BIO1130-L4: Microevolution PDF

Topic 4 MICROEVOLUTION 1  Learning objectives Distinguish between micro vs macroevolution and enumerate the processes causing the latter and the various ‘sources’ of genetic variance Contrast random vs. non-random mating and explain the effects of the different forms of these on...

Topic 4 MICROEVOLUTION 1  Learning objectives Distinguish between micro vs macroevolution and enumerate the processes causing the latter and the various ‘sources’ of genetic variance Contrast random vs. non-random mating and explain the effects of the different forms of these on allele and genotype frequencies Define inbreeding and inbreeding depression, outline the causes of the latter, and identify mechanism that have evolved to reduce the likelihood of inbreeding Identify the different types of mutations and how they are classified in terms of their effects on fitness (i.e., beneficial, neutral, deleterious); discuss the relative frequencies of these different types (i.e., fitness spectrum of new mutations) Discuss the effects of mutation on allele frequencies and its role in creating genetic variation Define gene flow and compare/contrast it with mutation in terms of their roles in altering allele frequencies and as sources of genetic variation in a population Outline how gene flow and spatially varying selection can interact to impact local adaptation Define genetic drift and explain how it varies with population size and the impacts it has on allele frequencies, genetic variation, and population divergence Summarize the effects of population bottlenecks/founder events Outline the human health implications of drift (via a founder event) on the frequency of a deleterious mutation and how subsequent gene flow (isolate breaking) can alter this Be comfortable interpreting the output of the drift simulation we discussed Define natural selection and fitness, outline the different components of fitness that can be involved, and the different forms natural selection can take Summarize the different approaches to detecting natural selection and the associated problem of correlated traits Outline how genetic variation can be maintained, including the role of the processes discussed in this topic 2  Introduction (micro)evolution: a change in allele frequency in a population or species across generations; focus is on variation within populations/species and evolutionary change over shorter time periods Macroevolution: evolution above the species level; focusses on variation among species and on questions related to diversification (e.g., origin of new species and higher order groups) across relatively long periods of time. Macroevolution is the result of microevolution writ large; i.e., the longer term and higher taxonomic consequences of microevolution within populations. Four processes can cause microevolution: mutation, gene flow, genetic drift, and natural selection Microevolution requires genetic variation (i.e., more than one allele segregating at a locus in a population) 3  The mathematics of microevolution Population and quantitative genetics provide rigorous mathematical frameworks to study the impacts of assortative mating and the microevolutionary processes of mutation, gene flow, drift, and selection on Mendelian variation and quantitative traits 2 pt wAA  pt qt wAa pt = p0(1-u)t p   R1   VA 1 Cov A (1, 2 )   1   t 1 w   Cov  VA2    2  2 pt qt wAa  qt waa  R2   A ( 1, 2 )  qt 1  w pt 1  (1  m) pt  mp V R  h2S  A S VP 1  1  Ft  (1  m ) 2  1  (1  m ) Ft 1 2 2N  2N  Interested? Go further in BIO3119 Introduction to population genetics; BIO3122 Evolutionary biology 4 Outline 4.1 Non-random mating 4.2 Mutation 4.3 Gene flow Mechanisms of (micro)evolution 4.4 Genetic drift 4.5 Natural selection 5  Random mating HW requires that individuals mate randomly with respect to their genotype at the locus of interest Random mating in a population is also termed panmixia Some species are panmictic; but most have some sort of geographic structuring into populations Oceanic eel larvae; Source: Kils at the English-language Wikipedia Adult American eel, Anguilla rostrata Source: Clinton & Charles Robertson Sargasso sea Source: U.S. Fish and Wildlife Service; http://www.fws.gov/northeast/images.html 6 Non-random mating Mating is often non-random because: – relatives may mate more often or less often than expected by chance (termed inbreeding and outbreeding respectively) – individuals may self-fertilized more or less often than expected by chance – individuals may mate more often with other individuals that are more or less similar to them in phenotype than expected by chance (termed assortative vs disassortative mating, or positive vs. negative assortative mating, respectively) Non-random mating affects how alleles are organized into genotypes (i.e., it alters genotype frequencies – homozygosity and heterozygosity) The assumption is that all individuals still mate (i.e., this is not sexual selection, which arises from differences in mating success). Therefore, allele frequencies are NOT affected and hence it is not a mechanism of evolution on its own. 7  Inbreeding Inbreeding is when mating takes place between