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    Mechanisms of evolution, natural selection, mutation

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    1. What are the mechanisms of evolution?
    2. How does natural selection result in biodiversity?
    3. Why is biodiversity important to continued evolution?
    4. What are mutations and sexual recombination?

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    Life: the Science of Biology by Purves, Sadava, Orians, Heller
    Evolution in genetic terms means changes in proportions of alleles present in a population for a particular trait across generations.
    Genes have different forms called alleles. If the two copies of a gene are identical alleles, this is a homozygous condition. If the two allele copies are different, this is a heterozygous condition.
    The alleles a person inherits are his or her genotype. Observable functional or physical traits, such as attached earlobes, are the phenotype. The gene pool is the total of all the alleles in a population.
    Population is a localized group of individuals that belong to the same biological species and who are capable of interbreeding and producing viable offspring. A population's gene pool is defined by its allele frequencies.
    A species is a group of potentially interbreeding organisms that produce fertile offspring and share similar characteristics. Speciation is the splitting of one species into two or more species or the transformation of one species into a new species over time; speciation is the final result of changes in gene pool allele and genotypic frequencies.

    1. What are the mechanisms of evolution?
    Evolution occurs through the process of descent with modification. All life is made up of cells. Cells arise from pre-existing cells. As life perpetuates itself over time, differences appear and these differences accumulate and cause different species to come into existence.
    It is not individuals that evolve, it is populations that do. In order for a population to evolve, there must be heritable genetic variation among its members. The mechanisms that change the genetic structure of a population and cause evolution are: Mutation, migration (gene flow), genetic drift, nonrandom mating, and natural selection.

    1) Mutation: Mutation is any change in the DNA of an organism. Any mutations in the reproductive cells (eggs and sperms) of an organism are inherited. Mutations can alter allele frequencies within a population by changing one allele into a different allele. The DNA affects the structure and functions of an organism. Thus any change in DNA can cause a change in an organism.

    2) Migration (gene flow): Gene flow (or gene migration) is the movement of alleles among populations by migration. Allele frequencies change when individuals leave a population (emigration causes a loss of alleles) or when new individuals enter a population (immigration causes a gain of alleles). Examples of gene flow are pollen being blown to a new destination or people moving into or out of cities or countries. Gene flow can be a very important source of genetic variation if genes are carried to a population where those genes previously did not exist.

    3) Genetic drift: Genetic drift is a change in allele frequencies of a gene pool due to chance. It happens more often in small populations. Genetic drift occurs when founders start a new population (Founder Effect), or after a genetic bottleneck (Population bottleneck).
    Founder Effect is the genetic drift that occurs when a few individuals separate from a large population and establish a new one. This happens because founding individuals contain only a fraction of total genetic diversity of the original population. The particular alleles that are carried by the founders is due to chance alone. An example: dwarfism is much higher in a Pennsylvania Amish community due to a few German founders.
    Population bottleneck is the genetic drift that results from the reduction of a population, due to causes such as natural disaster, predation, or habitat reduction, with the result that the surviving population is no longer genetically representative of the original population.

    4) Nonrandom mating: Random mating is individuals pairing by chance while nonrandom mating is individuals choosing a mate. Mating patterns may alter genotype frequencies if individuals in a population choose other individuals of certain genotypes as mates. For example, individuals may choose to mate preferentially with individuals of the same genotype, with the result that homozygous genotypes will be overrepresented and heterozygous genotypes underrepresented, in the next generation. Or, they may choose to mate primarily or exclusively with individuals of different genotypes.

    5) Natural Selection: Natural Selection occurs because members of a population exhibit variability in their traits and this variability is heritable. Individuals with traits that increase their chance of survival will have more opportunities to reproduce and their offspring will also benefit from the heritable, advantageous trait. Over time individuals with these variants will spread through the population.

