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5: Insect Microevolution Let’s start with a huge problem… The complex adaptive nature of ecosystems means that evolutionary forces are strongest at lower levels of organization; we have learned that the hard way in our continual battles with the evolution of resistance to pesticides and antibiotics, and the unwillingness of [insects and] microorganisms to take a reasonable approach to making things easy for us. (Levin 2001, p. 17) Why do insects evolve so rapidly? A major reason is that they can reproduce rapidly; in just one year, insects may have multiple generations. Some of the best-documented examples of contemporary evolution are those from insects; humans have rapidly changed the environment on this planet and insects have evolved rapidly in response (McKenzie 1996). But other groups of animals can also evolve rather rapidly, too… It has the menacing sound of an Alfred Hitchcock movie: Millions of rats aren't even getting sick from pesticide doses that once killed them. In one county in England, these "super rats" have built up such resistance to certain toxins that they can consume five times as much poison as rats in other counties before dying. From insect larvae that keep munching on pesticide-laden cotton in the U.S. to head lice that won't wash out of children's hair, pests are evolving genetic shields that enable them to survive whatever poisons humans give them. Rachel Carson predicted such resistance in her groundbreaking book Silent Spring, published soon after the chemical insecticide glory days of the 1950s. And the problem is getting worse. Farmers in the U.S. lost about seven percent of their crops to pests in the 1940s. Since the 1980s, some 13 percent of crops are being lost -and more pesticides are being used. Now 60% of crops are being lost due to pests in many areas. It's a huge problem, but the pests are only following the rules of evolution: the best-adapted survive. Every time chemicals are sprayed on a lawn to kill weeds or ants for example, a few naturally resistant members of the targeted population survive and create a new generation of pests that are poison-resistant. That generation breeds another more-resistant generation; eventually, the pesticide may be rendered ineffective or high doses may even kill other wildlife or the very crop it was designed to protect. In countless ways, human actions are hastening pests' evolution of resistance. Farmers spray higher doses of pesticide if the traditional dose doesn't kill, so genetic mechanisms that enable the pests to survive the stronger doses rapidly become widespread as the offspring of resistant individuals come to dominate the population. Farmers often also use only one pesticide, when using a combination would slow the evolution of resistance. Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. Pest management worldwide is currently dominated by broad-spectrum toxic chemicals. The limited adoption of other forms of pest management (we’ll talk about some of these later in the semester) suggests that concerns about toxic effects on humans and the environment are not as great as a desire to save money and control pests rapidly. However, recent initiatives in Europe have reduced agricultural pesticide usage by 50% because of concerns for human health. For the most part, though, chemical pesticides are applied liberally to crops, and one result has been the evolution of pesticide resistance. This phenomenon is similar to antibiotic resistance in bacteria. In 2001, the Food and Agriculture Organization reported more than 700 pest species were resistant to pesticides. A parallel may be drawn between the way that the presence of an antibiotic and the presence of a pesticide both select for resistance in their target species. Bacteria and insects often acquire resistance in the same way, through a single mutation. In the common fruit fly Drosophila, change in just one gene allows the fly to overproduce an enzyme that breaks down dichloro-diphenyl-trichloroethane (DDT) and other poisons. As seems to be the case in at least some instances of antibiotic resistance, removal of the selective agent (the global decline in the use of DDT in this case) does not necessarily cause the insects to revert back to a susceptible state (Major et al. 2003). What have we learned about pesticide resistance? Ways that we can minimize pesticide resistance include the following: 1) Rotating pesticides so that a single population of insects is not constantly bombarded with the same chemicals, 2) Using mixtures of chemicals, because the mutant insects that can survive one pesticide are unlikely to also survive the second or third pesticide, 3) “Spot spraying”, or spraying very carefully to only the affected areas, limits exposure for other organisms, including beneficial insects and pest insects not presently targeted. 