Evolutionary Plasticity

[Hyena]

Hi everyone,

 

I have a general question-ish/ proposition, but more or less I would like this thread to serve as a sort of disorganized compendium of thoughts related to specific questions of what I want to call evolutionary plasticity.

 

What I mean by evolutionary plasticity is basically an account of the differences in evolutionary histories (speed of evolution, outcomes, influences) of whatever lineage you may imagine. I'm interested in what drives evolution, and how these different driving forces may alter the outcome of a particular situation.

 

My question is sort of a long-winded one, and it's difficult for me to ask without giving some back-story first. I was thinking about the differences in life-histories of birds, and that some birds, after they hatch are basically ready to leave the nest (precocial, like ducklings) and other birds are born blind and helpless (altricial, like many songbirds). These birds thus have very different life-histories, related to feeding and predation and protection of the nest and everything else that accompanies these two strategies.

 

From my understanding of evolution, there was a common ancestor of these two types of strategies, that may have had a similar life-history pattern or may have been different (I'm not interested in the specifics of the basal lineage). However, at some point there must have been a divide between the precocial strategy and the altricial strategy. Perhaps it was some sort of intermediate strategy, of which which I'm sure there are examples present today. I'm assuming that slight variations in these strategies (a chick is born with slightly more feathers, or less developed, etc.) must have existed and over time diverged into what we see now. However, these two strategies are so different that they impact the behavior, ecology, etc. of the species that use them, and now it is to the point that a clutch of duck eggs isn't going to have any altricial ducklings, and vice versa. Essentially, there is little to no variation anymore on this trait, yet at one time it was a driving force of evolution that at some point must have had some phenotypic flexibility.

 

My main general question is this (assuming what I've said makes sense):

"What explains the variability of a trait in the past that is no longer variable?"

 

In other words, why don't we see variation in a trait that at one point must have been variable?

 

I'm sure there are exceptions, and really I don't think this is even a valid question, because it's probably presuming something that isn't always true. I don't know enough about birds to know whether or not the trait I've chosen is a good example to set the stage, but nevertheless, it got me thinking. I want you guys to think and ask questions too!

Edited April 21, 2015 by Hyena

[H.sapiens]

Different regimes of ballancing selection applied to two allopatric population is one route.

[MonDie]

That's *disruptive selection*, but it could just be directional. https://en.wikipedia.org/wiki/Directional_selection#/media/File:Genetic_Distribution.svg

Look up *epistasis*. Don't assume the ancestor had to be something inbetween.

 

To answer your question, I'm thinking changing environment. Otherwise, if the trait is continuous / graded, maybe it began as directional selection but eventually stabilized on an optimal point. I'm not that deep into evolution yet.

[Roamer]

The birds still have the same (approximate) "plasticity," it is just the speed of evolution is low(as opposed to revolution, which means a fast change)