Que Es La Seleccion Natural Y Artificial
Gro Bert
English naturalist Charles Darwin developed the idea of natural selection after a five-year voyage to study plants, animals, and fossils in South America and on islands in the Pacific. In 1859, he brought the idea of natural selection to the attention of the world in his best-selling book, On the Origin of Species.
Natural selection can lead to speciation, where one species gives rise to a new and distinctly different species. It is one of the processes that drives evolution and helps to explain the diversity of life on Earth.
Darwin and other scientists of his day argued that a process much like artificial selection happened in nature, without any human intervention. He argued that natural selection explained how a wide variety of life forms developed over time from a single common ancestor.
Mutations are changes in the structure of the molecules that make up genes, called DNA. The mutation of genes is an important source of genetic variation within a population. Mutations can be random (for example, when replicating cells make an error while copying DNA), or happen as a result of exposure to something in the environment, like harmful chemicals or radiation.
If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection. Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct. Dinosaurs are one example. An invasive species, a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species.
Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions. Extinctions seem to be occurring at a much faster rate today than they did in the past, as shown in the fossil record.
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La seleccin artificial en la mostaza salvaje
Por ejemplo, la coliflor, el brcoli, la col, la berza y el colinabo tienen aparentemente poco parecido con su pariente: la mostaza silvestre. Sin embargo, despus de muchas generaciones de seleccin artificial, se consiguieron estos cinco tipos de cosechas seleccionando caracteres distintos de un mismo ancestro comn.
Seleccin artificial en los perros domsticos
La domesticacin del perro nos da otro ejemplo espectacular del poder de la seleccin artificial. Los ancestros de los perros de compaa probablemente fueron domesticados a partir de mltiples linajes diferentes de lobos hace 20.000 a 40.000aos. Desde entonces, la seleccin humana ha producido una enorme variedad de razas (muchas de ellas seleccionadas artificialmente en los ltimos 200 aos). Por ejemplo, los perros salchicha se seleccionaron para levantar la caza y cazar animales pequeos, mientras que los pastores alemanes se seleccionaron para cuidar rebaos. Hay tal variedad de razas de perros domsticos actuales, tanto en tamao como en forma, que si no conocisemos su historia, podramos haberlos clasificado en docenas de especies diferentes.
Revista Chilena de Historia Natural COMMENTARY Evolution by natural selection: more evidence than ever before Evolucin por seleccin natural: ms evidencias que nunca ROBERTO F. NESPOLO Instituto de Ecologa y Evolucin, Facultad de Ciencias, Universidad Austral de Chile.
Casilla 567, Valdivia, Chile; e-mail: roberto...@uach.cl ABSTRACT
The modern evolutionary theory, understood as the integration of the empirically-demonstrated theoretical foundations 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 a mechanism that explains adaptive evolution. This review surveys the recent evidence for natural selection and its consequences on natural and artificial populations. In addition to the literature review, I present basic conceptual tools for the study of microevolution at the ecological scale, from a quantitative point of view. The outcome is clear: natural selection can be, is being, and has been quantified and demonstrated in both the field and in the laboratory,not many, but hundred of times during the past decades. The study of evolution by natural selection has attained maturity, which is demonstrated by the appearance of several syntheses and meta-analyses, as well as "evolutionary applications" where evolution by natural selection is used to resolve practical problems in disciplines other than pure biology. Caution is required when challening evolutionary theory. The abundant evidence supporting this conceptual body demands a careful examination of available evidence before dogmatically critizing its theoretical foundations.
