Recall the five factors that can cause a population to evolve: Let's continue to examine them.

2. Genetic Drift: The Effect of Non-infinite Population Size

It's quite simple, really. There is really no such thing as an infinitely large population.
And even in a large population, only a small subset of possible gametes will make it into the next generation.

  • The number of genetically unique gametes an individual can make is equal to 2n
  • n = the number of different genes the organism has.
  • The average mammal (you) has about 20,000 genes
  • That means you are capable of generating 220000 genetically unique gametes.

    But will each of your 220000 unique gametes make it into the next generation? That's where Genetic Drift comes in.

    Genetic Drift: Genetic Change due to Sampling Error

  • The smaller the population, the less genetic diversity it has.
  • In a very small population, alleles can be lost from one generation to the next, simply due to sampling error.
  • When a population evolves only because sampling error, then GENETIC DRIFT is taking place.

    There are two basic "flavors" of genetic drift:

    Consider the effects of inbreeding and outbreeding on homozygosity and heterozygosity.

    Small, isolated populations (such as island populations separated from the mainland) will eventually consist of members who are related to one another. This leads to inbreeding.

    Inbreeding greatly increases the likelihood of homozygosity at multiple loci.
    In a small population with only two alleles for a given gene, one or the other allele can be lost entirely,
    and the population will become fixed at that allele.
    Only a mutation of that gene (creating a new allele) will provide new genetic variability.


    Here's an overview of genetic drift and its consequences. These links should help clarify the concepts.

    Genetic Drift is likely a major force in microevolution and, ultimately, macroevolution.
    While this is especially true in small, genetically isolated populations, random sampling error contributes to evolution in any non-infinite population.

    3. Random Mating If a population is mating randomly, then the alleles at a given gene locus should combine with the same frequency predicted by their relative frequency in the population. This can be predicted by the Product Rule:

    Let's create an imaginary population in which a dominant allele (A) represents 60% (0.6) of the alleles and a recessive allele makes up the remaining 40% (0.6) at a given locus.

    Assortative mating can work two ways (with respect to a trait): Non-random Mating: Inbreeding vs. Outbreeding

  • inbreeding occurs in a population when matings between closely related individuals occurs more frequently than would be predicted by their relative frequency in the population. That is, individuals preferentially mate with relatives.

  • outbreeding occurs in a population when matings between unrelated individuals occurs more frequently than would be predicted by their relative frequencies in the population. That is, individuals preferentially mate with non-relatives.

    Inbreeding can change allele frequencies, as can be seen in this diagram of a brother/sister mating

    The Product Rule (The combined probability of any two independent events occurring together is equal to the product of their individual probabilities.) tells us that the probability that both Jethro and Ellie May inherited the A1 allele from Bob is

    0.5 x 0.5, or 0.25 (25%)

  • If Jethro and Ellie May reproduce together, then the chance of both of them passing the A1 allele to an offspring is the combined probability of both of them having inherited the A1 allele:

    0.25 x 0.25 (0.0625, or 6.25%)


    Now let's say that A1 is a relatively rare allele in this population: only one in every thousand individuals carries it. That means

    Note that small, isolated populations will much more quickly come to consist of members who are related to one another than a very large population. Small populations are much more likely to lose one allele and become fixed at the other for a given locus, simply due to random sampling error.

    Small populations and inbreeding can greatly increase the rate of evolution at any given gene locus.

    Criterion 4. No Migration

    Loss or addition of alleles from immigration or emigration will change the allele frequency in the population under study.

    Gene Flow is the process by which movement of genes takes place between populations or demes.

    A species whose demes tend not to become reproductively isolated is known as a cohesive species. There is no simple explanation as to why a particular species is cohesive or not.


    Forces that Drive Evolution: Redux If you're paying close attention, you will notice...

    These forces all result in (populational) genetic changes that are not direct responses to natural selection. Of all the forces that drive evolution, only natural selection results in organisms that are better suited to live and reproduce in their environment.

    5. Natural Selection A mutation can be one of three things to the organism that inherits it:

    In the "contest" of natural selection, organisms do not compete against their predators, parasites or pathogens. They compete against each other.

    The individuals in any given species who are best suited to leave the most offspring in a given environment are the "winners" of that round of natural selection. But note that a form well-suited to one environment may be completely inappropriate in another. A trait that's adaptive in one set of circumstances can become neutral or maladaptive if the environment changes.

