Thursday, July 17, 2014

On the mythology of natural selection: Part III. Organismal selection

The entrenched idea of natural selection that is clear from the way that most people have discussed the subject, from Darwin to the present, is that selection is inevitable in nature because all species can reproduce rapidly enough that they will inevitably press up against the ability of their environment to support their needs--they'll overeat their food supply, run out of territories for raising young, and so on.

Ultimately, the idea goes, this will (will, not just might!) lead to competition, in which organisms compete for the now-limited resources.  Since the idea is that there will always be relevant genetic variation, this competition will (will, not just might) inevitably lead to improved genomes for the competitively intense circumstances.  This change is called adaptation.

As we said yesterday, this logic seems fine if the assumptions are correct.  It invokes a rather vague sense in which the environment, broadly conceived, screens --selects-- organisms for their traits, only letting the better ones through to form a next generation.  This is of course a view of the world as coldly, remorselessly, impersonally, and relentlessly cruel.  However, that doesn't make the idea false, and Darwin was indeed inspired to see his ideas because of the cruelty he observed.

There are other inevitable issues, such as the element of chance (known as genetic drift), the stability of the environment relative to the rate of genetic change, and others that should force us to question more seriously the idea in its specific details and even its asserted ubiquity.  But in principle there is no problem with Darwinism of this sort as a possible reason for adaptive change.

An historically clear side effect of this reasoning, we believe, is that this idea has become a dogma or ideology.  Even that would only be somewhat bad, depending on how inaccurate it was for evolutionary change over eons of history in finite populations.  We will comment about our personal views on that subject at the end of this series.  But the more serious problem with unexceptioned, unquestioned Darwinism is that it has routinely been applied to humans, including leading to justification for discrimination of the worst kind in recorded history by some against others.  It's only one rationale that has been used to excuse societal rapine, of course, but if it is not as universally true as the dogma has it, then there is no reason to cling to it.  For those in positions of control who wish to discriminate against others, there will always be alternative reasons.

However, the point is that Darwinism was offered as a deeply insightful and highly plausible theory for explaining the origin of complex organisms without needing to invoke any immaterial creation phenomena, such as spontaneous generation or creation events due to God.

On the other hand, there are ways in which complex traits can evolve that are not based on competition, or the cold cruelty of Nature.

Organismal selection
Organisms, especially animals, explore their environments in many ways.  They have means to sense conditions and respond to them.  This is obvious.  But it follows that they can in this sense 'select' their environment, rather than their environment selecting them (as in Darwin's natural selection).

Individuals can identify environments for which they are suited.  Suitability is just a general term for what one might call 'adapted', that is, environments in which they can find food and survive and so on. Among the variants in a population (and, of course, the responsible genotypes), those that 'like' a particular place will go there.  There, they will encounter others similarly predisposed.  They will mate and the offspring will stay around.  Meanwhile, members of the same population who do not like this particular environment will congregate elsewhere, meeting like fellows, and so on.

We refer to this as organismal selection, because it is the organisms rather than the environment that is doing the selecting.  When this happens, genotypes that confer the reason for the preference will proliferate in the preferred respective areas.  Eventually, genomic changes can occur in the different environments so that speciation (reproductive isolation) has occurred.  Over time, if one examines variation, adaptive variants will be found, and can appear just as if they had been raised to high frequency by competitive natural selection.  But the process need not be based on differential reproduction nor on competition for scarce resources, and in that sense is adaptive evolution that is not 'Darwinian'.

Organismal selection can't immediately produce complex traits any more than classical Darwinian selection can.  It would be expected to work at a similarly slow pace, and require assortative reproduction among existing variation.  It need not, indeed would hardly be expected to, evolve in a straight line from state A to state B.  In that sense, it's not 'anti' to many key aspects of Darwin's ideas.

Plants are more restricted in their ability to choose, so it's not clear how much of these ideas can apply to them.  But they certainly apply to single-celled and other relatively simply organized organisms.  Genetic variants that lead them to move to or inhabit particular environments essentially assorting their genomic variation by location.  This is not a strange suggestion, nor would it challenge to our idea about the relationships between genomes and traits or behavior.

There can, of course, also be competitive evolution going on for other traits, or eventual for the traits in question.  But this need not be so and in any case the point is that complex traits can arise by organismal selection.  If the environmental niches are all in the same place (say, finding food high in trees compared to on the ground), and if the behavior eventually leads to forming new species, it is known as 'sympatric speciation'.  Whether sympatric speciation occurs has long been debated, but in recent decades examples have been found that are convincing.

