This may veer into GD territory but for now I think my question has a GQ answer (understanding that this is all theory anyway a GQ answer as those theories would have it).
I was recently watching a show detailing the fall of dinosaurs and the eventual rise of mamals on this planet. They were noting various adaptations that aided first the dinosaur’s preeminence and then those that ultimately aided mammals. Of these the one that got me thinking was the show displaying the fossil of an animal they believed to be the first to have a placenta and, therefore, live birth.
From a layman’s perspective I understand evolution and can go with much of it. Species over time adapting bone structures, lungs, musculature, brain size, etc., to slowly evolve into something different that is superior in surviving than its ancestors were.
But how the heck does an animal (or more to the point species) slowly develop a placenta and live birth? It seems to me it needs to be an all or nothing sort of thing…lay eggs or live birth. I have trouble envisioning a process where all the specialization necessary to manage a mother carrying a baby to term (womb, placenta, birth canal, hormone timings/changes, etc) just blink into existence from the previous generation’s egg laying behavior. Further, I cannot see how this would get anywhere if, say, only a single animal developed this trait. It has a survival benefit but I doubt one animal managing this would be enough to make that an obvious thing such that it became the “Eve” of that species branch. I would think you’d need a whole population to roll over to the live birth method in one go if you expected it to stick.
NOTE: This is an honest query to fill in obvious gaps in my understanding and not a backhanded attempt at slamming Evolution or supporting something like Intelligent Design. Indeed keep the ID stuff out of this…other threads for that around here. I am hoping for an answer based on how the current theories would explain the above even if uncertain or incomplete.
Look up “precocial vs. altricial birth” and how marsupials gestate. Then check out “ovoviviparity.” The process is not exactly an either-or one, contrary to what one might suspect. An animal belonging to a generally egg-laying taxon may naturally and normally retain the egg within itself until it hatches, “giving birth to live young,” in effect if not in actual result. It’s probably only a short step from retaining an egg including the yolk to nurture the unborn young to the next step, retaining the egg and supplementing the yolk with nutrients assimilated by the mother while carrying the egg within herself. Then a young born as essentially an embryo, brought to term in an exterior pouch instead of an internal womb. Finally the placental-mammal methodology, with the young born helpless and needing care (altricial, e.g. human babies), or retained until fully functional (precocial, e.g., fawns, foals).
Punctuated equilibrium, by the way, simply means that species and higher taxa evolve in discrete “quantum” steps, rather than a continuum, owing to mutations and similar phenomena. E.g., if a seed-eating bird can better crack seeds owing to a hook on the end of its bill, a mutation that causes such a hook to develop will be selected for, and “rapidly” (in evolutionary terms), the non-hook-bearing species will be replaced by the hook-bearing one. In terms of fossils, it looks like a sudden transition, with non-hook forms throughout the X-ian, followed by hook-bearing forms in the Y-ian and later.
The term for what the OP is describing is “saltation,” essentially one-step evolution of a complex trait due to mutation. It is generally believed to be rare if it occurs at all. As you say, “punctuated equilibrium” involves “standard” evolution by means of natural selection; it just says that the evolution of new species due to strong selection takes place over such short periods and in such small areas that a gradual transition between species is unlikely to be observed in the fossil record.
Thanks for clearing that up. If you want to change the title of the thread to more accurately describe what I am getting at feel free but it seems it has been answered anyway.
So saltation would be developing a feature due to several mutations, then? Like if my kid was born with wings or something? No one seriously believes that this is a major driver for evolutionary change, right?
I’m particularly curious because I’ve read references to it not in literature about biology but as a claim by Noam Chomsky about the development of the capacity for language. (I have lots of problems with Chomsky’s theories about language universals, but his suggestion that language developed in a sudden saltation struck me as particularly odd.)
Not necessarily. In theory unique features can equally come about rapidly as a result of a single mutation, such as an animal being born with two heads, no limbs , becoming sexually mature in the juvenile form and so forth. All are potentially saltatorial steps that don’t require multiple mutations.
That would be an extreme and ridiculous example. More realistically you might look at the neotenic form of something like an axolotl, or the devlopment of limblessness in several groups of reptiles as examples of a saltatorial jumps. An individual can become radically different from its parents as a result of mutation, but to posit something as exteme as a human growing functional wings is ludicrous.
It’s not currently held in high favour in the scientific community, but it still retains it s share of adherents in one form or another. Do a Google search on Richard Goldschmidt for one of the more emminent and vocal suporters of evolutionary saltation.
It’s one of those things that is imposible to prove of course, and it depends on exactly what was meant. Given the complexity of human language and the number of different and apparently unrelated brain centres involved it would be ridiculous to suggest that language itself was developed in a single step. However it may be that Chomsky was referring to “The Great Leap Forward” that occured around 40, 000 years ago. Something certainly happened then that radically altered the human species, and it happened fast. Chomsky isn’t the first person to suggest a rapid development of modern language as the cause. That doesn’t mean that language didn’t exist prior to that, but that the complexity and the capacity for abstract reference was so dispersed anatomically that the langauge wasn’t comparable to modern language. It’s conceivable, if unlikely, that a single step could lead to the integration necessary to produce modern language.
Or else the hooke-beak one will occupy an environmental niche with different food than the original and in time will split off into a new species as seems to be the case with Darwin’s Finches.
As for the punctuated equilibrium. Without selective pressure (at least on individual traits) variation among members of a species is continuously, if slowly, increasing. I think Punctuated Equilibrium suggests that there are long periods of time during which there are no “selection events” until something major happens to winnow down the population and select those with the advantageous traits. A competing theory is that Natural Selection is continuously occurring with selective pressure/competition all the time. As usual, I think most evolution scientists think nature is too complex to fit in to one mold all the time.
Okay, this makes a lot more sense than how I understood the term.
Chomskyan innateness is not based upon anthropology - or, if you ask me, much of anything beyond the logical requirements of his own theory of language universality; it’s not well-supported by direct evidence, in my opinion. The idea that language development caused rapid development of culture is of course a familiar one; one could easily suppose that it was instead though the result of a more minor change, maybe the development of one of the features that distinguish the human articulatory system from that of other primates. Chomsky instead suggests that an enormously complex and abstract “universal grammar” developed suddenly through saltation. Some linguists, on the other hand, go just as far in the other direction and claim that there’s no innate language ability whatsoever (a claim I find equally ludicrous.)
I’m just trying to contextualize part of the argument over innateness. Thanks for the illustration.