Natural Selection and Adaptation PDF Notes

A marvel of adaptation: the sword-billed hummingbird (Ensifera ensifera) is adapted to reach nectar deep in the long, tubular Natural Selection...

A marvel of adaptation: the sword-billed hummingbird (Ensifera ensifera) is adapted to reach nectar deep in the long, tubular Natural Selection flowers of passion vines (Passifloracee) and other plants. Its extraordinary bill evolved and Adaptation by natural selection. Ensifera ensifera is the only species in which the bill is longer than the rest of the body. https://www.istockphoto.com/search/2/image-film The bill is too long for preening and the bird must use its feet to remove ectoparasites and spread oil across its feathers. Snakes are fascinating animals. Depending on the species, they can crawl, burrow, swim, climb trees, and even glide, all without benefit of arms or legs. Perhaps most amazingly, they can swallow prey, whole, that are much larger than their heads. Figure 3.1 Amazing adaptations. Snakes can do this because, unlike The tooth-bearing maxilla bones of the humans, they have movable skull bones. upper jaw can be swung outward, further increasing the mouth opening. Each lower jaw is joined to a long, movable quadrate bone, so it can drop away from The upper and lower jaw bones on both the skull, and the front ends of the lower sides can be moved forward and jaws are not fused but joined by a backward independently to pull the prey stretchable ligament. into the throat. https://blogs.ubc.ca/mrpletsch/2017/03/03/class-reptilia/  Rattlesnakes and other vipers take this When the snake opens its mouth, the short apparatus a step further: their maxilla is maxilla is rotated 90° so that the fang is short and bears only a long, hollow fang— fully extended. a natural hypodermic needle—to which a duct leads from the poison gland (a modified salivary gland). The fang lies against the roof of the mouth when the mouth is closed. Figure 3.1 Amazing adaptations. Snakes' skulls are complex mechanisms that Cacti that grow in arid environments lack look as if they had been designed by leaves and have thick, sometimes globular engineers to perform a specified function. stems with a low ratio of surface area to They are inferred to be adapted to the volume, reducing water loss by strategy of swallowing large prey whole. evaporation. Every species has features—adaptations— In some cacti, such as the cholla, the coat that enhance survival or reproduction. Many features are adaptations to specific of hair that reflects sunlight reduces the environments. plant's temperature is an adaptation. An adaptation is a characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states. Adaptations have evolved by natural selection, which is the centerpiece of Darwin's On the Origin of Species and of evolutionary theory, and is perhaps the most important idea in Figure 3.2 Many plants that live in arid environments have adaptations to reduce biology. water loss, as in these cacti, which lack leaves. The stems, where photosynthesis takes place, are thick, with a low ratio of surface area to volume. Adaptation via natural selection is also one For hundreds of years, it seemed that of the most important ideas in the history adaptive design could be explained only by of human thought, for it explains the an intelligent designer. apparent "design" of the living world without recourse to an omnipotent, supernatural designer. In fact, this "argument from design" was considered one of the strongest proofs of the existence of God. In 1802, Reverend William Paley wrote in When Darwin offered a purely natural, Natural Theology that, just as the materialistic alternative to the argument from complexity of a watch implies an design, he not only shook the foundations of intelligent, purposeful watchmaker, so theology and philosophy, but also brought every aspect of the study of life into the every aspect of living nature, such as the realm of science. human eye, displays "every indication of contrivance, every manifestation of design, Darwin's alternative to intelligent design was which exists in the watch" and must, "design" by the completely mindless process likewise, have had a designer. of natural selection. Evolution by natural selection does not have Adaptive biological processes "appear" to a goal, any more than erosion has the goal have goals. of forming canyons, for the future cannot cause material events in the present. Animals engage in many adaptive behaviors, and a morphological feature, Thus the concepts of goals or purposes or such as a flower, develops toward a progress have no place in biology (or in any suitable shape and stops developing when of the other natural sciences), except in that shape is attained. studies of some animals' behavior. We may loosely describe such features by Rather, the apparent goal-directedness is teleological statements, which express caused by the operation of program-coded goals (e.g., "She studied in order to pass or prearranged information, residing in the exam” or "Wasps sting to defend DNA sequences—that, when activated by themselves from predators”). But no external or internal stimuli, controls a conscious anticipation of the future resides process. in the cell divisions that shape a flower or, as far as we can tell, in the behavior of A program can be encoded in a computer wasps. chip, but... https://www.shutterstock.com/search/wasp-sting... whereas that program has been shaped... resulting from interactions between by an intelligent designer, the information programmed instructions and in DNA has been shaped by the historical environmental conditions or triggers. process of natural selection. Modern biology views the development, physiology, and behavior of organisms as the results of purely mechanical processes... https://www.geeksforgeeks.org/energy-flow-of-ecosystem/ Adaptive Evolution Observed Adaptive evolution can be rapid, especially Darwin could not point to any cases in in species that have been introduced into which evolutionary change of a population new regions or subjected to human or species had actually been observed, and alterations of their environment. he supposed that evolution was much too slow for us to see it in action. But today we For instance, Carroll and Boyd (1992) can cite hundreds of cases of adaptive documted that soapberry bugs (Jadera evolution of morphological, physiological, haematoloma) have adapted rapidly to and behavioral traits that have been directly new food plants. studied.  