La teora moderna de la evolucin, entendida como la integracin del conocimiento terico y emprico de la evolucin orgnica, desarrollado desde Darwin hasta ahora, es uno de los cuerpos conceptuales ms importantes en biologa. Sin embargo, cierto debate ha surgido en el medio cientfico local en torno a la validez de la seleccin natural como mecanismo explicativo de la evolucin adaptativa. Este artculo revisa las evidencias recientes sobre el rol de la seleccin natural en poblaciones naturales y artificiales. Adems, se presentan algunas herramientas conceptuales bsicas necesarias para el estudio de la microevolucin a escala ecolgica, las que se discuten a la luz de la informacin mostrada desde un punto de vista cuantitativo. El resultado es claro: la seleccin natural puede ser, est siendo y ha sido medida y demostrada en el campo y en el laboratorio, no muchas, sino cientos de veces durante las ltimas dcadas. El estudio de la evolucin por seleccin natural ha alcanzado una fase de madurez que es demostrada por la aparicin de varias sntesis y metaanlisis as como tambin por el comienzo de "aplicaciones evolutivas", donde la evolucin por seleccin natural es utilizada para resolver problemas prcticos en disciplinas diferentes a la biologia bsica. Se concluye que se necesita cautela cuando se cuestiona la teora evolutiva. La gran cantidad de evidencia disponible exige un esfuerzo serio por leer y analizar dicho conocimiento antes de criticar sus fundamentos tericos.
In recent years there have been claims -in the daily press, on television, and by retired cosmologists- that Darwin may have been wrong However, to see Darwinism as being under serious threat would, I think, be a false perception. John Maynard-Smith The scientific method relies on skepticism, experiments and demonstration. To be accepted in the scientific community, new hypotheses must be based on strong proofs. Only then, the hypothesis becomes a theory. This is the way by which science progresses: upon a permanent and recursive self-validation (Sagan 1979, Levins & Lewontin 1985). One of the most pervasive theories in biological sciences is modern evolutionary theory1, with natural selection as the main mechanism explaining adaptations (Williams 1966, Stenseth 1999, Gould 2002). However, as with other theories in biology, the modern theory of evolution is a conceptual body of knowledge that integrates several interdisciplinary fields. This modern synthesis has been developed during more than 150 years, from Darwin to the present, and integrates Mendelian genetics, systematics, paleontology, and ecology into a coherent theory of evolution. More recently, modern synthesis also combines the theory of natural selection with the emerging understanding of how genes are transmitted from one generation to another (Stenseth 1999). This framework involves verbal propositions, metaphors, mathematical models and statistical methods (e.g., Michod 1999, Gould 2002). Depending on the timeframe, spatial and organizational level of study, the analysis of evolution takes different approaches, although common features persist. The main mechanism of adaptive evolutionary change (sensu Williams 1966) is natural selection, which can act at different organizational levels (Lewontin 1970, Vrba & Gould 1986, Nunney 1999, Weber 2000). In most cases, specially at the ecological time scale, the evidence suggests that the organism is the main unit of selection (Williams 1966, Maynard-Smith & Price 1973). Hence, the raw material for selection is intraspecific variability (Fisher 1930, Haldane 1932, Wright 1988). Felsenstein (1988) pointed out that: "Systematists and evolutionary geneticists don't often talk to each other, and they routinely disparage each other's work as being of little relevance to evolution. Systematists sometimes invoke the punctuationist argument that most evolutionary change does not occur by individual selection and hence that within-population phenomena are largely irrelevant to evolution Evolutionary geneticists in turn dismiss the idea that studies comparing species anciently diverged, using morphological characters far removed from the level of the gene and using nonquantitative methods, can either be sound in their inferences of pattern or can shed much light on evolutionary processes". Although this is a caricaturized view of two different schools in evolutionary biology (i.e., systematics versus evolutionary genetics), some of this confrontation is present in the Chilean style of teaching evolution where, I believe, the former (systematics) approach prevails. There is a long tradition of evolutionary thinking in Chile (Manrquez & Rotthammer 1997), however, undergraduate cources of evolution have been markedly biased to favor the systematic-taxonomical and historical view of evolution (Camus 1997, Manrquez & Rotthammer 1997). Popular topics in courses of evolution are the vitalism-evolutionist debate, the origin of life on Earth, biogeography, phylogenetics and comparative methods, and