Note for BSC 220 at UA-Chapter 5 Notes from Why Evolution is True
Note for BSC 220 at UA-Chapter 5 Notes from Why Evolution is True
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Date Created: 02/06/15
Chapter 5 WEIT notes for Januarv 31 2012 Hornets of Asia This is nature red in tooth and claw Homets are fearsome hunting machines and the introduced bees are defenseless But there are bees that can ght off the giant hornet honeybees that are native to Japan Their defense is stunningianother marvel of adaptive behavior When the hornet is inside the native honeybees nest it is mobbed and covered by a ball of bees Vibrating their abdomens the bees quickly raise the temperature inside the ball to about 117 degrees F Bees can survive this temperature but the hornet cannot In twenty minutes the hornet scout is cooked to deathiand the nest is usually saved There are several evolutionary lessons The most obvious is that the hornet is marvelously adapted to kill it looks as if it was designed for mass slaughter Many traits work together to make it a killing machine they include body form large size stinger deadly jaws big wings chemicals marking pheromones and deadly venom in the sting and behavior rapid ight coordinated attacks on bee nests and the larval I am hungry behavior prompts the hornet attacks Then there is the defense of the native honeybeesithe coordinated swarming and subsequent roasting of their enemyicertainly an evolved response to repeated attacks by hornets The recently introduced European honeybees are virtually defenseless against the hornet Those bees evolved in an area lacking giant predatory hornets and therefore natural selection did not build a defense We can predict though that if the hornets are strong predators the European bees will either die out or will find their own evolutionary response to the hornets Roundw orm Parasites of Central America The roundworm parasites infect ants When infected an ant undergoes a radical change in both behavior and appearance Its abdomen changes to a bright red color Ant becomes sluggish and raises its abdomen straight up into the air Can no longer produce alarm pheromones when attacked so it cannot alert others All of these changes are caused by the genes of the parasitic worm as an ingenious ploy to reproduce themselves The worm alters the appearance and behavior of the ant which advertises itself to birds as a tasty berry Birds gobble up the ants abdomens which are full of worm eggs The birds then pass the eggs in their droppings which ants scavenge and take back to their nests to feed the larvae Natural selection acting on a simple worm has caused it to commandeer its host and change the host s appearance behavior and structure turning it into a tempting mock fruit Zoe Bailey 1 Plants that look like animals When the frustrated insect departs without consummating his passion he unwittingly carries the pollen to the next orchid fertilizing it during the next fruitless psedocopulation Woodpecker The woodpecker has a whole group of traits that help it hunt and hammer Long tongue most of the time the tongue is retracted but it can be extended deep into a tree to probe for ants and beetles It is pointed and covered with sticky saliva to help extract insects from holes The woodpecker can strike up to fteen blows per second when it is drumming for communication each blow generates a force equivalent to banging your head into a wall at sixteen miles an hour Woodpeckers Skull specially shaped and reinforced with an extra bone The beak rests on a cushion of cartilage and the muscles around the beack contract an instant before each impact to divert the force of the blow away from the brain and into the reinforced base of the skull Eye1ids close to keep its eyes from popping out Fan of delicate feathers covering the nostrils so that the bird does inhale sawdust or woodchips when hammering Set of very stiff tail feathers to prop itself against the tree Xshaped fourtoed foot to grip the trunk o Everywhere we look in nature we see animals that seem beautifully designed to fit their environment whether that environment be the physical circumstances of life like temp and humidity or the other organismsicompetitors predators or prey Coat color in Mice Norma1colored or Oldfield mice have brown coats and burrow in dark soils On the pale sand dunes of Florida s Gulf Coast there is a lightcolored race of the same species called beach mice They are nearly all white with only a faint brown stripe down the back This pale adaption is to camou age the mice from predators Experiment results Mice whose coats contrasted most conspicuously with the soil were picked off more readily showing that camou aged mice really do survive better This experiment explains a general correlation that we see in nature darker soils harbor darker mice There is also a genetic difference between light and dark mice beach mice carry the light forms of several pigmentation genes that together give them their lightcolored coats while dark mice carry the dark alternative of the same gene Over time due to differential predation lighter mice would have left more copies of their light genes they have a higher chance of surviving to reproduce Population of beach mice would have evolved from dark 9 light Natural selction acting on coat color has simply changed the genetic composition os a population increasing the proportion of genetic variants the lightcolor genes that enhance survival and reproduction Zoe Bailey 2 0 Selection is not a mechanism imposed on a population from outside Rather is a process a description of how genes that produce better adaptations become more frequent over time o Adaptation to the environment is inevitable if a species has the right kind of genetic variation Three things are involved in creating an adaptation bV natural selection 1 The starting population has to be variable Mice within a population have to show some differences in their coat colors 2 Some proportion of that variation has to come