Bio 1101 Lecture 7 Part A Chapter 13: How Populations Evolve

3/12/2020Bio 1101 Lecture 13Chapter 13: How Populations EvolveEvolution Charles Darwin: developed the theory of Evolution byNatural Selection As a young man, Charles studied medicine (“longstupid lectures” And surgery? ugh!), then theology(ho-hum) before eventually following his interests innatural history Always interested in observing creatures in the naturalworld:– “One day, on tearing off some old bark, I saw two rarebeetles, and seized one in each hand; then I saw a thirdand new kind, which I could not bear to lose, so that Ipopped the one which I held in my right hand into mymouth. Alas! it ejected some intensely acrid fluid, whichburnt my tongue so that I was forced to spit the beetle out,which was lost, as was the third one.” CHARLES DARWIN, Autobiography, written 18761

3/12/2020 Became ship’snaturalist on theH.M.S. Beagle, andsailed around theworld from 1831-1836 Prior to Darwin’s publication of his researchon natural selection, the dominant world viewregarding species was that they had beencreated and have remained unchanged In the 1700s, some advances in geology werebeginning to challenge the idea of a staticworld2

3/12/2020 Rocks occur in layers, and different types offossils were found in different layers of rock– Fossils are preserved remnants or impressions leftby organisms that lived in the past– Over long periods of time, strata of rock areformed.– The farther down in the strata you dig, the furtherback in time you go 3

3/12/2020Fossils can take manyforms: Hard parts of animals,teeth and bones, areoften fossilized Thin tissues such asleaves may bepreserved as films Entire organisms may be preserved (e.g.frozen or in amber) Plant tissue is sometimes found petrified Imprints such as footprints of dinosaurs occur,but rarely.4

3/12/2020Frozen Wooly Mammoth. Source: WikipediaPetrified wood. Source: Wikipedia5

3/12/2020 Types of plants and animals that long ago hadgone extinct were discovered in the fossilrecord if species were created, why had somany gone extinct? And if species werecreated and remained fixed, why weren’t anyof today’s species found in the ancient fossilrecord? Geologists like Charles Lyell suggestedthe concept of uniformitarianism– The gradual, geological processes that areat work today were also at work in the past– Very gradual changes, over very longperiods of time, can result in very largephysical features of geology (such asmountains and canyons)– If slow processes like sedimentation builtdeposits of rock thousands of feet thick,the earth must be very old (4.6 billionyears!)6

3/12/2020 Before Darwin, there were somenaturalists who believed specieschanged over time– But the mechanisms they proposed weredifferent from natural selection– Jean Baptiste Lamarck: organismsevolved by inheritance of acquiredcharacteristics An organism changes during its lifetime insome way to adapt to its environment (e.g. agiraffe stretching its neck, making its necklonger, and then passing on that trait to itsoffspring) Darwin’s voyage on the Beagle7

3/12/2020– Observed plants and animals along coasts ofSouth America and the Galapagos Islands– Observations Plants and animals of the Galapagos Islands were similar tothose on the South American continent – why not unique? Plants and animals on the islands were adapted to eachother (tortoises were adapted with long necks to reach theleaves of taller plants to eat; different species of fincheswere adapted with different sizes and shapes of beaks foreating specific types of foods) Watch this video on natural selection (Natural Selection –Crash Course Biology #14):https://www.youtube.com/watch?v aTftyFboC M In addition to his own observations of plantsand animals, Darwin read books by Malthusand Lyell that influenced his theory of naturalselection– Charles Lyell’s writings indicated that the earthwas very old – old enough for slow processes toeventually result in big changes over long periodsof time8

3/12/2020– Thomas Malthus was an economist, andwrote about the exponential populationgrowth of humans, and the resultingfamines “The power of population is so superior to thepower of the earth to produce subsistence forman, that premature death must in some shapeor other visit the human race. “– Darwin extrapolated the ideas of Malthus toall creatures; left unchecked, all specieshave the capacity to reproduce faster thantheir food supplies This would set up competition betweenindividuals– Darwin was also aware of the use of artificialselection to create desirable varieties of plantsand breeds of animals for human use For example, vegetables of the species Brassicaoleracea have been modified, from a kale-like ancestor,to a variety of forms today Humans selected for desirable traits, creatingvegetables including broccoli, cabbage, collards,cauliflower, and Brussels sprouts9

3/12/2020 In the same way, artificial selection was usedto create all the breeds of dogs we have today10

3/12/2020 Putting all of this together 11

3/12/2020– Darwin developed his theory of evolutionby natural selection– Called it “Descent with Modification”(descendents were genetically modifiedthrough interaction with their environment)– Natural Selection “the differential survivaland reproduction of individuals withdifferent inheritable characteristics.” Theory of Natural Selection:– Every species is capable of producing more offspringthan can survive to maturity– But, population sizes don’t grow exponentially!Population sizes tend to remain fairly constant, at a levelthat can be supported by the environment– There is competition between individuals for survivaland reproduction– Individuals in populations vary in traits that affect theirchances for survival– THEREFORE, individuals with the best traits will surviveand reproduce better than other individuals12

