Biology 100/101
Lecture 25
Macroevolution: Speciation
(Print Version)

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Objectives

Web Resources

What is Macroevolution?

Evolution is a Fact

What is a Species?

How do New Species Arise?

Geographic Isolation

Geographic Speciation

Reproductive Isolation

Sympatric Speciation

Example of Speciation

Speciation and Time

Extinction

Lecture Syllabus

IB 100/101 Home Page

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Text Readings in
Lewis et al.		 Testing Your Knowledge Thinking Scientifically
Ch. 16, Speciation and Extinction Pg. 318, Questions 1, 4-6 Pg. 319, Questions 1-2

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 this chapter 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 Steve and Ed" discussion.

Objectives:

After studying this material you should be able to:

  1. Distinguish between macroevolution and microevolution.

  2. Explain why evolution is considered both a fact and a scientific theory.

  3. Discuss the limitations of the biological species concept, and why a species definition is important.

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

  5. Explain the concept of geographic isolation in terms of islands and barriers.

  6. Explain the different ways reproductive isolation can occur and provide examples.

  7. Explain how reproductive isolation is involved in the formation of a species.

  8. Distinguish among allopatric, parapatric, and sympatric speciation.

  9. Explain how speciation can occur within the same geographic region as the parental population (without geographic isolation).

  10. Describe what a phylogeny is.

  11. Know these terms and the relationships among them:

    12. species macroevolution natural selection
    geographic isolation reproductive isolation ecological isolation
    temporal isolation mechanical isolation behavioral isolation
    prezygotic reproductive isolation postzygotic reproductive isolation polyploid
    speciation theory hybridization
    adaptive radiation phylogeny mass extinction
    allopatric speciation parapatric speciation sympatric speciation

Web Resources:

What is Macroevolution?

  • The process by which new species are produced from earlier species (speciation). It also involves processes leading to the extinction of species.

  • Occurs at the level of the species or above.

  • Such changes span long periods of time (but can also happen rapidly).

  • Examples of macroevolution include: the origin of eukaryotic forms of life; the origin of humans; the origin of eukaryotic cells; and extinction of the dinosaurs.

  • In contrast, microevolution, involves evolutionary change at the level of the population, and is defined by changes in allele frequency within the population over time. Such changes take place over relatively short time periods. Accumulated gradual changes in two populations that preclude their interbreeding may lead to the formation of a new species.

    • Evolution as a Fact and a Theory

    Evolution--the process by which the genetic composition of a population changes over time--is a FACT.

    • This process is all that is required to produce the diversity and similarity of all life on this planet today.

    • Evolution has occurred; it still is occurring; it has been directly observed, documented, demonstrated, and described. Supporting evidence for it is overwhelming (and obtained from a wide range of scientific fields).

    The mechanisms by which evolution occurs (e.g., natural selection, mutation, genetic drift) are presented as SCIENTIFIC THEORIES.

    • Several theories have been proposed and debated. It is far from clear how evolution proceeds in every detail.

    Recall Lecture One, Science as a Way of Knowing the Natural World. Once a hypothesis has been supported by many experiments and/or observations it is considered by the community of scientists to be a theory. (Note that this is very different from the common use of the word, meaning an opinion or a guess.)

    In summary, Darwin established the FACT of evolution, and proposed a THEORY, natural selection, to explain the mechanism of evolution.

    What is a Species?

    This question is not readily answered, and many definitions exist. Historically, the concept has changed considerably.

    • The Biological Species Concept is the definition Lewis et al. use in your text.

      • Species are groups of actually or potentially interbreeding natural populations (i.e., producing fertile offspring) that are reproductively isolated from other such groups.

      • Defines species in terms of a gene pool isolated from other similar gene pools.
    • The biological species concept and some recently proposed alternatives. Another definition of species (the Phylogenetic Species Concept) describes species as being an "irreducible cluster of organisms that is diagnosably distinct."

    • Some Limitations to the Biological Species Concept (others exist)

      • Restricted to sexually reproducing organisms, so it does not apply to single celled organisms that reproduce by simple cell division (mitosis).

      • Some species are incredibly variable; different species can be virtually identical.

      • Does not fit many plant species (hybridization).

      • No clear application to the fossil record, since reproductive isolation does not show up in fossilized materials.
    • DNA sequence comparisons can be used to assess relatedness of organisms, and is useful in assigning organisms to species when no other information is available.

    • Our definitions of species can change as our ability to detect differences among types of organisms changes.

    How Do New Species Arise?

    Generally, there are two major stages that lead to speciation (the formation of new species):

    • 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).

