5. Speciation and mechanisms of speciation
(p. 571-573)
For your benefit, this topic has been broken down into more manageable pieces of information, using
question-answer format.
a) what is the widely accepted definition of a species? What is the problem with this definition?
b) Can all species be differentiated from one another on the basis of their physical appearances
or morphology?
c) What are reproductive isolating mechanisms? What is the value of such mechanisms to
scientists?
d) What causes the formation of a new species?
e) What is the major difference between prezygotic and post-zygotic isolating mechanisms?
f) What are the two different types of prezygotic isolating mechanisms?
g) Briefly describe each of the three prezygotic isolating mechanisms that prevent mating
between different species. Provide an example of each type of isolating mechanism.
h) Briefly describe each of the two prezygotic isolating mechanisms that prevent fertilization.
Provide an example of each type of isolating mechanism.
i) Briefly describe each of the three postzygotic isolating mechanisms. Provide an example of
each type of mechanism.
j) Try Questions a-j and question 6 on p. 576
k) State the necessary events in order that must occur in order for one population to become two
different species.
ANSWERS
a) species – members of groups or populations that interbreed or have the ability to interbreed under
natural conditions
problem – how do you determine evidence of interbreeding (between different species) e.g. in wild
animals.
b) No – species distinguished by behavioural or other biological methods
c) reproductive isolating mechanisms are behavioural, structural or biochemical traits that prevent
different species from reproducing successfully together
- they allow scientists to differentiate species
d) Speciation is caused by mechanisms that isolate reproduction
e) prezygotic - reproductive isolating mechanisms that prevent interspecies mating
postzygotic – reproductive isolating mechanisms that do not prevent mating but prevent maturation
and reproduction in offspring from interspecies reproduction
f) prezygotic isolating mechanisms – 1) prevention of mating; 2) prevention of fertilization
g) 3 prezygotic isolating mechanisms preventing mating:
i) ecological isolation
– occupying different habitats/niches
-no encounter
-e.g. different species of squirrels have different habitats
ii) temporal isolation
-plants blooming different times of year
-different mating season
-nocturnal vs. diurnal organisms
iii) behavioural isolation
-different signals in attracting a mate
-e.g. bird song
h) 2 prezygotic isolating mechanisms preventing fertilization:
i) mechanical
-structural differences of reproductive organs
-e.g. shape of penis in arthropods; orchid flowers prevent pollen
transfer
ii) gametic
-external fertilization: sperm and eggs in water have molecular
markers to recognize each other
i) 3 postzygotic mechanisms:
i) zygotic mortality – no zygotes develop
ii) hybrid mortality – hybrid offspring form but do not survive
iii) hybrid infertility – offspring sterile e.g. mule = horse + donkey; mules don’t reproduce
j) pg. 576
a) gametic isolation
b) ecological isolation
c) behavioural isolation
d) behavioural isolation
e) gametic/mechanical isolation
f) mechanical
g) zygotic mortality
h) hybrid inviability
i) temporal isolation
j) hybrid infertility
k) Events leading to speciation:
reproductive isolation  mutations  subsequent evolutionary changes