Speciation:
the formation of new species
from existing species
Same species or different species?
• Speciation occurs when some members of a sexually
reproducing population change so much that they
can no longer produce viable, fertile offspring with
members of the original population.
• Formation of new species is also called
macroevolution.
• Lack of gene flow between 2 populations may result
in the 2 populations becoming reproductively
isolated.
• A hybrid organism results from the breeding of 2
different species.
Reproductive Isolating Mechanisms
Class A: Pre-zygotic: a barrier that impedes mating
between species or prevents fertilization
Prevention of mating
1. Behavioural Isolation:
- Any special signals or behaviours that are species specific
prevent interbreeding with closely related species.
- Example: differences in the songs of the western and
eastern meadowlark
2. Temporal Isolation:
- Species may be kept separate by timing barriers.
- Example: 2 species may occupy in the same habitat but
mate or flower at different times of the day or in
different seasons
3. Ecological/Habitat Isolation:
- 2 species may live in the same general area but in
different habitats, therefore they rarely encounter
each other.
- Example: the common garter snake lives near water
and the northwest garter prefers open areas.
Prevention of Fertilization
1. Mechanical Isolation
- Closely related species try to mate but fail to achieve
fertilization because they are anatomically
incompatible.
- Example: genitals may not fit together
2. Gametic Isolation:
- If the gametes from 2 species meet they will rarely
fuse to make a zygote
- Could be due to how the eggs are fertilized within
the female reproductive tract.
- In plants, pollen of one species usually do not
germinate on the stigma of another species.
Class B: Post-zygotic: a barrier that prevents hybrid zygotes
from developing into viable fertile individuals.
- Prevention of Hybrids
1. Hybrid Inviability:
- Genetic incompatibility of the interbred species may stop
development of the hybrid zygote during its
development.
- Due to genetic incompatibility which prevents normal
mitosis after the fusion of the gametes.
2. Hybrid Sterility:
- 2 species mate and produce a viable offspring but the
offspring can not reproduce.
- Meiosis in these individuals fails to produce normal
gametes
- Example: mule (female horse x male donkey)
3. Hybrid Breakdown:
- The 1st generation hybrids are viable and
fertile but when they mate with each other or
an individual from the original species, they
produce sterile or weak offspring.
- Example: cotton
HW
• Read and make notes on pgs 363-366
• Include diagrams