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5. NO NATURAL SELECTION:

This means that all genotypes have an equal probability of leaving offspring to future generations. With respect to the particular trait you are studying (in our old example, the color of wing covers in beetles), AA, Aa, and aa are all equally adaptive genotypes.

Note that any heritable trait may be:

The bottom line of evolution by natural selection is that the individuals in a population that leave the most offspring are, by definition, the most "EVOLUTIONARILY FIT." And that term may not be what you've been told it is.

The Darwinian Revolution:
On the Origin of Species by Means of Natural Selection

First, let's discuss Darwin's revolutionary theory: Evolution by means of Natural Selection.

This type of evolution cannot truly be considered random change. It is, in a sense, "directed" change in gene frequencies due to the interaction of individuals in a population with their environment. Those individuals best suited to exploiting the various factors of the environment will, by definition, leave more genes to succeeding generations than their conspecifics (i.e., members of the same species).

NOTE THAT THIS DOES NOT MEAN THAT EVOLUTION HAS A "GOAL" OR THAT THERE IS A "MOST HIGHLY EVOLVED SPECIES." Evolution is not directional, nor does it have a value system.

Bottom line: in the game of natural selection, organisms do not compete against their predators or parasites or pathogens. They compete against EACH OTHER. (Recall the story of the bear!) And the organisms best suited to leave the most offspring in a given environment are the "winners" of that round of natural selection.

Darwin's four tenets of natural selection can be distilled down into FOUR MAIN IDEAS PERTINENT TO THE POPULATION GENETICIST...

Evolution via natural selection can occur only if there is variation in the population to begin with.

NATURAL SELECTION IS PROBABLY THE DOMINANT FORCE IN THE EVOLUTION OF SPECIES. IT IS NOT DIRECTIONAL, AND IT IS NOT AN INEXORABLE MARCH TO AN "IDEAL PINNACLE SPECIES" (which is most often defined as Homo sapiens by people who haven't a clue about biological realities...)


An individual's DARWINIAN (EVOLUTIONARY) FITNESS is a measure of the proportion of genes it contributes to succeeding generations. Nothing more, nothing less. Evolutionary fitness is defined by the environment. A phenotype which confers great fitness today in a particular environment could be a real liability if the environment changes tomorrow!

  • Fitness = W (= adaptive value of a particular genotype)
  • The genotype that produces the most offspring in a given population is said to have a fitness of 1.0. All other genotypes' W value is measured relative to the most successful genotype's W.
  • If you have three genotypes, AA, AA' and A'A' and over their lifetimes, AA genotypes produce an average of 10 offspring, AA' genotypes produce an average of 5 offspring and A'A' genotypes produce an average of 2 offspring, then...
    Care must be taken to carefully assign "fitness". An organism that produces the most eggs won't necessarily have the most offspring reared to reproductive maturity!

    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:
    AA: s = 1 - 1 = 0
    AA': s = 1 - 0.5 = 0.5
    A'A': s = 1 - 0.2 = 0.8
    Selection pressure is highest against the A'A' genotype, relative to the others.

    COMPONENTS OF EVOLUTIONARY FITNESS

    Natural selection can operate at any stage of an organism's life cycle, but usually in one of four ways...

  • zygotic selection (a.k.a. viability selection) - differential survival of genotypes, whether at the prenatal, juvenile or adult stage.
  • gametic selection (a.k.a. segregation distortion or meiotic drive) - the gametes of a heterozygote have differential success because of their genotypes. That is, one allele becomes part of a successful fertilization than the other allele.
  • sexual selection - organisms of a particular genotype mate more often than those of a different genotype (this usually occurs where there is competition for mates; you can no doubt think of plenty of examples.).
  • fecundity selection - one genotype is more fertile than other genotypes.

    EFFECTS OF NATURAL SELECTION

    At the start of a "selection cycle" the population is usually made up of individuals which express 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 selection: individuals at the average point on the curve are at a selective disadvantage; individuals with either extreme have a reproductive advantage.

    Example: Geospiza conirostris (Galapagos Cactus Finch)

    In drought periods, the birds don't have a wide variety of foods, and must resort to one of several feeding modes:

    In wet years, there's plenty of food everywhere, and birds with intermediate bill sizes can survive. But in drought, only the birds with one of the three bill sizes above can feed effectively. Disruptive selection ensues, and the population eventually is composed of individuals with

    1. deep, strong bills

    2. large, heavy bills

    3. very long bills.

    This production of distinct phenotypes in a population due to selective pressure is known as CHARACTER DISPLACEMENT.

    (a divergence of an equivalent character in a sympatric species (i.e, living in a single geographic area) due to competition for a resource. In this case, the resource is food.)

    There are considered to be THREE GENERAL MECHANISMS FOR SPECIATION

  • allopatric speciation
  • parapatric speciation
  • sympatric speciation

    Which can be diagrammed for ease of understanding like SO.

    Now that we've considered all of this, remember that...

  • Genetic variability in a population arises from many alleles at many different loci.

  • Evolution may be occuring with respect to one gene, and not others. And that's how populations change.

  • Ultimately, small changes in allele frequencies over time and non-lethal mutations may accumulate and result in reproductive isolation, and a new species is born!