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Biology LibreTexts

Microevolution

  • Page ID
    23994
  • 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.

    Macroevolution – Evolution on a Large Scale

    Macroevolution– evolutionary change on a grand scale: Origin of novel designs; Evolutionary trends; Adaptive radiation

    Microevolution – Evolution on a Small Scale

    Microevolution- a change in the genetic composition of a population over time. A change in the frequency of certain alleles in a population over several generations.

    Polymorphism

    Polymorphism occurs when there are different allelic forms of a gene in a population.

    Mojave (left) and Lesser (middle) are different alleles of the same gene. Wild type ball python is shown on the right.

    Screen Shot 2019-06-23 at 12.49.18 PM.png

    Figure \(\PageIndex{1}\). Lesser ball python (CC BY-NC-SA; Bailey & Bailey Reptiles.)

    Gene Pool

    All of the alleles of all of the genes possessed by all of the members of the population are contained in the gene poolof the population. ›We can measure the relative frequency of a particular allele in a population: Allelic frequency.

    Population Genetics

    Population Genetics– the study of how populations change over time. ›Dependent on both Darwin’s theory of natural selection and Mendel’s laws of inheritance. ›All heritable traits have a genetic basis, some are controlled by multiple genes – not as simple as in Mendel’s studies.

    Genetic Equilibrium

    According to Hardy-Weinberg equilibrium, the hereditary process alone does not produce evolutionary change. Allelic frequency will remain constant generation to generation unless disturbed by mutation, natural selection, migration, nonrandom mating, or genetic drift. These are sources of microevolutionary change.

    Frequency of Alleles

    Each allele has a frequency(proportion) in the population. ›Example population of 500 wildflowers.

    \(C^RC^R\)= red; \(C^RC^W\)= pink; \(C^WC^W\)= white

    250 red, 100 pink, 200 white

    Frequency of \(C^R\)= (250 x 2) + 100 / 1000 = 600/1000 =.6 = 60%

    is the frequency of the most common allele (CRin this case).

    p = 0.6 or 60%

    is the frequency of the less common allele (CWin this case).

    p + q = 1

    q = 1- p = 1 – 0.6 = 0.4 or 40%

    Hardy-Weinberg Theorem

    Populations that are not evolving are said to be in Hardy-Weinberg equilibrium. As long as Mendel’s laws are at work, the frequency of alleles will remain unchanged.

    Screen Shot 2019-06-23 at 12.55.05 PM.png

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

    The Hardy-Weinberg theorem assumes random mating. Generation after generation allele frequencies are the same.

    Conditions required for Hardy-Weinberg equilibrium to hold true:

    • Very large population
    • No gene flow into or out of the population
    • No mutations
    • Random mating
    • No natural selection

    Departure from these conditions results in a change in allele frequencies in the population. ›Evolution has occurred!

    Practice with Hardy Weinberg

    Frequency– the proportion of individuals in a category in relation to the total number of individuals.  

    100 cats, 75 black, 25 white – frequency of black = 75/100 = 0.75, white =0.25.

    Two alleles: p is common, q is less common.

    p+q= 1

    At a locus with two alleles, the three genotypes will appear in the following proportions: 

    (p + q) x (p + q) = \(p^2+ 2pq + q^2\)= 1

    Screen Shot 2019-06-23 at 12.59.15 PM.png

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

    We will use a population of 100 cats as a practice example: ›84 of the 100 cats are black; ›16 are white.

    We can use the equation and our color observations to calculate allele frequencies in our population of 100 cats.

    \(p^2+ 2pq + q^2\)= 1; 100 = population size

    84 of our 100 cats are black.

    Black is the dominant phenotype.

    Cats with the genotype Bb or BB will be black.

    The frequency of black cats is 84/100, but we can’t yet say anything about the B allele. 

    16 of our 100 cats are white. White is recessive (bb) and is represented by q2in our equation: p2+ 2pq + q21. So, q2 = 16/100 = 0.16

    q =square root of 0.16 = 0.40. Since p + q = 1p =1 – q = 0.60

    \(p^2\)= 0.36. \(p^2\) represents the proportion of individuals in the population with the homozygous dominant phenotype (BB).

    Remember population size = 100. Now we know how many of our cats have the BB genotype and the bb genotype. We can find the number of Bb cats using our equation: \(p^2+ 2pq + q^2\)= 1.

    2pq represents the proportion of cats with Bb. 2 x 0.6(p) x 0.4(q) = 0.48; 0.48 x 100 = 48 cats with Bb genotype.

     

    Creative Commons License

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

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