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Evolution
Individuals of different
species cannot
interbreed under natural
conditions
 Described as being
reproductively isolated
from one another
 Populations of different
species do NOT exchange
genetic information
 They have different gene
pools


Two types of isolating
mechanisms
 Pre-zygotic
▪ Impede mating between
species or prevent fertilization
of eggs if individuals from
different species try to mate
 Post-zygotic
▪ Occurs after fertilization occurs
▪ Mechanisms that prevent
hybrid zygotes from
developing into viable, fertile
individuals

1. Behavioural Isolating Mechanisms
 Different species use different mating signals
 Prevents wasted effort with a partner that will
not produce fertile offspring with you

2. Ecological Isolation
 Two species may live in
the same general region
but in different habitats
 Eg. Frogs – some breed
in fast moving streams
while other frogs breed
in ponds

3. Temporal Isolating Mechanisms
 Flowering or mating occurs at different times

4. Mechanical
Isolating
Mechanisms
 Structural
differences in
reproductive organs
prevent copulation
 Genital anatomy can
be used to classify
species based on
morphology

5. Gamete Isolating
Mechanisms
 Failure of gametes
from different species
to fuse
 Prevents fertilization at
the molecular level
 Ex. In plants, pollen
grains of one species
fail to germinate on the
stigma of another
species

1. Zygote Mortality
 Fertilized egg (zygote) fails to divide

2. Hybrid Inviability
 Genetic incompatibility of the interbred species
stops development of the zygote

3. Hybrid Sterility (Infertility)
 Offspring lives but gametes cannot undergo
meiosis (i.e. mule)


The process of species requires populations
of organisms to become, and largely to
remain, genetically isolated from one
another
Two types of Speciation – based on how gene
flow is disrupted within a population
 Sympatric Speciation
 Allopatric Speciation
Varieties of the same species living in the same area
at the same time
 Each variation adapts to reduce competition (as a
result of disruptive selection)

When a population is split
into two or more isolated
groups by a geographic
barrier
 Split populations become
so distinct that the two
groups are unable to
interbreed even if they are
brought back together

Situations where population size is reduced dramatically is
a called Genetic Bottleneck
 Results in a loss of genetic diversity
 Small populations that become isolated from the original
population are more likely to change enough to become
new species
 This is known as the founder effect
