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Inquiry Case 2 - Unit 3.docx

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BIOL 1070
Wright& Newmaster

Adaptation and Specialization • When we look at the natural world as biologists, we see two major patterns: 1) there is an enormous diversity of species occupying a wide range of very different habitats, and 2) organisms are generally well- suited to life in their particular habitat. Both of these are the result of evolution, the first relating to processes that generate new biological diversity (and those that eliminate existing diversity), and the second being the result of long periods of evolution by natural selection. • natural selection is not the only mechanism of evolution o Mutation (the origin of new genetic variation), genetic drift (changes due to chance, specifically founder effects and population bottlenecks), and gene flow (movement of genes among populations) also contribute to changes in the proportion of genes and heritable phenotypes in populations over time. • only natural selection — non-random differences in survival and reproduction among variable individuals — can result in the evolution of adaptations. • Natural selection: Non-random differences in survival and/or reproduction among individual entities on the basis of differences in heritable characteristics. • Adaptation: 1) a characteristic that enhances the survival and/or reproduction of organisms that bear it, relative to alternative (especially ancestral) character states; 2) a physical, physiological, behavioural, or other characteristic evolved through natural selection.Adaptation is NOT the change undergone by an individual organism during its lifetime in response to external conditions. • Population: for sexual species, a group of interbreeding individuals and their offspring. • Alleles: alternate (i.e., different and mutually exclusive) forms of a gene. e.g., “B” (brown eyes) versus “b” (blue eyes). • Genotype: the set of genes possessed by an organism. • Phenotype: the physical expression of the genotype (in combination with the environment). • Frequency: the proportional representation of a phenotype, genotype, gamete, or allele in a population. e.g., 6 out of 10 have blue eyes = 60% = a frequency of 0.6. The Basis of Natural Selection • In the Origin of Species, Darwin had two major objectives: the first was to provide convincing evidence that species are related through common ancestry, and the second was to argue that his mechanism of natural selection could account for changes through time and the resulting differences among species. • In principle, it is possible that species are related by common descent but that natural selection is not the dominant mechanism that explains this fact. However, when it comes specifically to adaptive changes among species and the traits of organisms that allow them to survive successfully in their environments, natural selection is indeed the only known mechanism that can provide a scientific explanation. • In explaining his theory of natural selection, Darwin made extensive reference to selective breeding of domesticated animals and plants, or what he called artificial selection. • Unlike artificial selection, which involves actual “selection” by a human breeder, natural selection does NOT involve choice by any particular agent. In other words, there is no “Mother Nature” actively examining and selecting the best among the available options. How, then, can natural selection occur? • In order to understand natural selection, we first need to grasp the basic postulates that Darwin presented. Natural selection is simply the necessary logical outcome of these postulates, assuming each one holds true. They are: • 1. Individuals within populations are variable. That not every individual is identical in a population is obvious for humans — simply look around you and you will see plenty of variation. As the mussels have shown us, the same is true of other species as well, even though we sometimes need to look closely to appreciate it. In light of these and countless other observations, we can conclude that this postulate holds true. 2. This variability among individuals is at least partly heritable. Offspring tend to look more like their parents than like unrelated members of the population, and this is due to the existence of a mechanism of inheritance by which traits are passed on from parent to offspring. Darwin had no understanding of how this works, but we now understand quite well that it is based on the transmission of genes encoded in the DNAmolecule.Again, we can conclude confidently that this postulate holds. 3. Not everyone survives and reproduces, and some individuals are more successful than others. Many more offspring are produced in each generation than could possibly survive, a phenomenon known as “overproduction”. To take just one example from our inquiry case, a single female zebra mussel can produce 30,000 eggs at a time, and she may reproduce multiple times. Just imagine what would happen if half of those grew into adult female mussels which in turn produced 30,000 eggs each, and so on over the generations. In just a few generations, there would be enough mussels to literally fill the Great Lakes to the brim. Obviously this has not happened, because the Great Lakes still contain water rather than wall to wall mussels. Not every egg that is laid can grow up to become an egg-laying adult — in fact, only a small minority manage to do so. Once again, we can safely conclude that this postulate applies in natural systems. 