Chapter 4: Speciation and Phylogeny

Chapter Outline

1. What Are Species?

1. Organisms that belong to a species are generally distinct from the members of other species.
2. Species are real biological categories, but some dissent exists in the true definition.
3. Biological Species
• The biological species concept defines a species as a group of organisms that interbreed.
• Reproductive isolation states that members of one group cannot successfully breed with members outside the group.
• The ability to breed leads to gene flow in the population.
4. Ecological species
• The ecological species concept emphasizes natural selection as a key force in creating and maintaining the species.
• Asexual species, in which gene flow does not occur, cannot be thought of as reproductively isolated.
• Selection would favor organisms of the same morphology, which would maintain their similarity even without gene flow.

2.  The Origin of Species

1. Allopatric speciation
• When a population is divided by a barrier, different parts of the population adapt to different environments.
• The longer the groups remain isolated, the greater the genetic difference is between the groups.
• When the separation results in complete reproductive isolation, a new species is formed.
• Character displacement refers to an occurrence after the two separated groups come together.  If there is some gene flow, such as through hybrid individuals, the competition between the groups over food or mates can still result in a divergence of characters.
• Reinforcement reduces gene flow between two formerly separated populations by selecting favorable adaptations that prevent mating between the members of the two populations.
• Allopatric speciation thus requires a physical barrier that interrupts gene flow and allows the two populations to diverge by means of natural selection.
2. Parapatric and sympatric speciation
• Parapatric speciation states that a new species can be formed if selection is combined with partial genetic isolation.
• At habitat boundaries, a hybrid zone can be formed.
• Sympatric speciation states that speciation can occur even in the absence of a physical barrier.
• Adaptive radiation occurs when a single species diversifies to fill numerous ecological niches.

3.  Tree of Life

1. Carolus Linnaeus created a standard scientific taxonomy system in 1735, including the binomial nomenclature to distinguish all species.
2. A phylogeny is a family tree that shows the common ancestor of two or more species.

4.  Why Reconstruct Phylogenies?

1. To classify and identify organisms.
2. To understand why an organism evolved a particular trait.
3. To understand morphology and behavior. 
• The technique used in this case is known as the comparative method.

5.  How to Reconstruct Phylogenies

1. Establishing comparisons between features, behavior, or genetics is the first step in creating a phylogenetic tree.
2. Convergence presents a problem in reconstructing phylogenies.
• Analogous traits are similar because of convergence.
• Homologous traits are similar because of descent from a common ancestor.
3. Ancestral and derived characteristics
• Ancestral traits characterize the last common ancestor of two or more species.  They appear early in the development of an organism, appear earlier in the fossil record, and are seen in out-groups.
• Derived traits have evolved since the branching off from the last common ancestor.  Only derived traits can be used in reconstructing phylogenies.
4. Genetic distance
• A computation of overall genetic similarity between pairs of species.
• Genetic distances with constant rates of change are known as molecular clocks.
• The neutral theory states that most changes in DNA have little to no effect.
• Knowing the distance between two species allows for estimation of how long ago the species diverged.

6.  Taxonomy

1. Taxonomic classification of Humans 
• Species: Homo sapiens
• Genus: Homo
• Family: Hominidae
• Superfamily: Hominoidea
  
2. Classification systems
• Cladistic taxonomy says that only descent matters.
• Evolutionary taxonomy says that both descent and overall similarity matter.

In This Chapter

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