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Transcript
Microevolution Evolution we can see Begins MICROEVOLUTION • changes in gene frequencies and phenotypic traits within populations and species Hi, I’m the narrator, and I’m here to help explain evolution to you. And I’m NEMESIS, I am going to question this narrator guy Previously, evolution evidence was examined and used to explain the world around us. Ya, I remember all that stuff We, however, must understand that EVOLUTION is a dynamic process and continues to occur. What are you talking about, I am not evolving! You are correct, individuals do not evolve, so you are stuck with the genetics that were given to you by your mother and father. The difference, is that populations of species actually are evolving. Now come on, the animals in the zoo aren’t changing! Yes, evolution is occurring right now as you read this presentation! The difference is that you need to look at populations that produce a lot of generations in a short period of time. Take for example bacteria that have a doubling time of one hour. These bacteria make a new generation in only one hour. Now think of how many generations they can produce in only one day. Ok that’s 24 generations, so what? Well comparing that to a 35 year generation time for humans, 24 generations would put you back in time to the middle ages. So how do bacteria apply to me? Wait, I heard people in the middle ages didn’t take baths What it means is that evolution can occur much more rapidly for the bacteria than for your species. Even though you may think you are very different from bacteria, the same natural forces act upon them, and their genetics, as act upon all living things. Ex. New bacteria are formed, grow, die and need food, stable environmental conditions, metabolic chemicals. Ya, but I don’t see bacteria evolving. What are they turning into, flies? Well, here is where you are wrong. Bacteria are evolving, its just that they are evolving in their biochemistry an not their body form. Take for example a problem that the medical field is experiencing. Every time an antibiotic is used to kill a population of bacteria, it is possible that one of the members of that population may have received a mutation in its DNA that causes the bacteria to survive even when the antibiotic is added. It may be that only one bacterium survives, but if it does, it will take only 24 hour for that one bacteria with a doubling time of 1 hour to turn into over 8 million antibiotic resistant bacteria! Sure wish I made money like that. Recognition of this problem has resulted in a tightening up of prescription antibiotics by doctors. It is directly connected to evolution as the incidence of bacteria with antibiotic resistance has dramatically increased in our modern world. I hate my doctor, he never gives me antibiotics when I have the flu! Newsflash: Antibiotics are useless against a virus like the flu. As a matter of fact, using antibiotics this way helps more bacteria gain resistance Hey wait a minute, these are just bacteria, they are prokaryotic, and are a lot different than our cells. Maybe this only applies to them. Okay, another microevolution study deals with insect pests in agricultural fields. It has been noted that specific pesticides applied to crops eventually become useless at killing the insect pests. Now some might think that it is because of a problem with the chemical, but it is not. The insect population is actually evolving resistance to the pesticide. Initially all the insect pests may have died, but after many times of application, a very small number of pests survived due to tolerance to the poison. Eventually only these survivors were reproducing, so the only genes passed on the next generation were those of pesticide resistance. Due to insect rapid generation time and the fact that one insect can lay thousands of eggs, the insects responded to the environmental pesticide. Now, you must understand that the insects are not doing any thinking, its simply evolution in action where the insect pests without resistance die, but the pests with resistance thrive. If this were to happen on a massive scale (the world) over a long time frame the genetics of the insect pest would change. So smart guy, what do the farmers do with these superbugs? Currently, farmers alternate between pesticides so that no insect that develops resistance one year can offer an advantage to its offspring in the next year because even though they may be resistant to the current pesticide, a new one will kill them in that next season. They better keep inventing new pesticides! Going back to what we discussed regarding bacteria, some bacteria have developed multiple drug resistance MDR, and some resistance to all known antibiotics. If you get sick with these, you have to fight the bacteria off by yourself. Incidentally, there are limited different antibiotics currently available with very little research into identifying new ones. Hah, you scientists are pretty short-sighted What a jab, but for your information, science has to be funded, and interest from the business and free market economy is centered on profit. From MDR bacteria to cures for Parkinson’s disease (both rare) little money (comparatively speaking) is allocated due to little return of profit to the business considering the great expense of research. So that’s why there are so many cold remedies. More money for the business with lots of sick people. STOP STOP STOP JUST WAIT A MINUTE. You haven’t sold me on this microevolution just yet! What about that Hardy-Weinberg principle I just learned about. It says that the genetics of the population does not change with time. You are telling me that it does! Let me explain. The Hardy-Weinberg principle is indeed correct, but if you recall there were certain conditions that had to be met before it would apply. All these conditions are very rarely met in a real world situation Lets take a look at some: Populations must be large says Hardy-Weinberg. Many populations are dwindling and are now very small Ex endangered species Many populations are separated by geography with small populations in isolated areas. Mate randomly in the population says Hardy-Weinberg. Some minority cultures historically did not mate into the general population and this may continue today Or Some animal species show a consistent preference for a particular body form or behaviour for their mate- sexual selection- leaving other potential mates out of the reproductive cycle no matter what their attributes No mutations occur says Hardy-Weinberg We are all exposed to mutations. Just look at the incidence of skin cancer, a mutation directly connected to the UV radiation of our cherished sun. The incidence of mutation to gametes, however is low. ---This is a good thing as most of the time mutations will lead to problems– but then again, maybe its worth the risk of problems if a mutation could lead to something like resistance to the HIV virus– Mutations happen rarely, but occur more in large populations and affect all life on the planet.