Why Didn't They Grow Up?

[seer]

Well, yes, because many populations would not be in that locale.

Ok, I get it - more luck. Thanks Sam...

[Ansgar Seraph]

OK. This looks pretty lousy but bear with me. Let's start here:

Start.jpg

Imagine that this is earth, devoid of life. Over time, however, life evolves and cyanobacteria populate the entire globe:

Cyan.jpg

Cyanobacteria populations are blue circles and the green tint means that those populations experience no morphological selection pressure. Now imagine that a significant change impacts earth's climate, creating a different ecological band towards the poles:


Change.jpg

Now, the red bands are areas that do create selection pressure for cyanobacteria to change morphologically and the yellow circles are populations that evolve to fit the new niches. There remain, however, populations of bacteria in the green-tinted band that are not under pressure to change morphology.

Reversion.jpg

In this image, a significant period of time has passed and the changes that created the red bands toward the poles are being reversed. Green tinting can be seen encroaching on what was once the domain of the yellow-circle populations. And, because these niches are once again similar to other niches occupied by the blue-circle populations of cyanobacteria, those populations are migrating back into those areas. During this event, not all cyanobacteria were exposed to selection pressure that necessitated a change in morphology. Now we'll add another event:

Greater Change.jpg

At this point in time, you can see that there has been an event which has caused the tropical band of the globe (tinted purple) to change; cyanobacteria in that band evolved into the teal-circle populations. Cyanobacteria retaining the original morphology still exist in the narrow pockets between the purple and red bands — these green-tinted environments still do not exert selection pressure that would necessitate morphological change.

Finally, the purple-tinted band recedes, as the global climate returns toward the original configuration:

End.jpg

Once again, the ancestral blue-circle cyanobacteria dominate the globe. Having ridden out global upheaval in bands or pockets still suited to the same morphology, there was no need for blue-circle bacteria to drastically change shape. However, we now have two new groups of bacteria, also — the yellow-circle bacteria and the teal-circle bacteria.

This is a gross (and poorly drawn) simplification. The only point is to show how a migrating population can remain in an ideal environment even while local environments affect other populations. So long as there were environments ideally suited to cyanobateria's morphology and populations with that morphology could successfully migrate to those environments, there is no necessary reason why a given species of cyanobacteria should not remain morphologically stable over time.

It's not luck, in other words. It's the same thing we see happen season-in and season-out among birds, fish, mammals and, indeed, all creatures. Some will respond to changes in environment by evolving while others will respond by moving. Still others will be able to do neither and their species will become extinct. So unless you can give good reasons why cyanobacteria could not successfully migrate to other areas during times of local crises, seer, your skepticism is not a valid criticism of evolutionary theory.

—Sam