2. Using the (older) typological definition of species,
these two groups of frogs were classified as one. Upon
further investigation, it was discovered that there are
two groups that differ only in their mating calls. They
call for mates at different frequencies.
3. Biologists rely on morphological features from fossil records
to distinguish thousands of different species.
For living populations that are morphologically similar,
behavioral or other biological methods are needed to
distinguish species.
One method used is to identify the species’ reproductive
isolating mechanisms (any behavioral, structural or
biochemical traits that prevent individuals of different species
from reproducing successfully together).
There are two types of isolating mechanisms
prezygotic isolating mechanisms
postzygotic isolating mechanisms.
4. Prezygotic mechanisms prevent interspecies mating and
fertilization. There are four types of isolation that prevent
mating from occurring, thus maintaining species isolation.
• Ecological isolation
• Temporal isolation
• Behavioural isolation
• Mechanical isolation
• Gametic isolation
5. Ecological isolation is when species occupy separate habitats
or niches and do not encounter one another to reproduce
due to some geographic or ecological barrier.
Example – ground squirrel species occupy different
habitats.
Woodchucks live in fields at low elevationWoodchucks live in fields at low elevation Marmots live in the RockyMarmots live in the Rocky
Mountains at high elevationsMountains at high elevations
6. Temporal isolation is when two species are found in the
same area, but are incapable of mating due to different
reproductive cycles for mating.
Example – Red and black sea urchins live in the same
location, but release their gametes at different times of the
year.
7. e.g. Drosophila pseudobscura and D. persimilis
are sympatric over a
large area in North America. They breed during
the same season of the year but at different
times of the day. D. Pseudobscura is sexually
active in the evening while D. persimilis in the
morning and hence don’t form hybrids.
8. Behavioural isolation is when distinct
mating rituals by one species may prevent
members of another species from
recognizing or selecting a mate.
Example: male jumping spiders dance
(shake their legs and wave their palps).
Females of different species do not
respond to the dance.
Example: Different species of fireflies do
not recognize each others' mating signals,
and as a result do not generally interbreed.
9. Mechanical isolation is when structural differences in
reproductive organs prevent successful fertilization. This
isolation comes about because of different and complex
structure of genital organs, in different organisms which,
in turn, doesn’t allow copulation to occur between
different species.
This is particularly true of insects, where according to
Dufour, genitalia are supp posed to work on “lock and
key” principle, like enzymes.
Example - .
10. The male and female genital organs are exactly fitted to
each other and copulation doesn’t occur even if there is
slight difference in the structure of either organs
e.g. Inter-specific crosses between different species of
Drosophila can cause injury or even death to the
female .Copulation between female Drosophila
pseudobscura and male D.melanogaster is simply
impossible.
11. Gametic isolation – may prevent reproduction at a molecular level.
Example – in coral reefs, many species with external fertilization
may release gametes simultaneously, so trillions of sperm and
eggs may be in the shallow water at one time. Sperm and eggs of
the same species recognize each other by molecular markers.
12. Postzygotic mechanisms prevent the hybrid zygote from developing into
healthy and fertile adults. There are three likely cases that will occur
to ensure that the hybrid does not reproduce:
1) Zygote mortality is a result of chromosomal incompatibility.
2) Hybrid inviability is when the embryo does develop but the hybrid
experiences reduced fitness and often an early death.
3) Hybrid infertility occurs when a hybrid develops into a mature adult
but is unable to undergo successful meiotic division, and is unable to
produce offspring. (seen in donkey-horse hybrids: mules)
13. Occurs when a population is divided by a
geographic barrier
◦ Barriers: mountains, rivers, regions excluding vital
resources (water, food) areas covered with volcanic lava
Interbreeding between populations not possible
(reproductive isolation)
Gene frequencies can diverge due to natural
selection, mutation, and genetic drift
14. The formation of new species without the
presence of a geographic barrier
Occurs by one of 3 ways:
◦ Balanced Polymorphism
◦ Polyploidy
◦ Hybridization
15. Suppose a population of insects possesses a
polymorphism for color. Each color provides a
camouflage to a different substrate (rock, tree
stump, etc..).
When not camouflaged, they are eaten
Thus, only insects with the same color can
associate and mate
Similarly colored insects are reproductively
isolated…so gene pools can diverge.
16. More than 2 sets of chromosomes found in diploid
(2n) cells.
Often occurs in plants (occasionally animals)
where triploid (3n), tetraploid (4n) and higher
chromosome numbers exist.
Caused by nondisjunction in meiosis
Tetraploid individuals will continue to produce
diploid gametes – making them reproductively
isolated very quickly.
17. Occurs when 2 different forms of a species mate
and produce offspring along a geographic
boundary called a hybrid zone.
The genetic variation of the hybrids is greater than
that of either parent
This permits hybrids to adapt to environmental
conditions beyond the range of either parent.
Hybrids can eventually diverge from parent forms
when faced with selective pressures
