermingling.
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And they often are self fertile so they can survive. I always found plants interesting because they did things genetically that were very unusual (like weird ploidy rules, multiple genomes from strange matings, etc).