related individuals and the resulting offspring are inbred Inbreeding causes an increase in the frequency of homozygotes (equivalently a decrease in heterozygosity) across the genome (i.e., at all loci) and thus a deviation from HW expected genotype frequencies The effects of inbreeding on genotype frequencies can be ephemeral: one or a few generations of random mating restore HW expected genotype frequencies Parents A1A1 A1A2 A2A2 Gametes A1 50% A1, 50% A2 A2 A1 (0.5) A2 (0.5) A1 A1 A1A1 A1A2 A2 Punnett (0.5) (0.25) (0.25) squares A1 A1A1 A2 A2A2 A2 A2A1 A2A2 (0.5) (0.25) (0.25) Offspring A1A1 ¼ A1A1, ½ A1A2, ¼ A2A2 A2A2  Inbreeding depression The increase in homozygosity as a result of inbreeding tends to decreases fitness This decrease in fitness as a consequence of inbreeding is called inbreeding depression Charles II of Spain Source: The Times, Dec. 2019; https://www.thetimes.co.uk/article/habsburg-royal-family-were-deformed-due-to-inbreeding-jw23xmmhf 9  Inbreeding depression Is widespread (not restricted to royal dynasties!) Can exacerbate the loss of genetic variation (allelic diversity) that can occur in small populations due to genetic drift Can impact conservation biology and human health litter size in mice Source: Freeman & Herron (2007) Evolutionary analysis. Source: Falconer & MacKay, 1995, Pearson Pearson Education 10  Mendelian causes of inbreeding depression Two, non-mutually exclusive, hypotheses: 1) Dominance hypothesis Alleles that decrease fitness (termed deleterious alleles) tend to be partially to completely recessive Such alleles are held at low frequency in a population by natural selection against them, and so when present are found primarily in heterozygotes (i.e., q2 mutation rates, so the effect of gene flow on allele frequencies is generally much greater than those of mutation Gene flow homogenizes populations, reducing genetic differences between them. If gene flow is sufficiently high, population differences are abolished in place of a single panmictic population. 24  Gene flow can impede local adaptation Local adaptation occurs when a population adapts to its local environment; populations inhabiting different local environments may therefore diverge due to the differences in selection they experience (termed divergent selection – meaning selection that favours different traits in different populations/environments) Gene flow can hinder local adaptation by constantly introducing maladaptive alleles from other populations (‘outbreeding depression’ – reduced fitness from matings between populations) Gene flow can instead promote adaptation by helping spread beneficial alleles among populations 25 Low gene flow High gene flow  Example: fugitive Atlantic salmon See also the example of Lake Erie water snakes in suggested reading in Campbell (Fig. 23.11 and accompanying text) Prof. Jeff Hutchings, Dalhousie University 26 Source: Jeff Hutchings; https://www.fishlifehistory.ca/index.php?page_id=38  Practice Hyperlink (click here), or: Q43: Which of the following statement about gene flow is incorrect? Q44: Outline a way in which gene flow may promote adaptation of a population to its environment, and a way in which it can hamper this. 27 Outline 4.1 Non-random mating 4.2 Mutation 4.3 Gene flow Mechanisms of (micro)evolution 4.4 Genetic drift 4.5 Natural selection 28  Finite populations All populations are finite, although some may be sufficiently large to behave as effectively infinite over at least relatively short time periods (i.e., such that HW expectations are met) Finite populations are subject, to varying degrees, to random changes in allele frequencies across generations This is the process known as genetic drift It occurs due to sampling variation: the difference between the value in a finite sample compared to the true value of a population A finite sample of alleles in one generation contribute to the next generation. That is, there is a true frequency of each allele in the parental generation, but the next generation can be thought of as a random sample of alleles from the parental gene pool, and the sampling variation inherent in this causes allele frequencies in the offspring to differ from those in the parents. 29  Effects of genetic drift Causes random (i.e., unpredictable) change in allele frequency across generations The magnitude of change is inversely related to population size; e.g., a smaller population experiences larger changes in allele frequency on average due to drift (i.e., ‘stronger’ drift) On average, drift reduces genetic variation because alleles are lost meaning heterozygosity decreases In small populations, drift can overwhelm selection such that deleterious alleles may rise in frequency and even fix Drift will cause populations to diverge from one another (in the absence of gene flow) If random mating occurs each generation, then drift-induced deviations from HW-expected genotype frequencies are usually small (especially in larger populations in which drift is weaker) 30  Simulating genetic drift Visit https://www.radford.edu/~rsheehy/Gen_flash/popgen/ to see a simulation of genetic drift in replicate independent populations Set the population