Evolucion Por Seleccion Natural Mas Evidencia Que Nunca (Nespolo, 2003) | Evolution

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  699 EVOLUTION BY NATURAL SELECTION   Revista Chilena de Historia Natural76: 699-716, 2003 Evolution by natural selection: more evidence than ever before Evolución por selección natural: más evidencias que nunca ROBERTO F. NESPOLOInstituto de Ecología y Evolución, Facultad de Ciencias, Universidad Austral de Chile.Casilla 567, Valdivia, Chile; e-mail: robertonespolo@uach.cl ABSTRACT The modern evolutionary theory, understood as the integration of the empirically-demonstrated theoreticalfoundations of organic evolution, is one of the most pervasive conceptual frameworks in biology. However,some debate has arisen in the Chilean scientific community regarding the legitimacy of natural selection as amechanism that explains adaptive evolution. This review surveys the recent evidence for natural selection andits consequences on natural and artificial populations. In addition to the literature review, I present basicconceptual tools for the study of microevolution at the ecological scale, from a quantitative point of view. Theoutcome is clear: natural selection can be, is being, and has been quantified and demonstrated in both the fieldand in the laboratory,not many, but hundred of times during the past decades. The study of evolution bynatural selection has attained maturity, which is demonstrated by the appearance of several syntheses andmeta-analyses, as well as “evolutionary applications” where evolution by natural selection is used to resolvepractical problems in disciplines other than pure biology. Caution is required when challenging evolutionarytheory. The abundant evidence supporting this conceptual body demands a careful examination of availableevidence before dogmatically critizing its theoretical foundations. Key words: evolution, natural selection, adaptations, heritability, directional selection differential, artificial selection. RESUMEN La teoría moderna de la evolución, entendida como la integración del conocimiento teórico y empírico de laevolución orgánica, desarrollado desde Darwin hasta ahora, es uno de los cuerpos conceptuales más impor-tantes en biología. Sin embargo, cierto debate ha surgido en el medio científico local en torno a la validezde la selección natural como mecanismo explicativo de la evolución adaptativa. Este artículo revisa lasevidencias recientes sobre el rol de la selección natural en poblaciones naturales y artificiales. Además, sepresentan algunas herramientas conceptuales básicas necesarias para el estudio de la microevolución aescala ecológica, las que se discuten a la luz de la información mostrada desde un punto de vista cuantitati-vo. El resultado es claro: la selección natural puede ser, está siendo y ha sido medida y demostrada en elcampo y en el laboratorio, no muchas, sino cientos de veces durante las últimas décadas. El estudio de laevolución por selección natural ha alcanzado una fase de madurez que es demostrada por la aparición devarias síntesis y metaanálisis así como también por el comienzo de “aplicaciones evolutivas”, donde laevolución por selección natural es utilizada para resolver problemas prácticos en disciplinas diferentes a labiologia básica. Se concluye que se necesita cautela cuando se cuestiona la teoría evolutiva. La grancantidad de evidencia disponible exige un esfuerzo serio por leer y analizar dicho conocimiento antes decriticar sus fundamentos teóricos. Palabras clave:  evolución, selección natural, adaptaciones, heredabilidad, diferencial de seleccióndireccional, selección artificial. INTRODUCTION In recent years there have been claims –in the dailypress, on television, and by retired cosmologists–that Darwin may have been wrong… However, tosee Darwinism as being under serious threatwould, I think, be a false perception.John Maynard-SmithThe scientific method relies on skepticism,experiments and demonstration. To be acceptedin the scientific community, new hypothesesmust be based on strong proofs. Only then, thehypothesis becomes a theory. This is the wayby which science progresses: upon a permanentand recursive self-validation (Sagan 1979,Levins & Lewontin 1985). One of the most COMMENTARY  700 NESPOLO pervasive theories in biological sciences ismodern evolutionary theory 1 , with naturalselection as the main mechanism explainingadaptations (Williams 1966, Stenseth 1999,Gould 2002). However, as with other theoriesin biology, the modern theory of evolution is aconceptual body of knowledge that integratesseveral interdisciplinary fields. This modernsynthesis has been developed during more than150 years, from Darwin to the present, andintegrates Mendelian genetics, systematics,paleontology, and ecology into a coherenttheory of evolution. More recently, modernsynthesis also combines the theory of naturalselection with the emerging understanding of how genes are transmitted from one generationto another (Stenseth 1999). This frameworkinvolves verbal propositions, metaphors,mathematical models and statistical methods(e.g., Michod 1999, Gould 2002). Dependingon the timeframe, spatial and organizationallevel of study, the analysis of evolution takesdifferent approaches, although commonfeatures persist. The main mechanism of adaptive evolutionary change (sensu Williams1966) is natural selection, which can act atdifferent organizational levels (Lewontin 1970,Vrba & Gould 1986, Nunney 1999, Weber2000). In most cases, specially at the ecologicaltime scale, the evidence suggests that theorganism is the main unit of selection(Williams 1966, Maynard-Smith & Price 