-- They are the source of new genetic diversity (new genes created) No migration says Hardy-Weinberg We see people enter and leave communities all the time. Animals, plant, they also are constantly in flux No natural selection says Hardy-Weinberg. Selection occurs all the time. Ex. Animals that are slow are the first eaten by predators– eliminating their genes for the future generations. Now hopefully you can see what I am getting at. By providing the set of conditions under which genetic change would not occur, the Hardy–Weinberg principle helps identify key factors that can cause evolution, a change to the gene pool of a population or a species. Can I swim in the gene pool? For our first analysis, let us look at the population issue. If a population dramatically is decreased in numbers, there is a good chance that, just by random choosing, many genes will be lost. Ex If all humans were to die in a nuclear war, but 10 survived they all could have a recessive allele (pretend its blonde hair) Now all future humans will have only blonde hair without exception. The population may expand to millions, yet everyone will have blonde hair. In a less radical situation, the allele frequency would change. Ex Of the 10 people left, one has brown eyes, the others blue. More blue eyes would be found in this new population than were found in the original human population. I like blue-eyed blondes. GENETIC DRIFT – is the name given to the process Do you realize that all cheetahs today are virtually identical genetically ? In technical lingo locus homozygosity or monomorphic You mean they are all like brothers and sisters? How could that happen? Related to Genetic Drift is something called the BOTTLENECK EFFECT usually caused by a dramatic, sometimes temporary, reduction in the size of a population . I’ve been an a traffic bottleneck before– only a few cars get through! It was very rapid, just like the one described because it was caused by an accident! This EVOLUTIONARY BOTTLENECK is similar as only a few individuals survive the “hard times” When the population recovers, there are few individuals that contribute genes to start the future generations. The result is that the species genetics are all nearly the same. Have you ever watched this sitcom? Its about seven stranded castaways trapped on a tropical Island by a severe storm. I’ve seen it on the retro channel. Well if these castaways never get off the island no matter how hard they try (the premise of the show), they are stuck here to reproduce only with those who are trapped here. Since it is a small sample, numerous genetics would be absent that would normally be found in the general population. The result of this would be a population different than the original because of the genetics of the original castaways. This genetic drift effect is called : This is called the FOUNDER EFFECT For our next analysis, we can look at the random mating scenario Do you think that your brothers and sisters would be insistent on marrying someone with the exact same genetics as your preferential mate? Uh, like I don’t think so You would expect that in large population, any selection you may require for your mate would be cancelled out by the selections made by other people like you with similar genetics. What, you mean everybody won’t marry blue-eyed blondes? NEWSFLASH-- SEXUAL SELECTION -- NON RANDOM MATING – H-W VIOLATED!! Sexual selection has resulted in some really obvious genetic preferences. Ex. Male peacocks –female choose to mate only with flashy feather males-result --males look a lot different than females– sexual dimorphism Male peacocks are the cool ones! Meet me at the club babe= SEXUAL DIMORPHISM Sexual dimorphism produces striking differences in physical appearance of males and females The non random mating may also involve closely related individuals preferentially mating. In ancient Egypt the pharaohs married their sisters. This would disrupt H-W if done on a large scale. Even Charles Darwin married his cousin. Mating of closely related individuals or individuals that isolate themselves to a small community will have a different genetic makeup than the population at large as a result of this long term practice. Ex Amish communities I heard it’s legal to marry your first cousin in Florida One last point regarding sexual selection. If the males take on these colourful feather patterns, it makes them stand out and thus more vulnerable to predation. Hold on, Sugar! Daddy's got a sweet tooth tonight!" ... So even though sexual selection favours inherited traits that enhance mating success they may also reduce an individual’s chances of survival. BURP Next, lets examine the mutation scenario Mutations are the only source of additional genetic material and new alleles. That’s okay God, give me the good genes, I’ll pass on the mutations. Mutations are random changes to the genetic code, and thus they are much more likely to be neutral or harmful than they are to be beneficial. See, I was right, no mutations in my genetics please–they could be harmful! Any mutation that occurs in a gamete has the potential to be passed on to later generations, and thus enter the gene pool. Maybe wearing my tight jeans could cause my sperm to mutate—Uh oh. Evidence suggests that the gene for blue eyes was a result of a mutation. Okay, I like blue eyes, maybe mutations can be good. Without mutations, only the brown eye characteristic of humans would exist NEWSFLASH -- One MUTATION= AN ENTIRELY NEW SPECIES--- POLYPLOIDY Polyploidy, the most dramatic form of mutation, provides an organism with