size to ‘finite’ and try these scenarios: – Freq. of A1 = 0.5, N = 5000 vs. 500 vs. 50 vs. 10 – Freq. of A1 = 0.1, N = 5000 vs. 500 vs. 50 vs. 10 What happens as N goes up? What happens as freq. of A1 gets closer to 0? The plot includes a line for an infinite population size. Does it make sense? Turn on migration and set it to ‘island’ (this means migrants move equally among all populations). Set N = 100, freq(A1) = 0.5, and compare what happens when you set the migration rate to 0 (equivalent to no migration), 0.01, and 0.1. (The rate is the proportion of individuals in a population each generation that came from another population, so higher values mean more gene flow.) Focus on how much divergence (i.e., differences in allele frequencies) you see among the populations under each migration rate. Do the results make sense? 31 Pr(allele eventually fixes by drift) = current allele frequency Pr(allele eventually lost by drift) = 1 - current freq.  Population bottlenecks A severe (generally rapid) decrease in population size which reduces genetic variation and enhances genetic drift Can be caused by environmental factors, human activities, disease, etc. Can also be caused by a founder event: when a small group of individuals colonizes a new geographic area, isolated from other populations (e.g., a remote island, a captive zoo population) Can have consequence for population persistence and future adaptation Fig. 23.9, Campbell ‘Yellow’ balls were lost by chance (representing a loss of genetic variation); all alleles have changed frequency due to this bottleneck. 33  Mutation-drift: human health implications Bottlenecks can result in increased frequency of a deleterious mutation in an isolated population This can have human health implication; e.g., Ellis-van Creveld (EvC) syndrome in Pennsylvania Amish, Myotonic dystrophy in Québecois(e) in Charlevoix/Saguenay-Lac-Saint-Jean region, Tay- Sachs disease in Ashkenazi Jews Polydactyly EvC patient: Source: Baujat G, Le Merrer M. 2007. Orphanet J. Myotonic dystrophy: Rare Diseases. 2: 27. CC BY 2.0 By Herbert L. Fred, MD, Hendrik A. van Dijk - http://cnx.org/content/m14898/latest/, CC BY 1.0, https://commons.wikimedia.org/w/index.php?curid= 34 4063371  Practice Hyperlink (click here), or: Q45: Which of the following statements about genetic drift is incorrect? Q46: How could a combination of mutation and drift help maintain genetic variation in a population? 35 Outline 4.1 Non-random mating 4.2 Mutation 4.3 Gene flow Mechanisms of (micro)evolution 4.4 Genetic drift 4.5 Natural selection 36  Natural selection Is a process that occurs when certain conditions are met i.e., a deduction: if these conditions exist, then this will happen If: 1) Individual vary in a trait 2) There is a non-random association between the trait and their reproductive success (i.e., their ability to survive and reproduce; also known as their ‘Darwinian fitness’) 3) The trait is heritable Then: The trait will evolve (i.e., the distribution of trait values in the population will change across generations); allele frequencies at the loci affecting the trait will change across generations 1 & 2 are necessary for natural selection to occur 1, 2 & 3 are necessary for natural selection to produce evolutionary change (i.e., for selection to cause allele frequencies to change across generations) 37  Fitness Natural selection is one mechanism of evolution: mutation migration genetic drift selection (natural/artificial) While natural selection sorts among random variation, its outcome is non-random (i.e., predictable); it is also the only cause of adaptation (= traits that increase survival and/or reproduction in a given environment) (Darwinian) fitness: the (absolute) contribution of an individual to the next generation; its reproductive success measured as the number of offspring in produces Natural selection arises from variation in relative fitness: an individual’s contribution to the next generation relative to that of other individuals in the population 38  Genotype - phenotype Natural selection acts on phenotypes To varying extents, genetic variation underlies this phenotypic variation Alleles associated with genotypes that code for advantageous (i.e., fitness-enhancing) phenotypes will be passed on more relative to other alleles The results is a change across generations in allele frequencies, and hence the distribution of trait values among individuals 39 Example – DDT resistance in insects France began spraying in the 1960’s Prof. Nicole Pasteur, U. Montpellier II Prof. Michel Raymond By 1972 populations began recovering Resistance due to a single mutation at a locus that encodes an esterase enzyme that breaks down a wide range of toxins, including DDT The mutation (ester1) rapidly spread ‘Mosquito resistance to insecticides’ from S317 Biological science: from genes to species. An OpenLearn chunk used/reworked by permission of The Open University ©2106.’ CC BY-NC-SA 4.0 40  Example – DDT resistance in insects ester1 confers high fitness in the presence of DDT, but has a cost of increased susceptibility to predators and reduced male reproductive success benefit > cost in presence of DDT; benefit