The Genetics of Adaptation: A Reassessment

H. Allen Orr and Terry A. Coyne

B11: Anthony Murphy

The Goal Review the theory and data supporting the

claim that adaptation results from accumulation of many alleles of small effect

Show that there is no real solid data to back up this claim

With the introduction of molecular approaches, there might be the ability to resolve this problem in the future

Micro-mutationismOriginated with Charles

DarwinDarwin and followers

believed evolution of adaptive systems happened in substitution of many genes of small effect

Darwin wrote “We have many slight difference which may be called individual differences…..these differences are important for us, as they afford materials for natural selection”

Macro-Mutationalism Originated with T.H. Huxley, a

colleague of Charles Darwin Huxley and followers believed

that adaptive evolution takes place in large steps with no intermediate steps

Huxley wrote: “Mr. Darwin’s position might..have been stronger than it is if he had not embarrassed himself with the aphorism, ‘Natura non facit saltum’…We believe that Nature does make jumps now and then, and recognition of the fact is of no small importance in disposing of many minor objections to the doctrine of transmutation.”

R.A FisherChampion of the Micro-

mutationism Theory Argued that adaptation

involves “conformity of parts” of the system

Large Random Change would cause “worsening of function”

He believed large mutations have a very small changes of being favorable

Fisher’s Microscope AnalogyConsider a

microscope a system for adaptation

A large change will almost always worsen the focus in an ideal microscope

However, small changes are less likely to harm the focus and might improve it

Fisher’s Sphere ExampleArrangement of parts

giving the highest fitness is the center of the sphere

A species’ present position can be represented by a part of the sphere surface

Any displacement inside the sphere improves adaptation

Any displacement outside the sphere worsens adaptation

Why Fisher is Incorrect Fisher shows only that small

mutations are more likely to be favorable, not that they are more likely to be substituted

The substitution rate of a class does not depend only on the chance of it being advantageous, but also on the mutation rate and fixation rate (According to Kimura)

Fisher’s argument does not apply to cases in which an adaptive landscape has more than one single peak

Fisher assumes that all mutations have a similar effect on fitness no matter how they interact in the body

Lande (1983)Concluded that, unless selection is very

strong and persistent, adaptation will usually result from substitution of micromutations.

Explains why artificial selection or selection in disturbed populations usually fix mutations of large affect while natural selection fixes polygenes

Problems with Lande’s ModelIt is considered, by the authors, to be an

improvement over Fisher, but still has problems

It is not clear that evolution in nature usually involves selection toward an ideal form

Lande’s theory also may have features that predispose it against the fixation of large mutations

However, it should be pointed out that no model can answer the question of relative importance between major or minor genes.

Frequency of Major and Minor MutationsThe rate of substitution of major genes

depends on how often they are produced by mutation

It is also said that mutations with small effects arise more often than those with major effects

There are many individuals who support the claim that mutations affecting viability

Only one (Gregory) investigated whether the same is true for genes affecting morphology

Gregory (1965)Compared the phenotypes of peanut plants

descended from seeds irradiated by X Ray and control plants

Concluded that mutations of large phenotypic effect are far rarer than small effect mutations

Problems with GregoryNo evidence that the mutations were genetic, Assessment of effects was not the same

between the two experimental variablesThe experiment was bias against the recovery

of large mutations

Genetic Analysis of AdaptationArtificial Selection- Selection for traits in

breedingDisturbed Populations in Nature- Pioneer

Species, Resistance to environment, Arms race

Adaptations in Natural Populations- Batesen and Mullerian Mimicry

Differences among Species and Subspecies- Genetic Difference between species of organisms

Future Research There are only a few genetic analyses of

species differences that are clear adaptations While some adaptations are based on genes

of small effect, sometimes major genes are involved

Traditional approaches can’t answer the central question

Must use genetic factors such as linkage and chromosome analysis

ConclusionThey found little support that adaptation

always involves genes of small effectIt has been found that major gene effects are

not as rare as they were once thoughtThere is a need in the near future to perform

more genetic studies of adaptation