4. The differential survival and reproduction of individuals is associated with the heritable variation among individuals (i.e., it is non-random). This is the key to natural selection, that the individuals who do manage to reach adulthood and have offspring of their own are, on average, better suited to surviving and reproducing in their particular environment because of traits that they inherited from their parents. In other words, which individuals succeed in leaving offspring and which do not is non-random: some individuals could be expected to be more successful because of their traits. This postulate is more difficult to establish than the others as it is not immediately obvious. However, it is easy to see how a slightly more camouflaged animal might escape predation more effectively, how a faster running predator might be more successful at catching prey, or how a plant with larger leaves might be more successful at photosynthesizing than members of the same species with smaller leaves. This also can be observed among bacteria exposed to antibiotics, and it has been documented many times in natural populations. • Assuming that all four of these postulates are true, then natural selection will inevitably result. That is, heritable traits that contribute to greater survival and reproductive success will be passed on more often than traits that do not, and from one generation to the next they will represent a larger proportion of the population. How Natural Selection Works NATURAL SELECTION DUE TO DIFFERENTIAL SURVIVAL • Blue dots were killed by bluebgone • Mutant red dots, which were already present in small proportions were not killed by blue b gone • Each generation, a higher portion of the population was made up of red dots as the blue ones were killed • No individual blue dot ever became resistant due to blue –b gone; they never turned into a resistant red dot. Only the population evolved resistance. • This process occurred over many generations. • ORGANISMS DO NOT EVOLVE, ONLY POPULATIONS DO. NATURAL SELECTION DUE TO DIFFERENTIAL REPRODUCTION • A mutation occurred which conferred a reproductive advantage in the specific environment (2:1 reproduction). • The new red types (with the advantage) left more offspring in each generation than the blue original type, though there was no difference in survival rate. • No individuals changed from blue to red; one was born red due to a mutation in the parent’s DNA. • After generations, red type allele became dominant. Adaptation in an evolutionary sense does NOT involve changes by individual organisms in response to pressures imposed by the environment. None of the dots chose to respond a certain way — in fact, they had any thoughts in the matter at all (they’re just dots!). Nor did the human who used the Blue-B-Gone choose which individuals would survive and which would not. Finally, note that exposure to the Blue-B-Gone did NOT cause the resistant phenotype to appear — the treatment merely changed the proportion of existing variants in the population from one generation to the next because some individuals (blue dots) were killed by the Blue-B-Gone whereas others (red dots) were not affected. This is how the evolution of antibiotic resistance works in bacteria. Individual bacteria vary in how sensitive they are to antibiotic treatment, such that the most susceptible are killed early in the treatment whereas more resistant individuals survive and reproduce. In each generation, the most resistant are the ones who leave the most offspring, until eventually the population is composed only of the descendants of these resistant individuals. Exposure to antibiotics does not “cause” individual bacteria to “become” resistant. Resistance does not “develop” in bacteria — resistance evolves in a population of bacteria. In the case of the Blue-B-Gone animation, the resistant red dots were already present in the population in low numbers. Before the population was treated with Blue-B-Gone, the difference between blue and red dots may have been irrelevant, but once the environment changed to include a substance toxic only to blue dots, the red phenotype became advantageous. Of course, it is also possible for a new phenotype to arise by mutation in the population, as happened in the second segment of the dot animation. Resistance genes can also arise by mutation, but it is essential to note that treatment with antibiotics does not cause these genes to arise — mutation is random with respect to any potential costs or benefits to the organism. It is also possible for resistance genes to move from one population of bacteria to another through gene flow. This is why it is a significant concern that resistant bacteria from one place might be introduced into a new area where the bacteria are not currently resistant. TheAccurate Language Challenge Bacteria that cause