• Chapter 18 (Introduction to evolution; Macroevolution vs. microevolution)

• Chapter 20 (How species arise and how they become extinct)

Consider the same questions provided in lecture outlines 22 and 23. The answers to many of these "to review" and "to think about" questions are provided in the book's web link. Click here to get there. :)

• Evolution is a Fact -- If you haven't read this paper yet, do so.

• Observed Instances of Speciation -- There is A LOT of pertinent information here dealing with today's lecture.

• The Talk/Origins Archive. A treasure trove of information on evolution.

• Evolution Homepage from the University of Toronto; a wonderful link to tons of information.

• A brief and idiosyncratic history of the concept of evolution from Anthropology 101 at the University of Alabama. This tutorial provides a good review of the history of evolution as well as some of the illustrations and text we provide below.

• Evolution and Natural Selection

After studying this material you should be able to:

1. Distinguish between macroevolution and microevolution.

2. Explain the importance of geographic isolation in the formation of a species.

3. Explain reproductive isolation and how it is involved in the formation of a species.

4. Explain how speciation can occur within the same geographic region as the parental population.

Be able to define these terms but, more importantly, know the relationships among them.

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How does it differ from microevolution?

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This question is not readily answered, and many definitions exist.

• The Biological Species Concept is the definition Lewis uses in your text.

• Some Limitations to the Biological Species Concept (many more exist)

Speciation depends upon many interacting factors (e.g., nonrandom mating, migration, genetic drift [founder effect, population bottlenecks], mutations, and natural selection).

There are generally two major stages that lead to speciation:

• Geographic isolation of populations (so that members of the two newly formed groups cannot interact)

• Reproductive isolation of these populations (so that members of the two separated groups cannot reproduce successfully with each other if the geographic barrier is lifted)

Stages in the formation of a new species (from Grant, 1963 and 1981, and the University of Alabama). This is basically the same illustration as provided below, with more detail.

• Illustrative example of geographic speciation from the University of Alabama. This poor quality figure shows the separation of two populations by some geographic barrier. Subsequent divergence leads to the formation of different species; they species are reproductively isolated when that barrier is removed.

• Darwin's finches

• The Amazon Basin's incredible diversity of organisms (Earth Magazine; Aug. 1997)

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New species arise when genetic differences accumulate to the point when the two groups can no longer successfully mate and reproduce (if and when they come back into contact). Remember: species can be defined as groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups.

Speciation events lead to the multiplication and diversification of species into higher taxa (e.g., genera, families, orders, classes, phyla, etc.). All species (animals, plants, fungi, and all major groups of microorganisms) can be traced back to a single origin of life on earth. Evolution is a continuing process that explains the history of life on earth, as well as the diversity of life today.

1. Geographical Isolation

• "Islands in the water"

• "Islands of forest in a sea of grass"

• "Islands of coolness in a sea of heat"

2. Reproductive Isolation from the University of Alabama

• Premating (some of these differences are the result of single gene mutations)
  • Ecological (Habitat), Seasonal and Temporal Isolation

  • Mechanical and Behaviorial (or Ethological) Isolation

• Postmating
  • Gametic and Zygotic mortality

  • Hybrid Inviability and Reduced Fertility

Speciation can occur within the range of the parent population (and sometimes quite rapidly). Gene flow is disrupted by:

• Chromosomal abnormalities such as polyploidy
  • autopolyploid (extra chromosome sets from the SAME species)

  • allopolyploid (chromosome sets from two or more DIFFERENT species)

  These polyploids can self-fertilize or breed among themselves. Remember, in humans and other animals, polyploidy is lethal. In plants, polyploidy is quite common and has given rise to many new species. It is estimated that as many as 50% of extant flowering plant species have evolved via polyploidy.

• and chromosome incompatibility (see page 413, text)

• and choice of host plant or habitat.
  • The natural host of the American fruit fly is a hawthorn tree; however, some flies live in apple trees. By eating, courting, mating, and laying their eggs on different host plants, the two groups of flies have become reproductively isolated from one another and are on their way to becoming different species. Genetic differences between these groups have been measured.

  • In many other organisms, shifts to new host plants or habitats trigger phenotypic changes that lead to new species.

An Example:

Below is a classic example of speciation by hybridization and polyploidy. The species belong to the genus Spartina(cord grass).

Species Extinctions: Causes and Consequences from the World Resources Institute.

• Extinction: the inability of a species to adapt to a particular environmental challenge.

• The history of the earth is punctuated by several mass extinctions (see Table 20.3, text).

• As we discussed in the biodiversity lectures, the number of organisms on Earth is now being reduced at a rate 1,000-10,000 times higher than any time prior to the evolution of humans (that is, a few decades or centuries rather than millions of years).

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