    The key to understanding the formation of new species is understanding how a population becomes reproductively isolated from other populations of the same species. Think of this as the isolation of gene pools (all the genes and their alleles in a population).

    If populations are not reproductively isolated, gene flow between the populations maintains their genetic similarity and they maintain the ability to interbreed, so new species do not form.

    Populations that are not reproductively isolated may change genetically over time--they evolve--but those changes are shared among the populations by gene flow so they remain as one species.

    Geographic Isolation

    Members of two newly formed populations cannot interbreed because they are geographically separated.

    Think of this as the concept of islands and barriers:

    • Islands of land in a sea of water
    • Islands of water in a sea of land
    • Islands of trees in a sea of grass
    • Islands of coolness in a sea of heat
    • Islands of warmth in a sea of cold
    • Islands of nature in a sea of humanity
    • Mountains as barriers
    • Rivers and canyons as barriers

    See opening discussion in Chapter 16, "Islands provide windows on evolution," for examples.

    Geographic Speciation and the Divergence of Populations

  • Stages in the formation of a new species (from Grant, 1963 and 1981, and the University of Alabama). This link leads to basically the same illustration as provided below, but has 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. The species are reproductively isolated when that barrier is removed.

  • If a population should become divided into two by a geographic barrier (or if some individuals are transported to a new area outside the parent population's range), evolution of each new population continues independently due to the forces of natural selection, genetic drift, migration, nonrandom mating, and mutation. With time, genetic differences between the two populations gradually accumulate. These genetic differences may result in different reproductive strategies, leading to the reproductive isolation of the populations.

  • Microevolution becomes macroevolution once a population divides and sufficient genetic divergence between the groups occurs so that if they once again come in contact, they could no longer produce fertile offspring (i.e., they are different, yet closely related species).

    • Allopatric speciation: The formation of new species when two populations are physically separated by a geographic barrier, such as this illustration of white and brown tamarin monkey populations on different sides of the Amazon River (Figure 16.5, text).

    • Parapatric speciation: The formation of a new species when populations inhabit neighboring areas but mate mostly among themselves, such as seen in these tropical little greenbul birds (Figure 16.6, text).

    Reproductive Isolation

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

    For new species to form, reproductive isolation is necessary. Genetic changes incurred lead to a variety of isolating mechanisms. Some of these differences are the result of single gene mutations.

    Reproductive Isolating Mechanisms: (from text, Fig. 16.4)

  • Premating or Prezygotic (isolating mechanisms that prevent the union of gametes; it occurs before or during fertilization)
    • Ecological (Habitat) Isolation. The two populations require different habitats.
    • Seasonal and Temporal Isolation. Active or fertile at different times of year or day.
    • Mechanical Isolation. Mating organs do not fit or are adapted for different pollinators.
    • Behaviorial (or Ethological) Isolation. Different behaviors or activities for mate selection.
    • Gametic Isolation. Gametes cannot combine.
    • Chromosomal Isolation. Chromosomes cannot pair.
  • Postmating or Postzygotic (mechanisms that reduce the viability or fertility of hybrid offspring)
    • Hybrid Inviability. Gametes combine, but development cannot produce a viable embryo.
    • Hybrid Infertility. Offspring lack the ability to make or deliver viable gametes. (Horse X Donkey = Sterile Mule, due to different chromosome numbers of parents; mitosis occurs normally, but meiosis is impossible)

    Sympatric Speciation

  • Geographic isolation is NOT always necessary for speciation to occur.

  • 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 through hybridization)

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

    • Chromosome incompatibility (see discussion on sunflower speciation in text chapter).

    • Choice of host plant or habitat (the utilization of different resources)

      • 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 can be measured.

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

    An Example of Speciation by Hybridization and Polyploidy

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

    Many of the crops we use for food are the result of hybridization and polyploidy.

    Speciation and Time

    An evolutionary tree (or phylogeny), depicting rates and times of speciation and extinction events. Evolution occurs in a branching pattern, with one species giving rise to others as they occupy and adapt to new habitats. They descend from an original ancestral form, much as the branches on a tree arise from the same trunk.

    A phylogeny depicts species' relationships based on descent from shared ancestors.

    Adaptive Radiation: the divergence of several new types of organisms from a single ancestral type. When a population faces an environment with abundant and diverse resources (such as the opening up of many new habitats), a burst of speciation can occur if members of a population inherit a structure or ability that gives them an advantage.

    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.

    Species Extinctions

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

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

  • Decreased genetic diversity may lead to extinction of populations and, eventually, the species.

  • The history of the earth is punctuated by several mass extinctions (see Table 16.1, text). Mass extinctions have periodically opened up vast new areas for adaptive radiation to occur.

  • As mentioned 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)