Speciation in Midas cichlids in different crater lakes in Nicaragua. Geiger et al., 2010

A skeptic wedded to the Darwinian party line might understandably ask how often the appropriate conditions occur.  The answer is that we don't know because nobody's looking (except for some instances of sympatric speciation), because they are mainly looking for classical selection or just assuming it and making up plausibility stories.  There is no reason that every situation should be expected to have the same explanation.  One feature about evolution that seems clear is that each case is different; that is close to being a fundamental attribute of evolution.

There may well be examples of adaptations that could serve to test the idea (one example might be human organismal selection for living at hypoxic high altitude, as in the Andes or Himalayas: how much did people die because of hypoxia when they somehow were forced to live there, as opposed to people settling there because they could do well).   Simplistic answers aren't likely to be found here any more than in the usual Darwinian cases. And the same skeptical question could rightly be asked of these latter, often simplistic tales: how often is the classical process the correct explanation--and how can we know?

This process in no way vitiates the possibility of adaptive evolution in a classical Darwinian competitive way, and nothing mystical or immaterial is involved.  It doesn't mean there is not also a war of all against all in Nature, it doesn't imply 'soft' (Lamarckian) selection.  It is as genetically based as classical Darwinism.  It does not 'overturn' Darwin or anything like that.  However, organismal selection is a means by which complex traits can arise without his idea of evolution driven by competitive natural selection.  Unless, of course, you just want to define any evolutionary change is due to natural selection.


Manoj Samanta said...

great series !!

I am wondering how natural selection explains the phenomenon of consciousness.

Ken Weiss said...

Reply to Manoj,
Nobody can explain this at present, because nobody knows what consciousness 'is', or what species have it. Going back to Wm James in the 19th century and up through Francis Crick (yes, the DNA guy) and colleagues, all we can do is study the 'neural correlates of consciousness'.

But selection by favoring whatever trait works can lead to the evolution of consciousness if the latter has had a history of conferring higher fitness on its bearers. It would not be inevitable, just if the right traits, from rudiment to what exists now, happen to have been present.

So I think selection per se is not the issue, indeed, the captivating and mesmerizing appeal of natural selection is just that: it can bring about whatever is presented as alternatives.

At the same time, so might the various other forms of differential proliferation that we try to discuss in this series.

My own view is that the existence of consciousness, is itself evidence of evolution having favored judgment vs hard-wiring, the ability (however it works) to assess sensory input, think about it, and make what appear to be the best decisions on how to respond. That is a form of flexibility that is the very opposite of the genomic hard-wiring that so many, in our day of hyper genomic determinism, seem to embrace.

One can construct scenarios by which organismal selection, rather than natural selection, led to consciousness in what eventually became our species (or those that have it). If we're the only ones, it may have to do with language or our form of abstract thinking, and those with similar bents aggregated (organismal selection).

I like the thought! But at this stage it is just the kind of Just-So story we should all try to avoid!

Anonymous said...

Are there traits that neither hinder nor help reproductive success (greying of hair) or were all traits originally adaptive but remain as "harmless" vestiges like the appendix? Is the "vestigialization" of traits adaptive or is it like "junk" DNA? What is junk DNA for that matter? It was presumably adaptive at one point or was it just a harmless and not helpful accident?

I'm sorry, too many questions; I'll just patiently read-on.

Ken Weiss said...

Reply to Anonymous:
Even Darwin recognized vestigial traits. Many traits, as many if not perhaps most individual nucleotides in genomes, have little if any function. Some may at one time have had a function and when environmental or other genomic etc circumstances changed, those traits became irrelevant to selection. Or, they could be the functionless by-product of some, say, developmental process that also yields an important structure or function.

There's no one rule--perhaps that IS the rule!

The appendix is, however, thought to have some immune and/or other functions (if you Google it you can find out); it's I think out of date to consider it vestigial. Our pheromone sensing vomeronasal system may be closer to that--perhaps.

As to 'junk' DNA, that idea is past its sell-by date. Much DNA has little or no known function, but there can be many subtle DNA functions that we simply don't know about. The Encode project is identifying transcribed (into RNA) bits of DNA whose actual function (if any) is currently unknown.

If you go to Evolutionary Anthropology and look for my article "Do we understand the genetic basis of evolution?" vol 23 88-92, this year, and my article in the prior issue on "What works works" I discuss some of these things.

Functions can come and go, be minor but not zero in strength, or be very indirect. The one thing I believe we can be sure of is that we can't be sure, that 'natural selection' is not the only criterion, especially as regards conservation, and that chance is important--topics coming up in this blog-post series.