disease exist in large populations, and not all individuals are alike. If some individuals happen to possess genetic features that make them resistant to antibiotics, these individuals will survive the treatment while the rest gradually are killed off. As a result of their greater survival, the resistant individuals will leave more offspring than susceptible individuals, such that the proportion of resistant individuals will increase each time a new generation is produced. When only the descendants of the resistant individuals are left, the population of bacteria can be said to have evolved resistance to the antibiotics. Ten really important points about natural selection 1. Natural selection by itself does not create new traits, it only changes the proportion of variation that is already present in the population. Most often, natural selection reduces the amount of variation in a population because some variants are eliminated. 2. Mutation, not natural selection, is the source of new variation. The repeated two- step interaction of mutation and natural selection is what leads to the evolution of new adaptive features. 3. Mutation is random with respect to fitness — that is, it occurs without regard for what happens to the organism. It is simply a genetic error. Natural selection is, by definition, non-random with respect to fitness. This means that, overall, it is a serious misconception to consider adaptation as happening “by chance”. 4. Mutations occur with all three possible outcomes: neutral (no phenotypic effect), deleterious (bad effects), and beneficial (positive effects). Beneficial mutations may be rare and deliver only a minor advantage, but these can nonetheless increase in proportion in the population over many generations by natural selection. The occurrence of any particular beneficial mutation may be very improbable, but natural selection is very effective at causing these individually unlikely improvements to accumulate. Natural selection is an improbability concentrator. 5. No organisms change as the population adapts. Rather, this involves changes in the proportion of beneficial traits across multiple generations. 6. The direction in which change occurs is dependent on the environment. A change in environment can make previously beneficial traits neutral or detrimental and vice versa. 7. Adaptation does not result in optimal characteristics. It is constrained by historical, genetic, and developmental limitations and by trade-offs among features. 8. It does not matter what an “ideal” adaptive feature might be – the only relevant factor is that variants that happen to result in greater survival and reproduction relative to alternative variants are passed on more frequently. As Darwin wrote in a letter to Joseph Hooker (11 Sept. 1857), “I have just been writing an audacious little discussion, to show that organic beings are not perfect, only perfect enough to struggle with their competitors.” 9. The process of adaptation by natural selection is not forward-looking, and it cannot produce features on the grounds that they might become beneficial sometime in the future. In fact, adaptations are always to the conditions experienced by generations in the past. 10. Natural selection is not the only mechanism of evolution. Mutation, genetic drift, and gene flow can also change the proportion of variants in a population. Natural Selection and Adaptation • natural selection requires heritable variation among individuals and environmental conditions that cause individuals with certain traits to leave more offspring than others. • In the case of our mussels, we have seen clear examples of intraspecific variation and interspecific disparity in shell morphology. This includes the sizes and numbers of bumps and ridges on the shells, which represent adaptations to remaining stationary in fast moving currents. • In order for bumps or ridges to evolve as adaptations to flowing water, there would have to be mutations that occur at random and result in slight changes in shell morphology. One might result in a small bump here, or a small divot there. If this variant manages to remain anchored in the substrate more effectively than other individuals in the population who lack it, then it may end up leaving more offspring than the other individuals — and therefore contributing a higher number of bumpy individuals to the next generation. More small changes by random mutation and more non-random differences in reproductive all the individuals have large bumps or ridges on their shells. This is adaptation by natural selection.h The Evolution of Complex Adaptations • the lures are modifications of tissue that already existed for other reasons — namely mantle tissue, which is the tissue that produces the shell. All mussels have mantle tissue, and it predates the evolution of lures. Male mussels have mantle tissue, and obviously they do not produce lures because they do not have brood larvae that need to be transferred to a fish host. • What good is half a lure? Well, 0% of a lure — i.e., an unmodified mantle — is good for something else: making shell. This process of “co-option” is a common one, whereby a feature tha
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