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Why Have Some Species "stopped Evolving"?


StewartP
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First, to nail my colours to the mast, I am an evolutionist. Old fashioned Darwinism.

 

But, I wqs thinking about those animals that the media always describe as "living fossils". The shark, crocodile, tortoise coelocanth etc. These animals look very much like, exactly like fossils from millions of years ago.

 

Given that there is always environmental pressure, from competitors, from their own species etc, why have these animals stopped evolving?

 

Is it because "stopped evolving" is the wrong phrase? Perhaps they are further evolved than their fossil record look-alikes but thaey have evolved in the soft-tissue arena which leaves no record?

 

or perhaps the fossils that they resemble are not the same species or even common ancestor, but just co-incidence that they are so similar.

 

or perhaps they have evolved from the fossil ancestor, and then due to environmental pressure, evolved "back" again.

 

It's just that the notion of a species "not evolving" over millions of years doesn't fit with my (limited) understanding of evolution.

 

Any biologists out there got some ideas?

 

regards

 

Stew

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Stewart, if amateur biologist will qualify then I will take a stab at answering your question.

 

My guess is that these organisms have "stopped evolving" because their niche has not changed enough to apply a different selective pressure on them. They have found and exploit a stable niche in the ecology and therefore there is no pressure to change.

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Stewart, if amateur biologist will qualify then I will take a stab at answering your question.

 

My guess is that these organisms have "stopped evolving" because their niche has not changed enough to apply a different selective pressure on them. They have found and exploit a stable niche in the ecology and therefore there is no pressure to change.

 

i thought of that, but then figured that even within their niche there would be pressure from their own species. ie: In a river with too many crocs there won't be enough prey, but a croc with an adaption to swim faster, or with quicker reflexes would be more fit to survive.

 

On the other hand those 2 examples are "soft-tissue" cases that wouldn't impact the fossil record?

 

It still just doesn't "feel" right that species stop evolving. It's to do with the fundie misunderstanding "if animals are evolving why are there still monkeys?"

 

Perhaps the answer lies partly in the rates of evolution. Some species because of their excellent adaption to their niche are under less pressure to evolve and so are evolving more slowly?

 

 

(edited for spelling)

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Perhaps the answer lies partly in the rates of evolution. Some species because of their excellent adaption to their niche are under less pressure to evolve and so are evolving more slowly?

Perhaps. Maybe if we were capable of directly comparing a living example of these ancient species against their modern counterparts then we might see some slight behavioral or soft tissue differences.

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Stewart, if amateur biologist will qualify then I will take a stab at answering your question.

 

My guess is that these organisms have "stopped evolving" because their niche has not changed enough to apply a different selective pressure on them. They have found and exploit a stable niche in the ecology and therefore there is no pressure to change.

 

I agree with this. Species that exist today such as sharks are not "prehistoric". All the species that exist today are modern, otherwise they wouldn't exist today.

Where is Mr. Neil or Crazy Tiger when you need them?

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First, to nail my colours to the mast, I am an evolutionist. Old fashioned Darwinism.

 

But, I wqs thinking about those animals that the media always describe as "living fossils". The shark, crocodile, tortoise coelocanth etc. These animals look very much like, exactly like fossils from millions of years ago.

You shouldn't expect the media to get it right. Those animals you mentioned are not exactly like they were millions of years ago.
Given that there is always environmental pressure, from competitors, from their own species etc, why have these animals stopped evolving?....
They have not stopped evolving. I can't go through your whole list there and explain how each one has changed, I just don't have that much time, but here is a good explanation of the coelacanth.

 

i thought of that, but then figured that even within their niche there would be pressure from their own species. ie: In a river with too many crocs there won't be enough prey, but a croc with an adaption to swim faster, or with quicker reflexes would be more fit to survive.

 

On the other hand those 2 examples are "soft-tissue" cases that wouldn't impact the fossil record?

With the swimming faster bit, the body would have been more streamlined or bigger muscles. Both would be noticable in fossils. A smaller river with less food and more competition could lead to a smaller croc. Todays crocs are about 1/10th the size of one of their ancestors.
It still just doesn't "feel" right that species stop evolving. It's to do with the fundie misunderstanding "if animals are evolving why are there still monkeys?"
That's because the funnymentalists need to misrepresent evolution. We didn't evolve from monkeys, we shared a common ancestor with apes.
Perhaps the answer lies partly in the rates of evolution. Some species because of their excellent adaption to their niche are under less pressure to evolve and so are evolving more slowly?
That's another problem with fundies, they cannot grasp the time it takes for evolution to happen. In "punctuated equilibrium" the fast part takes 50,000 years.
(edited for spelling)
Don't bother. It's fun to anoy the spelling police and the rest of us go by content. :grin:
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crocodiles have changed, the point is they havn't changed much. They didn't need to, evolution doesn't always occur at the same rate for different animals. The basic structure of a croc has stayed similar because there has always been warm/temperate waterways and pools. Look at the variety in todays crocodiles, some have ridiculous looking snouts, too thin to catch anything but fish, which is what they eat. Others have shorter muscular snouts and can bite with the force of a car crusher! Some freshwater sharks live in rivers and have snouts with teeth sticking out all around so they look like a chainsaw switched off. This is because they live in muddy rivers with low visibility and have adapted to swing their snout around and catch fish this way. The species have changed but they have not adapted into another complete species because their environment has not changed significantly. There have always been fish swimming around. Whatever they look like or feed on, they are very similar to hunt. The water is a far more stable environment than the land.

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Who says they've stopped evolving? And why do they think evolution has to be in leaps and bounds? Why can't it be just a little bit at a time, whatever is necessary for the species? Not all evolution is dramatic.

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Who says they've stopped evolving? And why do they think evolution has to be in leaps and bounds? Why can't it be just a little bit at a time, whatever is necessary for the species? Not all evolution is dramatic.
There are several ways that evolution occurs. There is not just one way. Punctuated equilibrium is just one way. Sexual selection is another. Survival of the fittest is another. Isolation is another. Then there's just plain old genetic drift. It could be a combination of all of the above too.
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Your understanding of cladistics is flawed,

You assume that all turtles are one species,

And you assume that there has ever only been one species of turtle.

 

Turtle are still evolving. There are hundreds of species of turtle.

NOT ONE of those species is older than a few million years old.

Of the many species of turtle alive in the late jurasic,,, NOT ONE remains alive today.

There are hundreds of species of shark, Same rules apply.

 

There are hundreds of species of crocodile, again, same rules,

 

A species stops evolving only when it becomes extinct,

 

but other related species of simmilar form can continue.

 

Evolution does not require drastic changes of form or function.

It is only a change of allel frequencys, of genetic information within a population.

 

The familys you mentioned are stable because there is no pressure for drastic change.

They are stable across many species within the family, genus, order, etc etc,

New species may evolve within the family as old ones die out, But the family is stable.

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Your understanding of cladistics is flawed,

You assume that all turtles are one species,

And you assume that there has ever only been one species of turtle.

 

I agree. I had a similar discussion with my father (fundie) the other day. He said that Darwin had wasted his time sailing around the globe when any farmer in England could have informed him about selective breeding.

 

he had not appreciated that the Finches that Darwin observed in the Galapagos were all differerent species of finch and could no longer interbreed. It was only when I showed him references to different species of zebra, that have different number of chromosomes did the penny drop that just because they are all called "zebra" doesn't make them all the same kind of animal.

 

Thank you all for the replies.

It seems that it is a combinantion of numerous factors that result in species "not evolving" not least of which is the bermuda-triangle-like non sequitur "not evolving"

 

regards

 

Stew

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