Great job explaining why genetic diversity is such an important quality of a vigorous population

201007252214.jpg by Ego Sum Daniel

It’s more than genes, it’s networks and systems
[Via Pharyngula]

Most of you don’t understand evolution. I mean this in the most charitable way; there’s a common conceptual model of how evolution occurs that I find everywhere, and that I particularly find common among bright young students who are just getting enthusiastic about biology. Let me give you the Standard Story, the one that I get all the time from supporters of biology.

Evolution proceeds by mutation and selection. A novel mutation occurs in a gene that gives the individual inheriting it an advantage, and that person passes it on to their children who also gets the advantage and do better than their peers, and leave more offspring. Given time, the advantageous mutation spreads through the population so the entire species has it.

One example is the human brain. An ape man millions of years ago acquired a mutation that made his or her brain slightly larger, and since those individuals were slightly smarter than other ape men, it spread through the population. Then later, other mutations occured and were selected for and so human brains gradually got larger and larger.

You either know what’s wrong here or you’re feeling a little uneasy—I gave you enough hints that you know I’m going to complain about that story, but if your knowledge is at the Evolutionary Biology 101 level, you may not be sure what it is.

Just to make you even more queasy, the misunderstanding here is one that creationists have, too. If you’ve ever encountered the cryptic phrase “RM+NS” (“random mutation + natural selection”) used as a pejorative on a creationist site, you’ve found someone with this affliction. They’ve got it completely wrong.

Here’s the problem, and also a brief introduction to Evolutionary Biology 201.

First, it’s not exactly wrong — it’s more like taking one good explanation of certain kinds of evolution and making it a sweeping claim that that is how all evolution works. By reducing it to this one scheme, though, it makes evolution far too plodding and linear, and reduces it all to a sort of personal narrative. It isn’t any of those things. What’s left out in the 101 story, and in creationist tales, is that: evolution is about populations, so many changes go on in parallel; selectable traits are usually the product of networks of genes, so there are rarely single alleles that can be categorized as the effector of change; and genes and gene networks are plastic or responsive to the environment. All of these complications make the actual story more complicated and interesting, and also, perhaps to your surprise, make evolutionary change faster and more powerful.

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This is a great discussion of something that probably many people who took biology many years ago (or even sooner) – most biological processes involve network effects. Vew proteins are so important that they are directly selected for. Most are parts of networks where their individual properties are somewhat buffered.

I have talked a lot, both here and at my other blog, SpreadingScience, about the need for genetic diversity and the ability of biological systems to continue to survive even when apparently “irreplaceable” proteins are removed. Genetic diversity allows a species to adapt rapidly to new environmental conditions.

In fact, I wrote about this for a column back in 2002 – an almost prehistoric time for the Web – about genetic buffering, a process where a tremendous amount of diversity can actually be hidden from selection until environmental stresses make them visible.

And an important thing to remember is that the selective pressures do not just come from the environment but from all sorts of other organisms in the ecosystem.

One thing to remember is that true and pure natural selection would tend to drive genes to the best possible – the most fit – actually removing diversity. To a first approximation, selection would seem to produce a single set of genes that are ‘best’ evolved for a particular environment. Any other set of genes would be less fit.

In reality, selection is often not that fine, there are a range of different gene products that can probably be almost equally fit and most biological systems are designed to support a wide range of diversity.

Most likely because organisms that can maintain diversity in the midst of selective pressure away from diversity will produce more offspring in a wide range of differing ecosystems. Organisms with multiple points of failure lost the evolution wars many, many years ago.

Being able to maintain high levels of genetic diversity is a benefit for any species and its individual members. Genetic networks and genetic buffering allow animals to be able to deal with a wide range of different environmental pressures while also providing for huge amounts of diversity, ready to respond to altered conditions very rapidly.

This is actually why some species can evolve very rapidly.

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