1.7+Ecological+Succession



= What is "ecological succession"? = "Ecological succession" is the observed process of change in the species structure of an ecological community over time. Within any community some species may become less abundant over some time interval, or they may even vanish from the ecosystem altogether. Similarly, over some time interval, other species within the community may become more abundant, or new species may even invade into the community from adjacent ecosystems. This observed change over time in what is living in a particular ecosystem is "ecological succession". = Why does "ecological succession" occur? = Every species has a set of environmental conditions under which it will grow and reproduce most optimally. In a given ecosystem, and under that ecosystem's set of environmental conditions, those species that can grow the most efficiently and produce the most viable offspring will become the most abundant organisms. As long as the ecosystem's set of environmental conditions remains constant, those species optimally adapted to those conditions will flourish. The "engine" of succession, the cause of ecosystem change, is the impact of established species have upon their own environments. A consequence of living is the sometimes subtle and sometimes overt alteration of one's own environment. The original environment may have been optimal for the first species of plant or animal, but the newly altered environment is often optimal for some other species of plant or animal. Under the changed conditions of the environment, the previously dominant species may fail and another species may become ascendant. Ecological succession may also occur when the conditions of an environment suddenly and drastically change. A forest fires, wind storms, and human activities like agriculture all greatly alter the conditions of an environment. These massive forces may also destroy species and thus alter the dynamics of the ecological community triggering a scramble for dominance among the species still present. Are there examples of "ecological succession" on the Nature Trail? Succession is one of the major themes of our Nature Trail. It is possible to observe both the on-going process of succession and the consequences of past succession events at almost any point along the trail. The rise and the decline of numerous species within our various communities illustrates both of the types of motive forces of succession: the impact of an established species to change a site's environmental conditions, and the impact of large external forces to suddenly alter the environmental nature of a site. Both of these forces necessarily select for new species to become ascendant and possibly dominant within the ecosystem. =Some specific examples of observable succession include: =  1. The growth of hardwood trees (including ash, poplar and oak) within the red pine planting area. The consequence of this hardwood tree growth is the increased shading and subsequent mortality of the sun loving red pines by the shade tolerant hardwood seedlings. The shaded forest floor conditions generated by the pines prohibits the growth of sun-loving pine seedlings and allows the growth of the hardwoods. The consequence of the growth of the hardwoods is the decline and senescence of the pine forest. (Observe the dead pine trees that have fallen. Observe the young hardwoods growing up beneath the still living pines). 2. The raspberry thickets growing in the sun lit forest sections beneath the gaps in the canopy generated by wind-thrown trees. Raspberry plants require sunlight to grow and thrive. Beneath the dense shade canopy particularly of the red pines but also beneath the dense stands of oaks, there is not sufficient sunlight for the raspberry's survival. However, in any place in which there has been a tree fall the raspberry canes have proliferated into dense thickets. You may observe this successional consequence of macro-ecosystem change within the red pine stand and all along the more open sections of the trail. Within these raspberry thickets, by the way, are dense growths of hardwood seedlings. The raspberry plants are generating a protected "nursery" for these seedlings and are preventing a major browser of tree seedlings (the white tailed deer) from eating and destroying the young trees. By providing these trees a shaded haven in which to grow the raspberry plants are setting up the future tree canopy which will extensively shade the future forest floor and consequently prevent the future growth of more raspberry plants! 3. The succession "garden" plot. This plot was established in April, 2000 (please see the series of photographs on the "Succession Garden Plot" page). The initial plant community that was established within the boundaries of this plot was made up of those species that could tolerate the periodic mowing that "controlled" this "grass" ecosystem. Soon, though, other plant species became established as a consequence of the removal of the stress of mowing. Over time, the increased shading of the soil surface and the increased moisture retention of the undisturbed soil-litter interface allowed an even greater diversity of plants to grow and thrive in the Succession Garden. Eventually, taller, woody plants became established which shaded out the sun-loving weed community. In the coming years we expect tree seedlings to grow up within the Succession Garden and slowly establish a new section of the forest.   = <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">How are humans affected by ecological succession? = <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Ecological succession is a force of nature. Ecosystems, because of the internal species dynamics and external forces mentioned above, are in a constant process of change and re-structuring. To appreciate how ecological succession affects humans and also to begin to appreciate the incredible time and monetary cost of ecological succession, one only has to visualize a freshly tilled garden plot. Clearing the land for the garden and preparing the soil for planting represents a major external event that radically re-structures and disrupts a previously stabilized ecosystem. The disturbed ecosystem will immediately begin a process of ecological succession. Plant species adapted to the sunny conditions and the broken soil will rapidly invade the site and will become quickly and densely established. These invading plants are what we call "weeds". Now "weeds" have very important ecological roles and functions (see, for example, the "Winter Birds" discussion), but weeds also compete with the garden plants for nutrients, water and physical space. If left unattended, a garden will quickly become a weed patch in which the weakly competitive garden plants are choked out and destroyed by the robustly productive weeds. A gardener's only course of action is to spend a great deal of time and energy weeding the garden. This energy input is directly proportional to the "energy" inherent in the force of ecological succession. If you extrapolate this very small scale scenario to all of the agricultural fields and systems on Earth and visualize all of the activities of all of the farmers and gardeners who are growing our foods, you begin to get an idea of the immense cost in terms of time, fuel, herbicides and pesticides that humans pay every growing season because of the force of ecological succession. <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Does ecological succession ever stop? <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">There is a concept in ecological succession called the "climax" community. The climax community represents a stable end product of the successional sequence. In the climate and landscape region of the Nature Trail, this climax community is the "Oak-Poplar Forest" subdivision of the Deciduous Forest Biome. An established Oak-Poplar Forest will maintain itself for a very long period of time. Its apparent species structure and composition will not appreciably change over observable time. To this degree, we could say that ecological succession has "stopped". We must recognize, however, that any ecosystem, no matter how inherently stable and persistent, could be subject to massive external disruptive forces (like fires and storms) that could re-set and re-trigger the successional process. As long as these random and potentially catastrophic events are possible, it is not absolutely accurate to say that succession has stopped. Also, over long periods of time ("geological time") the climate conditions and other fundamental aspects of an ecosystem change. These geological time scale changes are not observable in our "ecological" time, but their fundamental existence and historical reality cannot be disputed. No ecosystem, then, has existed or will exist unchanged or unchanging over a geological time scale. Primary succession is one of two types of [|ecological succession] and [|biological succession] of plant life, and occurs in an environment in which new [|substrate], devoid of vegetation and usually lacking soil, is deposited (for example a lava flow). (The other type of succession, [|secondary succession], occurs on substrate that previously supported vegetation before a [|disturbance] destroyed the plant life.) In primary succession [|pioneer species] like [|mosses], [|lichen], algae and fungus as well as other abiotic factors like wind and water start to "normalize" the [|habitat]. This creating conditions nearer optimum for vascular plant growth; [|pedogenesis] or the formation of soil is the most important process. These pioneer plants are then dominated and often replaced by plants better adapted to less austere conditions, these plants include vascular plants like [|grasses] and some [|shrubs] that are able to live in thin soils that are often mineral based. A good example of primary succession takes place after a [|volcano] has erupted. The barren land is first colonized by pioneer plants which pave the way for later, less hardy plants, such as [|hardwood trees], by facilitating pedogenesis, especially through the biotic acceleration of [|weathering] and the addition of organic debris to the surface [|regolith]. Primary succession occurs following an opening of a pristine habitat, for example, as previously stated a [|lava] flow, an area left from retreated [|glacier], or abandoned strip mine. In contrast, secondary succession is a response to a disturbance, for example, [|forest fire], [|tsunami], [|flood], or an abandoned field. Secondary succession is one of the two types of [|ecological succession] of plant life. As opposed to [|primary succession], secondary succession is a process started by an event[|[1]] (e.g. [|forest fire], [|harvesting], [|hurricane]) that reduces an already established [|ecosystem] (e.g. a forest or a wheat field) to a smaller population of species, and as such secondary succession occurs on preexisting [|soil] where as primary succession usually occurs in a place lacking soil. A harvested forest going back from being a cleared forest to its original state, the "[|climax community]" (a term to use cautiously), is an example of secondary succession. Each stage a community goes through on its way to the climax community in succession can be referred to as a "serial community." Simply put, secondary succession is the succession that occurs after the initial succession has been disrupted and some plants and animals still exist. =<span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; mso-outline-level: 2; text-align: center;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Climax Communities = <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">A climax community is one that has reached the stable stage. When extensive and well defined, the climax community is called a biome. Examples are [|tundra], grassland, [|desert] , and the deciduous, coniferous, and