Evolution: Abrupt Changes

[Gawdzilla Sama]

Change a pronghorn into a moose. I want to watch.

[Anomalocaris]

Sudden?  Over how many generations?   You might find the so called 'sudden' change actually occurred progressively over a very large number of generations.   tens of thousands.   Admittedly many species may appear to change little over millions of generations, so speciation over ten thousand generations is abrupt compared to the period of stasis.    But the process of speciation still takes place incrementally with little apparent change from generation to generation, but sufficient cumulative change over tens to hundreds of thousands of years.

The reason why species appear to remain little changed over many millions of years and then change significantly over hundreds of thousands of years usually have to do with stability of environment.   When the environment is stable, species evolve into an relatively stable form In adaquate equilibrium to the environment.   Once there, the species settle down into apparent stasis.   Genetic motive force behind the change still occur at the same rate.  But changes Now bring diminished possibility of further improvement in adaptation, but enhanced possibility of losing adaptation already achieved.  So most changes are weeded out.  The species over all appear to change little.

But if the environment changes, previously achieved adaptation losses their efficacy.  Now change bring increased chance of improving adaptation.  Improved adaptation gets selected, old obsolete adaptation does out.  So specie overall appear to change.

When you have just a few fossils of a specie, so 50.   The typical life of a mammalian species of 2 million years.  This means on average your 50 specimens are from time slices 40 thousand years apart.   Given this granualrity, a speciation event can indeed appear to us to be abrupt.   Slightly older, and the specimen looked this way.  For the last 49 specimens, the fossil all looked much the same.   Next specimen, it changed so much it's probably a new species.  Bang, abrupt speciation.  Yes?  No.   remember there are probably 40 thousand years between specimens.

So you're saying that a large amount of enviroment change will change a species significantly, but slow changes will happen afterwards?

Yes. New environmental conditions will encourage speciation in two ways: it opens up room for new adaptations better suited for the new environment to be selected, and it also ween old obsolete adaptations from the population. So rate of selection for significant changes goes up. But overtime, if the environment remains stable, then as better adaptation accumulate, there is reduced room for further improvements to adaptation, so the rate of selection for significant changes goes down.

[robvalue]

It makes sense to me. A species is always naturally tending towards the most efficient form for the environment (although a good degree of junk tends to sneak through the net along with it). So if there's a big change in the environment, there going to be a big gap between the current norm for the species and the new optimum characteristics. Useful changes are going to be more important and so will get factored in faster.

If the environment doesn't change or changes only slightly, the norm won't be very far away from optimum.

Or, y'know, reality is a big lie and it's all done by magicks.

[Gawdzilla Sama]

It makes sense to me. A species is always naturally tending towards the most efficient form for the environment (although a good degree of junk tends to sneak through the net along with it). So if there's a big change in the environment, there going to be a big gap between the current norm for the species and the new optimum characteristics. Useful changes are going to be more important and so will get factored in faster.

If the environment doesn't change or changes only slightly, the norm won't be very far away from optimum.

Or, y'know, reality is a big lie and it's all done by magicks.

The "next generation is different" idea has problems. First, how many animals have to have the same mutation to produce a successful breeding population? Second, the more likely response to a "big change in the environment" is a mass dying off. If savannah changes to tree-less plain the giraffes are going to have problems. Can they convert to "grazer" quickly enough? Nothing in the fossil record supports this. In fact the fossil records don't show "big change" at any time in one-half billion years. But there's still time, I suppose. 

[robvalue]

I don't understand your first question. Mutations that are beneficial are more likely to be promoted over time; that's the general principle of natural selection. They don't require everyone in each new generation to change at once, it will happen over time. It's just that it will happen faster if the mutation is more beneficial.

Indeed, a huge change could indeed lead to extinction instead. It will be adapt or die. Or maybe run off and find a better environment.

[Edwardo Piet]

Hello everyone.

Could someone please explain how evolution happens on an abrupt scale?

It doesn't.

[Anomalocaris]

It makes sense to me. A species is always naturally tending towards the most efficient form for the environment (although a good degree of junk tends to sneak through the net along with it). So if there's a big change in the environment, there going to be a big gap between the current norm for the species and the new optimum characteristics. Useful changes are going to be more important and so will get factored in faster.

If the environment doesn't change or changes only slightly, the norm won't be very far away from optimum.

Or, y'know, reality is a big lie and it's all done by magicks.

The "next generation is different" idea has problems. First, how many animals have to have the same mutation to produce a successful breeding population? Second, the more likely response to a "big change in the environment" is a mass dying off. If savannah changes to tree-less plain the giraffes are going to have problems. Can they convert to "grazer" quickly enough? Nothing in the fossil record supports this. In fact the fossil records don't show "big change" at any time in one-half billion years. But there's still time, I suppose. 

It is exceedingly unlikely that two animals, one male, one female, living at the same time in the same population, experience the same mutation, happen to come together and mate and out pops a new species. That's not how speciation works.

Instead,a heritable mutation occurs first in a single individual. If that mutation doesn't kill the individual before mating, prevent the individual from mating, and its impact sufficiently Limited so it can still produce fertile offspring with other members of its own population, then the mutation has a chance. Otherwise the mutation is self-eliminating.

If the mutation has a chance, then the first carrier of the mutation mates with a noncarrier and passes the mutation onto some of its offsprings. His mutation carrying offsprings then do the same. In time there would be a sizable number of mutation carriers, all can either mate with each other, or still with normal members of the population. Once you have big numbers, probability comes in, and survival of fittest is a game of probability.

If this mutation is harmful, then carriers of this gene would have less success in passing on their genes compared to the broader population. So percentage of population that carry this gene would reach some low threshold and not grow any further. If this mutation is beneficial, then its carriers would have better than average chance of passing it on. In time the entire population would be carrying this mutation.

So how does speciation occur? Well, not solely by the means described above. What is described above is gradual evolution of the gene pool of a given population. The entire population's gene pool shifts subtlety from generation to generation. Beneficial mutations gradually propagate to the entire gene pool, old obsolete adaptations graduallly disappear from the entire gene pool.

For clear speciation to occur, one more thing has to happen, that is the mutation only propagate through parts of the population, not other. If this happens, parts of the population through which mutations propagate will look increasingly different from the rest of the population. Keep this up for tens or hundreds of thousands of generations, the two parts could become so different they can no longer mate to produce fertile offsprings. If this happens, speciation occurs.

How could this happen? One way is geographic separation. For example, Parts of the population lives one one side of the river and the other part on the other side. They seldom are able to cross the river to mate. Mutation happens in one individual on this side of the river, in time the mutation sweeps through the entire population on this side. Eventually, different mutations accumulate this way on each side, until they become two different species. There are many other ways for similar confinement of mutation to sub populations to occur.

[Gawdzilla Sama]

Why did you quote me?

[Anomalocaris]

Because you made the statement about 'how many animals have to have the same mutation....".

[Alex K]

Two terms you might want to read up on before delving further into this conversation: Punctuated Equilibrium, and allopatric speciation.

That reminds me, on a recent SGU podcast, they mentioned that Orcas seem to be undergoing sympatric speciation, with two distict groups formig who hunt differently and different prey, and choose not to interbreed if they have a choice.