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The Chapter in Review

Macroevolution is evolution on a large scale because it considers the history of life on Earth. Macroevolution begins with speciation, the origin of new species. Without speciation and the extinction of species, life on Earth would not have a history.

Defining Species

The biological concept of species

  • recognizes a species by its inability to produce viable fertile offspring with members of another group.

  • is useful because species can look similar, and members of the same species can have different appearances.

  • has its limitations. Hybridization does occur between some species, and it is only relevant to extant * sexually reproducing organisms.

Reproductive Barriers

Prezygotic and postzygotic barriers keep species from reproducing with one another.

  • Prezygotic isolating mechanisms involve habitat isolation, temporal isolation, and behavioral isolation.

  • Postzygotic isolating mechanisms prevent hybrid offspring from developing or breeding if reproduction has been successful.

Models of Speciation

Allopatric speciation and sympatric speciation are two models of speciation.

  • The allopatric speciation model proposes that a geographic barrier keeps groups of populations apart. Meanwhile, prezygotic and postzygotic isolating mechanisms develop, and these prevent successful reproduction if these two groups come into future contact.

  • The sympatric speciation model proposes that a geographic barrier is not required for speciation to occur.

The Fossil Record

The fossil record, as outlined by the geological timescale, traces the history of life in broad terms. It has been possible to absolutely date fossils by using radioactive dating techniques.

The fossil record

  • can be used to support a gradualistic model: Two groups of organisms arise from an ancestral species and gradually become two different species.

  • also supports the punctuated equilibrium model: A period of equilibrium (no change) is interrupted by speciation within a relatively short period of time.

  • shows that at least five mass extinctions, including one significant mammalian extinction, have occurred during the history of life on Earth. Two major contributors to mass extinctions are the loss of habitats due to continental drift and the disastrous results from a meteorite impacting Earth.

  • Systematics is the study of the evolutionary relationships among all organisms, past and present. Systematics includes classification. In the Linnaean system of classification, every organism is assigned a scientific name, which immediately indicates its genus and specific epithet. Species are also assigned to a family, order, class, phylum, kingdom, and domain according to their molecular and structural similarities as well as evolutionary relationships to other species.

  • Phylogeny depicts the evolutionary history of a group of organisms. Systematics relies on the fossil record, homology, and molecular data to determine relationships among organisms.

  • Cladists use shared derived characters to construct cladograms. In a cladogram, a clade consisting of a common ancestor and all the species derived from that ancestor, the species have shared derived characteristics.

  • Linnaean classification has come under severe criticism because it does not always follow the principles of cladistics in the grouping of organisms.

Classification Systems
  • The three-domain system uses molecular data to designate three evolutionary domains: Bacteria, Archaea, and Eukarya:

  • Domains Bacteria and Archaea contain prokaryotes.

  • Domain Eukarya contains kingdoms for the protists, animals, fungi, and plants.

Key Terms
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Testing Yourself

Choose the best answer for each question.

  1. A biological species

    1. always looks different from other species.

    2. always has a different chromosome number from that of other species.

    3. is reproductively isolated from other species.

    4. never occupies the same niche as other species.

For questions 2–9, indicate the type of isolating mechanism described in each scenario.


  1. habitat isolation

  2. temporal isolation

  3. behavioral isolation

  4. mechanical isolation

  5. gamete isolation

  6. zygote mortality

  7. hybrid sterility

  8. low F2 fitness

  1. Males of one species do not recognize the courtship behaviors of females of another species.

  2. One species reproduces at a different time than another species.

  3. A cross between two species produces a zygote that always dies.

  4. Two species do not interbreed because they occupy different areas.

  5. A hybrid between two species produces gametes that are not viable.

  6. Two species of plants do not hybridize because they are visited by different pollinators.

  7. The sperm of one species cannot survive in the reproductive tract of another species.

  8. The offspring of two hybrid individuals exhibit poor vigor.

  9. Complete the following diagram illustrating allopatric speciation by using these phrases: genetic changes (used twice), geographic barrier, species 1, species 2, species 3.

  10. Transitional links are least likely to be found if evolution proceeds according to the

    1. gradualistic model.

    2. punctuated equilibrium model.