tropical rain [|forests]. Stability is attained through a process known as succession, whereby relatively simple communities are replaced by those more complex. Thus, on a lakefront, grass may invade a build-up of sand. Humus formed by the grass then gives root to oaks and pines and lesser vegetation, which displaces the grass and forms a further altered humus. That soil eventually nourishes maple and beech trees, which gradually crowd out the pines and oaks and form a climax community. In addition to trees, each successive community harbors many other life forms, with the greatest diversity populating the climax community. <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Similar ecological zonings occur among marine flora and fauna, dependent on such environmental factors as bottom composition, availability of light, and degree of salinity. In other respects, the capture by aquatic plants of solar energy and inorganic materials, as well as their transfer through food chains and cycling by means of microorganisms, parallels those processes on land. <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">The early 20th-century belief that the climax community could endure indefinitely is now rejected because climatic stability cannot be assumed over long periods of time. In addition nonclimatic factors, such as soil limitation, can influence the rate of development. It is clear that stable climax communities in most areas can coexist with human pressures on the ecosystem, such as deforestation, grazing, and urbanization. Polyclimax theories stress that plant development does not follow predictable outlines and that the evolution of ecosystems is subject to many variables. <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> = <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Species diversity = <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Species diversity <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> refers to the number and distribution of [|species] in one location. Simply the measure of the number of different species within a given area. Humans have a huge effect on species diversity, the main reasons are: <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> - Destruction, Modification, and/or Fragmentation of Habitat - Introduction of Exotic Species, - Overharvest - Global Climate Change <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Genetic diversity is a level of [|biodiversity] that refers to the total number of [|genetic] characteristics in the genetic makeup of a species. It is distinguished from [|genetic variability], which describes the tendency of genetic characteristics to vary. <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">The academic field of [|population genetics] includes several hypotheses regarding genetic diversity. The [|neutral theory of evolution] proposes that diversity is the result of the accumulation of neutral substitutions. [|Diversifying selection] is the hypothesis that two subpopulations of a species live in different environments that select for different [|alleles] at a particular locus. This may occur, for instance, if a species has a large range relative to the mobility of individuals within it. [|Frequency-dependent selection] is the hypothesis that as alleles become more common, they become less fit. This is often invoked in host-pathogen interactions, where a high frequency of a defensive allele among the host means that it is more likely that a pathogen will spread if it is able to overcome that allele. <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;"> =<span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; mso-outline-level: 2; text-align: center;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Importance of genetic diversity = <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">There are many different ways to measure genetic diversity. The modern causes for the loss of animal genetic diversity have also been studied and identified. [|[1]] [|[2]] A [|September 14], [|2007] study conducted by the [|National Science Foundation] found that genetic diversity and [|biodiversity] are dependent upon each other -- that diversity within a species is necessary to maintain diversity among species, and vice versa. According to the lead researcher in the study, Dr. Richard Lankauof, "If any one type is removed from the system, the cycle can break down, and the community becomes dominated by a single species." [|[3]] =<span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; mso-outline-level: 2; text-align: center;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Survival and adaptation = <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Genetic diversity plays a huge role in the survival and adaptability of a species. When a species’ environment changes, slight gene variations are necessary for it to adapt and survive. A species that has a large degree of genetic diversity among its individuals will have more variations from which to choose the most fitting allele. Species that have very little genetic variation are at a great risk. With very little gene variation within the species, healthy reproduction becomes increasingly difficult, and offspring often deal with similar problems to those of [|inbreeding]. [|[4]] =<span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; mso-outline-level: 2; text-align: center;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Agricultural Relevance = <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">When humans initially started farming, they used [|selective breeding] to pass on desirable traits of the crops while omitting the undesirable ones. Selective breeding leads to [|monocultures] : entire farms of nearly genetically identical plants. Little to no genetic diversity makes crops extremely susceptible to widespread disease. Bacteria morph and change constantly. When a disease causing bacteria changes to attack a specific genetic