Evolutionary Mechanisms

As we stated at the beginning of our discussion on evolutionary principles, evolution involves changes that occur in the frequency of a gene's alleles in a population from generation to generation. Each individual member of a population inherits a set of genes. He or she can not evolve or change the alleles inherited. But the contribution he or she makes to the population's gene pool through reproduction, relative to the contribution other members of the population make, can change the population's genetic composition from generation to generation. The collection of genes in a population is called the gene pool. For a population's gene pool to change, there must be some mechanism that promotes differential reproduction (or differential survival that is reflected in reproduction. When such change occurs, we have evolution.

To relate this to Darwin and Wallace, we can use the example of the single gene for the inheritance of flower color as discussed in your text. In a population of plants, there are two pre-existing alleles for flower color: purple and white. Purple is dominant. The two flower colors have been reproduced year after year. Then one year, a new predator happens into the environment. This predator sees the purple color as desirable and eats plants with purple flowers. The purple-flowered plants rarely get pollinated, since they are eaten before they can set seed. Within a few generations, only white flowers remain. The predator, as a grazer, has been the selection force behind the change in allele frequency for flower color in this plant.

We can see this and explain this today, because we know how genes and alleles are inherited. In the 1800's, they did not know this, and for about 50 years after Darwin's publications, scientists and others searched for mechanisms of evolution.

In the early 1900's Mendel's work was rediscovered by a number of researchers, who, at first, tried to use genetics to disprove Darwin's theory of gradual evolution through slow selection of beneficial characteristics. The "Mendel" group promoted the idea that changes would occur rapidly via mutations, and that natural selection had no role in changes that occurred in populations through time. It took many years to bring together genetics, population biology and natural selection as means of evolution.

The first real "breakthrough" came from the collaboration of a mathematician and a doctor in 1908, but as is common, took a while to gain acceptance.


The HARDY-WEINBERG principle for genetic equilibrium
Hardy and Weinberg demonstrated that the equation for a binomial expansion

(p2 + 2 pq + q2)

could be used to calculate gene frequencies within a population. They first showed the gene frequency for genetic equilibrium , the condition in which gene frequency would not change from generation to generation.

Genetic Equilibrium Formula 

Where:     p = frequency of 1 allele for the gene (A)
           q = frequency of the 2nd allele for the gene (a)
                          and:
                        p + q = 1

Where:    p2 = homozygous  (AA)
          q2 = homozygous alternative (aa)
         2pq = heterozygous (Aa)
                          and:
                    p2 + 2 pq + q2 = 1
  • The allele frequencies and genotype frequencies will be stable (genetic equilibrium) from generation to generation as shown by this equation.
  • In an ideal population there would be genetic equilibrium (no change in gene frequencies), and no evolution would occur.

    Hardy and Weinberg then proposed the conditions that would be needed in a population to have genetic equilibrium.

    The conditions needed for such genetic equilibrium are:

  • Population must be "infinitely" large (large enough to eliminate chance or random gene frequency fluctuations)
  • Population is isolated from other such populations (no immigration or emigration; no gene flow)
  • Mutation does not occur, or if mutation occurs, forward and reverse mutations are equal, so the gene pool is not modified
  • Mating is random
  • All genotypes are equally viable; natural selection is absent

    Any change in gene frequency from generation to generation can then be documented and we can look for the reasons or agents responsible for the change.

    As a result of the Hardy-Weinberg Equilibrium, biologists could search for the "agents" of evolution, or those factors that result in the change of gene frequency. You can see why evolution is now defined in genetic terms, since it is a biological phenomenon of population genetics.

    Based on their work, there are five major causes of change in populations:


    Agents of Evolution: Factors that bring about change
    1. Mutation
      • Inheritable changes in the DNA sequence
      • Can be induced for study
      • Original source for infinite numbers of small changes in genes

      Recombination
      • Natural occurrence in meiosis so that no two gametes are identical

    2. Gene Flow or Migration
      Migration is the flow of genes from one population of a species to another population. This is also called gene flow.

      Dispersal to new geographical areas

      • Fledging of young
      • Transport of pollen, spores, etc.

      Gene flow also maintains a gene pool over larger geographical areas with nomadic patterns of travel. Gene flow usually decreases genetic differences between populations by routinely adding and removing individuals.

      Gene flow tends to keep populations of species from varying too much by continually "mixing" the alleles of the species.

      In contrast, isolated gene pools are important factors in speciation, since they minimize gene flow.

    3. Random Drift and Small Populations A population can be subject to rapid and random changes which can be caused by chance events. When a population is large, chance events are less likely to impact the gene frequency (although over time they can). With small populations random events can have a much greater impact. Such random changes in gene frequencies are known as genetic or random drift.