Soapberry bugs feed on the seeds of plants Compared with the original native host plants, in the soapberry family (Sapindaceae). A the Asian tree species in Texas has a larger soapberry bug piercing the enveloping pod, and the Asian tree species in Florida has a seedpod with its slender beak. much smaller pod. Corresponding to this difference, soapberry In the last 50 years, related species of bug populations in Texas have evolved a longer Asian trees have been so abundantly beak, and those in Florida have evolved a planted in Texas and Florida that the bug shorter beak. Beak length has evolved rapidly populations feed mostly on these species. as an adaptation to the new host plants. Bacteria can evolve resistance to antibiotics very rapidly via horizontal gene transfer (e.g. MRSA). Similarly, resistance to chemical pesticides has evolved in hundreds of species of insects, and many species of weeds have evolved resistance to herbicides within 10 Figure 3.3 Soapberry bugs and the seedpods of their native and introduced host plants in Texas and Florida, drawn to scale. From: S. B. to 20 years of field exposure. Carroll and C. Boyd. 1992. Evolution 46: 1052-1069. Copper, zinc, and other heavy metals are toxic to plants, but in several species of grasses and other plants, metal-tolerant populations have evolved where soils have been contaminated by mine works less than 100 years old. Figure 3.4 Cumulative numbers of insect pest species known to have evolved resistance to five classes of insecticides. The number has increased since these data were tabulated.  Commercial overexploitation has severely When tolerant and non-tolerant genotypes depleted populations of many species of fish of a species are grown in competition with and has resulted in evolutionary changes as other plant species in the absence of the well. metal, the growth of the metal-tolerant genotypes is often much lower than that of In many species there has been a trend the non-tolerant genotypes, implying that toward earlier sexual maturation at a adaptation has costly side effects. smaller size, as we predict will happen when larger fish are more subject to predation. https://en.wikipedia.org/wiki/Noccaea_caerulescens In some species, such as Atlantic cod (Gadus morhua), these changes clearly have a genetic basis. Similarly, trophy hunting for those bighorn sheep (Ovis canadensis) with the largest horns has resulted in the evolution of smaller horns. Figure 3.5 The age at which 50% of Atlantic cod reached maturity declined until 1994, 2 years after the fishery in Canadian waters was closed because of overfishing. Body length at 5 years of age also declined. In both cod and bighorn sheep, the very quality that adds value to the resource has been diminished by the response to selection. Some species show adaptation to the ongoing climate change caused by human production of CO2, and other greenhouse gases. Figure 3.5 Mean horn length of 4-year-old bighorn sheep rams declined because of selection imposed by hunting. In many insects, day length is the cue for William Bradshaw and Christina Holzapfel entering diapause, a state of low metabolic (2001) sampled populations of the pitcher activity that is necessary for surviving the plant mosquito (Wyeomyia smithi) from winter. Northern populations are southern Canada to Florida four times genetically programmed to enter diapause between 1972 and 1996. at a longer day length than southern populations because cold weather arrives They measured the day length at which at northern latitudes sooner, when days the mosquitoe larvae entered diapause. are still long. Bradshaw and Holzapfel (2001) found that between 1988 and 1993, the critical photoperiod became shorter: the insects entered diapause later in autumn. Change was greatest in the most northern populations, as expected, because the increase in temperature (as climate changes) has been greater at higher latitudes. The Figure 3.6 In North America, the critical photoperiod for entering diapause, in relation to latitude, has decreased in the pitcher plant mosquito. This speed of evolution was amazing, having change is an adaptation to climate change. The larvae of this mosquito taken as little as 5 years. develop only in the water that collects in the tubular leaves of the pitcher plant. From: W. E. Bradshaw and C. M. Holzapfel. 2001. Proc Natl Acad Sci https://en.wikipedia.org/wiki/Sarracenia_purpurea USA 98: 14509-14511. The meaning of natural selection These evolutionary changes can be so rapid because populations in altered In The Origin of Species, Darwin introduced natural environments, especially those selection with these words: transformed by human activities, can experience stringent natural selection "Can it... be thought improbable, seeing that variations useful to man have undoubtedly occurred and because they contain genetic [in domesticated animals and plants], that other variation in many characteristics—the variations useful in some way to each being in the necessary ingredient of evolution. great and complex battle of life, should sometimes occur in the course of thousands of generations?...... If such do occur, can we doubt (remembering Among several slightly different definitions that many more individuals are born than can of natural selection used by biologists today, possibly survive), that individuals having any Futuyma & Kirkpatrick (2022) use this one: advantage, however slight, over others, would natural selection is any consistent have the best chance of surviving and procreating difference in fitness among different classes their kind? On the other hand, we may feel sure of biological entities. that any variation in the least degree injurious would be rigidly destroyed. This preservation of A simple way to think of fitness is as the favoured variations and the rejection of injurious number of offspring an individual leaves in variations, I call Natural Selection." the next generation. Suppose that in a species of