DOCSLIB.ORG

Population Genetics

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Population Genetics Population Genetics The “Modern Synthesis” of evolution is Darwinism enlightened by the understanding of molecular genetics which has been gained since Darwin. The key to understanding how evolution occurs is a move from viewing genetics in terms of individuals and their alleles to -- the frequencies of those alleles among the genes of all individuals comprising a population. We know about genes and particulate inheritance . Darwin did not. He was neither the first not the last to accept blended inheritance . He wrote before Mendel had described recessive traits. To explain evolution, he fell back into a second error: the inheritance of acquired traits. Most phenotypes, resulting from the influence of many genes, do seem to be inherited as if blended. Without a mechanism for particulate inheritance, it was hard to establish the concept. Mendel’s genetics disappeared into the literature until the beginning of the 20 th century. The rediscovery of Mendelian genetics led to a number of leading biologists claiming that evolution resulted from inheritance of mutations. Evolution, in this view, moved rapidly and by jumps, rather than gradually, as Darwin had believed. Failures to accept “ the modern synthesis ” of Mendelian genetics and Darwinian evolution persisted into and after WWII - e.g. Lysenko. To understand the modern synthesis, we need to consider the genetics of populations, rather than individuals. Consider a Punnett square for a single trait cross: (male) ½A ½a ½A AA Aa (female) ½a Aa aa in describing this cross, we have shown the effects of meiosis: 1/2 the sperm carry A, 1/2 a, and similarly for the eggs. Now recognize that the fractions of these two alleles in the population may not be equal, and there may be more than two alleles. The sum of all alleles for a trait in the population is the gene pool for that trait. We measure fraction p of the genes in this gene pool are of type A, and fraction q are type a (assuming males and females are genetically similar). Now the Punnett square looks like this: male pA qa pA p2AA pq Aa female qa pq Aa q2aa A mating like this does not change gene frequencies. Evolution is a change in the composition of the gene pool. Gradual change is called microevolution . In a population that had those allele frequencies, they would remain unchanged indefinitely if the conditions for Hardy-Weinberg equilibrium held. What are the conditions? 1. Large population size 2. No migration (gene flow occurring through immigration or emigration) 3. No mutation 4. Random mating (no assortative mating) 5. No natural selection Do the conditions often apply (or apply for long)? The Hardy-Weinberg Law is a null hypothesis . It holds (as what is called the Hardy-Weinberg equilibrium) when things don’t change, i.e. 1. In large populations there is no genetic drift. In small populations random events (mortality of a single individual) may materially affect gene frequency. This happens in small island popula- tions or populations of endo (internal) parasites. 2. There is no movement between populations, that would be gene flow. The genes moved would change the frequencies in both source and recipient populations. 3. There is no mutation. If one A mutated to a per 100 alleles, then what was 50% A in the starting population would become 49%A after mutation. Actual mutation rates are about 1/10 6 per gene, but that translates to about 1 mutation per gamete for us. We are, thus, each unique. 4. Mating (fertilization) occurs randomly. If blondes would only marry blondes (real ones) (blond hair being recessive), there would be a much higher frequency of the blond phenotype. Let’s look at an example of this: We can figure out gene frequencies in a population if we know the frequency of the recessive phenotype. For these individuals, knowing the phenotype frequency we also know the genotype and gene frequencies. The frequency of the recessive phenotype is q 2. That is also the frequency of the homozygous recessive genotype. Then the frequency of the recessive gene is the square root of q2 → q. Now for the example: We start with 100 people (50% male). 1 out of 10 is a natural blond. That means q 2 = .1, and q=.316. p = 1 – q = .684. Those would be the values indefinitely if mating were random, but… If blondes only mate with blondes, then the 5 blond males mate with the 5 blond females, and produce 10 blond children in the next generation. As to the other 90 (or 180 genes): p2 = (.684) 2 (·100) → 46 are homozygous for dark hair (or 92 dark-haired genes), and 2pq = 2(.684)(.316) ( ⋅100) → 44 are heterozygous (another 44 genes for dark hair) The overall frequency for the dark hair gene among the mating population of dark-haired individuals is: 136/180 = .755… Assuming that the dark haired individuals mate randomly: male gametes .75B .25b .75B .5625 BB .1875 Bb female gametes .25b .1875 Bb .0625 bb BB and Bb have the dark hair phenotype. Take these fractions and use them to correct to total 90 individuals to keep the population constant in size → 51BB + 34Bb are dark haired, 5bb are blonds Add these to the 10 blonds from assortative mating, and now there are 15 blonds instead of 10 out of 100, and 85 instead of 90 with dark hair. The phenotypic and genotypic frequencies have changed; microevolution has occurred. But, how often does assortative mating of the sort presented in this example occur in nature? 