      Characteristics of Genetic (Random) Drift

      • Rapid and random (chance) changes in gene frequencies of populations can result in a localized reduction in variation for that population.
      • More rapid when the gene pool is small and/or isolated
      • Genetic drift tends to reduce variation within one population, but increase genetic variation between different populations.

      Some Genetic Drift Methods

      • Population bottleneck
        Some catastrophic event causing a "bottleneck" (drastic reduction in size of a large population caused by some unfavorable condition). The few survivors' genotypes will be the source of the subsequent generations.

      • Founder Effect
        A small number of individuals disperse or move to an area isolated from the original population. The new population, with a small gene pool, will be established with a preponderance of a few genotypes. (like the bottleneck). The dispersed frequency will determine the character of the new population, which may differ significantly from the original population (which is how the founder effect differs from a bottleneck; in the bottleneck, the gene pool consists of the survivors from the original pool, rather than migrants). An example of this is the frequency of polydactyly in the Pennsylvania Dutch of the United States

    4. Non-Random Mating and Mate Selection
      For the Hardy Weinberg to work, each gamete produced by any male would have an equal chance of combining with any gamete made by any female (of that species). This occurs very rarely in populations. Differential reproduction is a significant part of the Darwin-Wallace theory, and mate selection. is an important part of natural selection.

      There are "bazillions" of examples of mate selection strategies, some of which will be discussed later, as well as species which use other means of non-random mating, such as those animals which are harem forming.

      Sexual (or Mate) Selection - A special case of Selection
      Sexual selection involves any trait (adaptation) that gives an individual a preferential advantage in mating. Such sexual selection is very important to non-random mating.

      • Some examples are:
      • Sexual dimorphism
      • Mating behaviors
          Competition for mates among harem animals
          Courtship Behavior (Display and Choice)
        One could give a course's worth of examples of mating behaviors
        Just a few are:
          Bower birds
          Gulls
          Mammal rutting behaviors for harem breeders

    5. Natural Selection (and selection, in general)
      In the Hardy-Weinberg equilibrium, all individuals must be equally adaptive in their environment for all of their genetic characteristics. In real populations this is not the case.
    How does Natural Selection Affect Population Patterns?
    There are three basic patterns that emerge in populations through time, as a result of selection pressures (or forces). They are:

    Directional Selection
     Stabilizing Selection
     Disruptive Selection
     Features (traits) that are subject to selection: Adaptations
    In general terms, adaptations are characteristics that help an individual survive and reproduce in its environment. The environment includes both the physical environment and the interactions with other organisms.

    This section overlaps with the study of ecology, the relationships of organisms to their surroundings. Many of the relationships of organisms deal with survival strategies: competition for resources, or predator and prey interactions, or symbiosis. A few examples will be discussed.

    Morphological Adaptations
    Shapes, patterns, size, colors
    Mechanical Protection

    Warning patterns
    "Protective Patterns" (A continuum of sophistication)
    Behavioral Adaptations
     Physiological Adaptations
     The Adaptive Strategy of Symbiosis
    In a symbiosis, two types of organisms become closely associated with each other so that survival depends on both organisms. Symbiosis can be
    Some examples of mutualisms are
    Some examples of commensal relationships are
    There are "thousands" of examples of parasitic relationships. For many humans, the debilitating effects of parasites reduces their survival so that common illnesses become life-threatening.

    It is obvious that we could discuss adaptations and successful survival mechanisms for many days, and by now we should be clear on how the selection of phenotypes which have beneficial adaptations for their surroundings can result in population changes from generation to generation, so that species can change over time.

    Remember, it's crucial to look at any adaptation in the context of its surroundings. Features with adaptive value in one habitat many be negative in a second.

    Extinction
    We have discussed a number of adaptations and given examples of successful adaptations. What about species which lack adaptations in their environment, and lose to those who are better adapted? Adaptations can affect not just populations of one species, but natural selection can also lead to the loss of a species by extinction.

    When we look at extinction, the most significant cause of extinction is change in habitat. Today we often discuss loss of habitat, but this loss is generally not a physical loss of geography, but a change in that habitat which results in an area no longer suitable for the individuals who originally lived there. Humans have done much in the past few centuries to alter habitat, to the detriment of thousands of species.

    Geographic changes and accompanying climate changes have also been responsible for loss of populations. Catastrophic geological events such as volcanic eruptions or massive earthquakes cause major environmental alterations. It is speculated that a meteor may be responsible for the extinction of dinosaurs, along with numbers of other organisms which lived in that era.



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