1973).Hence, the raw material for selection isintraspecific variability (Fisher 1930, Haldane1932, Wright 1988).Felsenstein (1988) pointed out that:“Systematists and evolutionary geneticistsdon’t often talk to each other, and theyroutinely disparage each other’s work as beingof little relevance to evolution. Systematistssometimes invoke the punctuationist argumentthat most evolutionary change does not occurby individual selection and hence that within-population phenomena are largely irrelevant toevolution… Evolutionary geneticists in turndismiss the idea that studies comparing speciesanciently diverged, using morphologicalcharacters far removed from the level of thegene and using nonquantitative methods, caneither be sound in their inferences of pattern orcan shed much light on evolutionaryprocesses”. Although this is a caricaturizedview of two different schools in evolutionarybiology (i.e., systematics versus evolutionarygenetics), some of this confrontation is presentin the Chilean style of teaching evolutionwhere, I believe, the former (systematics)approach prevails.There is a long tradition of evolutionarythinking in Chile (Manríquez & Rotthammer1997), however, undergraduate courses of evolution have been markedly biased to favorthe systematic-taxonomical and historical viewof evolution (Camus 1997, Manríquez &Rotthammer 1997). Popular topics in courses of evolution are the vitalism-evolutionist debate,the srcin of life on Earth, biogeography,phylogenetics and comparative methods, andphyletic gradualism versus saltationism. Thismay provide an adequate picture of the historyof systematic evolutionary thought, but it is nota realistic picture of current research inevolutionary biology. In these evolutionclasses, natural selection –the mechanism of adaptive evolutionary change–, and the analysisof variation –the raw material for naturalselection– are usually mentioned directly fromDarwin words or anecdotic and qualitativeexamples 2 . These concepts are not taught alongwith the well-established quantitative toolsdeveloped to measure them. In short, the proofsfor natural selection or evolution itself areusually not teached in Chilean undergraduatecourses. As a consequence, it is common tohear comments such as “…nobody can proveevolution…” or “…it is impossible to measurenatural selection…” in classrooms, and even inscientific meetings 2, 3 . Moreover, one may seepublications in local scientific media, whichgive naïve and qualitative examples, such asbirds feeding in suboptimal food patches, toclaim that evolutionary theory is obsoletebecause it does not explain such apparentlynon-adaptive behavior (Marone et al. 2002).Worse yet, some biologists appear to recall pastand expired controversies, such as the obsoletedogma “one gene, one trait” as actuallimitations to evolutionary theory (Maturana &Mpodozis 2000). In short, any course of  1  This theory is understood as the integration of the empi-rically demonstrated theoretical knowledge of organicevolution developed from Darwin to present. I do notagree with the idea of a single “evolutionary theory” ortheories that are mutually exclusive in explaining thesame evolutionary processes as Manríquez & Rothham-mer presented it (1997, see the criticism by Camus 1997).Similarly, changes in the names and adjectives (e.g.,“modern”, “neodarwinian”, “synthesis”) used to describethis knowledge are a matter of preference, but it does nothelp to avoid jargon. 2  Direct experience of the author in undergraduate coursesat P. Universidad Católica de Chile and Universidad deChile from 1995 to 2000. 3  Reunión conjunta de las sociedades de ecología de Ar-gentina y Chile, see also Marone et al. (2002).  701 EVOLUTION BY NATURAL SELECTION evolution should take care of the whole body of theoretical and empirical knowledgeaccumulated during the last 150 years, andit may include some less well knownmechanisms as long as some minimumevidence supports them.There is a general problem of ignorance of science and especially regarding the facts of evolution. Many people, including scientists andthe lay public, are unaware of the relevancy of evolutionary biology. Furthermore, the attacks tosupress the teaching of evolution have receivedwidespread support at the local level in the USA(Antolin & Herbers 2001, Bull & Whichman2001). This is just a consequence of a crisiswhich is affecting evolutionary biology and isevidenced in simple facts. For example, 35 % of American college graduates think that “theearliest humans lived at the same time asdinosaurs” and 42 % indicated that they did notthink “human beings, as we know them today,developed from earlier species of humans”(Alters & Nelson 2002). These authors suggestthat such ignorance follows from deficientmethods of undergraduate teaching.What the last paragraph, regardingmissconceptions of evolution facts in the USA,has to do with former discussions about theChilean teaching of evolution? I believe both areepiphenomena of the same general problem:incorrect teaching of evolution. For example, itis not surprising to find out that both graduateand undergraduate Chilean students of ecologyand evolution believe that evolution cannot betested experimentally 2 . Moreover, somebiologists strongly believe that natural selectionis not a mechanism that explain adaptations(Maturana & Mpodozis 2000, Marone et al.2002). In fact, some of them proposed a newevolutionary theory, the so-called “natural drift”(Maturana & Mpodozis 1992, 2000), which inpart, stimulated this commentary.Among other arguments, Maturana &Mpodozis (2000) claim that modernevolutionary theory fails to explain adaptations,or that it has been