from changes in the forms of genes that is the variation has to have some genetic basis called heritability a We know exactly which two genes have the largest effect on the darld light color difference One is called Agouti the same gene whose mutations produce black color in domestic cats The other is Mclr b Mutationsiaccidental changes in the sequence of DNA that usually occur as erroes when the molecule is copied during cell divisionicause of genetic variation c Mutations occur randomly Mutations occur regardless of whether they would be useful to the individual Most of the time they are harmful or neutral but sometimes can be useful The useful ones are raw material for evolution 3 The third aspect of natural selection is that the genetic variation must affect an individuals probability of leaving offspring 0 Evolution by natural selection is a combination of randomness and lawfulness o Chance alone cannot explain the marvelous fit between individuals and their environment I It is the ltering of that variation by natural selection that produces adaptations and natural selection is manifestly not random 0 It is a powerful molding force accumulating genes that have a greater chance of being passed on than others and in so doing making individuals ever better able to cope with their environment 0 Richard Dawkins provided the most concise definition of natural selection it is the nonrandom survival of random variants Selection to mold these features immune system braina agellum etc 0 First it has to create themimost often graduallyistepbystep form precursors As we have seen each newly evolved trait begins as a modification of an earlier feature I There is no going downhill in the evolution of an adaptation for selection by its very nature cannot create a step that doesn t benefit its possessor 0 An adaptation must evolve by increasing the reproductive output of its possessor For it is reproduction not survival that determines which genes make it to the next generation and cause evolution I On the other hand a gene that knocks you off after reproductive age incurs no evolutionary disadvantage It will remain in the gene pool Zoe Bailey 3 Harboring Ants Harboring ants gives a tree huge bene ts First herbivorous insects and mammals that stop by for a leafy treat are repelled by a ferocious ant horde The ants also cut down seedlings around the base of the treeiseedlings which when larger could compete with the tree for nutrients and light In every case when one species does something to help another it always helps itself This is a direct prediction of evolution 0 Adaptations always increase the tness of the individual not necessarily of the group or the species 0 In fact evolution can produce features that while helping an individual harm the species as a whole Evolution Without Selection Biologists define evolution as a change in the proportion of alleles different forms of a gene in a population Every individual has two copies of each gene which can be identical or different Every time sexual reproduction occurs one member of each pair of genes from a parent makes it into the offspring along with one from the other parent The upshot is that every generation the genes of parents take part in a lottery whose prize is representation in the next generation Eventually the resulting random walk can even cause genes to become fixed in the population that is rise to 100 frequency or alternatively get completely lost Such random change in the frequency of genes over time is called genetic drift Genetic Drift A legitimate type of evolution since it involves changes in the frequencies of alleles over time but it doesn t arise from natural selection Drift is a random process while selection is the antithesis of randomness Genetic Drift can change the frequencies of alleles regardless of how useful they are to their carrier Selection on the other hand always gets rid of harmful alleles and raises the frequencies of beneficial ones As a purely random process genetic drift can 1 cause the evolution of adaptations it could never build a wing or an eye What drift can do is cause the evolution of features that are neither useful nor harmful to the organism Genetic drift is not only powerless to create adaptations but can actually overpower natural selection especially in small populations This is almost certainly why we see a high incidence of genetically based diseases in isolated human communities TaySachs in Cajuns of Louisiana Many aspects of molecular evolution then such as certain changes in DNA sequence may re ect drift rather than selection Nevertheless we know that genetic drift must occur because in any population of finite size there are always sampling effects during reproduction And drift has probably played a substantial role in the evolution of small populations only a few examples tho Zoe Bailey 4 Animal and Plant Breeding Evidence for selection comes from many areas The most obvious is arti cial selectionianimal and plant breedingiwhich as Darwin realized is a good parallel to natural selection Breeding has wrought huge changes in a remarkably short period of time for animal and plant breeding has been practiced for only a few thousand years Domestic Dog Every single one purebred or mutt descends from a single ancestral speciesi most likely the Eurasia gray wolf Breeders have virtually sculpted these dogs to their liking changing the shade and thickness of their coats the length and pointiness of their ears the size and shape of their skeleton the quirks of their behaviors and temperaments and nearly everything else The variation among domestic dogs far exceeds that among wild dogs species Weight Domestic dogs range from 2 lbs to 180 lb Mastiff while the weight of wild dog species varies from 2 lbs to only about 60 lbs The success of breeding validates 2 of the 3 requirements for evol by selection 1 First there was ample variation in color size shape and behavior in the ancestral lineage of dogs to make possible the creation of