3/12/2020 Alfred Russel Wallace– Young British naturalist who was alsoinvestigating characteristics of variousorganisms and how they came to lookthe way they do; he was studyingorganisms in the East Indies– Wallace made similar observations toDarwin and ultimately developed theoryof natural selection independently– When Wallace told Darwin of hisdiscovery, it spurred Darwin to finallypublish his book On the Origin ofSpecies in 1859– Therefore, Wallace and Darwin are bothcredited with discovering the theory ofnatural selection, although Darwin haddiscovered it first Today, Darwin’s theory of natural selectionmakes even more sense, in light of ourknowledge of genetics The “Modern Synthesis” or “Synthetic Theoryof Evolution”– This is just the combination of Darwin’s theory ofnatural selection in combination with our modernunderstanding of the genetic basis of inheritance13

3/12/2020An example of naturalselection at work:As farmers spray theirfarm fields with the samepesticide over and over,the insect pests thathappen to have a genetictrait that allows them tosurvive reproduce moreOver time, the populationof insects contains moreand more individuals thatcan survive the pesticide,until the pesticide is nolonger effective In the news: MRSA infections– Our development of antibiotics in the 1940s allowedus to successfully treat many bacterial infections, likeStaphylococcus aureus (which causes skin infections)– By the 1950s, Staphylococcus aureus started to evolveresistance to antibiotics like penicillin and methicillinin response to over-use of antibiotics– MRSA (methicillin-resistant Staphylococcus aureus)infections have now become common, spreading inplaces like schools, gyms, and hospitals14

3/12/2020Evidence for Evolution Fossils The oldest known fossils are from 3.5 billion yearsago, and are prokaryotic cells Fossils trace macroevolutionary changes in achronology of events Fossil evidence for evolution of vertebrates:–––––The oldest fossil vertebrates are fishesthen the amphibians then the reptiles then mammals then birds This is the chronology, as shown by the fossil record15

3/12/2020Missing Link from Sea to Land: Tiktaalik 375 mya transitional fish predecessor ofamphibians Comparative Anatomy The comparison of body structures between species– Example: the forelimbs of different types of mammals– Although they may perform different functions, they arederived from the same bones they have just beenmodified for use in different habitats– Homologous structures – structures that are similar indifferent species and evolved from the same ancestralstructure (similarity due to common ancestry)16

3/12/2020 Comparative Embryology– Comparison of structures that appear in earlydevelopment of different organisms– All vertebrates have similarities in their embryonicdevelopment e.g., gill pouches and post-anal tail17

3/12/2020 Exploring comparative embryology: Guess theEmbryo interactive website!– .html18

3/12/2020 5-minute break Molecular Biology– Comparing the genes and proteins of differentspecies– The more similar the genetic code is between twospecies, the more closely they are related– All species use the same genetic code19

3/12/2020 Biogeography– The study of the geographical distribution ofspecies– Some of the most important evidence Darwin hadto work with For example, he noted the animals looked more likeanimals on the South American continent than animalson other, more distant islands Species that are geographically close to each other aremore likely to be more closely related than species thatare very distant geographically20

3/12/2020Microevolution and Population Genetics What is evolution, and how does it happen?– Evolution a change in allele frequencies in apopulation over time A population is defined as all the individuals of a singlespecies living in a particular area– Microevolution (evolution on its smallest scale) a change in the genetic makeup of a populationfrom generation to generation Note that evolution occurs at the populationlevel (NOT the individual level).– If an individual changes over its lifetime, that is anacquired trait and isn’t genetically coded– Only genetically coded traits are passed on tooffspring and influence future gene pools– A population is the smallest biological unit thatcan evolve.– Again, Evolution is the change in allele frequenciesin a population21

3/12/2020 Mutations are the source of all geneticvariation– Single genes may mutate– Entire segments of chromosomes may be deletedor added, or entire chromosomes may beduplicated or omitted from the genome, resultingin more drastic forms of mutation The Hardy-Weinberg Equilibrium– A theoretical state in which a population does notevolve. It is purely theoretical; natural populationsare never truly in Hardy-Weinberg Equilibrium– 5 Conditions must be met for a population to be inHardy-Weinberg Equilibrium (and therefore notevolve): The population is infinitely large (no genetic drift)There is no mutation (no creation of new alleles)There is no gene flow between populationsRandom matingNo natural selection– If all these conditions are met, the frequencies ofalleles in the population will stay the same over time22

3/12/2020 So what causes evolution?– Any deviation from any of the five conditions ofHardy-Weinberg equilibrium– So, how often do populations meet these? NEVER in nature However, it can be modeled using computer simulations AND, the Hardy-Weinberg Theorem has providedmathematical equations that can be used to calculateapproximate allele and genotype frequencies in realpopulations Calculating Allele Frequencies– “The gene pool consists of all alleles in all theindividuals making up a population.”– Consider a population of 100 wildflowers, wherepurple flowers are dominant to white flowers In those 100 wildflowers, there will be 200 copies ofany gene, because individuals are diploid If 20 plants are “PP”, 60 are “Pp” and 20 are “pp” then: There are 100 copies of the “P” allele and 100 copies ofthe “p” allele in this gene pool23