5. No natural selection occurs. When natural selection occurs the survival and reproduction of different phenotypes differs. Some have higher survival and/or reproduction; they leave behind a larger fraction of the offspring that form the next generation (differential reproductive success ). Their genes represent a greater fraction of the gene pool in the next generation. A numerical example: selection against the sickle cell gene. We will conveniently forget the advantageous effects of being heterozygous. An example of selection: Sickle cell anemia Begin with 50% of the genes S and 50% s. The initial, randomly mated cross is: .5S .5s .5S .25 SS .25 Ss .5s .25 Ss .25 ss We will assume the .25ss die without reproducing. Now calculate new gene frequencies. The 75% of the population of offspring surviving to reproduce are the ‘whole’ population. Now 66% of the genes are S and .33 are s The cross in the 2nd generation is: .66S .33s .66S .44SS .22Ss .33s .22Ss .11ss Natural selection against the homozygous recessives has reduced the fraction from 25% to 11% in one generation. It would further reduce the fraction each generation, but since there are fewer of them, fewer would be selected against, as well. N.B. natural selection - acts on phenotypes - selects only among variants present Natural selection acts on phenotypic variation. Where does the variation come from? Ultimately, all genetic variation in living organisms originates as mutations. The variation we observe in a population is also determined by: 1) recombination (sexual reproduction) 2) the spread of variants in a population due to drift, and 3) the effects of environmental variation on the relative success of different phenotypes. One view of the amount of genetic variation in a species is the fraction of its genes that are hetero- zygous. That fraction in part indicates the amount of outcrossing (breeding with unrelated members of the species) and in part reflects the history of the species. Cheetahs went through a severe bottleneck within the last 10,000 years; only 0.07% of their genes are heterozygous. Humans have not gone through a bottleneck like that; 7% of our genes are heterozygous. Why are so many genes not heterozygous? - because altered alleles are not as ‘good’ as the ones that persist. Others have been removed by selection. While examples indicate how gene frequencies can change, the most common cause of genetic change (microevolution) in natural populations is natural selection… Natural selection can occur in different ways. We categorize the basic types of natural selection into three forms: stabilizing selection, directional selection, and diversifying (or disruptive) selection. Modes of Natural Selection 1) Stabilizing selection - - acts against extreme forms, favors intermediates - one example: human birth weights 2) diversifying (or disruptive) selection - acts against intermediates, favors extremes - example - selection of different coloration patterns in Papilio to resemble noxious but unrelated butterflies 3) directional selection - favors one extreme, selects against the opposite extreme - shifts the phenotype distribution curve in one direction. Numerous examples: industrial melanism pesticide or drug resistance There is another form of selection: 4) sexual selection - leads to evolution of secondary sexual characters - results in sexual dimorphism - usually males evolve showy characters, e.g. : a) tails of peacocks; peahens are drably colored b) antlers of deer or caribou - females lack antlers c) colors of male mallards at breeding time,... - Why? usually females choose mates, showiest or most dominant male gets a large harem, others remain generally unmated So, to take a human view, imagine John Travolta in Saturday Night Fever, or... (Sorry, I couldn’t find a good copy of the classic pose in a white polyester suit, strutting his stuff) These are Wodaabe men from Niger in a pose off, where the women select the most beautiful men. They are wearing lipstick and other makeup, where the males of many animal species are naturally decorated (e.g. cardinals, peacocks, birds of paradise). Questions about selection: 1) Are most genes subject to the intense natural of (most) of these examples? No! These extreme examples make evolution more apparent, and occurring more rapidly. 2) Are some genes strongly conserved through the varieties of living things? Yes! For example, there have been only a handful of changes in the base sequence of cytochrome C from bacteria to man.
