STUDY QUESTIONS #5 ANSWERS

Write brief but complete answers to three of the following questions.

1. A prominent evolutionary biologist (Dr. Jerry Coyne, formerly of UMCP) has said, "If we understand the evolution of reproductive isolation, we understand speciation". Say why this makes sense under the Biological Species Concept.

In the Biological Species Concept, two groups are called different species when they cannot interbreed successfully. Thus, characters that prevent or lessen successful interbreeding (i.e., cause reproductive isolation) are central to the evolution of new species. Thus, if we understand how these characters evolve, we understand how species (in this species concept, at least) form.

2. Compare and contrast "allopatric" and "sympatric" speciation.

Allopatric speciation requires a physical barrier to gene flow, sympatric speciation does not. In allopatric speciation, reproductive isolation occurs as a BYPRODUCT of independent evolution of two groups, in sympatric speciation, reproductive isolation is more likely to be a direct result of natural selection. Allopatric speciation is thought to take a very long time to occur, sympatric speciation is probably quite rapid.

3. What is premating reproductive isolation? Describe one of the types of premating isolation discussed in class, and say how it serves as a barrier to gene flow.

Premating reproductive isolation is caused by characters that prevent or disrupt mating. Examples include changes in courtship calls, displays or pheromones that affect whether an individual is recognized as a potential mate, change in timing or reproduction so that individuals are not mating at the same time as other groups, change of habitat preference for taxa that mate exclusively within the habitat (individuals in another habitat are not encountered during mating time).

4. What is postmating reproductive isolation? Describe one of the types of premating isolation discussed in class, and say how it serves as a barrier to gene flow.

Postmating isolation is caused by disruption of gene flow after mating has occurred. Examples include gametic incompatibility (gametes don't recognize one another, or fertilization is not successful), developmental incompatibility (genes from two parents don't work well together in development and lead to death of embryos or juveniles), hybrid sterility (genetic incompatibility leads to problems in meiosis).

5. What is a phylogeny? There are three types of characters that could be used to make a phylogeny-- shared ancestral, shared derived, and unique derived. Which of these three is used in cladistic phylogenies and why?

A phylogeny is a hypothesized picture of the historical relationships among organisms. Shared derived characters are the most useful in constructing phylogenies, because they permit relationships to be determined. In contrast, shared ancestral characters are not useful because all taxa under consideration have them (so, cant be used to determine relationships). Unique derived characters don't permit determination of relationships because only one group has them.

6. What is parsimony, and how is it used when making a phylogeny?

The most parsimonious phylogenetic tree requires the fewest hypothesized evolutionary changes of character states. This is considered to be the best tree, and is used as the standard in phylogenetic reconstruction.

7. Using the phylogeny of HIV that you were given, is HIV-1 derived from HIV-2?

No. HIV-1 is on a different lineage from HIV-2, and they probably evolved after these lineages split. If they were each other's closest relative on the phylogeny, there might be some evidence that one evolved from the other, but the phylogeny that you were given suggests that they are not all that closely related.

8. How does examination of the phylogeny of fungi provided on your handout tell us something about the evolution of the lichen symbiosis?

When the characteristic "has symbiotic relationship with alga" is mapped onto the fungal phylogeny, it appears that the ability to form a lichen has evolved 5 times, and has occurred independently in both the Ascomycetes and the Basidiomycetes.