The Origin of Species
CHAPTER 24
24.1
The biological species concept emphasizes
reproductive isolation
Microevolution
 Focuses on the change in genetic frequencies
for a trait in a population over time (HWE)
What is a species?
 Organisms of the same group that can
interbreed and produce fertile offspring
Biological Species Concept
 A population whose members have the
potential to:
 Interbreed with one another and
 Produce viable, fertile offspring but
 Who cannot produce viable, fertile offspring with
members of other species
What does this mean?
 Reproductive isolation is the key
 once similar organisms are no longer able to
interbreed in nature, they are considered to be
another species
 Speciation has occurred
Reproductive Isolation
 Barriers that prevent two species from
producing viable, fertile hybrids
Pre-zygotic Barriers
 Prevent successful mating
 No fertilized eggs (zygotes)
Habitat Isolation
 Species live in different locations may never
meet and thus will never mate
Behavioral Isolation
 If mating behaviors are dissimilar among
species, mating may not occur
 Unique behaviors allow own species
recognition
 Blue footed booby dance
Temporal Isolation
 Breed or flower at different times
Mechanical Isolation
 The reproductive
organs of two
species are
incompatible
 Galearis diantha and Ponerorchis
chusua orchids pollinated by
different species of bees
Gametic Isolation
 Gametes of 2 species may not meet due to
chemical barriers preventing survival of
sperm
 Important to aquatic species
Post-zygotic Barriers
 Barriers that prevent the hybrid zygote from
becoming a viable, fertile adult.
Reduced Hybrid Viability
 Genetic
incompatibility may
force miscarriage of
hybrid embryo
Zygote (blue) forms but
soon aborts (clear)
Reduced Hybrid Fertility
 Robust, viable offspring
produced
 Adult hybrid organism
is sterile
Hybrid Breakdown
 1st generation
hybrids are fertile
but subsequent
generations are
not
Rice strains whose offspring are fertile but
those offspring are not; accumulation of
mutations over generations cause sterility
Limitations to Biological
Species Concept
 Does not account for
asexually reproducing
organisms, like bacteria
 Does not account for
interbreeding that may
have happened in fossil
species