Load more

Recommended publications
  • The Nature of Inheritance
    I THE NATURE OF INHERITANCE The consequences of the blending theory, as drawn by Darwin. Difficulties felt by Darwin. Particulate inheritance. Conservation of the variance. Theories of evolution worked by mutations. Is all inheritance particulate ? Nature and frequency of observed mutations. But at present, after drawing up a rough copy on this subject, my conclusion is that external conditions do extremely little, except in causing mere variability. This mere variability (causing the child not closely to resemble its parent) I look at as very different from the formation of a marked variety or new species. DARWIN, 1856. (Life and Letters, ii, 87.) As Samuel Butler so truly said: 'To me it seems that the "Origin of " Variation ", whatever it is, is the only true Origin of Species 'V w. BATESON, 1909. The consequences of the blending theory THAT Charles Darwin accepted the fusion or blending theory of inheritance, just as all men accept many of the undisputed beliefs of their time, is universally admitted. That his acceptance of this theory had an important influence on his views respecting variation, and consequently on the views developed by himself and others on the possible causes of organic evolution, was not, I think, apparent to himself, nor is it sufficiently appreciated in our own times. In the course of the present chapter I hope to make clear the logical con- sequences of the blending theory, and to show their influence, not only on the development of Darwin's views, but on the change of attitude towards these, and other suppositions, necessitated by the acceptance of the opposite theory of particulate inheritance.
    [Show full text]
  • Proceedings of the California Academy of Sciences, 4Th Series
    PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES Vol. 48, No. 6, pp. 131-140 December 21, 1993 FIFTY YEARS OF PROGRESS IN RESEARCH ON SPECIES AND SPECIATIONanne Biological Labo *oods Hole OceiograSc Stion Ub By «7. Ernst Mayr Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 Woods Hole, MA 02543 Adapted from a lecture delivered at the Golden Jubilee Celebration of the publication of Systematics and the Origin of Species at the California Academy of Sciences on October 16, 1992. Received December 31, 1992. Accepted February 11, 1993. Historians of science have taught us how much the nature of the genetic material, resulting in one can learn from studying the history of a field the theory of particulate inheritance. However, of science. This is excellently illustrated by the they drew from this the wrong conclusion as far history of evolutionary biology as a whole, and as evolution is concerned, claiming that new spe- by our growing understanding of species and spe- cies were produced by new mutations in a single ciation, in particular. saltation, completely rejecting Darwin's theory After 1859, two of Darwin's theories were ac- of gradualism. Their opponents were the biom- cepted almost at once. First, evolution as such, etricians, such as Pearson and Weldon, who cor- and secondly, the branching theory of common rectly insisted on the gradualness of evolution descent. Natural selection was with almost equal but incorrectly claimed that inheritance was unanimity rejected, being accepted only by a small equally gradual, that is, blending. As far as ge- group of naturalists. This was not too surprising netics is concerned, the Mendelians were right; since at that time no one understood variation as far as evolution is concerned, the biometri- and its origin.
    [Show full text]
  • Contents More Information
    Cambridge University Press 978-0-521-72877-5 - Modular Evolution: How Natural Selection Produces Biological Complexity Lucio Vinicius Table of Contents More information Contents Preface page xi 1 From natural selection to the history of nature 1 Nature after natural law 2 Reductionism and emergence 4 History and inheritance 7 Darwinian progress: order at the macroevolutionary scale 10 Macroevolution as history 12 Contingency as the startling consensus 15 Convergence 18 The denial of ‘progress’: Darwinism’s prejudice against Darwin 20 Biological complexity and information 22 Organisms as DNA programs 25 Major transitions: the aggregational mode of evolution 27 Convergent aggregation, levels of selection and levels of organisation 30 The evolution of biological order 32 The evolution of order: macroevolutionary consequences 35 2 From the units of inheritance to the origin of species 38 The gene as the module of inheritance 40 The fundamental principle of natural selection 43 Gradualism: the cost of biological complexity 46 The adaptive walk in the real world 51 Epistasis as a main legacy of particulate inheritance 55 The shifting balance and the structure of real populations 58 vii © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-72877-5 - Modular Evolution: How Natural Selection Produces Biological Complexity Lucio Vinicius Table of Contents More information viii Contents Sex and the origins of species 60 Speciation as adaptation: the evolution of xenophobia 62 Speciation as an accident
    [Show full text]
  • 1 Population Biology, Test 1. Name
    Population Biology, Test 1. name _____________________ •Most people will finish well before the 75 minutes are up, but those few who don't may without speaking follow me to the room next to my office where more time will be provided. If you want to use my dictionary or to say something to me, raise your hand. •Be very careful to not even look like you are peaking at other people's tests or your notes. If you even look suspicious I'll have to move you and it's disruptive. Also, the sequence of questions is different on neighboring tests, so you won't get very far cheating even if you try. •Don't worry tonight if you were unable to answer every question. This is a hard test meant to allow great students to show their stuff, but the grading system is lenient. You can miss quite a few questions and still get an okay grade. When two or more answers seem possible to you, I suggest guessing among them and moving on. 1. Populations that have plenty of resources a. are at carrying capacity. b. grow arithmetically (linearly). c. grow exponentially (geometrically). d. have discrete generations rather than continuous generations. e. are at Hardy-Weinberg equilibrium. 2. The study of quantitative trait loci (QTLs) a. is bringing together the ecology of population growth (focusing on N) and the study of population genetics (p's and q's). b. is a way of showing that hitchhiking has occurred whereby a neutral allele has changed in frequency by being linked to a mutation that swept through the population by selection.