variation, it can easily wipe out vast quantities of the species. If the genetic variation that the bacterium is best at attacking happens to be that which humans have selectively bred to use for harvest, the entire crop will be wiped out. [|[5]] <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">A very similar occurrence is the cause of the infamous [|Potato Famine] in Ireland. Since new potato plants do not come as a result of reproduction but rather from pieces of the parent plant, no genetic diversity is developed, and the entire crop is essentially a clone of one potato, it is especially susceptible to an epidemic. In the 1840s, much of Ireland’s population depended on potatoes for food. They planted namely the “lumper” variety of potato, which was susceptible to a rot-causing mold called //Phytophthora infestans//. [|[6]] This mold destroyed the vast majority of the potato crop, and left thousands of people to starve to death. =<span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; mso-outline-level: 2; text-align: center;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">Coping with Poor Genetic Diversity = <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">The natural world has several ways of preserving or increasing genetic diversity. Among oceanic [|plankton], viruses aid in the genetic shifting process. Ocean viruses, which infect the plankton, carry genes of other organisms in addition their own. When a virus containing the genes of one cell infects another, the genetic makeup of the latter changes. This constant shift of genetic make-up helps to maintain a healthy population of plankton despite complex and unpredictable environmental changes. [|[7]] <span style="line-height: 200%; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; text-align: justify;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;"> [|Cheetahs] are a threatened species. Extremely low genetic diversity and resulting poor sperm quality has made breeding and survivorship difficult for Cheetahs – only about 5% of cheetahs make it to adulthood. [|[8]] About 10,000 years ago, all but the jubatus species of cheetahs died out. The species encountered a [|population bottleneck] and close family relatives were forced to mate with each other, resulting in inbreeding. [|[9]] However, it has been recently discovered that female cheetahs can mate with more than one male per litter of cubs. They undergo induced ovulation, which means that a new egg is produced every time a female mates. By mating with multiple males, the mother increases the genetic diversity within a single litter of cubs. [|[10]] <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> = What is evolution? Evolution is technically defined as: "a gradual process in which something changes into a different and usually more complex or better form." As it is most famously used, "evolution" is the process by which an organism becomes more sophisticated over time and in response to its environment. The Theory of Evolution is currently the most popular concept of how life reached its current state. Evolution as a biological mechanism is driven by natural selection. This theory is favored by many scientists to explain phenomena in nature, so much so that it is generally assumed as factual in most studies. = Natural selection is the process by which favorable [|heritable] [|traits] become more common in successive [|generations] of a [|population] of [|reproducing] [|organisms], and unfavorable heritable traits become less common, due to differential reproduction of [|genotypes]. Natural selection acts on the [|phenotype], or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and [|reproduce] than those with less favorable phenotypes. The phenotype's [|genetic] basis, [|genotype] associated with the favorable phenotype, will increase in [|frequency] over the following generations. Over time, this process may result in [|adaptations] that specialize organisms for particular [|ecological niches] and may eventually result in the [|emergence of new species]. In other words, natural selection is the mechanism by which evolution may take place in a population of a specific organism.

<span style="color: windowtext; font-family: 'Times New Roman',serif; font-size: 12pt; font-weight: normal; line-height: 200%;">
= <span style="color: windowtext; font-family: 'Times New Roman',serif; font-size: 12pt; font-weight: normal; line-height: 200%;">Darwin's Theory = <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">Darwin's theory of evolution has four main parts:
 * 1) <span style="line-height: 200%; mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">Organisms have changed over time, and the ones living today are different from those that lived in the past. Furthermore, many organisms that once lived are now extinct. The world is not constant, but changing. The fossil record provided ample evidence for this view.
 * 2) <span style="line-height: 200%; mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">All organisms are derived from common ancestors by a process of branching. Over time, populations split into different species, which are related because they are descended from a common ancestor. Thus, if one goes far enough back in time, any pair of organisms has a common ancestor. This explained the similarities of organisms that were classified together -- they were similar because of shared traits inherited from their common ancestor. It also explained why similar species tended to occur in the same geographic region.
 * 3) <span style="line-height: 200%; mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">Change is gradual and slow, taking place over a long time. This was supported by the fossil record, and was consistent with the fact that no naturalist had observed the sudden appearance of a new species. [This is now contested by a view of episodes of rapid change and long periods of stasis, known as //[|punctuated equilibrium]//].