phyletic gradualism versus saltationism. This may provide an adequate picture of the history of systematic evolutionary thought, but it is not a realistic picture of current research in evolutionary biology. In these evolution classes, natural selection -the mechanism of adaptive evolutionary change-, and the analysis of variation -the raw material for natural selection- are usually mentioned directly from Darwin words or anecdotic and qualitative examples2. These concepts are not taught along with the well-established quantitative tools developed to measure them. In short, the proofs for natural selection or evolution itself are usually not teached in Chilean undergraduate courses. As a consequence, it is common to hear comments such as "nobody can prove evolution" or "it is impossible to measure natural selection" in classrooms, and even in scientific meetings2, 3. Moreover, one may see publications in local scientific media, which give nave and qualitative examples, such as birds feeding in suboptimal food patches, to claim that evolutionary theory is obsolete because it does not explain such apparently non-adaptive behavior (Marone et al. 2002). Worse yet, some biologists appear to recall past and expired controversies, such as the obsolete dogma "one gene, one trait" as actual limitations to evolutionary theory (Maturana & Mpodozis 2000). In short, any course of evolution should take care of the whole body of theoretical and empirical knowledge accumulated during the last 150 years, and it may include some less well known mechasnisms as long as some minimum evidence supports them. There is general a problem of ignorance of science and especially regarding the facts of evolution. Many people, including scientists and the lay public, are unaware of the relevancy of evolutionary biology. Furthermore, the attacks to supress the teaching of evolution have received widespread support at the local level in the USA (Antolin & Herbers 2001, Bull & Whichman 2001). This is just a consequence of a crisis which is affecting evolutionary biology and is evidenced in simple facts. For example, 35 % of American college graduates think that "the earliest humans lived at the same time as dinosaurs" and 42 % indicated that they did not think "human beings, as we know them today, developed from earlier species of humans" (Alters & Nelson 2002). These authors suggest that such ignorance follows from deficient methods of undergraduate teaching. What the last paragraph, regarding missconceptions of evolution facts in the USA, has to do with former discussions about the Chilean teaching of evolution? I believe both are epiphenomena of the same general problem: incorrect teaching of evolution. For example, it is is not surprising to find out that both graduate and undergraduate Chilean students of ecology and evolution believe that evolution cannot be tested experimentally2. Moreover, some biologists strongly believe that natural selection is not a mechanism that explain adaptations (Maturana & Mpodozis 2000, Marone et al. 2002). In fact, some of them proposed a new evolutionary theory, the so-called "natural drift" (Maturana & Mpodozis 1992, 2000), which in part, stimulated this commentary. Among other arguments, Maturana & Mpodozis (2000) claim that modern evolutionary theory fails to explain adaptations, or that it has been misinterpreted. Unfortunately, the work of Maturana & Mpodozis (2000) is weak in at least three basic aspects of any new hypothesis. First, the poor and tautological writing makes it hard to follow, a point that has been criticized in detail elsewhere (Gallardo 1997, Manrquez & Rothhammer 1997). Second, it ignores at least 30 years of ecological-evolutionary research, which explains why they find so many facts that modern evolutionary theory cannot account for (e.g., non-adaptive traits, constraints to evolution, neutral change). These criticisms are mentioned by Gallardo (1997) and Manrquez & Rothhammer (1997), but mostly from the systematic perspective. Third, in nearly 50 pages (and 28 references, six of which are self citations), they do not present a single case that supports the natural drift (Maturana & Mpodozis 2000). This last point has not been discussed in detail before. Obviously, empirical evidence is the most important structural support for any hypothesis that is posed to become a theory. This review is directed to students and young biologists in Chile, it has been motivated by the general problem of a lack of knowledge about evolution, and the challenge placed by Maturana & Mpodozis (2000) to modern evolutionary theory. In science all new ideas must be open to debate. Moreover, students of science need to be presented with proofs of what they are learning. Many new ideas are interesting and appealing, but if not subjected to verification by systematic research, they are no 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 established theory, such as evolution by natural selection, is simply wrong (Antolin & Herbers 2001), as it occurs in Chile (Maturana & Mpodozis 2000), could have devastating consequences in the formation of new scientists. Here, I offer a short, representative review of the recent evidence for natural selection from the perspective of quantitative genetics and phenotypic selection. In this review I defend that (1) a conceptual framework to study 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 and demonstrated both in the field and in the laboratory, not a few times, but hundreds of times during the past decades. CONCEPTUAL FRAMEWORK Three critical elements must be kept in mind when studying evolution by natural selection: (i) that a trait exhibits intraspecific variation, (ii) that this variation is heritable, and (iii) that the trait is the target of natural selection. To characterize these processes, some formalizations are needed. In a large enough population, a metric trait is distributed in a continuous fashion. These kinds of traits are usually codified by several genes of small effects (Roff 1997). Assuming that natural (or sexual) selection acts directionally over this trait (Fig. 1A), there will be a number of individuals surviving the selective event. The important point here is that this process modifies both the variance and the mean of the distribution. Both effects have profound consequences to the population: the mean is changed by a value, "S", and the variance is reduced. In the next generation, the offspring of selected individuals will present the changed mean only if resemblance in the trait exists between parents and offspring. Fig. 1: (A) Directional selection acting on the right tail of a distribution of a metric trait in a large population. Large and small curves represent the distribution of the trait before and after selection. Similarly, o and represent the mean populational values before and after selection. (B) Directional selection differential (S) and response to selection (R) in a hypotetical trait with narrow-sense heritability (h2) close to one. The mean populational values before and after selection are represented as o and , respectively. The mean of the trait in the descendents of the selected individuals is 1. (C) Directional selection differential (S) and response to selection (R) in a hypothetical trait with narrow-sense heritability (h2) of intermediate value. (D) Directional selection differential (S) and response to selection (R) in a hypothetical trait with narrow-sense heritability (h2) close to zero. (A) Seleccin direccional actuando en la cola derecha de la distribucin de un rasgo mtrico en una gran poblacin. Las curvas grandes y pequeas representan la distribucin del rasgo antes y despus de la seleccin. Similarmente, o y representan las medias poblacionales antes y despus de la seleccin. (B) Diferencial de seleccin direccional (S) y respuesta a la seleccin (R) en un rasgo hipottico con heredabilidad en sentido estricto (h2) cercana a uno. La media poblacional antes y despus de la seleccin son representadas como o y , respectivamente. La media poblacional del rasgo en los descendientes de los individuos seleccionados es 1. (C) Diferencial de seleccin direccional (S) y respuesta a la seleccin (R) en un rasgo hipottico con heredabilidad en sentido estricto (h2) de valor intermedio. (D) Diferencial de seleccin direccional (S) y respuesta a la seleccin (R) en un rasgo hipottico con heredabilidad en sentido estricto (h2) cercana a cero. Formally, S is defined as the selection differential, such that: S = - o (1) where o and are the population means before and after selection, respectively (Fig. 1A). If the trait is completely inherited, 1, the population mean of the offspring of selected individuals will be close to (Fig. 1B). On the contrary, a trait for which the genetic contribution is too low (i.e., only environmentally determined) will have a 1 very close to o (Fig. 1D). This measureis called the narrow-sense heritability (h2) (0 h2 1). Hence, if h2 1, then 1 ; similarly if h2 0, 1 o (see Fig. 1B, 1C, and 1D). Narrow-sense heritability indicates the relative proportion of additive genetic variance to phenotypic variance. Additivity of gene effects relates to the fact that each gene is inherited individually. In evolutionary terms the important fact is the individual contribution of each gene to the phenotype. Effects that depend on the interaction among genes (e.g., dominance, epistasis) are less important in the long term (and in large, panmitic populations) because these are properties of genotypes, not of genes. The additive genetic variance (VA) contains the variance of breeding values which are those properties of individual genes. Together, VA and VP make up h2: h2 = VA / VP (2) Phenotypic variance (Vp) becomes VP = VA + VD + VI + VE (3) where VD = dominance variance, VI = variance from interaction among loci (epistasis) and VE = environmental variance. Variance components can be visualized graphically as in Fig. 2, where VA is usually a small fraction of VP.
