Terms to know:  species, according to the phylogenetic species concept; species, according to the biological species concept; hybrid zone; cline; polymorphism; disruptive selection; allopatric speciation; parapatric speciation; sympatric speciation; vicariance event; dispersal event; founder effect speciation; chromosomal mutation; polyploidy

Questions:

1. Describe ways in which variation can occur within a species.  Give an example of each.
2. What are the two main ways a hybrid zone can form?  What can make a hybrid zone stable?  How is it possible for a hybrid zone to exist without having the characteristics of the species on one side of the zone merge into the characteristics of the species on the other side of the zone?  Explain this with reference to gene flow.
3. Distinguish between the biological and phylogenetic species concepts.  Give a point in favor of and a point against using each concept.  Then give an argument that using either concept to define species is irrelevant to evolutionary biology.
4. For each of the following situations, state whether the two groups would be considered separate species or the same species according to the biological species concept and according to the phylogenetic species concept Explain why.
1.  Group 1: A population of mice in Mexico that has large ears relative to the size of its head.  Group 2: A population of mice in Alaska that has small ears relative to the size of its head.  If you travel from Mexico to Alaska, you find that ear size relative to head size decreases gradually. Individuals from all populations can and sometimes do interbreed with populations farther north or farther south.
2.  Group 1: horses. Group 2: donkeys.  Horses and donkeys will sometimes mate with each other and produce offspring; the offspring (mules or hinnies) are healthy but sterile.
3.  Group 1: Red Oak trees.  Group 2: Black Oak trees.  The two kinds of oak sometimes interbreed in natural populations to produce healthy, fertile offspring.  Each kind of oak (Black Oak and Red Oak) has several unique characteristics of bark structure, acorn shape, and allozymes that are different from the characteristics found in the other form of oak, or in other oaks or other trees generally; these have remained unique to the two groups over long periods of time despite the occasional hybridization that occurs.
4.  Group 1: A population of towhee (a kind of bird) in California and a population of towhee in Michigan.  In California, towhees have white spots on their wings; in Michigan, they do not.  In between California and Michigan, there is gradual change in appearance from the spotted form to the non-spotted form; there is no place where one can clearly draw a line between the forms.  All forms are capable of interbreeding with each other and produce healthy, fertile offspring.
5.  Group 1:  A population of flickers (a kind of bird) in California.  Group 2:  a population of flickers in Tennessee.  Each kind of flicker has several unique derived characteristics that allow it to be clearly distinguished from the other.  Between California and Tennessee, there is a region of less than 200 miles wide within which the two forms both occur and produce healthy, fertile hybrids.  East of this zone, all flickers have characteristics like the Tennessee flickers; west of this zone, all flickers have characteristics like the California flickers.
5. Consider the following four situations:
1. as you travel from east to west in Europe, you discover that, in a species of green woodpecker, there is clinal variation in the shade of green on the back such that eastern woodpecker populations have pale green backs and back color becomes gradually darker, from population to population, as you go to the west.
2. as you travel through Italy, you discover that frogs from the north and frogs from the south reproduce with each other, but that tadpoles that result from reproduction of northern and southern frogs do not survive to metamorphose into frogs, so they die before reproduction.
3. travelling through Scandanavia, you discover that two very distinct forms of maple tree, each of which has its own unique characteristics, sometimes reproduce with each other and produce offspring that are healthy and fertile.  Despite this, the different forms of maple retain their own unique characteristics.
4. in England, you find within many populations of a snail species that some have orange stripes and others have brown stripes; these individuals reproduce with each other freely and have healthy fertile offspring so that most snail families include both brown striped and orange striped individuals.

 

In which ONE of these situations would the biological species concept and the phylogenetic species concept differ with regard to whether the groups described belonged to the same species or to different species?  Which concept would group them as one species, and which would consider different groups to be a different species?  Based on what criterion are they grouped into one species by one concept, and based on what criterion are they considered different species by the other concept?  State one advantage to grouping groups such as this as a single species, and one advantage to splitting groups such as these into separate species.  Finally, for each of the OTHER situations, state whether the organisms described belong to the same species (by both concepts) or to more than one species (by both concepts.)
 

6. Discuss the role of gene flow, genetic drift, and natural selection in speciation.  Explain how the importance of these different forms of evolution may differ depending on whether speciation is occurring through allopatric, parapatric, or sympatric speciation.
7. Sympatric speciation is generally considered to be less likely than is allopatric speciation.  Why?
8. Relate the concept of disruptive selection to speciation.  In which form(s) of speciation (allopatric, sympatric, parapatric, speciation related to chromosomal mutation) is disruptive selection most crucial?  In which is it not required?  Why?
9. What form(s) of evolution are likely to be occurring during founder effect speciation?  Give the argument that founder effects are likely to result in speciation.  Give the counter-argument that speciation is unlikely to occur through a founder effect.
10. Relate the concept of founder effect speciation to the concept of a population bottleneck.  How are these concepts similar?  What is the main form of evolution occurring in each?  How do the concepts differ?
11. Discuss how likely it would be for each of the following species to undergo sympatric speciation.  (a) A parasitic insect that breeds on a specific kind of host. (b) A parasitoid wasp in which males and females mate in flight, nowhere near the host.  (c) A species of plant that is found only in wet habitats in which pollen is dispersed by the wind.  (d) A species of plant that is found only in wet habitats in which pollen is dispersed by one specific kind of pollinator.
12. What kinds of chromosomal mutation (large-scale change in chromosomal number or structure)  may result in speciation?  Why do these result in speciation?
13. Suppose a chromosomal mutation occurs such that the number of chromosomes is duplicated.  Why are individuals with the mutation in a different species from individuals without the mutation, based on the biological species concept?
14. Changes in chromosome are thought to have accompanied a high proportion of speciation events in plants, but a lower proportion of speciation events in animals.  What is the proposed explanation for this difference between plants and animals?
15. Within a population of strawberry plants, an individual has a chromosomal duplication, so that it has twice as many chromosomes as all the other individuals.  Describe the steps through which this individual could give rise to a population of a new species of strawberry.  Would individuals of this species be able to reproduce with the original species? Why/why not?  Would it be possible to have gene flow between the original species and the new species?  Why/why not?
16. Suppose that a species of wild garlic has developed as a result of a chromosomal duplication.  Individuals of this new wild garlic species sometimes reproduce with individuals of the original species of wild garlic (with half as many chromosomes).  These hybrid offspring are healthy.  How is this possible, given that the two species have different numbers of chromosomes?  Will the offspring be able to reproduce?  Why/why not?
17. In a diploid population of blackberry plants, a chromosomal duplication occurs such that the individual produced has 4 chromosome sets (is 4N.)  Is this individual the same species or a different species from the original diploid plants according to the biological species concept?  Clearly explain why.
18. What is QTL analysis?  How can it be applied to understanding mechanisms of speciation?
19. You are studying two closely related species of Drosophila (fruit fly).  While these species have different mating behavior and will not mate in nature, you can artificially inseminate them in the lab and obtain crosses between species.  You suspect that speciation between them has occurred through the evolution of different mating behaviors, and want to test whether these behavioral changes have occurred through a large number of mutations, each with a relatively small impact on the behavioral phenotype, or through a small number of mutations, each with a large phenotypic impact.  Explain how to test between these hypotheses.