    [Show full text]
  • The Modern Synthesis Huxley Coined the Phrase, the “Evolutionary
    The Modern Synthesis Huxley coined the phrase, the “evolutionary synthesis” to refer to the acceptance by a vast majority of biologists in the mid-20th Century of a “synthetic” view of evolution. According to this view, natural selection acting on minor hereditary variation was the primary cause of both adaptive change within populations and major changes, such as speciation and the evolution of higher taxa, such as families and genera. This was, roughly, a synthesis of Mendelian genetics and Darwinian evolutionary theory; it was a demonstration that prior barriers to understanding between various subdisciplines in the life sciences could be removed. The relevance of different domains in biology to one another was established under a common research program. The evolutionary synthesis may be broken down into two periods, the “early” synthesis from 1918 through 1932, and what is more often called the “modern synthesis” from 1936-1947. The authors most commonly associated with the early synthesis are J.B.S. Haldane, R.A. Fisher, and S. Wright. These three figures authored a number of important synthetic advances; first, they demonstrated the compatibility of a Mendelian, particulate theory of inheritance with the results of Biometry, a study of the correlations of measures of traits between relatives. Second, they developed the theoretical framework for evolutionary biology, classical population genetics. This is a family of mathematical models representing evolution as change in genotype frequencies, from one generation to the next, as a product of selection, mutation, migration, and drift, or chance. Third, there was a broader synthesis of population genetics with cytology (cell biology), genetics, and biochemistry, as well as both empirical and mathematical demonstrations to the effect that very small selective forces acting over a relatively long time were able to generate substantial evolutionary change, a novel and surprising result to many skeptics of Darwinian gradualist views.
    [Show full text]
  • The Role of Transgenerational Epigenetic Inheritance In
    NoN-GeNetic iNheritaNce Abstract David W. Pfennig*, Inheritance is crucial to the evolutionary process. Although most Maria R. Servedio evolutionary biologists assume that inheritance occurs exclusively through changes in DNA base sequence, it has long been known that inheritance can also occur through epigenetic mechanisms, such as chromatin Department of Biology, marking, maternal effects, parasite transmission, or learning. In recent University of North Carolina, years, the possibility that such transgenerational epigenetic inheritance Chapel Hill, NC, USA mechanisms can mediate long-term evolutionary change has received increased attention. Here, we consider the potential contribution of speciation. As we describe, a growing body of theoretical and empirical studies suggests that epigenetic inheritance can accelerate the likelihood that genetic change will occur and thereby facilitate speciation. Additionally, inherited environmental or learned effects, completely independent of any changes in DNA base sequence. Generally, clarifying whether (and speciation remains a key frontier of evolutionary biology. Keywords Received 30 May 2013 © 2013 David W. Pfennig et al., licensee Versita Sp. z o. o. Accepted 21 July 2013 This work is licensed under the Creative Commons Attribution-NonCommercial- NoDerivs license (http://creativecommons.org/licenses/by-nc-nd/3.0/), which means that the text may be used for non-commercial purposes, provided credit is given to the author. sensu Epigenetic effects Inheritance Transgenerational epigenetic inheritance: a challenge Isolating mechanism for evolutionary biology sensu 1.1 The importance of inheritance for evolution Phenotypic plasticity On the Origin of Species 5 Speciation Species sensu Transgenerational epigenetic inheritance * E-mail: [email protected] Unauthenticated | 65.87.174.230 Download Date | 8/18/13 12:20 PM D.W.