misinterpreted.Unfortunately, the work of Maturana &Mpodozis (2000) is weak in at least three basicaspects of any new hypothesis. First, the poorand tautological writing makes it hard tofollow, a point that has been criticized in detailelsewhere (Gallardo 1997, Manríquez &Rothhammer 1997). Second, it ignores at least30 years of ecological-evolutionary research,which explains why they find so many factsthat modern evolutionary theory cannot accountfor (e.g., non-adaptive traits, constraints toevolution, neutral change). These criticisms arementioned by Gallardo (1997) and Manríquez& Rothhammer (1997), but mostly from thesystematic perspective. Third, in nearly 50pages (and 28 references, six of which are self citations), they do not present a single case thatsupports the natural drift (Maturana &Mpodozis 2000). This last point has not beendiscussed in detail before. Obviously, empiricalevidence is the most important structuralsupport for any hypothesis that is posed tobecome a theory.This review is directed to students andyoung biologists in Chile, and was motivatedby the general problem of a lack of knowledgeabout evolution, and the challenge placed byMaturana & Mpodozis (2000) to modernevolutionary theory. In science all new ideasmust be open to debate. However, students of science need to be presented with proofs of what they are learning. Many new ideas areinteresting and appealing, but if not subjectedto verification by systematic research, they areno longer scientific and become dogmatic (e.g.,Fischer 2001, see review in Gallardo 2001).Dogmas are dangerous when taught as truths.Even worse, teaching that a well establishedtheory, such as evolution by natural selection,is simply wrong (Antolin & Herbers 2001), asit occurs in Chile (Maturana & Mpodozis2000), could have devastating consequences inthe formation of new scientists.Here, I offer a short, representative reviewof the recent evidence for natural selectionfrom the perspective of quantitative geneticsand phenotypic selection. In this review Idefend that (1) a conceptual framework tostudy evolution experimentally does exist; (2)that natural selection is the main force of adaptive change in natural populations, and that(3) both natural selection and evolution can be,are being and have been measured anddemonstrated both in the field and in thelaboratory, not a few times, but hundreds of times during the past decades. CONCEPTUAL FRAMEWORK Three critical elements must be kept in mindwhen studying evolution by natural selection: (i)that a trait exhibits intraspecific variation, (ii) thatthis variation is heritable, and (iii) that the trait isthe target of natural selection. To characterizethese processes, some formalizations are needed.In a large enough population, a metric traitis distributed in a continuous fashion. Thesekinds of traits are usually codified by severalgenes of small effects (Roff 1997). Assuming  702 NESPOLO that natural (or sexual) selection actsdirectionally over this trait (Fig. 1A), there willbe a number of individuals surviving theselective event. The important point here is thatthis process modifies both the variance and themean of the distribution. Both effects haveprofound consequences to the population: themean is changed by a value, “S”, and thevariance is reduced. In the next generation, theoffspring of selected individuals will presentthe changed mean only if resemblance in thetrait exists between parents and offspring.Formally, S is defined as the selectiondifferential, such that:S = µ  - µ o (1)where µ o  and µ  are the population means beforeand after selection, respectively (Fig. 1A). If thetrait is completely inherited, µ 1 , the population Fig. 1:  (A) Directional selection acting on the right tail of a distribution of a metric trait in a largepopulation. Large and small curves represent the distribution of the trait before and after selection.Similarly, µ o  and µ  represent the mean populational values before and after selection. (B) Directio-nal selection differential (S) and response to selection (R) in a hypotetical trait with narrow-senseheritability (h 2 ) close to one. The mean populational values before and after selection are represen-ted as µ o  and µ , respectively. The mean of the trait in the descendents of the selected individuals is µ 1 . (C) Directional selection differential (S) and response to selection (R) in a hypothetical traitwith narrow-sense heritability (h 2 ) of intermediate value. (D) Directional selection differential (S)and response to selection (R) in a hypothetical trait with narrow-sense heritability (h 2 ) close to zero. (A) Selección direccional actuando en la cola derecha de la distribución de un rasgo métrico en una gran población. Lascurvas grandes y pequeñas representan la distribución del rasgo antes y después de la selección. Similarmente, µ o  y µ representan las medias poblacionales antes y después de la selección. (B) Diferencial de selección direccional (S) yrespuesta a la selección (R) en un rasgo hipotético con heredabilidad en sentido estricto (h 2 ) cercana a uno. La mediapoblacional antes y después de la selección son representadas como µ o  y µ , respectivamente. La media poblacional delrasgo en los descendientes de los individuos seleccionados es µ 1 . (C) Diferencial de selección direccional (S) y respuesta ala selección (R) en un rasgo hipotético con heredabilidad en sentido estricto (h 2 ) de valor intermedio. (D) Diferencial deselección direccional (S) y respuesta a la selección (R) en un rasgo hipotético con heredabilidad en sentido estricto (h2)cercana a cero.
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