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Speciation
The evolution of species
Speciation

Occurs when members of similar
populations no longer interbreed to
produce fertile offspring within their
natural environment
Geographic Isolation
Occurs whenever a physical barrier
divides a population
 The divided groups progress differently in
response to their environments
 Ex. Finches on different Galapagos islands

Reproductive Isolation
Occurs when
formerly
interbreeding
organisms can no
longer mate and
produce fertile
offspring
 Ex. One population
of tree frog mates in
the spring and the
other in the summer

Chromosomes
Chromosome numbers play a role in
speciation
 Any species with a multiple of the normal
set of chromosomes is known as a polyploid
 Can be caused by mistakes in meiosis or
mitosis
 If their offspring survive, they are usually
unable to reproduce, but some can with
other polyploids
 Ex. Wheat, cotton, apples, and bananas

Timeline
Speciation can occur quickly or slowly
 Gradualism – the idea that species
originate through a gradual change of
adaptations
 Punctuated equilibrium – argues that
speciation occurs relativley quickly, in
rapid burst followed by long periods of
genetic equilibrium
 Both theories are supported by different
fossil evidence

Patterns of Evolution
Support the idea that natural selection is
an important agent of evolution
 Three main patterns

◦ Adaptive radiation
◦ Divergent evolution
◦ Convergent evolution
Adaptive Radiation
An ancestral species
divides into many
species to fit a
number of diverse
habitats
 Ex. Honeycreepers
on the Hawaiian
Islands

Divergent Evolution
Species that were once similar to each
other become increasingly distinct
 Caused by different populations adapting
to different environmental conditions, and
they become more different as they adapt,
eventually becoming separate species

Convergent Evolution

Occurs when
unrelated species
occupy similar
environments in
different parts of the
world, but because
they share a similar
environment, these
two different species
adapt to be very
similar to each other