Example: confidence

Limits to natural selection - UFSCar

Limits to natural selectionNick Barton1* and Linda Partridge2 SummaryWe review the various factors that limit adaptation bynatural selection . Recent discussion of constraints onselection and, conversely, of the factors that enhance``evolvability'', have concentrated on the kinds of varia-tion that can be produced. Here, we emphasise thatadaptation depends on how the various evolutionaryprocesses shape variation in populations. We survey thelimits that population genetics places on adaptive evolu-tion, and discuss the relationship between :1075 1084, John Wiley & Sons, selection follows inevitably from inherited variationin the ability to reproduce. The systematic accumulation ofchance variations is the only process that can lead to biologicaladaptations, and has produced impressive results: organismsthat can live in extreme environments and on diverse energysources,(1)finely honed organs such as the ribosome, the eyeor the human brain, and developmental programs that canreliably construct such

Limits to natural selection Nick Barton1* and Linda Partridge2 Summary We review the various factors that limit adaptation by natural selection. Recent discussion of constraints on

Tags:

  Selection, Natural, Limits, Limits to natural selection

Information

Domain:

Source:

Link to this page:

Please notify us if you found a problem with this document:

Other abuse

Transcription of Limits to natural selection - UFSCar

1 Limits to natural selectionNick Barton1* and Linda Partridge2 SummaryWe review the various factors that limit adaptation bynatural selection . Recent discussion of constraints onselection and, conversely, of the factors that enhance``evolvability'', have concentrated on the kinds of varia-tion that can be produced. Here, we emphasise thatadaptation depends on how the various evolutionaryprocesses shape variation in populations. We survey thelimits that population genetics places on adaptive evolu-tion, and discuss the relationship between :1075 1084, John Wiley & Sons, selection follows inevitably from inherited variationin the ability to reproduce. The systematic accumulation ofchance variations is the only process that can lead to biologicaladaptations, and has produced impressive results: organismsthat can live in extreme environments and on diverse energysources,(1)finely honed organs such as the ribosome, the eyeor the human brain, and developmental programs that canreliably construct such organs from the limited informationencoded in the genome.

2 Over much shorter times, artificialselection has dramatically increased yields from domesticatedplants and animals; applied to populations of RNA moleculesin vitro, it has produced novel catalytic activities ( ).In this essay, we consider what Limits the power of selec-tion to produce such adaptations. We can ask how rapidlypopulations respond to selection and what Limits the rate ofresponse, whether any absolute limit is reached, and, if so,what determines it. These are practical questions for thedesign of artificial selection schemes, either in vivo or in applied to natural evolution, we may also ask whetheractual rates of adaptation are close to any limit, and whetheractual genetic systems have evolved so as to improve theresponse to selection .

3 This last question is especially delicate,since species do not in general evolve so as to optimise theirown evolvability. In order to show that populations haveevolved so as to facilitate further evolution, one would need toshow either that selection within populations favours individualalleles that modify the genetic system appropriately, or thatselection between groups can overcome the intrinsicallystronger force of selection between individuals. Such argu-ments require the greatest care.(3)There have been recurrent arguments that straightforwardnatural selection is in some way unable to explain variousfeatures of evolution. Following Darwin and Wallace'sproposal of adaptation by natural selection , it was argued thatthere had not been enough time since the formation of theearth for natural selection to act (Ref.)

4 4 p206), and that sexualreproduction and blending inheritance would eliminate thevariation on which selection depends.(5)These doubts weresettled at the turn of the century by the discovery of radioactivedecay, which established the antiquity of the earth, and by therediscovery of Mendelian heredity, which maintains variationfrom generation to generation. The early Mendelian geneti-cists argued that natural selection could not generate novelty,since it could only sift amongst genotypes that already existedin the population (Ref. 4, p238). However, selection does notoccur in a single step: it is the accumulation of successivefavourable variants that produces genotypes that are, overall,exceedingly improbable.