Fig. 2: Diagramatic representation of components of variance in a hypothetic trait (and a large population) with mean and variance VP. Additive genetic variance (VA) accounts for variation in breeding values and, hence, is included as a small proportion of the genotypic variance (VG). The areas representing variance components are only for diagramatic purposes and cannot be taken as quantitative representations since variances are squared deviations (which technically cannot be represented diagramatically). Representacin esquemtica de los componentes de varianza en un rasgo hipottico (y una poblacin grande) con media y varianza VP. La varianza gentica aditiva (VA) da cuenta de la variacin en los valores de cra y, por lo tanto, est incluida como una pequea proporcin de la varianza genotpica (VG). Las reas representando los componentes de varianza son solo para propstitos diagramticos y no pueden ser consideradas como representaciones cuantitativas dado que una varianza es una desviacin al cuadrado (que tcnicamente no pueden ser representadas diagramticamente). From equation (1), and considering the introduced notation, it is possible to establish a measure of evolutionary change. We define S as the selection differential that measures the strength of natural selection, R is the difference between the mean trait value of the offspring of the selected individuals (1) and the mean before selection (); R = - 1 (Fig. 1B, 1C, 1D). R = h2S (4) Equation (4), known as the "breeders equation", shows that natural selection translates into evolution only if there is some degree of inheritance in the selected trait. The breeders equation has been empirically demonstrated and its components in many cases are different from zero (i.e., evolution by natural selection is occuring) (Falconer & Mackay 1997). Two of the most important advances in evolutionary theory are the Fundamental Theorem of Natural Selection (Fisher 1930) and the Robertson-Price Identity (Robertson 1966, Price 1970), also referred as the Secondary Theorem of Natural Selection (Caswell 1989)4. The important contributions of these models are that S can be equated to the covariance between a trait of interest (z), and to relative fitness (w) (see Appendix), S = Cov (z,w) (5) The usefulness of this equation is clear from the fact that it is not necessary to measure the trait before and after natural selection, which can be very difficult, especially when selection occurs continuously over time and with overlapping generations. Equation (6) indicates that it is sufficient to measure the trait in each individual, along with a measure of the reproductive contribution of individuals to the population, and to standardize these measures according to the average contribution of the total population (e.g., the mean number of offspring that attain sexual maturity). This univariate reasoning was extended to multiple traits and multivariate selection by Lande & Arnold (1983, see also Arnold 1983, 1988). When multivariate selection and inheritance are considered, h2 is transformed into an additive genetic variance-covariance matrix (the G matrix, which includes genetic correlations among traits), S is changed into a vector of partial selection coefficients, termed the directional selection gradient (the vector), and R is transformed into a vector of partial responses to selection (the Dz vector). Although it is not necessary to repeat this derivation here, it is enough to make the point that G, and Dz are equivalent to h2, S and R, respectively, when dealing with several traits (Lande & Arnold 1983). I found that at least ninety studies carried out during the last five years demonstrate the existence of strong selection differentials, high heritabilities, differential fitness and/or large responses to selection in natural and artificial populations of living organisms. Any person who is not convinced of the evidence that natural selection is an adaptive evolutionary agent, should analyze and criticize, in detail, each of these studies, not to mention the intensive research done and published by paleontolists, population geneticists and ecologists during last century after Darwin (see Haldane 1932, Provine 1985). DISCUSSION Natural selection, artificial selection and quantitative genetics The evidence for natural selection is neither new nor scarce (Hoekstra et al. 2001, Kingsolver et al. 2001). In fact, Haldane in 1932 mentioned at least ten cases of phenotypic divergence due to natural selection in both wild and domestic species of