CH 24
What is a species?
 Group of organisms that can breed
and produce fertile offspring
 Members of a species DO NOT
need to be near each other
 How do we know if two VERY
similar BUT separated populations
are members of the same
II. Reproductive Isolation and the
formation of new species
 Reproductive isolation:
biological barriers that prevent
formation of viable fertile
 Can be prezygotic or
 Prezygotic barriers:Prevent the
formation of a zygote (fertilized
o Ecological Isolation: if 2
populations occupy
different habitats, they
don’t encounter each other
to mate
o Behavioral Isolation:
Differences in behavior
prevent mating
o Temporal Isolation:
Different breeding times
prevent mating
o Mechanical Isolation:
Genitalia are incompatible
o Gamete isolation: sperm
doesn’t fertilize egg
Two species of garter
snakes: one lives
mainly in water while
the other is mainly
The eastern spotted
skunk mates in late
winter; the western
spotted skunk mates in
late summer.
Blue-footed boobies of
the Galapagos perform a
courtship display
unique to the species.
These 2 species of
snails have opposite
spirals in their shells so
their genital openings
are not aligned.
 Postzygotic Barriers: Prevents the
formation of normal fertile
o Embryo formed is not
o Offspring produced are
weak and die
o Offspring produced are
Postzygotic barriers prevent a hybrid zygote from developing into a fertile adult.
Gametes of red and Some salamander subspecies A mule is the
Hybrids of two rice
robust but sterile
strains are vigorous and
purple sea urchins of the genus Ensatina can
are released into the hybridize, but hybrids do not hybrid between a
fertile, but the next
water, but are unable complete development or are male donkey and a
generation (center) may
to fuse.
female horse.
be sterile.
III. Mechanisms of Speciation
 Allopatric speciation
 Sympatric speciation
A. Allopatric Speciation
 Results from lack of gene flow
between populations that have
been geographically isolated
 Populations evolve independently
due to mutation, natural
selection, genetic drift
 Where would there be a greater
diversity of species?
B. Sympatric speciation
 Evolution of different species
WITHOUT geographic
 Can be due to:
o Polyploidy in plants is the
doubling of chromosomes
from one generation to
the next resulting in
reproductive isolation
o Appearance of new
ecological niches in an area
can result in sympatric
speciation too
IV. Hybrid Zones
 Regions where two species with
incomplete reproductive barriers
meet and produce hybrid offspring
 Usually a band where 2 species
 3 possible outcomes in the hybrid
o Reinforcement of reproductive
barriers: hybrid offspring are
less fit so rate of hybridization
o Fusion: if hybrids are as fit as
parents there could be enuf
mating and gene flow between
the populations that they fuse
into 1 species
o Stability: extensive gene flow
prevents the continued selection
of reproductive barriers