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Lecture 4

BIOL 150 - Week 4 Lecture Notes

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University of Waterloo
BIOL 150
Rebecca Rooney

Disruptive Natural Selection in Sticklebacks • field study carried out by Robinson lakes of coastal BC wherever two stickleback species occur in the same lake, they occupy different • habitats and make use of different sources • limnetic form feeds on zooplankton • benthic form feeds on larger invertebrate prey from sediments and submersed aquatic vegetation • differ morphologically - ex. gill rakers (bony structure that diverts solids from the gills) smaller in limnetic form than benthic • most lakes have one species and most will be of intermediate morphology and habitat use - sometimes feeds in limnetic zone and sometimes in the benthic zone • some individuals will mirror the difference in species pair: limnetic form and benthic form: different phenotypes Hypothesis: • disruptive selection is driving evolution because optimization for one form entails costs to adopting the other form Prediction: • difference between two forms are heritable - descendants of either form will remain true to it, even in neutral habitat • divergence in morphology will be reflected in foraging efficiency for limnetic or benthic prey - variation in trait must lead to differences in fitness Experiment 1 • reared offspring of both forms one species under identical lab conditions and diet - do differences persist? Results: yes, traits are heritable • Experiment 2 • feeding trails of foraging efficiency in artificial limnetic and benthic habitats using two food types • released fish of one phenotype into either a limnetic or benthic and counted how many prey it caught intake rate: # of prey captures/min capture effort: # bites per prey caught • Results: limnetic form: better at eating limnetic prey in limnetic aquarium benthic form: better at eating benthic prey in benthic aquarium Therefore, foraging efficiency is related to phenotype September 30, 2013 - Lecture 10 Speciation and Hybridization Speciation: evolution of a new species when gene flow is reduced between populations, they may then diverge genetically as a result of • mutations, natural selection and genetic drift • genetic divergence may eventually lead to speciation - creation of a new species usually creates two or more distinct species from a single ancestral group • finches some common ancestor populated all islands, eventually populations diverged (evolved in different • environments) Adaptive radiation: process in which one species gives rise to multiple species that exploit different features of the environment 14 BIOL 150 Fall 2013 To make distinct species, we need: • reproductive isolation: new species created when the diverging population can no longer reproduce • sympatric speciation: occurs without geographic isolation - usually due to disruptive selection • polymorphism forms, likely due to patchy habitat • polymorphism: existence of more than one distinct form of individuals (phenotypes) in a population • may cause speciation is differences in form affect reproductive morphology, reproductive timing or reproductive behavior to cause reproductive isolation of the two polymorphs • ecological speciation: special case of sympatric • morphology, behavior, timing (traits changes due to natural selection pressure - targets of selection) • divergent natural selection • ex. soapberry bugs • ex. mosquito fish • use beaks to reach seeds inside fruits • inhabiting blue holes in the bahamas has • native plant fruits (large fruits), non- evolved larger caudal region and smaller native plant fruits (small fruit) heads in the presence of predators than in • evidence for disruptive selection on their absence beak length found that short beaked • populations growing on non-native plants and long beaked populations growing on native plants • speciation is allopatric: occurs as a result of geographic separation of a population into two or more subpopulations with no movement between them • ex. Diane Dodd and fruit flies - divided population into 2, fed one starch and the other maltose • brought the two together, only starch flies wanted to mate with other starch, and maltose with maltose • ex. lizards - one continuous population, island population begins to diverge due to drift and selection • river changes course, runs through population of lizards, population begins to diverge due to drift Sympatric vs. Allopatric Speciation Sympatric Allopatric Similarities • both involve hee ormaatonn ofa neww species via epproducttve solatonn offhe genee pooll fomm existnggsppecies • both occuurwhhen naaurallselecton creaaessgeenettcdivergeence betweeen neew anddanncesttall poppulatonso Differences • involves a reproductive or behavioral • involves the physical separation of separation populations • populations occupy same geographical • populations occupy different geographical areas areas • ex. polyploidy in wheat strains • ex. adaptive radiation of Galapagos finches Hybridization: if they are so genetically and phenotypically distinct, they may not be able to interbreed - reproductively isolated - offspring have lower fitness hybrid offspring do not develop or reproduce normally • called reinforcement, because the traits that isolate populations reproductively are selected for, so the speciation is reinforced sometimes the two species can mate successfully (fertile offspring that survive) • • may reverse speciation - parents no longer reproductively isolated 15 • may create new species - if hybrid cannot back-cross with either parent; if it can meet with other hybrids, you have created a whole new species may animal hybrids are sterile because of an uneven number of chromosomes • • ex. horse: 64 chromosomes; donkey: 62; mule: 63 - odd number, cannot reproduce • lots of plant hybrids (~70% of flowering plants) because it can reproduce asexually (apomixis and vegetative reproduction) even if hybrid is sterile, it can persist • polyploidy: multiplication of chromosome number • offspring cannot back-cross with parents - speciation through reproductive isolation • how does it happen? spontaneous doubling after fertilization; union of unreduced gametes • if they can: i. mate with other polyploids of the same chromosome number ii. reproduce vegetatively iii. reproduce by apomixis (asexual reproduction in which seeds are formed without meiosis or sexual recombination) • then they may be a new species! • no species is even perfectly adapted to its environment • trade-offs - traits of benefit for one environment will have a cost - energy allocation • environment is in flux - not constant • natural selection acts only on available variation in the gene pool: just a filter; not creative • rand chance - some degree of extinction is not related to fitness: eg. volcano, ice storm, etc. • correlation among genes on chromosomes - a beneficial gene may be on the same chromosome as a deleterious one or neutral one Phenotypic plasticity: ability of the genotype to give rise to different phenotypes under different environmental conditions • individuals can respond to temporal and spatial changes of environment by moving to a more suitable location and by a direct influence of the environment on gene expression - often see phenotypic plasticity in plants because they can’t move • norm of reaction: the set of phenotypes expressed by a single genotype across a range of environmental conditions • same phenotype in different environments • different phenotypes in the same environment • if lines intersect - at the place where they intersect, you have the expression of the same phenotype (but still two different genotypes acting) developmental plasticity: occurs during growth - irreversible • • common in plants (ex. Polygonum persicaria) • acclimation: reversible changes in physiology (bullhead catfish in summer vs. winter temperature ranges), morphology or behaviour • ex. fish - as long as temperature changes slowly, it can move its optimal temperature range • even though these changes are not heritable, natural selection can act on the capacity for plasticity • essential plasticity is a way of being adapted to variability in the environment by altering phenotypic expression October 2, 2013 - Lecture 11 Simbio Lab Review Frequency histogram - graphical representation of how common a particular value is • distribution of variable is the arrangement of its values that indicates their frequency of occurrence • bar graphs - not always bell shaped 16 BIOL 150 Fall 2013 Evolution by natural selection • change in allele frequencies within the populations between generations • requires: • variation in ph
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