4) Avoiding pesticides except when absolutely necessary. Pesticide resistance will not evolve if the selective forces (i.e. pesticides) are not sprayed! Evaluating the results of treatment can also help farmers to choose the most effective treatment. All of this parallels the plans to slow the evolution of antibiotic resistance… understanding evolution in one context helps us to understand it in another context. The overuse of pesticides has caused some problems that were not expected. For example, broadcast spraying of synthetic organic insecticides has induced resistance not only in their intended targets—crop pests—but also in vectors of disease. Agricultural spraying, especially on cotton and rice, has contributed greatly to the evolution of pesticide resistance in the Anopheles mosquitos that transmit malaria, and thus to Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. the worldwide resurgence of that disease. The continuing battle against resistance is analogous to the evolutionary concept of arms races, in this case pitting human ingenuity in discovering new toxins and using them appropriately against the adaptive capacity of pest species. DDT, the powerful synthetic pesticide once thought to be harmless unless ingested, was used liberally on boll weevils and humans alike. Typhus outbreaks in the countries entered by the U.S. army during World War II were stopped cold by DDT, sprayed directly on people, clothing and bedding -- even sprayed by air over entire cities. REFERENCES This reading draws directly from the following sources: Denholm I, Rowland MW. 1992. Tactics for managing pesticide resistance in arthropods: theory and practice. Annu. Rev. Entomol. 37:91–112 Levin SA. 2001. Immune systems and ecosystems. Conserv. Ecol. 5:17 http://www.consecol.org/vol5/iss1/art17 Major J, Bogwitz M, Perry T. 2003. Lone gene could force rethink on pest insect control. Presented at Int. Congr. Genet. Poster, 19th, July 6–11. Referenced in http://news.bio-medicine.org/biology-news2/ Lone-gene-could-force-re-think-on-pest-insect-control-4295-1/ Orzech KM, Nichter M. 2008. From resilience to resistance: political ecological lessons from antibiotic and pesticide resistance. Annual Review of Anthropology 37: 267–282. http://www.pbs.org/wgbh/evolution/library/10/1/l_101_02.html Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. How does microevolution differ from macroevolution? Earlier in the semester we went over macroevolution: changes at or above the species level. Macroevolution is “big picture” evolution, including the formation of new species. Now we will discuss microevolution, or genetic change in populations. Microevolution happens all the time – nearly all populations experience genetic change on an almost daily basis as members of that population are born and die. Some of these genetic changes result in obvious differences (e.g. size, color, and pesticide resistance), while other genetic changes may not change the phenotype of organisms. Macroevolution also involves genetic changes (though they are usually more massive). Important Definitions Theory The common use of the term theory implies speculation or assumption that has not been verified or has limited proofs. However, in science, a theory is a well-substantiated explanation or a set of statements that have been confirmed over the course of many independent experiments. In comparison, theories are more certain than hypotheses but less certain than laws. And in science, an unproven idea or a mere theoretical speculation is regarded as a hypothesis rather than a scientific theory. Hypothesis A supposition or tentative explanation for (a group of) phenomena, (a set of) facts, or a scientific inquiry that may be tested, verified or answered by further investigation or methodological experiment. A scientific hypothesis that has been verified through multiple scientific experiments and other research may be later considered a scientific theory. Contemporary evolution Evolution often occurs on contemporary timescales, often within decades, or even more quickly for organisms with short generation times, such as insects and (especially) bacteria. Contemporary evolution is evolutionary change occurring now or within a human lifetime. Human activity is generating selection across the planet, so much that rapid evolution is becoming the new normal. Until recently it seemed that science progressed quickly while nature stood still, but now it seems the reverse is true. In today’s world, it is crucial for people to understand how evolution functions. Huge problems loom including antibiotic resistant bacteria, pesticide resistance in insects, and species extinction. These Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. problems can only be solved (or at least mitigated) by using the insights and tools of evolutionary biology. Forces that can result in evolution (genetic changes in populations) are the following: 1. 2. 3. 4. 5. Natural selection Sexual selection Genetic drift Gene flow Mutation NATURAL SELECTION Evolution by natural and sexual selection was Darwin’s big idea. Change in organisms over time was already a known fact (animal and plant breeding for desirable characteristics had occurred for centuries), but natural mechanisms responsible for these changes were not known. Natural selection is selection resulting from differences in the survival of organisms in a population. Examples of natural selection pressures include predation, extreme temperatures, and food limitation. Three elements need to be present for natural selection to result in evolution: 1) There is phenotypic variation, and these differences affect survival. • E.g. Big animals die while small animals live when food is in short supply. 