    2. How does natural selection results in biodiversity?
    Natural selection requires that
    a) There is variation among individuals within a population in some trait.
    b) This variation is heritable (i.e., there is a genetic basis to the variation, such that offspring tend to resemble their parents in this trait).
    c) Variation in this trait is associated with variation in fitness. Variations that are better adapted to the mode of life of that species or to the environment in which that species lives confers on the individuals possessing them an advantage and they tend to survive longer and leave more offspring than individuals that do not possess these variations. Similarly there are variations that are harmful to the organism possessing them. Such organisms may die before they reach reproductive age. Thus the harmful variations are not passed on. The fact that poorly adapted organisms die and the well adapted survive and transmit their beneficial characteristics to their offspring is known as natural selection- nature selecting the fit and rejecting the unfit.
    Thus fitness is associated with Differential adaptedness (some variations affect how well an organism is adapted to its environment), and Differential reproduction (better adapted individuals are more likely to reproduce). Thus fitness is the extent to which an individual contributes fertile offspring to the next generation. Relative fitness is comparing the fitness of one phenotype over another.
    All organisms have the biological imperative to perpetuate their existence. Thus, one of the prime motives for all species is to survive and reproduce, passing on the genetic information of the species from generation to generation. When they do this, they tend to produce more offspring than the limited resources of food, water and shelter can support. This results in a competition for survival. Some individuals are relatively fitter and survive. Others do not. Over time the characteristics that confer fitness accumulate in the population while those characteristics that diminish fitness tend to die out. When reproductive isolation occurs new species will form as ultimately the organisms differ so much from their predecessors that they no longer interbreed with them. This new variety is a new species.

    3. Why is biodiversity important to continued evolution?
    Biodiversity or biological diversity refers to the variety of life forms occurring in nature. It has come into existence as a result of evolution. The biodiversity of the earth constantly changes with the extinction of species and formation of new species.
    Biodiversity is the planet's genetic raw material for future evolution in response to changing environmental conditions. If the environment changes rapidly and if the species living in these environments do not already possess genes which enable them to survive in the changed environment and if random mutations do not accumulate quickly enough then, all the members of these species will die. Therefore, biodiversity is necessary for evolution to continue and for life to exist on earth.
    There is growing evidence that humans have become a major selective force leading to premature extinction of a growing number of species. Habitat destruction, over-harvesting, pollution and the inappropriate introduction of foreign plants and animals, are the main causes of current biodiversity loss. At current rate one-fifth of all species will be lost by 2030, and by the end of the century about one- half will be lost. Evolution is an extremely slow process. On such a short time scale, the formation of new species will not be able to recoup such major losses.

    4. What are mutations and sexual recombination?
    Mutation and sexual recombination produce variations in gene pools which are what cause differences among individuals in a population. New genes and new alleles can originate only by mutation. However, on a generation-to-generation timescale, sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible.
    1) Mutations: Mutations are changes in the nucleotide sequence of DNA. Mutations cause new genes and alleles to arise. Most mutations occur in body cells (somatic cells) and are lost when the individual dies. Only mutations in cells that produce gametes (sperms and eggs) can be passed to offspring. Only a small fraction of these mutations spread through populations and become fixed. A new mutation that is transmitted in a gamete to an offspring can immediately change the gene pool of a population by introducing a new allele.
    Mutations may occur because DNA fails to copy accurately (occurs naturally) or because of external influences (such as exposure to specific chemicals or radiation). Mutation rates are low in animals and plants (average is about one mutation in every 100,000 genes per generation). Microorganisms have more rapid mutations.
    Mutations are one of many mechanisms that bring change in the gene pool of a species. Species as a whole evolves when mutation brings about changes in trait that enable the organism to better fit the changing environment. The organism survives and passes down the trait through reproduction.
    2) Sexual Recombination: Sex and recombination produce the genetic differences that make adaptation possible for natural selection to operate. In organisms that reproduce sexually, most of the genetic variation in a population results from the unique combination of alleles that each individual receives. Though all the differences in these alleles are originally from past mutations, during sexual reproduction some genetic "shuffling" occurs, bringing together new combinations of genes. There are three mechanisms that contribute to this shuffling: 1) Crossing over, 2) Independent assortment of chromosomes during meiosis and 3) Random fertilization. All three combine to ensure that sexual reproduction rearranges existing alleles into new combinations which providing much of the genetic variation that makes evolution possible.

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