Question 18 from test: one tree has four steps, not three'

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Question 18 from test one tree has four steps,
not three.
Section from exam data printout, the question
was scored correctly.
18 2 0 2 20 3 28 9
2 0 0 0 0 .00 .03
.31 .05 .44 .14 .03 .00 .00 .00
.00 .00 -.11 .47 -.39 -.55
-.51 -.18 .00 .00 .00 .00
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WE WILL ATTEMPT TO HAVE 2 REVIEW SESSIONS FOR
THE FINAL EXAM (room tba by e-mail). The
first one will be this Friday at 1200 noon.
This one is definite. The second one is
tentatively scheduled for Monday night at 800
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TEACHING EVALUATION FORMS WILL NOW BE
DISTRIBUTED. I WILL TAKE A 10 MIN BREAK WHILE
YOU FILL THESE OUT. PLEASE MAKE SURE TO PASS
THEM TO THE DESIGNATED COLLECTOR
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Gene flow from crops to closely related weeds
is an important practical issue. What would
happen, for example, if the crop had genes
engineered for herbicide resistance? This may be
one of the most important obstacles to
the widespread use of genetically engineered
crops, and it is an area of much contemporary
concern, not limited only to plant species.
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Polyploidy, which we have already studied, can be
a form of instant speciation. Use the diagram
at left to refresh your memory of
allopolyploidy. Then trace through the events
shown at right, noting the roles of unreduced
gametes, backcrossing to one of the parent
species, etc.
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There are two ways by which geographical
isolation can happen
Vicariance division of a continuous
distribution by some (geo)physical barrier.
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The glaciers of the Ice Ages were major vicariant
events that affected many species. Here, three
successive glaciations apparently resulted in
the formation of three species of western
warblers, each derived from the eastern
Black-throated green warbler, Dendroica virens.
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The text uses Snapping Shrimp (Altheus spp) to
illustrate vicariant speciation In this
cladogram C Caribbean and P Pacific The nos.
1 - 7 refer to seven pairs of species. These
were first recognized by morphological
comparisons, and here confirmed with DNA
sequences.
Notice that each species in the Pacific has a
sister species in the Caribbean.
The vicariant event in this case was the rise and
closing of the Isthmus of Panama, completed
about 3 mybp.
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A widely-cited case of dispersal
Some Hawaian Drosophila species sequentially
dispersed and diverged by island hopping as new
islands appeared
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Dispersal implies the founding of an isolated
population by a small number of migrant
individuals who manage to survive and reproduce
at the new location. It adds a feature to
allopatric speciation that is usually not
encountered in the vicariant model
CHANCE VARIATION can have a significant
influence on the genetic composition of the
founding population and can continue to play a
role while the effective population size remains
small.
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• CHANCE VARIATION AND DISPERSAL
• A small sample of any population is almost always
  
• biased with respect to the variation of the
  population
• as a whole.
• A founding population of organisms in a new
  habitat
• (e.g., an island as compared to the
  mainland) is therefore
• likely to have different gene frequencies
  than the original
• population.
• This is known as the Founder Effect.

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Heres an example of how the Founder Effect can
work Suppose a mainland population of mice is
at HW for a particular gene with 2 alleles,
and that p . 9 and q .1 The prob of AA
.81, Aa . 18 and aa .01
Notice that the a allele is lost in the most
probable founder popln.
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• CHANCE VARIATION AND DISPERSAL, contd
• 2. While the population remains small, drift can
• be important even after the founding event.
• Summary the most likely effects of chance on
• the populations founded by dispersal
  include
• biased sampling of the variation in the
  parent
• population (including the loss of alleles),
  
• changes in gene frequency and reduction in
• heterozygosity due to subsequent drift.

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Something to think about If a founding
population is small, the sex ratio is unlikely
to be even. For example, if there are 6
founders, the probability that there will be 3
males and 3 females is 20(1/2)6 approx 0.3, so
the prob. of an uneven sex ratio is about 0.7.
We have already seen that uneven sex ratios
reduce effective population size. In this
example, the effective size could be as low as
3.3.
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bottlenecks
The so-called flush-crash cycle may induce
multiple bottlenecks in newly - established,
isolated populations, further enhancing genic
divergence. This cycle may be relevant to the
quantum or dispersal model of geographic
speciation.
This cycle has been used in laboratory
experiments some of the resultant populations
displayed some reproductive isolation
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A note on the roles of chance and natural
selection We often emphasize the differences
between chance and natural selection in
determining the genetic composition
of populations. This dichotomy is valid as far
as it goes. But we need to keep reminding
ourselves that their effects are seldom really
independent.
In fact, their effects are often interlaced and
interdependent. For example, suppose that a
mainland population had allele a but that its
island derivative did not because of the
Founder Effect. The climate of the entire region
is changing, and a encodes a product that
provides higher fitness in the new environment.
Lacking a (except perhaps for mutation)
the island population cannot adapt to the new
climate by the same route as the mainland. In
effect, natural selection has to find another way
to enable this population to adapt.
The late Stephen J. Gould had this interplay in
mind when he referred to chance as providing the
contingent factors in evolutionary biology.
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Secondary
Contact Previously isolated populations
re-establish contact after a period of
separation and divergence. They might do this
via range extension, elimination of a vicariant
barrier, etc.
Reinforcement Upon
secondary contact, hybrids between the two
populations are of reduced fitness, and natural
selection acts to reduce the frequency of hybrid
formation. This is called reinforcement. It
completes the process of allopatric speciation.
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Important note reinforcement is only one of
several possible outcomes of secondary contact
As a general rule, secondary contact probably
results initially in the formation of hybrid
zones. The size and fate of these zones is a
function of the relative fitnesses of parentals
and hybrids and several other factors. Note In
general, hybridization, even between good
species, is more frequent in natural systems
than many of us have been taught to expect.
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Does reinforcement really occur?
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Contrasting the conventional (vicariance) and
dispersal (quantum) models of speciation
vicariance
dispersal or quantum Popln sizes
can be large probably
small Role of selection important
may not be initially impt Role of
drift not important
probably important Founder effects or
not important important bottlenecking

Secondary contact Likely
important in both models
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Sympatric and parapatric speciation have been
much more controversial than either allopatric
model
Both require isolating mechanisms to evolve in
the face of ongoing gene flow. Recall that gene
flow works against divergence, and that even low
levels of gene flow tend to homogenize
populations. Natural selection could counteract
the results of gene flow, but its intensity would
have to be very great. For a long time, this was
considered unrealistic.
Recently, however, both theoretical work and
discoveries in the field have changed this view,
and it now appears that sympatric sspeciation
is not only possible, but that it can occur
rapidly under certain circumstances. This is a
major paradigm shift within the field of
evolutionary biology.
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Host races of parasitic species provide good
candidates for sympatric speciation. At
present, the best-studied cases are those of the
maggot flies of the genus Rhagoletis, The
basic observation is that many of the host fruit
trees are introduced species (e.g., apple,
cherry), yet every host has its own race of
maggot flies, reproductively isolated from other
such races. In addition to the material
presented in class, see your text for additional
perspective and for discussion of several
important experiments
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The popular notion is that evolution, including
speciation, occurs very slowly and is discernable
only over geological time scales. Recently,
however, a growing number of cases of
speciation in action have been identified, and
it is now clear that reproductive isolation can
sometimes evolve very quickly.