2) The differences are heritable • E.g. Small adults tend to produce small offspring 3) Reproduction must occur Artificial selection is a type of natural selection, but humans are responsible for creating the selection. For example, humans have bred wild dogs to result in everything from Chihuahuas, Boxers, and Terriers, to Huskies and Great Danes. Similarly, Native Americans bred a type of grass that was good to eat, and by selecting only those with big grains to reproduce, created corn. Mustard plants were bred to produce broccoli, cauliflower, cabbage, and kale (they are all the same species!). SEXUAL SELECTION Sexual selection is selection due to differences in mating success. Sexual selection consists of two components: male-male competition, which results in the evolution of weapons, and mate choice, which results in the evolution of ornaments. Sexual selection is generally stronger in males than in females because of great variation in mating success – some males mate many times, while most males mate only a few times or not at all. This difference in selective pressures occurs because most animals are not monogamous. Because sexual selection can be so strong, elaborate ornaments and weapons can Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. evolve, even if they are bad for survival (i.e. natural selection). The long eye-stalks on stalk-eyed flies and the long tails of birds of paradise are just a couple of examples of such ornaments and weapons. GENETIC DRIFT Genetic drift is simply random genetic changes in populations and is more likely to occur in small populations. One example of genetic drift occurs when populations go through a “bottle neck” where population size becomes tiny, and the number of genotypes represented is small. For example, if a couple of insects get trapped inside a cargo ship and are transported to another continent, they may breed and colonize the new habitat. However, the genetic composition of the new population will be different from the old population. E.g. insects in the native habitat may be red, grey, and white, while in the new habitat, they are only white (only white ones caught a ride on the boat) GENE FLOW Gene flow is simply mating among individuals from different populations. Gene flow occurs when seeds are dispersed, animals immigrate into a new population, or when a lovebug catches a ride in your car to a new location where it then finds a mate. MUTATION Mutation is the alteration of a DNA sequence (thus, genetic change). Mutations are fairly rare, though exposure to certain types of radiation and toxic chemicals (carcinogens) can increase the rate of mutations. It is fairly common to come across statements like “insects reproduce at such a high reproductive rate to ensure the survival of the species,” or “male beetles rarely fight to the death because such aggressive interactions might lead to the extinction of the species.” However, these statements show an important misunderstanding of what evolution is and how it operates. When insects are stressed (due to pesticides, food limitation, competition, etc.) some will succeed and produce lots of offspring while others will die before producing offspring (this is individual-level selection). If the difference between those that succeed (i.e. those with high fitness) and those that fail (i.e. those with low fitness) is genetically heritable, then offspring will resemble parents, and the genetic make-up of the population will change (this is evolution). An altruistic trait – a feature that reduces the fitness of an individual that bears it for the benefit of the population or species – cannot evolve by individual selection. An altruistic genotype amid other genotypes that were not altruistic would necessarily decline in frequency, simply because it would leave fewer offspring per capita than the others. Likewise, if a population were to consist of altruistic Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. genotypes, a selfish mutant – a “cheater” – would increase to fixation, even if a population of such selfish organisms had a higher risk of extinction. Image to the right: Lemmings once had the reputation of being altruistic. When population densities got too high, as the story goes, then some lemmings would drown themselves to save others in the population. But, as the cartoon shows, cheaters (here, the one with the inflatable tube) will survive, produce offspring, and if the cheating phenotype is heritable, the cheater adults will produce cheater babies. Soon, nearly everyone in the population will be watching out for themselves, and altruism will disappear. So what about behaviors that appear altruistic? Some insects and other animals do have behaviors that benefit their immediate group – such as alarm pheromones that alert others of danger. Why would they do this? One major cause for such behaviors is kin selection – that individuals should have behaviors