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NATURAL SELECTION

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NATURAL SELECTION

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    1. NATURAL SELECTION

    3. SELECTION WORKS ON THE PHENOTYPE      The biotic and abiotic factors within the environment apply selection pressure to the phenotype, not the genotype.  However, the environmental factors working on the phenotype will result in certain phenotypes having greater reproductive success than other phenotypes, thus the genotype ultimately changes.  Therefore, the gene pool changes - thus evolution will occur as a result of selection pressures.

    4. ARTIFICIAL SELECTION      Darwin studied the artificial selection practiced by breeders of animals and plants to determine if and how particular traits (admired by humans, for agricultural purposes, beauty, etc.) were passed on to offspring (inherited).  Of course, as animal and plant breeders had demonstrated, certain desirable traits of animals and plants (as perceived by humans) were heritable and passed on to offspring, thus undesirable traits could be weeded out.  "This weeding out of organisms by humans for human purposes is artificial selection." (Kardong, 2005). Dogs and Cats Agriculture - corn, cabbage family, etc. Flowers (like the tulip, rose, etc.)

    5. Diversity of dogs Artificial selection has produced numerous breeds of dogs, which diverged from wolves about 15,000 years ago.

    6. Corn Artificial selection through the centuries evolved the modern male tassel and female ears of corn from wild grass.

    7. Corn varieties From left to right: popcorn, sweet corn, flint corn, dent corn, and pod corn.

    8. Tulips Tulips come in a variety of colors and stripes, but never in a deep color of completely black. This variety does not exist, at least so far, within the species.

    9. NATURAL SELECTION       "The weeding out of organisms by biological processes, without deliberate or directed human intervention, is natural selection." (Kardong, 2005). Generally, natural selection is an average process that works on the phenotypes in the population, ultimately resulting in the survival and reproductive success of phenotypes that are more fit for their environment. Thus natural selection leads to a change in the genotypic frequencies in a population over time.  By human standards, natural selection is a very slow process, acting over hundreds of generations.  However, viruses and bacteria (because of very rapid reproduction) may evolve significantly in human lifetimes (examples, the AIDs virus, antibiotic resistance in certain strains of bacteria).  There are also examples of more complex organisms that have recently evolved by natural selection, such as the following:

    10. DARWIN’S FINCHES Darwin reasoned that the finches of the Galapagos Islands had evolved from an ancestral species from the mainland of South America and with the passage of time the descendants on each of the islands had been modified by natural selection to occupy distinctive niches.  In particular, the beaks of the various finches are specialized for a specific diet, such as soft or hard seeds, insects, etc.

    11. DARWIN’S FINCHES

    12. MODERN STUDIES OF DARWIN’S FINCHES A modern study of the natural selection of beak size and beak depth of one of Darwin's Finches, Geospiza fortis, by Peter and Rosemary Grant of Princeton University on a small island, Daphne Major, in the Galapagos Archipelgo shows how a changing enivironment can cause natural selection over a relatively short period of time.

    13. MODERN STUDIES OF DARWIN’S FINCHES

    14. Galápagos finches—natural selection on beak shape Finch beaks are their tools for opening seeds. Different seeds open best with different beak size and features, which are inherited characteristics. When all seed sizes were easily available in normal seasons, ground finches (Geospiza fortis), shown here, with large and small beaks survived about equally well. But in dry seasons, small, easy-to-open seeds were scarce. Large, hard-to-open seeds were more plentiful. Predominantly, finches with large, strong beaks could open the large seeds and survive (red), but those with small beaks did less well (green).

    15. THE PEPPERED MOTH (Biston betularia) and Industrial Melanism

    16. Natural selection on snails Birds, such as this song thrush, hunt snails and break their shells open against “anvil rocks” where debris collects. The snail (Cepaea normalis) has several distinct color morphs, which are camouflaged against different natural backgrounds.