plants and animals. Other known examples of early measurements of natural selection include the storm sparrows of Bumpus, and the industrial melanism of Kettlewell (Bumpus 1899, Kettlewell 1955, Grant 1999). More recently, several syntheses, reviews and meta-analyses have been published that include hundreds of studies dealing with heritability, natural selection, artificial selection and experimental evolution in both, artificial and natural settings (Endler 1986, Roff & Mousseau 1986, Brodie et al. 1995, Roff 1997, Gibbs 1999, Conner 2001, Fairbairn & Reeve 2001, Hoekstra et al. 2001, Kingsolver et al. 2001, Reznick & Travis 2001, Stirling et al. 2002). Classical works such as those by Haldane (1932), Endler (1986) and Lande & Arnold (1983) have shown how natural selection can be measured in the field via a number of procedures. Statistical methodologies to determine natural selection dealing mostly with correlational statistics and fitness surfaces that make use of the Robertson-Price identity (equation 5, Appendix 1) have received a thorough analysis elsewhere (Lande & Arnold 1983, Brodie et al. 1995, Fairbain & Reeve 2001, Reznick & Travis 2001, Appendix 1). A summary of recent evidences of natural selection using these applications is shown in Table 1, along with studies where the evidence of natural selection has been detected from mapping molecular structures and then inferring periods of adaptive change (Chen et al. 2000, Grossman et al. 2001). These studies are restricted to taxa where individuals are available in large numbers and amenable to experimental manipulation (e.g., flies, birds, humans, plants, reptiles, fishes and small mammals, see Table 1). The traits studied are similarly biased, with researchers focusing on either easy-to-measure traits (e.g., morphology) or survival-related traits (e.g., life histories) (Table 1, see also Hoekstra et al. 2001). However, natural or sexual selection can only be measured when significant variation in a trait can be detected. Since populations and organisms have existed during long time periods before present day, and since selection reduces genetic variation (see Appendix 1), the measurability of current natural selection should be low. In fact, the meta-analysis by Kingsolver et al. (2001) shows that the power of directional selection analyses is rather low, which means that a sample size below 135 individuals will yield poor estimates of selection differentials or gradients. Strong directional selection is not common in natural populations either, as compared with sexual selection (Hoekstra et al. 2001, Kingsolver et al. 2001). Despite these limitations, it is notable that natural selection can indeed be measured and demonstrated.
Quantitative genetic studies that account for the existence of additive genetic variance or narrow-sense heritability are quite common, probably more so than any other kind of evidence (Table 2). This is due, in part, to historical reasons: domestic animal and plant breeding programs began long before evolutionary studies (Haldane 1932, Falconer & Mackay 1997, Roff 1997, Lynch & Walsh 1998). Additionally, quantitative genetic analyses can be done in experimental settings, which makes easier such research. From Table 2, it is clear that birds, insects, fishes and plants are the preferred models for measuring h2 in wild populations. As seen in natural selection studies, easy-to-measure traits in the laboratory also are the norm in studies of heritability (morphology, behavior and life histories). Yet low statistical power (i.e., large sample size is needed) is also a common problem of these studies. In quantitative genetic analyses the null hypothesis is that h2 = 0. Hence, when the real value of h2 is near to zero (e.g., as in life history, behavioral and physiological traits, see Roff 1997), hundreds or even thousands, of data records (individuals) are needed to demonstrate significant heritabilities. Physiological traits are a especially dramatic case since these traits are difficult to measure, and generally exhibit low values of h2 (Tsuji et al. 1989, Hoffman 2000, Dohm et al. 2001, Nespolo et al. 2003). In spite of these limitations, hundreds of high values of h2 have been reported to date (see Table 2 and references therein).
A fourth line of evidence that supports the occurrence of natural selection comes from artifical selection experiments (Table 3) where the researcher selects the parental individuals that will produce the next generation according to their extreme phenotypes