    It's all about context.


    Unlike genetic drift, evolution via natural selection cannot be considered random change. It is directed change in gene frequencies due to the interaction of organisms with each other and with their environment.

    Recall the tenets of Evolution by Natural Selection:

    An individual's Darwinian (evolutionary) fitness is a measure of the proportion of genes it contributes to succeeding generations. Evolutionary fitness is defined by the environment. A phenotype conferring fitness in a particular environment could be a liability if the environment changes.

    Quantifying Relative Darwinian Fitness: Fitness and Selection Coefficients In a population where individuals expressing a particular genotype have a selective advantage over those expressing a different genotype, certain coefficients can give an indication of just how much selective pressure is operating on each genotype.

    The fitness coefficient (W) is an expression of the adaptive value of a particular genotype relative to other genotypes. The genotype that produces the most offspring in a given population is assigned a fitness value of 1.0.

  • Example: There are three genotypes, AA, AA' and A'A' in a population.
    Over their lifetimes, the three genotypes produce the following numbers of offspring:

    The fitness coefficents for each genotype are measured relative to the most successful genotype.

  • AA: W = 10/10 = 1.0
  • AA': W = 5/10 = 0.5
  • A'A': W = 2/10 = 0.2

    The AA genotype has the highest fitness for this gene locus in this environmental/selective context.


    Conversely, the selection coefficient (s) is a measure of selective pressure against a particular genotype, relative to the other genotypes in the population. It is calculated as 1 - W.

    In our example, for each of our genotypes:

    Sexual Selection
    Sexual selection is a special case of natural selection based upon an individual's relative ability to attract and mate with members of the opposite sex. Individuals exhibiting characters that make them more likely to gain mating opportunities gain a selective advantage. Sexual selection is most likely to change allele frequencies when there is competition for mates.

    Sexual selection can result in sexual dimorphism.

    Sexual selection can operate in two ways:

    The Effects of Natural Selection

    At the start of a "selection cycle" the population is usually made up of individuals expressing a particular trait along a continuum, which can be expressed as a bell-shaped curve

    stabilizing selection: selective forces at work on a population favor greatest reproduction by individuals exhibiting the average state of a particular character. In this instance, the composition of the population doesn't change.

    directional selection: the individuals at one extreme or the other of the bell shaped curve have a reproductive advantage over the rest.

    (e.g., in drought years in the Galapagos, insects become scarce and seeds relatively abundant. Finches with deep, thick bills have an advantage in that they can more effectively crack seeds. The narrow-billed birds die out or have lower reproductive success because of the scarcity of food.)

    disruptive (= diversifying) selection: individuals at the average point on the curve are at a selective disadvantage; individuals with either extreme have a reproductive advantage.

    Why Can't Polar Bears Just Evolve Gills? You've probably heard the bad news that polar bears will likely be extinct before the end of the century, largely because climate change is drastically changing their habitat.

    (Click on the image, and then click on "THREATS")

    So why can't polar bears just evolve gills and get over it?

    As you probably have figured out, evolution doesn't work that way.

    Natural Selection Does not Make Perfect Organisms Though natural selection results in populations that are evolutionarily adapted to a their particular environment, there are plenty of examples of a lack of "intelligent design." Why?

  • Natural Selection acts only on polymorphism that already exists.

  • Natural Selection works only on traits that already exist.

  • Natural Selection results in traits that are sometimes compromises.

  • Random events govern evolution, too.

    These four constraints mean that evolution doesn't necessarily build ideal organisms.

    We're just as good as we can be, given the circumstances. And that's...okay.

    Population Genetics: Measuring Evolutionary Change In an idealized, model population that is not evolving, none of the five factors we have just examined are at work, and allele frequencies for a given locus will not change over time.

    Such idealized populations rarely, if ever, exist in nature.

    By monitoring changes in allele and genotype frequencies over generations, the population geneticist can quantify microevolutionary change.

    The population geneticist translates evolution into precise, genetic terms by studying

    Genetic variability can exist:

    Microevolution involves change in the relative frequency of the various alleles of gene loci in a population.
    How can we tell if this is happening? Measuring Heterozygosity

    A measure of genetic variabity in populations is heterozygosity: the proportion of genes in the genome that are present in heterozygous condition.