5 (6)Wright(7)emphasised that selec-tion on a single population cannot establish variants that arefavourable in combination, but individually deleterious. Heproposed that this limitation could be circumvented by a``shifting balance'', involving selection among populationscarrying different gene combinations. The discovery in mid-century of extensive variation in protein and DNA sequence,both within and between species, stimulated Kimura(8)topropose that most of this variation has no effect on fitness, onthe grounds that it could not all be maintained by , the striking stasis of species both across wide geo-graphic regions, and over long geological timespans, hassuggested that special mechanisms such as founder eventsare required for new species to form,(9)and that macroevolu-tion is uncoupled from microevolution.

6 (10)Here, we try to make sense of these disparate argumentsby classifying and clarifying the various kinds of Limits toselection. We consider (1) constraints on variation that arisefrom limitations on what can be built, (2) lack of a chain of fittransitional forms leading to an optimal phenotype, (3) limitsinherent in the process of selection , set by the number ofselective deaths and by interference between linked variants,(4) Limits due to all the other evolutionary processes, whichBioEssays 22:1075 1084,fl2000 John Wiley & Sons, of Cell, Animal and Population Biology, University ofEdinburgh, of Biology, University College London, agencies: BBSRC and NERC, and the Darwin Trust ofEdinburgh.

7 *Correspondence to: Dr. Nick Barton, Institute of Cell, Animal andPopulation Biology, University of Edinburgh, Kings' Buildings,Edinburgh EH9 articlesnecessarily tend to oppose adaptation by natural selection ,and (5) Limits caused by conflicting natural discussion has tended to be of the first kind of limit:throughout the history of evolutionary biology, there has beena tension over whether the course of evolution is determinedprimarily by selection among slight variants, or by constraintson the kinds of major variant that can be produced. This isapparent in Jenkin's(5)concerns over blending inheritance, inthe bitter conflict between biometricians and Mendelians at theturn of the century, and on through to current debates over theevolutionary consequences of developmental and geneticmechanisms.

8 ``Developmental constraints'' have been muchdiscussed elsewhere ( Refs 3,11,12) and we do not dis-cuss these further. In this article, we take a population-geneticview, and survey a variety of Limits on the evolution of wholepopulations. selection acts on variation that has accumulatedin populations and that is influenced by a variety of processes:it is not simply a matter of whether or not a particular favourablevariant can be to variationThe most obvious limit to natural selection is that suitablevariation may not be available. This may be because certainphenotypes cannot be built, being ruled out either by physicallaw or by the properties of biological materials. Someexamples of such limitations are: (1) enzyme catalysis by therate of diffusion of substrates to the active site, (2) mutationrate by the thermodynamics of base pairing, (3) reaction timesby the speed of propagation of nerve impulses, and (4) theacuity of the eye by spherical aberration.

9 These Limits dependon assumptions about what is possible, and may be circ-umvented by the ingenuity of natural selection . For example,(1) sparse molecules diffusing in three dimensions may beconcentrated by absorption onto two- or one-dimensionalsurfaces, as on a moth's antenna,(13)(2) mutation rates maybe reduced by successive proof-reading mechanisms,(14)and(3) spherical aberration can be abolished by a pupil that closesdown to the diffraction limit. Nevertheless, we can supposethat there is a set of possible phenotypes, while bearing in mindthat this set occupies a space with more dimensions than wecan imagine. There is necessarily no variation across theboundary of this set, and hence no response to selection forphenotypes that cannot be realised (Ref.)

10 3 Chapters 6,11).Long-term artificial selection is a powerful way of exploringthe space of possible phenotypes. Darwin's strongest evi-dence for the power of natural selection was by analogy withthe dramatic success of artificial selection ,(Ref. 15, Chapter 1)and studies since Darwin's time have confirmed his view. Whatis remarkable is that almost all traits respond to selection ,and that selection on large sexual populations causes asustained response over many generations.(16,17)For exam-ple, Weber(18)selected on the angle between two wing veins inDrosophila melanogasterfor 20 generations, and achieveddivergence of 20 phenotypic standard deviations. There wasno sign of a decrease in response over time, suggesting thatthe response is due to variants of slight effect.


Related search queries