Comments on: The Modes of Natural Selection

By: shleep

shleep — Fri, 18 May 2018 17:39:22 +0000

im shleep

By: Sandy

Sandy — Wed, 27 Mar 2013 14:06:55 +0000

Evolution denier for life? It’s so sad to me, and immeasurably more scary than Darwin’s position, that a Christian(?) would think that a humanly derived theory could destroy Christianity! Is the healing/saving message of the Christ so weak and conquerable? Where is your faith?

By: Mike

Mike — Thu, 27 Aug 2009 01:53:14 +0000

Thanks for that blog-entry – I think it’s about time that the fact that a particular trait might be spread so that no selection pressure is exerted on any value of it gets the attention it deserves. In addition, we might even consider situations where a trait is uniformly distributed so that concerning this trait, there is null selection. Or we might consider traits with have a flat fitness landscape (presuming we can – as some suggest, work with a fitness-concept for traits) and thus null selection.

By: Michael Ralston

Michael Ralston — Wed, 26 Aug 2009 21:39:11 +0000

@2: A bit of thought should indicate that stabilizing selection should be the most common form of selection (well, of the classical three) – directional and disruptive selection are inherently self-limiting, as they’ll drive the population into regimes where they don’t affect a trait anymore, or where some other force comes up on the other end and turns it into stabilizing selection.

By: Jim Thomerson

Jim Thomerson — Wed, 26 Aug 2009 21:23:15 +0000

Jared, Good points. As I understand it, the Hardy-Weinberg condition vis mutation is either (1) no mutation, or (2) mutation in equilibrium. I was using the simplest case to make the point that mutation frequencies are not a matter of magic, but rather a matter of mathmatically describable interactions. I will also say that my example was about as complex as my mathematics can deal with.

By: Jared

Jared — Wed, 26 Aug 2009 19:15:15 +0000

Jim, mutations don’t exist in equillibrium like the dissociation constant of a weak acid. Hardy-Weinberg equilibrium is a result of allelic frequencies in a population not subject to pressure, not mutation from one to the other. Differential rates of mutation do exist, but not from A to A1 and A1 to A, but from A to A1, A to A2, A to A3… and A1 to A1A, A1A to A1B, A1B… and A9Z9Z to A9Z9Z1…

There are some “back mutations,” but these are mostly compensatory mutations rather than reverting exactly to the parent genetic sequence.

By: Jim Thomerson

Jim Thomerson — Wed, 26 Aug 2009 19:03:52 +0000

I take your “null selection” to mean that we have a Hardy-Weinberg situation except with respect to neutral mutations. To take the simplest case: We have a gene which exists in a single form and is neutral, ie. p=1. A mutation, similarly neutral, occurs, so we now have a small q and p is less than 1. Call the genes A and A1. A is going to have a rate of mutation to A1. Similarly A1 is going to have a rate of back mutation to A. Eventually, equilibrium will be reached, where the number of A to A1 mutation equals the number of A1 to A mutations during a period of time. p and q will reflect the ratio of A to A1 at equilibrium and will not change into the future unless we change a paramater. The “null curve” is not a random curve, but is rather determined relative mutation rates.

By: Greg Laden

Greg Laden — Wed, 26 Aug 2009 11:33:07 +0000

qbsmd: I have done it in the manner you suggest, and it does work very nicely. Or, really, one animation for each mode.

In fact, these pictures often fail in a classroom unless the instructor gives a LOT of support as to how these graphs are constructed and what they mean.

I have used this post almost exactly as it is as a supplement to pre-existing course material that talks bout modes. It works pretty well in that case as well.

By: qbsmd

qbsmd — Wed, 26 Aug 2009 11:21:56 +0000

Your explanation is inconsistent with respect to including mutations; you started out without them, and then included them.

“Note that the upper end of the distribution does not move up … in other words, directional selection does not simply move the bell curve along in one direction.”

“In null selection there are no Arrows of Selection happening to the bell curve, but there is still the constant introduction of mutations, so over time the distribution goes wacky and essentially becomes random.”

To avoid confusion, I would recommend including three pictures for each mode: bell curve before, bell curve after selection, and bell curve after multiple generations of selection and mutation.

By: Greg Laden

Greg Laden — Wed, 26 Aug 2009 07:44:28 +0000

If Darwin looks like anybody it’s William Jennings Bryan.