    Genetic variability in a population can be quantified as average heterozygosity: the average percent of loci that are heterozygous in that population.

    The Hardy-Weinberg Equation: Testing Whether a Population is Evolving In 1908, Godfrey H. Hardy, a British mathematician, and Wilhelm Weinberg, a German physician...

    ...independently reported a mathematical rule that predicts relative genotype frequencies--given starting frequencies of two alleles--in a population that is not evolving.

    It's an expansion of the binomial equation, (p + q)2

    In brief:
    For a population segregating two alleles at a particular locus in which A is the dominant allele and a is the recessive allele

    Hardy and Weinberg independently noted that the expected relative genotype frequencies (AA, Aa and aa) in an idealized, non-evolving population could be predicted by the equation:

    p2 + 2pq + q2

    in which

    The model assumes...

    If a population's relative genotype frequencies (at a particular locus) match those predicted by the HW equation, the population is NOT EVOLVING at that locus. It is said to be in Hardy Weinberg equilibrium with respect to that locus.

    If relative genotype frequencies are significantly different from the prediction, or if they change significantly over generations, then the population IS EVOLVING at that locus, and is NOT in Hardy-Weinberg equilibrium.

    Why do these numbers make sense?

  • If the dominant allele (A) is present in known frequency (p) in both eggs and sperm, then its likelihood of being inherited from both an egg and sperm by the next generation can be expressed by the Product Rule (The probability of two independent events happening is the product of their individual probabilities):

    p x p = p2
  • The same is true for the recessive allele:

    q x q = q2

  • And Product Rule also gives us the expected frequency of heterozygotes in the multiplied frequency of both alleles (p x q)(p x q):


    (Each offspring inherits two alleles for a given locus, and each of these two "inheritance events" will give either A or a).

    Hence, given any initial relative starting frequencies of A (p) and a (q), the three genotypes should be present in the predicted relative frequencies in the next generation: p2 : 2pq : q2 long as the five conditions are met:

    A Sample Hardy-Weinberg calculation Species: Sciurus carolinensis (Grey Squirrel)

    Gene Locus: A
    Alleles: A (dominant; wild type agouti fur) and a (recessive; melanistic black fur)

    We are studying a population of 1000 squirrels. Of these, 60 (60/1000, or 0.06) are melanistic.
    If each of these melanistic squirrels carries two recessive alleles, we can use this to calculate the expected frequency of q, since q2 is the frequency of the alleles in the homozygous recessive individuals.

  • The square root of q2 is equal to q. (duh)
    In our example, the square root of 0.06 = .25.

  • Since p + q = 1.0, you can now solve for p

  • 1 - 0.25 = 0.75

  • Our predicted frequencies, based on the assumption that the squirrel population is in HW equilibrium, are p = 0.75 and q = 0.25

  • plug these values into the HW equation to calculate expected relative genotype frequencies:

    0.752 + 2(0.75)(0.25) + 0.252

    This means that if our population of 1000 squirrels is in HW equilibrium, then

    (Notice that these three frequencies add up to 1.0, 100% of the 1000 squirrels in the population.)

    Heterozygosity and Deleterious Alleles

  • Can you think of reasons why it might be advantageous to be heterozygous, rather than homozygous for a particular trait?

  • Recall the example of Sickle Cell Anemia.

    Here's another example.

    The major histocompatibility complex The MHC, or Major Histocompatibility Complex is a relatively large gene family found in most vertebrates. The MHC genes encode MHC polypeptides, which are constructed into important players in the immune system.

    The significantly longer survival of MHC heterozygotes over MHC homozygotes is another bit of evidence that heterozygosity at certain gene loci can be adaptive. The more heterozygous the individual, the longer that patient staves off full-blown AIDS.

    This trend is seen not only in HIV+ human patients, but also in other species.

  • Hybrid individuals usually exhibit a high degree of heterozygosity, which results in HYBRID VIGOR.
  • The more closely related parents are (i.e. the more inbred the offspring), the LESS heterozygous their offspring are likely to be.

    While inbreeding does not always produce misfit offspring, it often does result in reduced vigor of the products of inbreeding.

    Reduction in biological (as opposed to evolutionary) fitness in an inbred organism is known as inbreeding depression.

    Humans' breeding of animals for specific traits has led to some striking examples of inbreeding depression.
    Meet Kenny the White Tiger.

    (click on pic)

    You might reasonably ask: