Biology 100/101
Lectures 23 and 24
Microevolution: The Forces of Evolutionary Change
(Print Version)

Announcements

Objectives

Web Resources

Lecture Activity

What is Evolution?

Darwin's Ideas

Artificial Selection

Examples of Microevolution

Hardy-Weinberg

Conditions causing Change

Nonrandom Mating

Migration

Genetic Drift

Mutation

Natural Selection

Balanced Polymorphism

Lecture Syllabus

IB 100/101 Home Page

Announcements

Text Readings in
Lewis et al.		 Testing Your Knowledge Thinking Scientifically
Ch. 14, The Evolution of Evolutionary Thought
Ch. 15, The Forces of Evolutionary Change		 Pg. 283, Questions 2, 4, 6
Pg. 298, Questions 1, 5, 6		 Pg. 283, Questions 1, 4, 6
Pg. 298, Questions 1-3

Much of the material cited in this lecture outline came from your textbook (Lewis et al., 2004, Life Fifth Edition). It is highly beneficial to read these chapters carefully before your final exam.

Answers to many of these questions can be found at the Text On-Line Learning Center

You may also ask questions and see answers to your classmates' questions in Web Crossing in the "Talk to Jim, and Ed" discussion.

Objectives:

After studying this material you should be able to:

  1. Describe biological evolution in terms of change in allele frequency in a population.

  2. Explain Darwin's main ideas concerning evolution by natural selection.

  3. Describe what is meant by artificial selection and how it compares to natural selection.

  4. Explain the factors that must be in play for a population to exhibit no change in allele frequencies over several generations.

  5. Explain how genetic drift (including bottleneck events and founder effects) and gene flow via migration could change allele frequencies of a population.

  6. Describe the evolutionary mechanism leading to the rise of antibiotic resistant strains of bacteria, or the development of heavy metal-tolerant invertebrates in Foundry Cove, NY, or industrial melanism.

  7. Describe an example in which natural selection has affected the virulence and/or spread of a human disease.

  8. Describe the results of directional, disruptive, and stabilizing selection.

  9. Explain the concept of balanced polymorphism and give an example of it.

  10. Understand the relationships among these terms:

    11. biological evolution artificial selection natural selection
    microevolution macroevolution allele frequencies
    directional selection stabilizing selection disruptive selection
    genetic variation balanced polymorphisms mutation
    genetic drift Hardy-Weinberg equilibrium bottleneck
    founder effect gene flow random mating

Web Resources:

Lecture Activity

  1. Get together in small groups.
  2. Each person print and sign your name on one piece of paper. Also include your TA's name(s) and/or section(s).
  3. Discuss and answer (briefly!) the following questions:

    What is (biological) evolution?

    What factors act to increase genetic variation in a natural population?

    What factors act to decrease genetic variation in a natural population?

    Give one example of evolutionary change.

What is Evolution?

  • Evolution is genetic change in a population over time. Specifically, a change in allele frequency (percentage) from one generation to the next. This has been called microevolution, and such changes can take place over a relatively short time periods.

  • An individual cannot change his or her alleles. So, the target of evolution is the population. Recall, a population is "A group of interbreeding organisms living in the same area."

  • Macroevolution, the subject of our last set of lectures, represents accumulated changes in allele frequency in two populations that preclude their interbreeding leading to the formation of new species (or their extinction). Often, macroevolutionary changes tend to span very long time periods.

  • The "raw material" of evolution is inherited variation (through sexual reproduction and mutation, with the latter having potential to introduce new alleles/traits into the population).

  • All the genes and their alleles in a population constitute the population's gene pool.

  • The proportion of different alleles for each gene determines the characteristics of that population.

  • Changing allele frequencies (percentages) within populations over time "are the small steps of change that collectively drive evolution."

  • Many factors can alter allele frequencies.

  • Evolution is an ongoing process.

    • Darwin's Main Ideas on Evolution

    or, Natural Selection as the Mechanism for Biological Evolution

    Darwin's Finches (14 of them, each slightly different, on islands that differ slightly in habitat).
    Fig. 14.5, Finch beak shape reflects natural selection.