    17. Snail Selection The shell of the snail Cepaea occurs in three color phases: brown, pink, and green. In different habitats-beech woodlands, meadows-different-colored shells are common or rare. In deciduous woodlands, the frequency of the color phases changes from spring to summer.

    18. Natural Selection of Northern Water Snakes The northern water snake (Nerodia) occurs throughout eastern North America. It is darkly patterned, except on some islands in Lake Erie where many are light colored.

    19. Water Snake Differential Survival (a) Scored color phases A (light) to D (banded). In young born on the islands, most are C or D (dark and banded). But by the time the snakes reach adult stage, most are A or B (light colored). (b) On the islands, predaceous gulls feed on young snakes, usually spotting and eating the more conspicuous banded snakes, producing differential survival of mostly unbanded snakes. Occasional immigrants from the mainland return some of the genes for banded color.

    20. TYPES OF NATURAL SELECTION Stabilizing Selection: The extremes of certain characters (traits) within a population are disadvantageous and eliminated. Directional Selection: One extreme for certain characters within a population are disadvantageous and are selected against. (The Giraffe neck length example we talked about before.) Disruptive Selection: Selection favors the extremes for particular traits rather than the central tendency, which is selected against.

    21. The bell-shaped curve represents the distribution of a character in a population. The shading indicates where in that variation selection acts to eliminate individuals. (a) In stabilizing selection, extremes are eliminated, leading to a narrowing of the variation. (b) In directional selection, one extreme is eliminated, shifting the curve. (c) In disruptive selection, individuals with intermediate variation are eliminated, producing two bell shaped curves at the extremes.

    22. Stabilizing Selection Natural Selection tends to favor the average (or norm) of the individuals within a population. Much deviation from the average (or norm) is selected against. This does lead to less genetic variability in a population. Natural selection is most commonly stabilizing (as compared to directional and disruptive selection).

    23. Directional Selection In an environment that is changing in a certain direction, members at one tail of the population distribution may be selected for and members at the other tail selected against. Our discussion of the evolution of the long neck in the giraffe would be an example.

    24. Disruptive Selection In disruptive selection, individuals with a certain trait(s) at the extreme ends of the population distribution for that trait(s) are selected for and selection against the average trait value(s) of the population distribution. This may often result in divergence of the population into two separate populations.

    25. SEXUAL SELECTION Sexual selection is a form of natural selection wherein the male or female members of a population select their mate based on secondary sexual characteristics. Secondary sexual characteristics are distinguishing features that differ between the males and females within a population. Such distinguishing features between males and females results in sexual dimorphism between the sexes. Most often, at least in animals, females do the choosing and males vie for the female choice. Thus males are often larger than the females and/or support other secondary sexual characteristics (such as bright or flashy coloration; large horns, tusks, or antlers; manes or more elaborate manes; long or elaborate and colorful tail feathers; etc.)

    26. Secondary sexual characteristics—kudu This male kudu, a resident of Africa, sports spiraled horns which are part of its social display during the reproductive season.

    27. Sexual dimorphism The large, male California sea lion, is distinctive from the surrounding, smaller females.

    28. Red winged blackbird This male red winged blackbird illustrates the colorful shoulder feathers, which are displayed during courtship and territorial defense.

    29. Peacock The peacock has a luxuriant tail and bright body used to attract the attentions of the female, the peahen.

    30. Barn Swallows The solid boxes represent the average number of young each group of males produced during a summer; the vertical lines express the range of variation of the results (standard deviation). The control groups did not differ significantly from each other, but both differed from shortened males, which produced significantly fewer offspring, and from elongated males, which produced significantly more offspring. (After MØller, 1988)

    31. Assignment on Sexual Selection Go to the following website to do an activity called the Flashy Fish: http://www.pbs.org/wgbh/evolution/educators/lessons/lesson4/act2.html . Print out the data sheet (pdf) and discussion questions (pdf). You will want to answer all the questions on both of these documents to complete the activity. Follow all instructions given at the PBS Flash Fish website to complete the simulation activity.

    32. THE END

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