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Natural Selection Charles Darwin observed that the individual members of any species show a great deal of variation in their characteristics. They show differences in size, color, strength, behavior, and many other features. Some of these characteristics are passed on from parents to offspring. For centuries, in fact, plant and animal breeders have bred organisms to emphasize or increase certain prized characteristics. Plant breeders select plants with high resistance to diseases and mate them, hoping to obtain offspring with even higher resistance. Likewise, horse breeders might select mating pairs to try to increase speed, stamina, or both. Sometimes these techniques result in significant differences from one breeding group to another. The figure of various types of kale illustrates six different types of vegetables that humans eat. All six plants are the same species. Humans have bred them for different agricultural uses and to suit different human tastes. cauliflower cabbage kale broccoli brussels sprouts kohlrabi Six types of kale. These kale plants differ primarily in the part of the plant that stores most of the starch. Darwin wondered. He thought that if humans could change plants, then natural pressures could cause changes in species and organisms. The process that occurs in nature is similar to selective breeding. The difference is that natural selection, not humans, determines which organisms survive. A new species can result from natural selection occurring across millions of years. Selective breeding has not produced a new species. Remember that a species is a group of organisms whose members closely resemble each. That is because they share many key characteristics. For example, all humans are one species because we share many fundamental characteristics. Organisms belonging to different species usually vary in key characteristics. One characteristic that separates species is the ability to mate and produce fertile offspring. A horse and a donkey can mate and produce offspring called mules. But because Copyright © by BSCS. All rights reserved. Copymaster 6.5 Page 1 of 4 horses and donkeys belong to two different (although very closely related) species, the offspring mules are sterile. How does the process of natural selection take place? This figure shows the logic of evolution. Evolutionary biologist Ernst Mayr proposed this summary of Darwin’s thinking. The summary is a way to understand the evidence and inferences that underlie evolution. Let’s follow the summary and try to understand some of the thinking that led Darwin to his conclusions. ,ISTOF-AYRSPOINTS &ACT !LL SPECIES HAVE SUCH GREAT POTENTIAL TO PRODUCE LARGE NUMBERS OF OFFSPRING THAT THEIR POPULATION SIZE WOULD INCREASE EXPONENTIALLY IF ALL INDIVIDUALSTHATAREBORNWOULDREPRODUCESUCCESSFULLY &ACT %XCEPT FOR SEASONAL FLUCTUATIONS MOST POPULATIONS ARE NORMALLY STABLEINSIZE &ACT .ATURAL RESOURCES ARE LIMITED AND IN A STABLE ENVIRONMENT THEY REMAINRELATIVELYCONSTANT )NFERENCE "ECAUSE MORE INDIVIDUALS ARE PRODUCED THAN THE AVAILABLE RESOURCESCANSUPPORTANDTHEPOPULATIONSIZEREMAINSSTABLETHEREMUST BEAFIERCESTRUGGLEFOREXISTENCEAMONGTHEINDIVIDUALSOFAPOPULATION 4HIS RESULTSINTHESURVIVALOFONLYAPARTOFTENAVERY SMALLPARTOFTHEOFFSPRING OFEACHGENERATION &ACT .OTWOINDIVIDUALSINAPOPULATIONAREEXACTLYTHESAME RATHEREVERY POPULATIONDISPLAYSANENORMOUSVARIETYOFCHARACTERISTICS &ACT -UCHOFTHISVARIATIONCANBEINHERITED )NFERENCE 3URVIVALINTHESTRUGGLEFOREXISTENCEISNOTRANDOMBUTDEPENDS IN PART ON THE CHARACTERISTICS THAT THE SURVIVING INDIVIDUALS INHERITED 4HIS UNEQUALSURVIVALISAPROCESSOFNATURALSELECTIONTHATFAVORSINDIVIDUALSWITH CHARACTERISTICSTHATFITTHEMBESTINTHEIRENVIRONMENT )NFERENCE /VERTHEGENERATIONSTHISPROCESSOFNATURALSELECTIONWILLLEAD TOACONTINUINGGRADUALCHANGEINPOPULATIONSTHATISTOEVOLUTIONANDTHE PRODUCTIONOFNEWSPECIES The logic of the theory of evolution. Read each fact and determine how evidence and inference are used to establish a coherent theory of evolution. The first three facts in the logic of evolution describe the situation that inevitably exists in any environment: • Without limits to population growth, organisms reproduce very rapidly. • Nevertheless, our observations tell us that the size of most populations in the wild stays more or less the same. Copyright © by BSCS. All rights reserved. Copymaster 6.5 Page 2 of 4 • Our observations also tell us that the resources available in any natural environment are limited. One important factor that influenced Darwin’s views was an essay written in 1798 by Thomas Malthus. Malthus was a member of the English clergy. He argued that much of the human suffering we see is the result of human population to growth. Human population sometimes grows beyond the resources available to support it. Overpopulation seemed to Darwin to be a characteristic of every population. It led him to infer that individuals within a population must face a struggle for survival, and that only a few individuals need to survive to pass on their characteristics to the next generation. The rest fail to develop; die of starvation, predation, or other causes before they reproduce; or do not reproduce for other reasons. The loss of reproductive individuals is one factor that causes wild populations to be relatively stable in size. The remaining two facts suggested to Darwin how this struggle for survival plays out among the members of a natural population: • Individual organisms within a population are different in particular characteristics from one another. • Most, although not all, of this variation can be inherited from one organism to the next. Darwin thought about the struggle for existence. He reasoned that particular characteristics can help organisms survive. These characteristics must be inherited. Successful organisms inherit characteristics that best equip them to survive in a particular environment. These organisms are most likely to reproduce. They are likely to leave behind offspring with the same beneficial characteristics. We call these beneficial characteristics adaptations. By contrast, some organisms inherit characteristics that make it more difficult to survive. They do not tend to reproduce in an environment. As a result, their particular characteristics would be less likely to be passed on to surviving offspring. For example, suppose a population of rabbits lived in a moderately warm climate. Within that population, a small proportion of the individuals have thick fur. Now suppose these rabbits have thick fur because their genetic material is different from that of other rabbits living in the same area. This difference might be passed on to their offspring. Imagine first that the thicker fur is a disadvantage to the rabbits in this environment because they overheat when coyotes chase them. As a consequence, these thick-furred rabbits do not survive and reproduce as well as the rabbits with thin fur. You probably can see, in this case, that thick fur would not be a common trait in this population. Now imagine that over several decades the climate in this environment gets colder. In the colder climate, the rabbits with the thicker fur now have an advantage. That is, the thick fur would be an adaptation that helps them survive the periods of cold weather. Thick-furred rabbits would be more likely to survive. They would reproduce more successfully. As a result, they are likely to pass on their characteristics to the next generation. As the figure of rabbit population shows, the number of rabbits with thick fur increases in the population. This is the process of natural selection. Copyright © by BSCS. All rights reserved. Copymaster 6.5 Page 3 of 4 Relative number of rabbits in population Warm Climate Thin fur Thick fur Cold Climate (40 years later) Thin fur Thick fur Characteristics of a population change due to natural selection. Why do the heights of each bar change? What do the changes have to do with evolution? There is a key difference between selective breeding and natural selection. In selective breeding, humans choose how to allow organisms to reproduce. In natural selection, interactions with the natural environment determine which individual organisms survive and reproduce. In natural environments, many pressures affect survival and reproduction. Changes in the environment can alter the balance of life. That is, some characteristics are more beneficial than others. Sometimes these changes are catastrophic, like the effect a volcanic eruption might have on both local and very distant populations of organisms. Copyright © by BSCS. All rights reserved. Copymaster 6.5 Page 4 of 4