annual plant, Fitness is sometimes called reproductive only 1 of every 1000 seeds survives to success, which includes survival because reproductive age and that those that dead organisms do not reproduce. survive produce an average of 3000 seeds. The average fitness of that type of If evolution by natural selection is to occur, individual is 0.001 × 3000 = 3. there must be a change in the population across generations, and this requires that The components of fitness are survival and the phenotypic differences among the reproduction. entities be inherited. Thus, evolution by natural selection occurs For example, susceptibility vs. resistance to if (1) there is a correlation between an a herbicide, and that their average fitnesses individual's phenotype and its fitness and are 3 and 4, respectively. (2) variation in the phenotype is correlated between parents and their offspring. If these values are constant from generation Suppose, for example, that in an asexually to generation, genotype B increases in reproducing annual plant, genotypes A and number much faster than A and will make B differ in a characteristic that affects their up the great majority of the population fitness. within a few generations (see Figure 3.7). Figure 3.7 Two genotypes of a plant are growing together. Genotype A has Figure 3.7 The frequency of genotype B increases as the frequency of a fitness of 3, while genotype B has a fitness of 4. Both genotypes start genotype A decreases. The result is that, while both genotypes start with a with 10 individuals. The population size of genotype B grows much more frequency of 0.5, genotype B makes up almost 90% of the population just rapidly. seven generations later. We observe that the frequency In sexual species, moreover, individual (proportion) of genotype B has increased genes undergo replication, but genotypes (and that the frequency of A has declined). do not because they are broken up by recombination and segregation during In sexually reproducing organisms, such as meiosis. animals, fitness is more complicated. In general, males compete for mating Accordingly, it can be useful to think about success, which Darwin called sexual the fitness of an allele, and consequently selection. of selection among alleles. https://bccampusbiology.pressbooks.tru.ca/chapter/the-process-of-meiosis/ We consider the fitness of an allele, and Evolution by natural selection in sexually consequently of selection among alleles, reproducing populations entails changes even though the "entities" that differ in in the frequencies of alleles at the locus survival and reproduction in most (or loci) that underlie the variation in the discourse about evolution are individual phenotypic characteristic that influences organisms with different phenotypes fitness. (individual selection). Most evolutionary biologists discuss One must be very careful to understand that natural selection among alleles and among natural selection is not an agent or active heritable individual phenotypes because power, and certainly not a purposeful one, selection has no evolutionary effect even though the language we use often without inheritance. seems to personify it or to suggest that it is an agent. Darwin coined the term natural Most of the subsequent discussion will selection as analogy to the selection that assume that inheritance of a trait is based breeders of crops and domestic animals use on genes. to improve desirable characteristics. https://flexbooks.ck12.org/cbook/ck-12-advanced-biology/section/10.36/ primary/lesson/natural-selection-and-populations-advanced-bio-adv/ In later editions of The Origin of Species, he Likewise, evolutionary biologists often say wrote that "it has been said that I speak of that selection "favors" a certain natural selection as an active power or characteristic, or they refer to selection as Deity; but who objects to an author a "force." This is metaphorical language, speaking of the attraction of gravity as used for brevity. Natural selection is a ruling the movements of the planets? Every name for statistical differences in one knows what is meant and is implied by reproductive success among genes, such metaphorical expressions; and they organisms, or populations—and nothing are almost necessary for brevity." more Natural selection and chance One of those processes is genetic drift: random fluctuations in the frequencies of Natural selection is not synonymous with genotypes within a population. evolution. Natural selection can occur without any evolutionary change, as when Neutral alleles are those that do not alter natural selection maintains the status quo fitness: the average reproductive success by eliminating deviants from the optimal does not differ between individuals that phenotype. And processes other than carry one neutral allele (say, A1) or another natural selection can cause evolution. (A2). https://www.allassignmenthelp.com/blog/genetic-drift-complete-explanation/  So the frequencies of these neutral alleles Purely by random accidents, a higher fluctuate: they change by genetic drift, and percentage of A1 individuals may be killed eventually one allele may completely replace before they reproduce in one generation, the other. If this occurs, the bearers of one allele but a higher percentage of A2 individuals have had a greater rate of increase than the might not survive in the following bearers of the other allele. generation. The frequency of A1 declines But natural selection has not occurred, because slightly in one generation and increases the genotypes do not differ consistently in slightly in the next, but the increase need fitness: the alternative allele could just as well not exactly equal the decrease. have been the one to increase. Natural selection is the antithesis of There is no average difference between chance. the alleles, no bias toward the increase of one relative to the other. In practice, we can ascribe genetic changes to natural selection rather than random Fitness differences, in contrast, are average genetic drift only if we measure numerous differences, biases, and/or differences in individuals of each genotype or phenotype the probability of reproductive success. and find an average difference in reproductive success.