Speciation: How Species Form
Modes of Speciation
A species is a population of organisms that can interbreed and produce a viable offspring. Speciation is the
formation of a new species (______________). In order for a new species to form, individuals from the original
species must evolve to become reproductively _________ from the remainder of the population. The creation of a
new species is continuous (_____________) and it is often difficult to pinpoint exactly when a new species forms.
Reproductive Isolation biological barriers that keep
species from reproducing
Pre-zygotic isolating mechanisms either ____________ between species or prevent __________ of the eggs.
These include:
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Behavioral isolation – different species use different _______ and other __________ clues to find and
attract a mate. Examples: songs of birds and species specific pheromones
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Ecological/Habitat isolation - two species may live in the same region but in different habitats and may
never ___________ each other. Example: elevation within a mountain range
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Temporal isolation - many species are kept separate by ________________ (mate at different times of
the day, different seasons, or even different years)
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Mechanical isolation - may attempt to mate but are ___________________________ (lock and key
genitals of some insects)
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Gametic isolation - if gametes of different species do meet, male gametes may not be able to __________
and fertilize an egg of a different species. Example: chemical markers on the surface of fish eggs.
Post-zygotic barriers are when sometimes a sperm from one species is able to fertilize the egg of another
species and a zygote is produced, but post-zygotic barriers prevent these hybrid zygotes from developing into
___________, _____________ individuals. These include:
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Zygotic Mortality – mating and fertilization is possible, but genetic differences result in a zygote that is
unable to develop past an __________________. Example: goat and a sheep
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Hybrid Inviability – the hybrid individual develops and either ______ before birth or if born alive cannot
survive to _______________. Example: tiger and leopard
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Hybrid Infertility - offspring produced is __________. Example: donkey and horse = mule
Types of Speciation
Sympatric Speciation – populations within the _______________________ and become reproductively isolated.
Example: a mutation called polyploidy, where an organism has three or more sets of chromosomes; common in plants
that self-pollinate - potatoes
Example: two different species breed to produce a sterile offspring that then reproduces asexually - wheat.
Allopatric Speciation – formation of a new species as a
result of evolutionary changes ___________________
_________________. i.e. mountains, rivers or roadways.
Eventually the split populations become so distinct that
they are unable to breed even if brought back together.
Example: Darwin's Finches
Parapatric Speciation – the formation of a new species in
an area where an organisms range does not significantly
______. Individuals at the periphery of a population tend
to have a slightly different __________ than that of the
parent population and are subjected to the founder effect
and genetic drift.
The Speed of Evolution
Current models of evolution suggest that main types of change are at work:
Gradualism – a model of evolution that views evolutionary change as _________________________. Big changes
occur by the accumulation of many small changes.
Punctuated Equilibrium – a model of evolution that views history as long periods of ___________ (equilibrium)
interrupted by periods of __________. It seems that a lot of species undergo changes
when they first diverge from the parent species. After that, the change is relative slow.
Human Impact on Speciation and Mass Extinctions
Human activities can affect the genetic diversity of a population in various ways:
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In addition to the affect humans have on species, there have been ____ notable ____________ events identified
throughout history where ____________ in the environment caused by a major shift in the ___________ on the
planet. The most recent of these was the extinction of the dinosaurs thought to have been caused by the ice age
triggered by the impact of a large asteroid. This event was the catalyst for the adaptive radiation of _________.