    This is a pulitzer-prize winning book on Darwin's finches: evolution in our time. A Christmas present??

    Adaptive radiation--the divergence of several new types of organisms from a single ancestral type.

    Table 14.1, Darwin's Main Ideas (Listed below)

    Darwin's Observations of Nature

    • Organisms vary and some variations are inherited. Within a species, no two individuals are exactly alike

    • More individuals are born than survive to reproduce

    • Individuals compete with one another for limited resources that enable them to survive

    Darwin's Inferences Based on these Observations

    • Within populations, the inherited traits of some individuals make them more able to survive and reproduce than others under certain environmental conditions

    • As a result of the environment's selection against nonadaptive traits, only individuals with adaptive traits live long enough to transmit traits to the next generation. Individuals with adaptive traits are more likely to reproduce and increase the frequency of adaptive alleles in the population. Over time, natural selection can change the characteristics of populations (and even mold new species)

    • Lewis et al. (page 935) define natural selection as "the differential survival and reproduction of organisms whose genetic traits better adapt them to a particular environment"

    Artificial Selection: Proof of the Power of Selection

  • Also called selective breeding, domestication, or selection by humans.

  • The process of intentional or unintentional modification of a species through human actions which encourage the breeding of certain inherited traits over others. The breeding potential of individuals who possessed desirable characteristics is intentionally encouraged, whereas the breeding of individuals with less desirable characteristics is discouraged.

  • The many breeds of domestic dogs and cats existing today are a consequence of artificial selection. Darwin raised pigeons and artificially selected several new breeds.

  • The same process can occur naturally. "Individuals in the wild who possess characteristics that enhance their prospects for having offspring would then undergo a similar process of change over time; although in this case "desirable" characteristics would be not those which specifically satisfy human needs, but those which enhance survivability. This natural process forms the basis of Darwinian evolution." From Wikipedia, the Free Encyclopedia.

  • Artificial selection underscores the power of selection in generating evolutionary change.

    • Some Examples of Microevolutionary Change

    1. Resistance to Cadmium. Evolution in a polluted river.

    2. Industrial Melanism

    3. Epidemiology

    When does microevolution NOT occur?

    Only when there is no Change in Allele Frequencies (from one generation to another)

  • This is only possible if:
    • the mating population is large
    • mating is entirely random
    • there is no migration
    • there is no mutation
    • there is no natural selection

  • If allele frequencies remain unchanged, then evolution IS NOT occurring.

  • Hardy-Weinberg Equilibrium is a theoretical state in which the allele frequencies of a population remain constant over many generations. It serves as a basis of comparision to reveal when evolution is occurring.

  • The conditions necessary for Hardy-Weinberg Equilibrium are rarely, if ever, met in natural populations.

  • If allele frequencies change from one generation to the next, which they do, evolution IS occurring.

    • Conditions that Cause Evolutionary Change in Natural Populations

    Microevolution occurs when the frequency of an allele in a population changes. This may happen through:

    1. Nonrandom mating

    2. Migration

    3. Genetic drift

    4. Mutation

    5. Natural selection

    Nonrandom Mating

  • Completely random matings (where each individual has as equal chance of mating with every other member of the population) are nearly impossible to achieve.

  • Individuals seek mates within similar subpopulations within a larger population.

  • Isolated populations, such as those on islands, have no choice but to mate among themselves.

  • Sexual selection: the natural selection of traits that increase an individual's reproductive success. These traits contribute to attraction, courtship, or mating. Most species exhibit some sort of preferences in mate choice; the alleles for these desired traits will become more common in future generations.

      • Examples: Stag's anthlers; rhinoceros's horn; male insect's genitalia; elaborate feathers in male birds; courtship songs of birds.

  • Figure 15.6b, Nonrandom mating

    • Migration

  • Individuals migrate between populations.

  • Immigrating individuals introduce new alleles.

  • Emigrating individuals remove alleles.

  • Gene Flow is the movement of alleles from population to population.

  • Migration is very common in natural populations.

  • Any advantage given to members with new alleles will change the population due to natural selection.

  • Because geographic barriers greatly influence migration patterns, allele frequencies may differ between adjacent but separated geographic regions.