that benefit their relatives (since their relatives share many of the same alleles). Another possibility is that even though an individual’s behaviors benefit the group, the individual itself also achieves many selfish benefits. Some species with highly evolved social networks (such as primates) commonly help one another. In these groups “reciprocal altruism” is common – one individual helps another, and this behavior is later reciprocated. If individuals do not reciprocate, they are commonly ignored or shunned (and thus achieve lower fitness). Reference: Futuyma, D.J. 2005. Evolution. Sinauer Associates: Sunderland, MA. In the Sensory Systems lecture we discussed how pheromones can result in nymphal grasshoppers developing into locusts. Just because of the presence of pheromones and other cues during development, the color, morphology, and behavior of these insects can change. This is an example of phenotypic plasticity, not evolution. Evolution requires genetic change, but here, these Orthoperans may be genetically identical. Phenotypic plasticity is the capacity of a single genotype to produce more than one phenotype. Phenotypic plasticity is very common. For instance, if you had an identical twin who was raised in a far-off country, you two could speak different languages, choose different clothes to wear, prefer different foods… all because of your environment. You would be different not because of genetic differences, but because you were raised differently. Granted, some things would be the same! Some traits are phenotypically plastic while others are “canalized”, where differences among individuals are due to genetic differences. Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. 5: Insect MicroEv M volution Stud dy Questio ons and Ob bjectives Draw w a diagram m to explain n how insec cts evolve pesticide p re esistance. N Now label th he diagram and provvide a shortt essay in support of th hat diagram m. In your la abels and e explanation,, answer the following quesstions: wha at is the sele ective press sure involve ed? How m many pest sspecies are resistant to o pestticides? If an a insect ge ets sprayed with pestic cide, what tthree eleme ents are needed for se election to resu ult in evolutiion? Finally y, provide fo our ways th hat growerss can minim mize pesticid de resistancce. Base ed on the CBS C news reading, r ho ow long did bednets re esult in a dro op in malarria attacks? ? What is the p pesticide us sed in the bednets? b Base ed on the NPR N reading and radio o story (visit http://www w.npr.org/2011/01/19/133057071 1/bed-buggeno ome-revealls-pesticide e-resistance efor the story), describ be the evide ence that be edbugs havve evolved pestticide resisttance? Wha at was the mechanism m m thought to o be respon nsible for the evolution n of pestticide resisttance (i.e. how h did the e resistant bedbugs b tolerate the p pesticide)? S Some bedb bugs can tolerrate high levels of pyre ethroids. Th hey can withstand a do ose of up to o how many times the e dose that wou uld usually be b lethal? Explain the diffference betw ween selec ction and ev volution. Exxplain a hyp pothetical ccase where selection doess not resultt in evolution and why.. Explain how ge enetic drift is responsib ble for the invasive “su uper-colonie es” of the A Argentine A Ant. From the rradio broad dcast, what is one way y that huma ans might co ontrol these e super-colonies witho out typical pestticides? Explain group selection, s based b on the course note es, and why y it is largely y a faulty co oncept. Whyy do most in ndividuals rarely r do an nything “for the good off the specie es” unless it is likely to diirectly bene efit them as s well? Stud dy Terms Theo ory, hypoth hesis, Biolog gical Evolution Pestticide resisttance, Natu ural Selectio on, Artifficial Selecttion, Sexual Selection,, Genetic Driftt Gen ne, Allele, Locus Gen notype, Phe enotype, Fittness, Herittability Con ntemporary evolution, Adaptation A Grou up selection n, Phenotypic plasticitty Dr. M Miller ENY300 05/5006 Princciples of Entom mology, University of Floriida Includees material from the intro. entomolo ogy course taughtt by Dr. Douglas Em mlen and from Thee Insects, Authors : Gullan & Cransto on. Malaria fears up as mosquitoes show pesticide resistance - - CBS News Page 1 of 1 August 18, 2011 12:27 PM Malaria fears up as mosquitoes show pesticide resistance By David W Freeman (CBS) Malaria experts are worried in the wake of an ominous new study from Senegal showing that the mosquitoes that spread the deadly disease can develop resistance to insecticide-treated nets. Researchers studied malaria infections in a village in the West African nation and found that the disease may be rebounding because the insects are becoming resistant to the increasingly resistant to the insecticide deltamethrin. Malaria-carrying mosquito, Anopheles gambiae (Credit: CDC Public Health Image Library) Despite decades of efforts to beat malaria with insecticides, indoor spraying, bednets, and drugs, the disease kills nearly 800,000 people a year, Reuters reported. Most of the victims are babies and young children in sub-Saharan Africa. In recent years, the nets have