2 - Nothing in Biology Makes Sense Except - Tagged.pdf

“NOTHING IN BIOLOGY MAKES SENSE EXCEPT IN THE LIGHT OF EVOLUTION.” Theodosious Dobzhansky 1  Periodical cicadas (Magicicada spp.) emerge every 13 or 17 years Why do they show this very unusual life cycle – what’s the benefit? 2  Why have such a long genera...

“NOTHING IN BIOLOGY MAKES SENSE EXCEPT IN THE LIGHT OF EVOLUTION.” Theodosious Dobzhansky 1  Periodical cicadas (Magicicada spp.) emerge every 13 or 17 years Why do they show this very unusual life cycle – what’s the benefit? 2  Why have such a long generation time? Why emerge all The Scientific Method at once? Observation – cicada broods emerge in spring every 13 or 17 years; adults live for Why only a few weeks 13 or 17 Hypothesis(es) – causal explanation(s) years? Predictions – logical consequences Tests – additional experiments or observations Use SEE when you answer! Why are there different broods? Why did periodicity arise at all? 3  Evolutionary thinking enables us to answer the ‘why’ questions Darwin’s theories are based of 4 observations and their logical conclusion 1. Organisms vary in their characteristics Morphology, physiology, behavior Variation 2. Some of that variation is genetic (heritable) Passed relatively faithfully among generations Heritability Fitness 3. More offspring are produced than can survive Competition – driven by the environment 4. Organisms differ in their ability to survive Competition and/or reproduce Differential success – random or non- random 4  Evolution is an outcome driven by interaction of these processes It’s most appropriate to say “The Theory of Evolution via…” 1. Variation - individual cicadas differ in larval period and emergence time XX X 2. Heritability – larval period/emergence time is a genetic trait passed from parents to offspring XX X 3. Competition – cicadas are subject to predation or other type of death upon emergence 4. Differential success – larval period/emergence time influences the likelihood of survival Those cicadas with larval periods/emergence times that increase survival will reproduce at 5 a higher rate, and that trait will increase in  WHAT ARE THE WAYS THAT EVOLUTION CAN OCCUR? That is, what are the different mechanisms of evolution? 6  Natural selection leads to the evolution of adaptations It’s important to see the distinction among several related terms Evolution – the change in the genetic structure of a population over time; outcome XX X Natural selection – differential, non-random survival and/or reproduction of certain individuals based on their genetically- determined characteristics; deterministic XX X process Adaptation – (n) a trait that increases the ability of an individual to survive or reproduce; (v) an increase in the average fitness of a population; specific type of outcome Fitness – the contribution of an individual to the next generation; measurable currency 7  Natural selection comes in 4 different forms The way in which a population evolves differs depending on which individuals have the highest fitness The Environment Fitness Directional selection – individuals at one end 12 of the spectrum have the highest fitness; 8 population shifts in one direction, but 4 variation stays the same 0 Se rie s1 12 8 4 0 Se rie s1 12 8 4 0 Se rie s1 8  Natural selection comes in 4 different forms The way in which a population evolves differs depending on which individuals have the highest fitness 12 Directional selection – individuals at one end 8 Fitness of the spectrum have the highest fitness; 4 population shifts in one direction, but 0 variation stays the same Se rie s1 Stabilizing selection – intermediate 15 individuals have the highest fitness; 10 population doesn’t shift, but variation 5 decreases 0 Se rie s1 15 10 5 0 Se rie s1 9  Natural selection comes in 4 different forms The way in which a population evolves differs depending on which individuals have the highest fitness Fitness 12 Directional selection – individuals at one end 8 of the spectrum have the highest fitness; 4 population shifts in one direction, but 0 variation stays the same Se rie s1 Stabilizing selection – intermediate 15 individuals have the highest fitness; 10 population doesn’t shift, but variation 5 decreases 0 Se rie s1 Disruptive selection – individuals at both 12 ends have higher fitness than intermediates; 8 population doesn’t shift, but variation 4 increases 0 Se rie s1 10  Natural selection comes in 4 different forms The way in which a population evolves differs depending on which individuals have the highest fitness Fitness Directional selection – individuals at one end 12 of the spectrum have the highest fitness; 8 population shifts in one direction, but 4 variation stays the same 0 Se Stabilizing selection – intermediate rie s1 12 individuals have the highest fitness; 8 population doesn’t shift, but variation 4 decreases Fitness 0 Se Disruptive selection – individuals at both rie 12 s1 ends have higher fitness than intermediates; 8 population doesn’t shift, but variation 4 increases 0 Se Frequency-dependent selection –the fitness rie s1 of an individual depends on how common it 11 is in the population; population changes in  What type(s) of selection explain(s) periodical cicadas? How did we get to the patterns that we observe today? Long Generation Time Prime Number Cycling Mass Emergence Predator Satiation The Selfish Herd Multiple Broods Adaptive Landscapes https://www.smartickmethod.com http://compbio.ethz.ch/research.html 12  NOT ALL THINGS ARE ADAPTATIONS So not everything evolves through natural selection 13  The Sentinelese are one of about 100 uncontacted tribes The Indian government prevents contact, and the Sentinelese are