    [Show full text]
  • Stable Equilibrium
    Evolution II.2. After The Origin. ”Some of the great scientists, carefully ciphering the evi- dences furnished by geology, have arrived at the convic- tion that our world is prodigiously old, and they may be right but Lord Kelvin is not of their opinion. He takes the cautious, conservative view, in order to be on the safe side, and feels sure it is not so old as they think. As Lord Kelvin is the highest authority in science now living, I think we must yield to him and accept his views.” [Mark Twain quoted by Burchfield (1990)] 1 Part I: The Eclipse of Darwinism. Between Scylla and Charybdis. a. 1st edition of The Origin published 1859; 6th, 1872. b. By 1867, Darwin’s argument had been shredded: 1. Selection requires herita- ble variation, and herita- ble variation would be lost at a rate of – ~50% per generation – due to “blending” – Hat tip: Fleeming Jenkin. 2. Gradual evolution re- quires earth ancient, but both the age of the habitable earth and that Fleeming Jenkin and his of the sun ≤ 100 million telphers. years – consequence of 2nd Law – Hat tip: William Thomson, later Lord Kelvin. 2 By the end of the 19th century: a. Many biologists confidently predicted the imminent demise, if it was not already dead, of what by this time was called “Darwinism”. b. There was renewed interest in both an inherent ten- dency to progress and IAC – so-called “Neo- Lamarckism”. c. What kept the idea of evolution alive was increasing evidence – anatomical, developmental and especially paleontological – for descent with modification.
    [Show full text]
  • 4. Forces of Evolution
    4. Forces of Evolution Andrea J. Alveshere, Ph.D., Western Illinois University Learning Objectives • Describe the history and contributions of the Modern Synthesis • Define populations and population genetics as well as the methods used to study them • Identify the forces of evolution and become familiar with examples of each • Discuss the evolutionary significance of mutation, genetic drift, gene flow, and natural selection • Explain how allele frequencies can be used to study evolution as it happens • Contrast micro- and macroevolution It’s hard for us, with our typical human life spans of less than 100 years, to imagine all the way back, 3.8 billion years ago, to the origins of life. Scientists still study and debate how life came into being and whether it originated on Earth or in some other region of the universe (including some scientists who believe that studying evolution can reveal the complex processes that were set in motion by God or a higher power). What we do know is that a living single-celled organism was present on Earth during the early stages of our planet’s existence. This organism had the potential to reproduce by making copies of itself, just like bacteria, many amoebae, and our own living cells today. In fact, with today’s genetic and genomic technologies, we can now trace genetic lineages, or phylogenies, and determine the relationships between all of today’s living organisms—eukaryotes (animals, plants, fungi, etc.), archaea, and bacteria—on the branches of the phylogenetic tree of life (Figure 4.1). Looking at the common sequences in modern genomes, we can even make educated guesses about what the genetic sequence of the first organism, or universal ancestor of all living things, would likely have been.
    [Show full text]
  • PERSPECTIVES on Hardy-Weinberg in Introductory Biology: Teaching This Fundamental Principle in an Authentic, Engaging, and Accur
    PERSPECTIVES On Hardy-Weinberg in Introductory Biology: Teaching this Fundamental Principle in an Authentic, Engaging, and Accurate Manner Michael J. Wise* 1Department of Biology, Roanoke College, Salem, VA 24153 *Corresponding author: [email protected] Abstract: The Hardy-Weinberg principle (HWP) is a fundamental model upon which much of the discipline of population genetics is based. Despite its significance, students often leave introductory biology courses with only a shallow understanding of the use and implications of the HWP. I contend that this deficiency in student comprehension is too-often a consequence of teachers of introductory biology having an insufficient mastery of the HWP, as well as a lack of proficiency in quantitative analysis in general. The purpose of this Perspectives paper is to correct some common misconceptions about the HWP so that teachers of introductory biology will have more confidence that they are communicating this important principle correctly to their students. A companion Innovations paper provides a problem-set of six real-world Hardy-Weinberg scenarios, along with explicit instructions on quantitatively analyzing the scenarios. Keywords: evolutionary mechanisms, Hardy-Weinberg equilibrium, introductory biology, population genetics The Centrality of the Hardy-Weinberg Principle to the macro-scale than the cellular and molecular in Evolutionary Biology perspectives that constitute much of the course matter The Hardy-Weinberg principle (HWP) often in introductory biology. Second, the HW perspective serves as the foundation for students to build an provides students with an appreciation that evolution understanding of how evolution works in populations is a phenomenon that happens continuously and that of organisms. Nevertheless, applying the HWP tends can be studied in real time (Winterer, 2001).