been widely distributed in Africa, and the World Health Organization says that when properly deployed they can cut malaria rates by half, the BBC reported. But will the nets lose their effectiveness if mosquitoes are resistant to the pesticide used to treat them? To find out, researchers looked at malaria cases in the Senegalese village of Dielmo before and after the bednets were distributed there. During the two years from August 2008 to August 2010 after bednets were distributed, there was a sharp drop in malaria attacks, according to Reuters. But between September and December 2010 - 27 to 30 months after the nets had been given out - malaria attacks in adults and older children rose to even higher levels than before. The rise in cases seemed to mirror the increasing proportions of malaria-carrying mosquitoes resistant to deltamethrin. What does it all mean? "Strategies to address the problem of insecticide resistance and to mitigate its effects must be urgently defined and implemented," concluded the authors of the study, which was published in The Lancet Infectious Diseases. The CDC has more on malaria. http://www.cbsnews.com/2102-504763_162-20094086.html?tag=contentMain;contentBody 9/30/2011 Bedbug Genome Reveals Pesticide Resistance : NPR Page 1 of 2 Bedbug Genome Reveals Pesticide Resistance by JON HAMILTON January 19, 2011 text size A A A Scientists have found some new genetic hints that could help explain why bedbugs are so hard to kill. Some bedbugs appear to have evolved a mechanism that helps them break down toxins, including the ones in many pesticides, a team from The Ohio State University reports in the journal PLoS ONE. Enlarge Orkin LLC/AP Modern bedbugs are increasingly resistant to pesticides. Some populations, in fact, can survive 1,000 times the amount of pesticide that would be needed to kill a traditional bug. The finding comes after earlier research found genetic changes in bedbugs that help protect nerve cells from specific pesticides. Together, the findings suggest that current efforts to curb bedbug infestations will be more difficult than in the past, says Susan Jones, an urban entomologist at Ohio State University in Columbus. "We're dealing with a different bug than what we were decades ago," Jones says, one that's harder to exterminate. The latest genetic explanation for bedbugs' ability to resist pesticides comes from a study comparing genes from modern bedbugs with those from a colony started decades ago by a military bug expert named Harold Harlan. Harlan's colony has been kept completely isolated, Jones says. "So it has had absolutely no exposure to insecticides," she says." When you expose it to insecticides, the bugs just keel over." Breaking Down Pesticides Bugs from Harlan's colony were compared with bugs from an apartment complex in Columbus that had been treated repeatedly with insecticide, Jones says. We're dealing with a different bug than what we were decades ago. - Susan Jones, urban entomologist, Ohio State University Researchers focused on genes known to be involved in breaking down toxins and removing them from the body, says Omprakash Mittapalli, an entomologist from The Ohio State University in Wooster. Mittapalli suspected modern bedbugs had genes that encouraged their bodies to produce more of the enzymes that break down pesticides. And, he says, that's just what the team found. http://www.npr.org/2011/01/19/133057071/bed-bug-genome-reveals-pesticide-resistance 9/30/2011 Bedbug Genome Reveals Pesticide Resistance : NPR Page 2 of 2 "These enzymes are indeed higher in the pesticide-exposed populations compared to the pesticidesusceptible population," Mittapalli says. Bedbugs' ability to eliminate toxins and protect nerve cells has become quite common, says Ken Haynes, an entomologist at the University of Kentucky who has studied many populations of modern bedbugs. Related NPR Stories Extreme Fight: Dogs, Chemicals Take On Bedbugs Exterminators and desperate families are going to extreme lengths to eradicate the nasty critters. The World Of Undercover Bedbug Sleuths Nov. 18, 2010 Bedbugs Aren't Just Back, They're Spreading Aug. 21, 2010 He says nearly all of the populations he's studied have some resistance to common pesticides known as pyrethroids. And many "have a level of resistance that's quite extraordinary," he says, meaning they can withstand up to 1,000 times the dose that would usually be lethal. A New Approach To Killing Bedbugs All the new information about resistance suggests it may be time to try a different approach to killing bedbugs, Haynes says. "Instead of relying on the same insecticide generation after generation of the bedbugs," Haynes says, "you'd rotate to a different class of pyrethroid or a different class of insecticide altogether with a different mode of action." Bedbug control also should include nonchemical approaches like heat to kill the bugs and vacuuming to remove them from a room, says Louis Sorkin, an entomologist at the American Museum of Natural History in New York. "It's not just one chemical approach and that's the end of it," Sorkin says. http://www.npr.org/2011/01/19/133057071/bed-bug-genome-reveals-pesticide-resistance 9/30/2011