Download Hi These questions are about the evolution of species (speciation

Hi
These questions are about the evolution of species (speciation). There are a number
of models (theories) to describe events leading to the evolutionary emergence of new
species. In order to examine how speciation occurs, we need first to define species: most
often this is said to be a group of individuals that actually or potentially interbreed in nature.
In this sense, a species is the biggest gene pool possible under natural conditions.”
Speciation is a lineage-splitting event that produces two or more separate species. The
species are separate when they can no longer mate with each other, thus they are said to have
reached reproductive isolation - for a good summary description of this see
http://evolution.berkeley.edu/evosite/evo101/VC1gReproIsolation.shtml. Reproductive
isolation can include occasional matings that produce offspring that do not survive long or
that cannot themselves reproduce.
From: http://evolution.berkeley.edu/evosite/evo101/VC1aModesSpeciation.shtml: “The
key to speciation is the evolution of genetic differences between the incipient species. For a
lineage to split once and for all, the two incipient species must have genetic differences that
are expressed in some way that causes matings between them to either not happen or to be
unsuccessful. These need not be huge genetic differences. A small change in the timing,
location, or rituals of mating could be enough. But still, some difference is necessary. This
change might evolve by natural selection or genetic drift. Reduced gene flow probably plays
a critical role in speciation. Modes of speciation are often classified according to how much
the geographic separation of incipient species can contribute to reduced gene flow”.
And according to http://en.wikipedia.org/wiki/Speciation :
“Speciation refers to the evolutionary process by which new biological species arise.
There are three main ideas concerning the emergence of new species (Modes of Speciation),
each based on the degree to which populations undergoing this process are geographically
isolated from one another (allopatric speciation, sympatric speciation, parapatric speciation,
polyploidy speciation”.
“Allopatric speciation is speciation by geographic isolation. In this mode of speciation,
something extrinsic to the organisms prevents two or more groups from mating with each
other regularly, eventually causing that lineage to speciate. Isolation might occur because
of great distance or a physical barrier, such as a desert or river.
In parapatric speciation there is no specific extrinsic barrier to gene flow. The population
is continuous, but nonetheless, the population does not mate randomly. Individuals are
more likely to mate with their geographic neighbors than with individuals in a different
part of the population’s range. In this mode, divergence may happen because of reduced
gene flow within the population and varying selection pressures across the population’s
range.
Sympatric speciation does not require large-scale geographic distance to reduce gene
flow between parts of a population. How could a randomly mating population reduce
gene flow and speciate? Merely exploiting a new niche may automatically reduce gene
flow with individuals exploiting the other niche. This may occasionally happen when, for
example, herbivorous insects try out a new host plant.”
In the first part of question 1, you have a reproductive isolation mechanism
driving an allopatric speciation event. In part b, your island species is reproductively
isolated from the distant species but can mate with nearby members to form a hybrid
species (for more on this see
http://biomed.brown.edu/Courses/BIO48/21.Models.HTML) .
In your second question, you are looking again at isolated populations with limited
interbreeding. These populations are expected to diverge over time and form different
species if they become unable to breed with each other through genetic drift alone. You are
interested in whether or not the given migration rate generates enough gene flow between
the populations to drive evolution away from divergence of the populations into two or
more species. Migration tends to increase variability within a population and at the
same time prevent populations from diverging to the extent that they become new
species by introducing gene flow. The migration rate you have been given is 0.5 breeding
individuals every five generations. This turns out to be too low to prevent divergence of
allele frequencies within the populations because it requires an average of one immigrant
every other generation to counterbalance drift (see
http://darwin.eeb.uconn.edu/eeb348/lecture-notes/mutation-drift/node4.html and
http://www.ucl.ac.uk/~ucbhdjm/courses/b242/MigEvol/Fst4Nm.pdf.
I hope this helps, good luck with your studies.