Slide
1
There are some basic steps that must take place for a speciation event to
occur.
Step 1:† gene flow between two populations is interrupted
and
populations become genetically isolated from each other
Step 2:† genetic
differences gradually accumulate between the two populations and the
populations diverge genetically
Step 3:† reproductive isolation evolves as
a consequence of this divergence and a reproductive isolating mechanism
evolves
Slide 2
This image from the tutorial illustrates the possible
ways that populations can become reproductively isolated from each other.
In general, these mechanisms can be described as either pre-zygotic (occuring before a zygote forms) or post-zygotic (occuring after a zygote forms).
The prezygotic isolating mechanisms are habitat isolation, temporal isolation, behavioral isolation, mechanical isolation and gametic isolation.
Slide 3
An example of habitat or geographical
isolation are stickleback fish species.
Pungitius pungitius is a brackish water form of the 9-spine stickleback (brackish water is water that is a mix of fresh water and salt water). These fish build their nests and reproduce in water with a salinity of 13.5% and an average temperature of 13.3 oC.
Pungitius tymensis is a freshwater form of a stickleback known as the Sakhalin stickleback. They overlap in range with the 9-spine stickleback but they build their nests and reproduce in water with a salinity of less than 1% and average temperature of 11.3 oC.
These reproductive preferences keep the two species separate.
Slide 4
An example of
temporal isolation - or mating at different times - are the toad species Bufo
americanus and Bufo fowleri. These toads overlap in geographic range and
when in a laboratory they will mate and produce fertile offspring.
In the wild, however, Bufo americanus mates in the early summer while Bufo fowleri mates in late summer.
These differences in mating time keep the two species separate.
Slide 5
Behavioral differences
between species can serve as reproductive isolating mechanisms.
Birds, fish, and mammals are well known for their elaborate courtship displays and mating calls.
Many organisms also release chemicals known as pheromones - these pheromones are very specific and the pheromones of one species do not attract individuals of a different species.
Slide 6
This
series of pictures shows the mating dance of the male blue footed booby -
apparently she liked his dance!
Slide 7
Mechanical isolation
refers to males and females of different species being unable to mate because
of mechanical incompatibility between their reproductive organs.
This is well documented in insects - insects have hard exoskeletons and their genitalia must fit together very precisely for mating to occur. Differences in genital structure serve as a reproductive isolating mechanism between species.
Slide 8
Gamete incompatibility refers to
crosses where no offspring are produced because the gametes are incompatible
and fertilization does not occur. This happens in many closely related
species.
In some species, the female reproductive tract is inhospitable - this has been observed in crosses between different species of fruit flies.
Marine invertebrates, like coral for example, have external fertilization - they release their sperm and eggs into the seawater and the gametes need to find each other. They need to be compatible and of the same species for fertilization to take place.
This is also common in plants - the pollen from one species of plant cannot fertilize the ovules of a different species.