Speciation Worksheet
Name ________________
Work Partners _________________________
Use your book, notes and your partners to answer the following questions. Each
of you will turn in your own worksheet.
Biological Species Concept:
“A species is a group or groups of organisms that can actually or potentially
interbreed in nature and are reproductively isolated from other such groups.”
This is our working definition of a species. However, as is true with many things
in biology, there are few absolutes and this definition of species is not without its
problems.
1. Identify 2 problems with this definition.
2. How else could biologists define species. What is one advantage and
what is one disadvantage of this alternative definition?
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3. Classify each of the following situations as either a pre- or postzygotic
isolating mechanism (first space) and then specifically which type of preor postzygotic isolating mechanism (second space):
a) In flowering plants, the pollen is often transferred from the pistil of one
plant to the stigma of another by some type of animal vector (eg., bee,
hummingbird). As a result, there are times when the pollen of one species
ends up on the stigma of another. But of fertilization to occur, a pollen
tube must form that allows the pollen to reach and fertilize the ovum. In
most cases, this will not occur when the wrong pollen reaches the stigma
due to differences in the structure of the pollen.
__________________________
__________________________
b) A short hike through the Sandia Mountains from early spring to late
summer will often reveal several different species of penstemon. The
Rocky Mountain penstemon blooms in the spring while the very similar
inflated penstemon blooms in late summer. As a result, hybrids are rarely
encountered.
__________________________
__________________________
c) Ligers are the result of a cross between a male lion and a female tiger.
Almost all known male offspring of these crosses have been infertile.
__________________________
__________________________
d) Norway rats and black rats are very similar. Attempts to cross the two
species (why?) have led to pups being born, but they die within hours.
__________________________
__________________________
e) When male fruit flies of one species (Drosophila melanogaster) attempt to
mate with females of another (D. pseudoobscura), the act generally leads
to severe injury or even death to both flies.
__________________________
__________________________
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4. Thinking in terms of natural selection, which isolating mechanism,
prezygotic or postzygotic, would you expect to be most common in nature
and why?
5. Ever since Darwin published his book, On the Origins of Species, there
has been significant debate about the relative importance of the two main
patterns of speciation: allopatric and sympatric speciation. Darwin actually
believed, with little evidence to support his position, that sympatric
speciation was probably most important, while most biologists since the
early 1900’s favored allopatric speciation.
a) First, distinguish between allopatric and sympatric speciation.
b) Why do you think it has been so difficult to establish which pattern of
speciation is most important in nature?
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6. One concept that your book does not
talk about is the idea of “ring species.”
A ring species occurs “when a single
species becomes geographically
distributed in a circular pattern over a
large area. Immediately adjacent or
neighboring populations of the species
vary slightly but can interbreed. But at
the extremes of the distribution -- the
opposite ends of the pattern that link to
form a circle -- natural variation has
produced so much difference between
the populations that they function as
though they were two separate, non-interbreeding species”
(http://www.pbs.org/wgbh/evolution/library/05/2/l_052_05.html). In the diagram to
the right, imagine that each differently colored segment represents
different populations of the same species with some degree of gene flow
occurring where they come in contact. If you look at figure C, where the
two ends of the circle come in contact, there is no gene flow, suggesting
that these two populations are in fact two different species. Hmmmm….
One specific example of this in nature is the greenish warbler of Asia. If
you examine the
distribution map to the
right (http://www.zoology.
ubc.ca/~irwin/GreenishWarblers
.html), you will note that
there are 8 different
populations of this species
of bird, each with a distinct
song. Even within each
population, from north to
south in its distribution,
there can be differences in
the songs sung by the
males.
What effect does this pattern have on our working definition of species?
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7. In our whole discussion of speciation, gene flow is a key concept that must
always be considered. Define gene flow and explain its significance in
terms of its role in a) determining if speciation has occurred and b) in
determining the rate at which speciation will occur.
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