    [Show full text]
  • A History of Genetics and Genomics
    A History of Genetics and Genomics Phil McClean September 2012 Genomics is a recent convergence of many sciences including genetics, molecular biology, biochemistry, statistics and computer sciences. Before scientists even uttered the word genomics, these other fields were richly developed. Of these fields, the history of genetics and molecular biology are particularly relevant to the techniques, experimental designs, and intellectual approaches used in genomics. The development of computers and the internet has provided researchers ready access to the large body of information generated throughout the world. Table 1 is an extensive history of the major developments in these fields. The narrative will try to unify some of these discoveries into major findings. Mid to Late 19th Century: Evolution, Natural Selection, Particulate Inheritance and Nuclein Understanding origins is a constant pursuit of man. In the 1858, our understanding of the origin of species and how species variability arose was revolutionized by the research of Darwin and Wallace. They described how new species arose via evolution and how natural selection uses natural variation to evolve new forms. The importance of this discovery was reflected in the now famous quote of Dobzhansky: “Nothing in biology makes sense except in the light of evolution.” - Theodore Dobzhansky, The American Biology Teacher, March 1973 A few years later, Gregor Mendel, an Austrian monk, summarized his years of research on peas in his famous publication. In that paper, he described the unit of heredity as a particle that does not change. This was in contrast to the prevailing “blending theory of inheritance.” Equally important, Mendel formalized the importance of developing pure (genotypically homozygous) lines, keeping careful notes, and statistically analyzing the data.
    [Show full text]
  • Cultural Inheritance in Generalized Darwinism
    Cultural Inheritance in Generalized Darwinism Christian J. Feldbacher-Escamilla and Karim Baraghith*y Generalized Darwinism models cultural development as an evolutionary process, where traits evolve through variation, selection, and inheritance. Inheritance describes either a discrete unit’s transmission or a mixing of traits (i.e., blending inheritance). In this arti- cle, we compare classical models of cultural evolution and generalized population dy- namics with respect to blending inheritance. We identify problems of these models and introduce our model, which combines relevant features of both. Blending is imple- mented as success-based social learning, which can be shown to be an optimal strategy. 1. Introduction. This article deals with a special kind of inheritance in cul- tural evolution (i.e., within the framework of generalized Darwinism). This framework is postulated by scientists and philosophers from different fields of research as a new and interdisciplinary theoretical structure or paradigm (e.g., Richerson and Boyd 2001; Reydon and Scholz 2014). An extensive overview of the generalized-Darwinism approach is, for example, provided by Schurz (2011). For a strong defense of generalized Darwinism and a care- fully worked out core of Darwinian principles, see Aldrich et al. (2008). Dif- ferent approaches and some methodological problems are discussed by Witt Received November 2017; revised August 2019. *To contact the authors, please write to: Christian J. Feldbacher-Escamilla, Department of Philosophy, Düsseldorf Center for
    [Show full text]
  • 6.047 / 6.878 Computational Biology: Genomes, Networks, Evolution Fall 2008
    MIT OpenCourseWare http://ocw.mit.edu 6.047 / 6.878 Computational Biology: Genomes, Networks, Evolution Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. 6.047 LECTURE 11, MOLECULAR EVOLUTION/COALESCENCE/SELECTION/KAKS OCTOBER 09, 2008 1. Introduction Evolution is the shaping of phenotypic and genotypic variation by natural selec­ tion. The relative importance of selection and mutation has been a long standing question in population genetics. And, we’d like to understand the relative impor­ tance of each of these forces. In particular, we’d like to know which genes are undergoing active selection. neutrality tests are a key tool for addressing this ques­ tion. There are many different evolutionary forces that might cause deviations from neutrality such as differential mutation rates, recombination, population structure, drift in addition selection and others. Therefore, it is easier to develop test of neu­ trality rather than directly searching for signatures of selection. From a historical perspective, most of these ideas were developed theoretically in the last century. And only in the last few decades that we were able to gather data to directly test these theories. 1983 is the first time that we have molecular polymorphism data. First, we need to understand the neutral model, focusing on inheritance alone. Historically, this area of study dates back to Darwin’s time. Scientists in the 1860s believed in blending inheritance which stated that each organ was determined by a different gemmule. In this model, children inherit a blending of their parents’ corresponding gemmules, so if mom had a “yellow” gemmule and dad had a “blue” gemmule, then baby would inherit a “greenish” gemmule.
    [Show full text]