Evolutionary threshold

[Baby Astronaut]

Has a name been given for the evolutionary "halt" when an organism is so well adapted to its environment that any genetic mutation is detrimental or unnecessary?

 

A prime example: the fossils of certain diatoms and mosses species found in Antarctica...

 

 

"To be able to identify living species amongst the fossils is phenomenal. To think that modern counterparts have survived 14 million years on Earth without any significant changes in the details of their appearances is striking. It must mean that these organisms are so well-adapted to their habitats that in spite of repeated climate changes and isolation of populations for millions of years they have not become extinct but have survived" said Ashworth, the principal paleoecologist in the research.

Other examples might be turtles, certain insects, plants, etc.

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I visualize it as so...

 

The evolutionary branching for such an organism had gone on as usual, then at some point later, the branches reach a platform from which they start looping back to the platform. There's not going to be any further evolution -- at least not in that range of environments.

 

 

(also, if the thread could be moved under Evolution it'd be appreciated)

[Mokele]

You do have a point, however, I feel compelled to make a few notes:

 

First, no organism can ever be that well adapted, and even if it is, no environment will stay static long enough. If you want absolute stasis, you won't find it, especially at the genetic level (though morphological stasis may occur).

 

Secondly, you need to distinguish between species which remain unchanged (a good example being the Australian lungfish, which is found in Australian sediments dating back 100 million years with no obvious morphological changes) and *lineages* which retain the same body form (such as bats, birds, etc.)

 

Thirdly, what counts as "unchanged"? No genetic changes at all? Minimal enough changes that, given a time machine, a modern individual might, as it was once so eloquently put, "do the nasty in the pasty" and produce offspring? "Invisible" genetic changes but constant morphology? How constant? Scale pattern changes in crocodiles? Jaw shape changes?

 

Fourthly, what about descendant lineages. For instance, consider crocodiles and sharks. Modern crocodiles and reef sharks strongly resemble fossil forms far back in time, but in each case, there are numerous divergent lineages with radically different body forms (all extinct in crocodiles, some extant in sharks).

 

 

 

That said, it is known that under certain circumstances, species will persist unchanged for very long periods of time, often garnering the term "living fossil". I'm unaware of a specific scientific term for these taxa - I suspect most of the time they're dealt with individually in scientific texts, so a collective term has never been needed.

 

For whole lineages, there's a term "phylogenetic conservatism", which generally refers to a trend to remain in the ancestral state (not necessarily in terms of morphology).

[CharonY]

Isn't the OP essentially describing a Hardy-Weinberg situation, only with selective forces that move towards rather than away from equilibrium?

[Baby Astronaut]

First, no organism can ever be that well adapted, and even if it is, no environment will stay static long enough.

Agreed, but I did mention a range of environments that it'd be adapted to. Enough to survive overall if a few environments "go bad"

 

Secondly, you need to distinguish between species which remain unchanged (a good example being the Australian lungfish, which is found in Australian sediments dating back 100 million years with no obvious morphological changes) and *lineages* which retain the same body form (such as bats, birds, etc.)

Definitely the first one. No significant changes whatsoever, as mentioned in the article.

 

Thirdly, what counts as "unchanged"? No genetic changes at all? Minimal enough changes that, given a time machine, a modern individual might, as it was once so eloquently put, "do the nasty in the pasty" and produce offspring? "Invisible" genetic changes but constant morphology? How constant? Scale pattern changes in crocodiles? Jaw shape changes?

Pretty easy: if the jaw changed, or anything significant did, the organism's no longer unchanged.

 

That said, it is known that under certain circumstances, species will persist unchanged for very long periods of time, often garnering the term "living fossil". I'm unaware of a specific scientific term for these taxa - I suspect most of the time they're dealt with individually in scientific texts, so a collective term has never been needed.

Living fossil is a perfect enough term far as I'm concerned.

 

For whole lineages, there's a term "phylogenetic conservatism", which generally refers to a trend to remain in the ancestral state (not necessarily in terms of morphology).

Interesting to know. Though I'm referring to no significant evolution from morphology as well.

 

Isn't the OP essentially describing a Hardy-Weinberg situation, only with selective forces that move towards rather than away from equilibrium?

A good description I'd say. But more precisely, I just found it thought provoking to consider a scenario where an organism's best chance for survival would be to not evolve any further.

 

Past a certain stage it'd work against the organism. So from then on, an environment it must survive against is evolution itself.

[dttom]

I would rather treat it as a long punctuated equilibrium, that any mutation would be reverted by the eqm. But provided with a change in conditions, the equilibrium would shift.