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Natural Selection

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    23992
  • Evolution– includes all of the changes in the characteristics and diversity of life that occur throughout time.

    Evolution can occur on both large and small scales.

    • Microevolution
    • Macroevolution

    ›The concept of evolution, that organisms may change over time, was not new in Darwin’s time. ›However, it was not a widely accepted concept because no one understood how it could work. ›A mechanism was missing.

    Natural Selection

    Darwin provided that mechanism with his theory of Natural Selection.

    Screen Shot 2019-06-23 at 11.43.47 AM.png

    Figure \(\PageIndex{1}\). Charles Darwin (CC BY-NC-SA; Wikipedia)

    In any population of organisms there is natural variation. ›Some of these variations will allow the organisms possessing them to survive and reproduce better than those without these particular traits.

    Screen Shot 2019-06-23 at 11.44.58 AM.png

    Figure \(\PageIndex{2}\). (CC BY-NC-SA; N. Wheat)

    The successful traits will spread through the population. This change in the frequency of alleles in the population is evolution.

    Natural Selection – High Reproductive Potential

    Darwin observed that organisms have the potential for very high fertility. ›Organisms have the potential to produce, and often do produce large numbers of offspring. Population size would quickly become unmanageable if all of the offspring survived.

    Screen Shot 2019-06-23 at 11.47.12 AM.png

    Figure \(\PageIndex{3}\). (CC BY-NC-SA; N. Wheat)

    Natural Selection – Population Size Remains Constant

    Despite this high potential fertility, natural populations usually remain constant in size, except for small fluctuations. Not all of the potential offspring survive.

    Natural Selection – Limited Resources

    Resources that organisms need to survive are limited. Food, water, shelter, nesting sites, etc.

    Natural Selection – Competition

    If there are not enough resources for all of the individuals, there will be competition for those resources. Survivors represent a small part of the individuals produced each generation.

    Natural Selection – Populations Show Variation

    Which individuals will survive is often not a matter of luck. Populations show variation– individuals are not identical. They differ in many different traits.

    Natural Selection – Variation is Heritable

    Some of the variation between individuals in the population is heritable. It can be passed down from one generation to the next.

    Natural Selection – Some traits Enhance Survival

    Some of the traits found in the population enhance the survival and reproduction of the organisms possessing them.

    Natural Selection – Adaptation

    The favored traits will spread through the population. Over many generations, the species will become adapted to its environment. Over time, these changes can lead to the formation of a new species.

    Adaptation

    A species may become adapted to its environment in response to environmental pressures. A trait may be favored due to enhanced survival or reproduction when faced with a particular aspect of the environment.

    When an environment changes, or when individuals move to a new environment, natural selection may result in adaptation to the new conditions. Sometimes this results in a new species.

    Populations Evolve

    Individuals do not evolve; populations evolve. ›Evolution is measured as changes in relative proportions of heritable variations in a population over several generations.

    Natural Selection – Important Points

    • Natural selection can only work on heritable traits. Acquired traits are not heritable and are not subject to natural selection.
    • Environmental factors are variable. A trait that is beneficial in one place or time may be detrimental in another place or time.
    • Natural selection is not random. It occurs in response to environmental pressures and results in adaptation.
    • When natural selection is occurring, some individuals are having better reproductive success than others. Alleles are being passed to the next generation in frequencies that are different from the current generation.

    Upsetting Genetic Equilibrium

    Natural selection is not the only way that allele frequencies can change from one generation to the next.

    • Genetic Drift– a random loss of alleles.
    • Mutation– a new mutation can add alleles.
    • Nonrandom mating – inbreeding increases the number of homozygous traits.
    • Migration– shuffles alleles between populations; can prevent speciation.

    Genetic Drift

    The smaller the sample, the greater the chance of deviation from expected results. These random deviations from expected frequencies are called genetic drift. Allele frequencies are more likely to deviate from the expected in small populations. Which allele gets lost is due to random chance. Over time, drift tends to reduce genetic variation through random loss of alleles.

    \(C^RC^R\) = red

    \(C^RC^W\)= pink

    \(C^WC^W\)= white

    Screen Shot 2019-06-23 at 12.19.55 PM.png

    Figure \(\PageIndex{4}\). (CC BY-NC-SA; N. Wheat)

    The Bottleneck Effect›

    Sometimes a catastrophic event can severely reduce the size of a population. The random assortment of survivors may have different allele frequencies. This is a type of genetic drift called the bottleneck effect. 

    Screen Shot 2019-06-23 at 12.21.53 PM.png

    Figure \(\PageIndex{5}\). (CC BY-NC-SA; N. Wheat)

    The actions of people sometimes cause bottlenecks in other species. N. California elephant seal population reduced to 20-100 individuals in the 1890s. ›

    • Current population> 30,000.
    • Variation drastically reduced – 24 genes with 1 allele.

    Screen Shot 2019-06-23 at 12.23.49 PM.png

    Figure \(\PageIndex{6}\). (CC BY-NC-SA; pdphoto.org)

    The Founder Effect

    Founder effect– Another type of genetic drift occurs when a small group of individuals becomes separated from the population and form a new population. The allele frequencies in their gene pool may be different than the original population.

    Screen Shot 2019-06-23 at 12.26.50 PM.png

    Figure \(\PageIndex{7}\). (CC BY-NC-SA; N. Wheat)

     

    Creative Commons License

    This tutorial was funded by the Title V-STEM Grant #P031S090007.

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