I. REPRODUCTIVE ISOLATING MECHANISMS
in order for one population to become very different
from another, they must be reproductively isolated,
this means that there will no longer be a free exchange
of alleles between the two populations
there are Prezygotic mechanisms that prevent mating
or fertilization and Postzygotic mechanisms that prevent
development of a zygote
1. PREZYGOTIC MECHANISMS
a. Ecological Isolation
two populations do not exchange alleles with each other
because they are in different geographic places or at
different places within the same ecosystem
eg. cheetah and tiger do not meet because
one is in Africa and one in Asia
cheetah range
tiger range
eg. Asiatic lion and Bengal tiger are both in northern
India, but lion is on savannah and tiger in forest
b. Temporal Isolation
two populations do not exchange alleles because
they are only available to exchange alleles at
different times of year or even of the day
eg. morning glory opens its flower at sunrise;
cactus opens its flowers at sunset
eg. purple finch mates in June so babies have access
to lots of berry seeds
goldfinch mate in August so that babies have
access to lots of thistle seeds
c. Behavioural isolation
two populations do not exchange alleles because they
do not respond to each others mating rituals
eg. male grey crickets rub legs at 25 times a second
male black crickets at 45 times a second the females of
each species only responds to the sound made by the
male of that same species
d. Mechanical Isolation
two populations do not exchange alleles because of
some physical barrier that prevents this
eg. many insects have modifications on their
exoskeletons such that the male and female
parts are a perfect 'lock-and-key' fit
eg. orchids are shaped so that only certain beetles can
reach the nectar and therefore pick up the pollen,
that beetle will the go to another orchid of the same
type to deposit the pollen
e. Gametic isolation
two populations exchange sperm and eggs but
chemical markers prevent the eggs from being
fertilized by the 'wrong' sperm and so no alleles are
exchanged
eg. wind blows the pollen of corn onto
the flowers of milkweed, but the
pollen can not grow down
through the stigma because it
does not possess the correct
enzyme
eg. clams and fish both shed eggs and sperm into the
same water, but the clam sperm can not penetrate
the fish eggs and vice versa, due to enzymes not
being able to eat through zona pellucida
REPRODUCTIVE ISOLATING MECHANISMS # 11-17
work on these HW
show is not over!!
2. POST-ZYGOTIC ISOLATING MECHANISMS
a. Zygotic mortality - even though the zygote is created,
it fails to develop to maturity
b. Hybrid inviability- even though the hybrid is born, it does
not live long or is not as healthy
c. Hybrid infertility - even though the hybrid is healthy and
vigourous, it is not able to reproduce
eg. donkey X horse  mule
eg. lion X tiger  liger
healthy and strong
but mules are sterile
healthy & strong;
but ligers are sterile
3. SPECIATION
when two populations become completely isolated and
no longer exchange alleles, they are said to have
formed separate species
a. Allopatric speciation - this is a situation in which the
two populations are geographically isolated
prior to them becoming separate species
the population is isolated and
then the changes occur due to
new environments
b. Sympatric speciation - this is a situation where the two
populations remain in physical contact with each
other but still stop exchanging alleles and become
separate species
although not separated,
the population undergoes
different selection
pressures and changes
occur over time in allele
frequencies