Checklist Week 8: The Origin of Species and the History of Life on Earth

 

In Part A of this week’s module, you will explore definitions of species, and what mechanisms contribute to the formation of new species. The definition of species is often relative to the sub-discipline of biology involved. Historically, the concept of a biological species has been widely accepted as a group of organisms able to interbreed and produce viable offspring. This is often referred to as reproductive isolation. Reproductive isolation can occur before or after fertilization occurs, in the forms of prezygotic and postzygotic barriers, respectively. There are limitations to this concept, however, as it does not apply to organisms reproducing asexually, nor does it address organisms only known through the fossil record. To address this limitation, other species concepts that are based on morphology or genetics are also used.

Once speciesis defined, you will begin to investigate the mechanisms that contribute to species formation. One such mechanism is geographic separation, where two populations of the same parent population are physically separated, and one or both populations undergo evolutionary changes. If the populations are reunited and are unable to successfully mate, they are deemed separate species. This is a form of allopatric speciation, where physical separation has occurred.

Speciation can also occur when sub-populations still remain in contact: this is referred to as sympatric speciation. Mechanisms that contribute to sympatric speciation include sexual selection (mate choice), habitat shifts, and polyploidy(change in chromosome number).

Where divergent species overlap, mating can occur, producing hybrids of mixed ancestry. Resulting hybrid offspring can have a variety of effects on the species’. They may reinforce prezygotic barriers if they are unfit (are not reproductively viable); they may fuse the parental species’ gene pools (reversing the speciation process); or, they may form a stable hybrid zone, not directly affecting either parental population.

The rate at which new species form is variable, and can occur over anywhere from a few thousand years to tens of millions of years. Recent developments in genetics are providing clues to why there is such a range in rates. This has allowed researchers to identify specific genes involved in speciation, demonstrating that two species may differ by just a single gene alteration in some cases, or by many gene alterations in others.

In Part B of this week’s module you will be introduced to the history of life on Earth. Key events in the 4.6 billion year history of our planet have enabled life to form, diversify and flourish. Though the early formation of organic molecules into living cells is not well understood, the fossil record and modern living cells together provide many clues to how those simple unicellular prokaryotes may have evolved into the plethora of organisms that populate the Earth today.

Conditions on Earth during the first billion years of our planet’s existence were not conducive to life as we know it. It wasn’t until the presence of oxygen-producing prokaryotes appeared--perhaps 3.5 billion years ago—that the atmosphere began to change. Oxygen levels were increased further over this period, likely owing to the evolution of eukaryotic cells containing chloroplasts. The resulting massive increase in atmospheric oxygen was not a positive change for many of the prokaryotes, as oxygen readily breaks down bonds and damages unprotected cells. Those prokaryotes that survived likely did so only in the anaerobic habitats where we continue to find their descendents today. Other cells adapted by means of cellular respiration.

The next landmark in evolution was multicellularity (~1.5 billion years ago). Soft bodied, multicellular organisms flourished as an extensive ice age ended about 565 million years ago. Thirty million years later, a sudden burst in evolutionary change occurred, coined “The Cambrian Explosion.” Prior to this, there was little record of predator-prey interaction, but during this time, organisms in the fossil record suddenly show evidence of claws and armament.

Land was colonized about 500 million years ago, and is associated with adaptations that allowed organisms to prevent dehydration, and to reproduce on land. Such adaptations include vascular structures in plants, and waxy coatings on leaves to retain water.

The fossil record has also provided evidence of continental drift— the moving of continents that float on the Earth’s crust across its mantel. This movement of land masses, as well as the other consequences of geologic structures such as mountains and islands, have had a substantial effect on speciation and evolution. Labrador, on the east coast of Canada, was at one time positioned at the equator. Imagine the change in environments as the supercontinent Pangaea separated and the continents moved to their present day locations!

The species we observe today are not only a result of moving land masses. Mass extinctionsby means of large environmental disruptions have also shaped present day species. When large numbers of species are removed, ecological communities collapse. This changes the course of evolution. As gaps in niches occur, opportunities arise in which new groups of organisms can rise to prominence. Adaptive radiations are periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different niches. Evidence of adaptive radiation has occurred after each of the five mass extinctions identified in the fossil record.

 

Part A

Learning objectives

 

At the end of this module you should be able to:

1. Define the biological species concept, and discuss how and why it differs from other species concepts. Understand the limitations of each.

2. Discuss the various prezygotic and postzygotic reproductive barriers.

3. Discuss various mechanisms of speciation.

4. Understand the effects hybrids may have on populations and species.

5. Describe examples that illustrate rapid and gradual speciation events.

6. Explain how a small number of genetic changes may lead to speciation in plants and animals.

Check List

• Read Chapter 24: The Origin of Species, in Campbell and Reece’s Biology, 8^th Ed.

• As you are reading, address each of the learning objectives listed above.

• Make flash cards for the terminology list provided. This will be beneficial for studying for the midterm and final exams later in the semester.

• Review the Power Point Lecture “Bio 102 Lecture 8a: The Origin of Species”

• Discussion Post:

  • Choose a species that possesses what you consider “unusual” features. Research and summarize how these features make this species particularly well adapted to its environment. Hypothesize how this phenotype might be affected if the environment were to change suddenly (i.e. from dry to wet, or hot to cold). Do you think it is best to be a generalist (well adapted to a variety of environments) or a specialist (well adapted to a particular niche)? Post your response and read and comment on two of your classmates’ posts.

• For extra practice, try the Self Quiz or Practice Test on the Mastering Biology Website. To log onto the website, use the access code provided in your textbook. You will also find other resources, such as downloadable MP3 tutorials for each chapter, a glossary, and an electronic copy of your text—you can catch up on your reading anywhere!

Key Terms

 

allopatric speciation

allopolyploid

autopolyploid

biological species concept

ecological species concept

hybrid

hybrid zone

macroevolution

morphological species concept

phylogenetic species concept

postzygotic barrier

prezygotic barrier

punctuated equilibrium

reinforcement

reproductive isolation

speciation

species

sympatric speciation

adaptive radiation

amino acid

ammonite

Cambrian explosion

endosymbiosis

geologic record

half-life

heterochrony

macroevolution

marsupial

mass extinction

paedomorphosis

Pangaea

physiology

protobiont

radioactive isotope

radiometric dating

ribozyme

serial endosymbiosis

stromatolite