all breeds 2 Second some of that variation was produced by genetic mutations that could be inheritedifor if not breeders could make no progress If artificial selection can produce such canine diversity so quickly it becomes easier to accept that the lesser diversity of wild dogs arose by natural selection acting over a period a thousand times longer Onlv 1 difference between Arti cal Selection and Natural Selection Arti cial selection it is the breeder rather than nature who sorts out which variants are good and which are bad In other words the criterion of reproductive success is human desire rather than adaptation to a natural environment Sometimes the criterion coincides greyhound was selected for speed and wound up shaped much like a cheetah An example of convergent evolution similar selective pressures give similar outcomes Evolution in the Test Tube Bacteria can divide as often as once every twenty minutes allowing us to observe evolutionary change over thousands of generations in real time This is genuine evolutionary change demonstrating all three requirements of evolution via selection variation heritability and the differential survival and reproduction of variants Microbes can adapt to virtually anything that scientists throw at them in the lab But lab adaptations can also be more complex involving the evolution of whole new biochemical systems Zoe Bailey 5 Resistance to Drugs and Poisons Given their huge populations sizes and short generation timesifeatures that make bacteria ideal for studies of evolution in the labithe chance of a mutation producing antibiotic resistance is high Those bacteria that are resistant to a drug will be those that survive leaving behind genetically identical offspring that are also drugresistant Resistance comes from evolution os the microbe not habituation of patients to the drugs Viruses the smallest form of evolvable life also evolved resistance to antiviral drugs Insects have become resistant to DDT and other pesticides plants have adapted to herbicides and fungi worms and algae have evolved resistance to heavy metals that have polluted their environment There almost always seem to be a few individuals with lucky mutations that allow them to survive and reproduce quickly evolving a sensitive population into a resistant one Selection in the Wild We want to see a natural population meet a natural challenge we want to know what that challenge is and we want to see the population evolve to meet it before our eyes For one thing natural selection in the wild is often incredibly slow The evolution of feathers probably took hundreds of thousands of years If we are to see natural selection at all it must be strong selection causing rapid change and we d best look at animals or plants having short generation times so that the evolutionary change can be seen over several generations Further we shouldn t expect to see more than small changes in one or a few features of a speciesiwhat is known as microevolutionary change Given the gradual pace of evolution it s unreasonable to expect to see selection transforming one type of plant or animal into anotherisocalled macroevolutioni within a human lifetime If a species of bird for example has evolved the optimum body size for its environment and that environment doesn t change selection will act only to cull birds that are larger or smaller that the optimum But this kind of selection called stabilizing selection won t change the average body size Can Selection Build Complexity Sure complexity can change the beaks of birds or the owering period of plants but can it build complexity Complex features take a long time to evolve and most of them did so in the distant past when we weren t around to see how it happened There is no alternative theory that can build a complex adaptation only natural can We know of no adaptations whose origin could not have involved natural selection How can we be sure For anatomical traits we can simply trace their evolution when possible in the fossil record and see in what order different changes took place Dogged Scientific research is beginning to give plausible and testable scenarios for how complex biochemical features could have evolved The bloodclotting pathway of vertebrates involves a sequence of events that begins when one protein sticks to another in the vicinity of an open wound Zoe Bailey 6 Many of the bloodclotting proteins are made by related genes that arose by duplication a form of mutation in which an ancestral gene and later its descendants becomes duplicated in full along a strand of DNA because of a mistake during cell division Since fibrinogen occurs in all vertebrates as a bloodclotting protein it preseumably evolved from a protein that had a different function in ancestral invertebrates who were around earlier but lacked a clotting pathway A protein was discovered in the sea cucumber an invertebrate Sea cucumbers branched from the vertebrate lineage at least 500 mya yet they have a protein that while clearly related to bloodclotting proteins of vertebrates is not used to clot blood This means that the common ancestor of sea cucumbers and vertebrates had a gene that was later coopted in vertebrates for a new function It appears then that in principle there is no real problem with evoltuoin building compleX biochemical systems Rates of Evolution in fossil record vs those seen in lab Experiments Showed rates of change in animal size and shape during laboratory and colonization studies are actually much faster than rates of fossil change 500 X faster 1 mill X faster Even the fastest rates of evolution in the fossil record are nowhere near as fast as the slowest rates seen when humans practice selection in the laboratory The lesson is that selection is perfectly adequate to explain changes that we see in the fossil record One reason why people raise this question is bc they don t grasp the immense spans of time that selection has had to work Zoe Bailey 7
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