aggressive towards outsiders The Sentinelese Most isolated tribe on Earth North Sentinel Island – 27 square miles Thought to be last remaining pre-Neolithic tribe Likely descendants of first migrants out of Africa Virtually uncontacted for 30,000-60,000 years Population estimated to be 50-200 individuals Believed to live in 3 small bands Live in long communal huts, temporary shelters Forest hunter-gatherers 14 Fish in coastal waters in narrow dugout  Why should we continue to leave the Sentinelese alone? From a biological perspective, what are the risks to them of contact with outsiders? Conquest of the Aztecs Cortes 1519 15  MHC proteins are key to immunity and resistance to pathogens MHC proteins are coded for by HLA loci in the human genome MHC Proteins Present fragments of proteins synthesized within the cell – including Locu Commo Rare Total s n Alleles Alleles viruses (Class I) - or by bacteria Alleles outside the cell (Class II) A 68 425 493 B 125 658 783 Allows immune system to recognize www.sciencephoto.com C 44 231 275 and destroy infected cells/infectious DRB 79 339 418 agents 1 DRB 5 - 5 3 HLA loci are among the most DRB 6 - 6 polymorphic and fastest-evolving 4 coding regions DRB 5 - 5 5 ‘Common’ alleles occur in frequencies DQA 15 16 31 1 of at least 0.001 (0.1%) DQB 22 68 90 16 1  Why should we continue to leave the Sentinelese alone? What conditions would you predict to exist that make uncontacted tribes susceptible to disease? The Scientific Method Observation – uncontacted tribes usually Variation suffer severe epidemics and population declines following contact Hypothesis – those who make contact bring infectious diseases with them and expose Heritability Fitness the population Predictions – logical consequences Tests – additional experiments or observations Competition Use SEE when you answer! 17  If the hypothesis that the Sentinelese would be susceptible to diseases introduced by those who might make contact were true… Then we would predict the Sentinelese have little immunity and S resistance to these diseases. That is, we would expect to observe reduced levels of genetic E variability among individuals within the Sentinelese population. For example, we would expect to see a low number of MHC E alleles relative to other, non-isolated populations. 18  Small populations are subject to evolution via genetic drift Differential success occurs by chance alone, so does not yield a deterministic outcome 16/64 = 25% 8/64 = 12.5% 4/64 = 6.25% 16/64 = 25% 16/64 = 25% 16/64 = 25% 8/64 = 12.5% 2/64 = 3.125% 16/64 = 25% 16/64 = 25% 4/64 = 6.25% 4/64 = 6.25% 2/64 = 3.125% 16/64 = 25% 8/64 = 12.5% 8/64 = 12.5% 8/64 = 12.5% 16/64 = 25% 8/64 = 12.5% 19  Genetic drift causes populations to evolve randomly We can’t predict what specific evolutionary changes will occur in a given population https://biologydictionary.net/ But we can expect that: 1. Small populations will drift more than large ones 2. Over time, genetic variation will be lost 20  BUT WHERE DOES ALL THE VARIATION COME FROM? How and how often do new alleles arise in populations? 21  Mutation is the ultimate source of genetic variation Without mutation there is no genetic variation, and so there can be no evolution Mutations – changes in the DNA sequence that occur during DNA replication Mutations that are important are evolution are those that occur in the germ line In humans, it is estimated that each individual may carry an average of 1.6 phenotypically-detectable mutations Most mutations in coding regions are probably mildly deleterious, but many are https://www.youtube.com/watch?v=yybsSqcB7mE also neutral (at least initially) We measure the amount of genetic variation in a population by calculating heterozygosity 22  PURE MATHEMATICS IS ON THE WHOLE DISTINCTLY MORE USEFUL THAN APPLIED. G.H. Hardy, 1941 23  The Hardy-Weinberg Theorem allows us to model populations HW allows us to estimate and predict frequencies of alleles and genotypes, given certain If we have a gene with 3 alleles A, B and C, assumptions If p = 0.5, q = 0.3, and r= 0.2, then and pA = frequency of A qB = frequency of B rC = frequency of C then (pA + qB + rC)2 = 1 p2 +2pq + 2pr + q2 + 2qr + r2 = 1 where p2 = expected frequency of AA 2pq = expected frequency of AB 24 etc.  The Hardy-Weinberg Theorem allows us to model populations HW allows us to estimate and predict frequencies of alleles and genotypes, given certain The HW Theorem shows that under certain No mutation assumptions assumptions: n No migratio 1. Allele frequencies in a population will not change over time. 2. Genotype frequencies will also remain No selection constant over time. p u la ti on si ze That is, the population will be in Hardy- Infinite po Weinberg Equilibrium. ting This means that the population will not Random ma evolve. Hardy-Weinberg serves as a null But what are the assumptions? hypothesis! 25  Populations evolve when the assumptions of HW are not met Some evolutionary forces introduce variation, and some sort the variation that exists Introduction of Variation Sorting of Variation Mutation – generates new DNA Selection – preserves or eliminates new sequences (alleles) in populations, alleles based on their effect on fitness which may or may not have an obvious effect on fitness Genetic Drift – preserves or eliminates alleles randomly with respect to fitness Migration – carries (introduces) alleles from one population to another; gene Inbreeding – mating between relatives flow counteracts the effects of drift that increases frequency of homozygotes for all genes Non-Random Mating – positive or negative assortative mating changes frequency of homozygotes for traits linked to the traits of choice 26