  11. Which is the scientific name of an organism?

    1. Rosa rugosa

    2. Rosa

    3. rugosa

    4. rugosa rugosa

    5. Both a and d are correct.

  12. Which of these statements best pertains to taxonomy? Species

    1. always have three-part names, such as Homo sapiens sapiens.

    2. are always reproductively isolated from other species.

    3. share ancestral traits, but may have their own unique derived traits.

    4. always look exactly alike.

    5. Both c and d are correct.

  13. Which of the following groups are domains? Choose more than one answer if correct.

    1. bacteria

    2. archaea

    3. eukarya

    4. animals

    5. plants

  14. Which pair is mismatched?

    1. homology—character similarity due to a common ancestor

    2. molecular data—DNA strands match

    3. fossil record—bones and teeth

    4. homology—functions always differ

    5. molecular data—DNA and RNA data

  15. One benefit of the fossil record is

    1. that hard parts are more likely to fossilize.

    2. that fossils can be dated.

    3. its completeness.

    4. that fossils congregate in one place.

    5. All of these are correct.

  16. The discovery of common ancestors in the fossil record, the presence of homologies, and molecular data similarities help scientists determine

    1. how to classify organisms.

    2. the proper cladogram.

    3. how to construct phylogenetic trees.

    4. how evolution occurred.

    5. All of these are correct.

  17. In cladistics,

    1. a clade must contain the common ancestor plus all its descendants.

    2. shared derived characters help construct cladograms.

    3. the principle of parsimony states that the simpler hypothesis is preferred.

    4. the species in a clade share homologous structures.

    5. All of these are correct.

  18. Answer these questions about the following cladogram.

    1. This cladogram contains how many clades? How are they designated in the diagram?

    2. What character is shared by all animals in the study group? What characters are shared by only snakes and lizards?

    3. Which animals share the most recent common ancestor? How do you know?

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  20. Allopatric, but not sympatric, speciation requires

    1. reproductive isolation.

    2. geographic isolation.

    3. spontaneous differences in males and females.

    4. prior hybridization.

    5. rapid rate of mutation.

  21. Which kingdom is mismatched?

    1. Protista—domain Bacteria

    2. Protista—single-celled algae

    3. Plantae—flowers and mosses

    4. Animalia—arthropods and humans

    5. Fungi—molds and mushrooms

  22. Many new species evolving in various environments from a common ancestor is called

    1. cladistics.

    2. the gradualistic model of evolution.

    3. adaptive radiation.

    4. convergent evolution.

    5. the PhyloCode.

Thinking Scientifically
  1. Using as many terms as necessary (from both X and Y axes), fill in the proposed phylogenetic tree for vascular plants.

  2. The Hawaiian Islands are located thousands of kilometers from any mainland. Each island arose from the sea bottom and was colonized by plants and animals that drifted in on ocean currents or winds. Each island has a unique environment in which its inhabitants have evolved. Consequently, most of the plant and animal species on the islands do not exist anywhere else in the world.

    In contrast, on the islands of the Florida Keys, there are no unique or indigenous species. All of the species on those islands also exist on the mainland. Suggest an explanation for these two different patterns of speciation.

Bioethical Issue
Funding for Phylogenies

Reconstructing evolutionary relationships can have its benefits. For example, an emerging virus is apt to jump to a related species rather than to an unrelated species. We now know, for example, that the HIV virus jumped from the chimpanzee—with whom we share 95% of our DNA sequence—to humans. Chimpanzees don't become ill from the virus; therefore, studying their immune system might help us develop strategies to combat AIDS in humans.

Cladists want to acquire more information about how genetic and environmental factors influence bone development. Recently, cladograms based on bone shape and arrangement do not match up well with those based on DNA base-pair sequencing. If we knew more about bone development under different environmental conditions, we might be able to discover the reason why they don't match and develop evolutionary (phylogenetic) trees in which all have confidence. To acquire the necessary data could cost millions of dollars of research funding.

Should the public be willing to fund all types of research or only research that has immediate medical benefits, as was described for HIV research? Would you be willing to fund research that helps us understand our evolutionary past, even if the medical benefit is not immediately known?

Essentials of Biology Website

The companion website for Essentials of Biology provides a wealth of information organized and integrated by chapter. You will find practice tests, animations, videos, and much more that will complement your learning and understanding of general biology.