Recall...
Speciation occurs when two populations become
reproductively isolated.
Speciation
It is produced and maintained
by reproductive isolating mechanisms
(pre-zygotic and post-zygotic).
Section 9.2
Homework
Read 9.2 (pg. 360-363)
Pg. 363 #13, 15, 17
Pg. 373 #1-4
worksheet: Speciation
Reproductive isolation requires
barriers to gene flow:
Reproductive Isolating
Mechanisms
Pre-zygotic
Post-zygotic
aka “pre-fertilization”
aka “post-fertilization”
prevention of:
• mating
• fertilization
affect the survival
or viability
of hybrid offspring
Pre-zygotic mechanisms act
before fertilization.
Prevention of mating:
• behavioural isolation
• ecological/habitat isolation
• temporal isolation
• mechanical isolation
Prevention of fertilization:
• gametic isolation
Behavioural isolation
Pre-zygotic mechanisms
• behavioural isolation
•ecological/habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Sexual cues/mating rituals are not recognized by
individuals of different species.
Example:
Songs of different
species of birds or
insects; mating
“dances”
Habitat isolation
Pre-zygotic mechanisms
• behavioural isolation
•habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Geographical isolation prevents
individuals from
meeting and mating.
Temporal isolation
Different species are fertile
at different times, preventing mating.
 different seasons, different times of day
Can occur even within the same area,
if the populations occupy
different habitats.
Example:
B. americanus and
B. fowleri can produce
fertile hybrids in a lab;
in nature they are fertile
during different seasons.
Example:
Two species of garter snakes
(genus Thamnophis) occur in the
same geographic area. One lives
mainly in water, and the other is
terrestrial.
Mechanical isolation
Pre-zygotic mechanisms
• behavioural isolation
•ecological/habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Genitals are mechanically incompatible
(though copulation may be attempted).
Gametic isolation
• zygotic mortality/hybrid inviability
• hybrid sterility
• hybrid breakdown
Pre-zygotic mechanisms
• behavioural isolation
•ecological/habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Mating and sperm transfer may
take place, but gametes are incompatible.
sperm may not survive in female reproductive tract
fusion is prevented by some biochemical mechanism
Example:
Insects
Plants (mode of pollination)
Post-zygotic mechanisms act
after fertilization.
Pre-zygotic mechanisms
• behavioural isolation
•ecological/habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Example:
Common in marine
invertebrates, which
utilize external
fertilization.
Zygotic mortality or
Hybrid inviability
Post-zygotic mechanisms
• hybrid inviability
• hybrid sterility
• hybrid breakdown
Zygote does not survive to birth, or
surviving hybrid is frail.
Example:
Goat/sheep hybrid
zygote mortality
Hybrid sterility
Post-zygotic mechanisms
• hybrid inviability
• hybrid sterility
• hybrid breakdown
The hybrid cannot produce normal gametes,
so it cannot reproduce.
Hybrid breakdown
Post-zygotic mechanisms
• hybrid inviability
• hybrid sterility
• hybrid breakdown
F1 hybrids are viable and fertile,
but subsequent generations are not.
When parents differ in
chromosome number,
meiosis cannot occur
normally.
Example:
Some rice cultivars
produce robust hybrids,
but the next generation
is small and sterile.
Example:
Mules
(donkey/horse hybrid).
Four species of leopard frog:
What keeps these
species isolated?
• mating calls differ
• defective hybrid
embryos
1) Rana pipiens
2) R. blairi
3) R. utricularia
4) R. berlandieri
Summary
• Speciation involves reproductive isolation of
populations.
Pre-zygotic or
post-zygotic?
• Reproductive isolating mechanisms maintain
species by preventing gene flow and
hybridization.
Which specific
mechanism?
▫ Can be categorized as either pre-zygotic or
post-zygotic.
• Pre-zygotic mechanisms prevent the
production of hybrids, by preventing either
mating or fertilization.
• Post-zygotic mechanisms act after fertilization.
Hybrids are produced, but with reduced viability
or fertility.