 * 4) <span style="line-height: 200%; mso-list: l2 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">The mechanism of evolutionary change was natural selection. This was the most important and revolutionary part of Darwin's theory, and it deserves to be considered in greater detail.

<span style="color: windowtext; font-family: 'Times New Roman',serif; font-size: 12pt; font-weight: normal; line-height: 200%;">The Process of Natural Selection
<span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">Natural selection is a process that occurs over successive generations. The following is a summary of Darwin's line of reasoning for how it works <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">"The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when 30 years old and goes on breeding until 90 years old; if this be so, after a period from 740 to 750 years there would be nearly 19 million elephants descended from this first pair." <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> This unbounded population growth resembles a simple geometric series (2-4-8-16-32-64..) and quickly reaches infinity. <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> As a consequence, there is a "struggle" (metaphorically) to survive and reproduce, in which only a few individuals succeed in leaving progeny. <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> Organisms show variation in characters that influence their success in this struggle for existence. Individuals within a population vary from one another in many traits. (Animal behavioralists making long-term studies of chimps or elephants soon recognize every individual by its size, coloration, and distinctive markings.) <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> Offspring tend to resemble parents, including in characters that influence success in the struggle to survive and reproduce. <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> Parents possessing certain traits that enable them to survive and reproduce will contribute disproportionately to the offspring that make up the next generation. To the extent that offspring resemble their parents, the population in the next generation will consist of a higher proportion of individuals that possess whatever adaptation enabled their parents to survive and reproduce. The well-known example of camouflage coloration in an insect makes for a very powerful, logical argument for adaptation by natural selection. Development of such coloration, which differs according to the insect's environment, requires variation. The variation must influence survival and reproduction (fitness), and it must be inherited. During the early and middle 20th Century, genetics became incorporated into evolution, allowing us to define natural selection this way:
 * <span style="line-height: 200%; mso-list: l1 level1 lfo2; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">If all the offspring that organisms can produce were to survive and reproduce, they would soon overrun the earth. Darwin illustrated this point by a calculation using elephants. He wrote:
 * <span style="line-height: 200%; mso-list: l1 level1 lfo2; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 200%;">[[image:file:///C:/DOCUME~1/JONBUL~1/LOCALS~1/Temp/msohtmlclip1/01/clip_image001.gif width="507" height="399" caption="Process of Natural Selection"]] <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> ||
 * //<span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">Figure 2: The Process of Natural Selection //<span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;"> ||

= <span style="color: windowtext; font-family: 'Times New Roman',serif; font-size: 12pt; font-weight: normal; line-height: 200%;">Natural Selection Requires... = <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">For natural selection to occur, two requirements are essential: <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">Unless both these requirements are met, adaptation by natural selection cannot occur. Some examples: <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">In addition, natural selection can only choose among existing varieties in a population. It might be very useful for polar bears to have white noses, and then they wouldn't have to cover their noses with their paws when they stalk their prey. The panda could have a much nicer thumb than the clumsy device that it does have. When we incorporate genetics into our story, it becomes more obvious why the generation of new variations is a chance process. Variants do not arise because they are needed. They arise by random processes governed by the laws of genetics. For today, the central point is the chance occurrence of variation, some of which is adaptive, and the weeding out by natural selection of the best adapted varieties. <span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">
 * 1) <span style="line-height: 200%; mso-list: l0 level1 lfo3; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">There must be heritable variation for some trait. Examples: beak size, color pattern, thickness of skin, fleetness.
 * 2) <span style="line-height: 200%; mso-list: l0 level1 lfo3; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">There must be differential survival and reproduction associated with the possession of that trait.
 * <span style="line-height: 200%; mso-list: l3 level1 lfo4; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">If some plants grow taller than others and so are better able to avoid shading by others, they will produce more offspring. However, if the reason they grow tall is because of the soil in which their seeds happened to land, and not because they have the genes to grow tall, than no evolution will occur.
 * <span style="line-height: 200%; mso-list: l3 level1 lfo4; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 200%;">If some individuals are fleeter than others because of differences in their genes, but the predator is so much faster that it does not matter, then no evolution will occur (e.g. if cheetahs ate snails).