  • Figure 15.6c, Migration

    • Genetic Drift

  • Occurs within a small group of individuals when it is separated from a larger population and establishes a new gene pool.

  • Changes in allele frequencies in the isolated, smaller population are random and unpredictable.

  • A new population is formed from a subset of genotypes in the original population.

    • Population Bottleneck: A type of genetic drift occurring when many members of a population die, and a few remaining individuals mate, eventually restoring their numbers. The new population has lost much of the genetic diversity that was present in the larger ancestral population.

    Founder Effect: A type of genetic drift resulting in the establishment of a new, geographically isolated population from a single or very few individuals. It is very unlikely that the gene pool of a founding population is representative of the original population.

      A solitary penguin

      Figure 15.2a, The Founder Effect

      Ellis-van Creveld Syndrome. Small groups of people founding new settlements may have different allele frequencies than the original population, and may also have higher incidents of certain traits (such as genetic disorders) because they marry within the group.

      Ellis-van Creveld is an autosomal recessive disease and occurs in 7% of the people in the Amish community of Lancaster County, Pennsylvania. The occurrence of the disease is high because these Amish marry among themselves. See page 291, text, for more information.

    Mutation

  • Review our lecture on mutations. Remember, most mutations are neither beneficial nor useful, with no effect on phenotype. Some are harmful, resulting in defects in protein production that can lead to disease.

  • Sickle cell disease results from a single base change (See Figure 13.17, in Lewis et al., page 256) and Hemoglobin mutants.

  • Mutations introduce new alleles (new traits) into a population by altering old alleles.

  • Figure 15.6e, Mutation

  • "The genetic makeup of populations, and ultimately species, changes as natural selection permits differential survival of variants that are adapted to a particular environment."

  • Other examples: A random mutation in the DNA of one bacterium that confers antibiotic resistance will permit that bacterium to live and reproduce. Eventually, all bacteria without the mutation die, and the mutant offspring thrive and reproduce. Mutations in the genes that encode certain receptors on T cells in humans protect against HIV infection.

    • Natural Selection

  • Natural Selection is "the differential survival and/or reproductive success of individuals with particular genotypes in response to environmental challenges." (Lewis et al., page 270)

      • If a population contains variation, and
      if the variation is at least partly heritable, and
      if some variants survive to reproduce at higher rates than others,
      then the population will evolve. (EvoDots Tutorial, Jon C. Herron 2002)

  • Simply, some phenotypes are better adapted to a particular environment than others. Natural selection favors some phenotypes and the alleles that produce them and removes others from the population. Therefore, allele frequencies will change in response to environmental change.

  • Two fundamental forces are operating: genetic variation and environmental change. Both are constantly occurring at random in every natural population. Those with more adaptive traits survive in the new environment.

  • Natural selection reflects adaptation to a prevailing environmental condition. The direction of natural selection can change. A phenotype that is adaptive in one set of circumstances may be a liability in another.

  • Over time, the population would change so that it could no longer breed with the original group. Eventually, a new species would arise.

  • Figure 15.6f, Natural Selection

    • Types of Natural Selection:

    • Figure 15.4a, Directional Selection. This is selection against one extreme phenotype, allowing another to gradually become more prevalent. An example of this is industrial melanism, where some 100 species of insects have undergone color changes enabling them to blend into polluted backgrounds. Another example is the rise of antibiotic resistance.

    • Figure 15.4b, Disruptive Selection. Two extreme expressions of a trait each have a selective advantage, so both persist. An example is white and tan snails living among white barnacles on tan colored rocks; green colored snails are more often seen and eaten by predatory shorebirds.

    • Figure 15.4c, Stabilizing Selection. Selection is for an intermediate form of a trait, as it has greater survival and reproductive success. Extreme phenotypes are less adaptive.

    Balanced Polymorphisms

  • Is a form of stablizing selection that maintains deleterious recessive alleles because heterozygotes are protected against another medical condition.

  • Maintains a potentially lethal genetic disease in a population even though the illness diminishes the fitness of affected individuals.

  • The inherited disease persists because carriers (heterozygotes) have some health advantage over those who are homozygous dominant (and don't have the disease).