an immediate doubling of genetic material. This means that in one generation, it is different and forms a new species from the original --They are genetically incompatible. This only occurs in plants and has played a major role in the evolution of plants. Crazy stuff; Isn’t corn polyploid? I’m eating double DNA • Harmful mutations occur frequently but they are selected against and, therefore, these mutant alleles remain extremely rare. • Beneficial mutations are rare but they are selected for and, therefore, these mutant alleles accumulate over time. • Lethal mutations are least likely to influence the evolution of a species (they kill the individual) Cowabunga Dudes! Lets look at migration next. Hey, that’s like what the Canada goose does. Not exactly, migration means you move into or out of a population, you just don’t fly south for the winter. I know, my dad immigrated to Canada. We know that it is virtually impossible to separate populations, even though there may be vast boundaries between them. In humans, look at the example of vast oceans separating continents and isolating various subpopulations of humans. Humans explored across them, and now the world population is really one big population, where migration to or from anywhere is possible. If you think that other forms of life can’t do the same thing, think again! Ya, I watched that Jurassic Park Movie. You just can’t stop those dinosaurs MIGRATE WITH THE FLOW—THE GENE FLOW Gene flow is the movement of alleles from one population to another through the movement of individuals or gametes. So go where your flow may take you, find a mate reproduce and move your genes to some exotic location, or if you are a homebody, wait until they come to you from somewhere far away. Either way, you would contribute to gene flow – changing the genes of the original population. Lastly, an analysis of natural selection. However, before we approach this topic we have to understand something about genetic variations . Hey this is like Darwin’s book “The Origin of Species by Natural Selection” I actually know something! NEWSFLASH: DIVERSITY WITHIN A SPECIES-KEY TO GENETIC HEALTH OF POPULATIONS “I want to luxuriate in the racial deliciousness of our country...We are a nation that celebrates racial diversity.” Cory Booker - Mayor Newark, New Jersey Does he want to compliment us or eat us? The general idea is variation within species (aka diversity) is a great attribute. Evolutionary speaking, it is a genetic strength – not a weakness. Why and what’s aka? Diversity within a species better prepares the whole species for survival. Selective forces can change and traits that may seem advantageous today are not necessarily adaptive in the future. (aka is also known as) You mean the scrawny guy’s genetics are important too? Exactly, If the whole population rid itself of particular genetics (Ex fur) to adopt those that are preferred because of some new selective pressure (Ex hot temperatures) it would leave the population vulnerable to being wiped out if conditions changed (Ex sudden Ice age) Fur keeps you warm but PETA hates it! NEWSFLASH: HETEROZYGOTES IN A POPULATION MAINTAIN RECESSIVE ALLELES Now for the natural selection discussion It’s about time Within a population, there is a vast possible number of genes and gene combinations. These lead to differences that we call variations. In ostrich, for example, you have some tall, some short, but most are average size. As you approach either height extreme, there are very few ostrich. Only a few giants or “tom thumb” sized ostrich. The graph representing the population would be bell shaped. Kind of like our class average is supposed to be! You would not expect the population to change if it was in H-W equilibrium. This bell shaped curve would be constant, but natural selective pressures could change this population dramatically. If the ostrich were forced into dense jungle, larger ostrich would be at a disadvantage (taller individuals hit more branches slowing their speed) avoiding predatory tigers. This means that ostrich may not reproduce, thus eliminating its tall genes from the population. Over time and many tiger meals, the population would shift to a shorter variety ostrich. . I prefer steak. On the other hand, if the ostrich were placed in open savannah, (where it actually lives) it would be an advantage to be a fast runner. Taller ostrich with longer legs would be able to avoid the predators. Small ostrich would become the meals and small ostrich genes would be lost. The population would shift to taller ostriches. I heard ostrich are faster runners than humans. These ostrich examples offer three different mechanisms of speciation (the creation of new species) Hey, where did they all go? I’m not going to remember them if they move so fast! Stabilizing selection Directional selection Disruptive selection Stabilizing Selection means that the extremes on the curve are selected against. That’s a polite way to say they die The population becomes more uniform in their genetics. DIRECTIONAL SELECTION results in a shift of the population genetics because individuals with more extreme conditions are favoured. Sounds kind of like the tall basketball player that gets all the girls. The population phenotype average has shifted to the right. This usually occurs when a population experiences a change in habitat or environmental conditions. DISRUPTIVE SELECTION favours individuals at opposite extremes Disruptive selection is a significant evolutionary mechanism for the formation of distinctive forms within a population. Distinctive groups may eventually become isolated breeding populations with separate gene pools. Hey that means new species can be created once they separate! This usually occurs when a population experiences a change in habitat or environmental conditions. I think I’m starting to understand now. I just have one more thought. Can’t behaviours, just like body forms be inherited? I know that a lot of animals can be identified by their mating rituals. You are correct. Behaviours also can be passed via genetics. It’s a little different than in humans because we learn behaviours –This is not something we get from our genetics. However, in the case of the animal behaviour we are talking about, it is due to something we call instinct. That’s all folks! The end