THE ORIGIN OF SPECIES
THE ORIGIN OF SPECIES
• HOW DOES EVOLUTION LEAD TO THE
FORMATION OF ALL THE DIFFERENT
ORGANISMS, OR SPECIES, WE SEE ON THE
PLANET?
• FIRST WE MUST DEFINE A “SPECIES”
THE ORIGIN OF SPECIES
• SPECIES
– BIOLOGICAL SPECIES CONCEPT
• A SPECIES AS A POPULATION OR GROUP OF POPULATIONS
WHOSE MEMBERS HAVE THE POTENTIAL TO INTERBREED
AND PRODUCE FERTILE OFFSPRING
– MORPHOLOGICAL SPECIES CONCEPT
• CLASSIFICATION IS BASED MAINLY ON OBSERVABLE AND
MEASURABLE PHENOTYPIC TRAITS
– GENEALOGICAL SPECIES CONCEPT
• A SET OF ORGANISMS WITH A UNIQUE GENETIC HISTORY
– ECOLOGICAL SPECIES CONCEPT
• IDENTIFIES SPECIES IN TERMS OF ECOLOGICAL NICHES
THE ORIGIN OF SPECIES
• REPRODUCTIVE BARRIERS KEEP SPECIES
SEPARATE
– A BIOLOGICAL FEATURE
OF THE ORGANISMS THEMSELVES
TO PREVENT
POPULATIONS
BELONGING TO
CLOSELY
RELATED
SPECIES FROM
INTERBREEDING EVEN WHEN RANGES OVERLAP
THE ORIGIN OF SPECIES
• REPRODUCTIVE BARRIERS
THE ORIGIN OF SPECIES
• REPRODUCTIVE
BARRIERS
REPRODUCTIVE BARRIERS BETWEEN SPECIES
Prezygotic Barriers: Prevent Mating or Fertilization
Temporal Isolation
Mating or flowering occurs at different seasons or times of day
Habitat Isolation
Populations live in different habitats and do not meet
Behavioral Isolation
There is little or no sexual attraction between males and females
Mechanical Isolation
Structural differences in genitalia or flowers prevent copulation or
pollen transfer
Gametic Isolation
Male and/or female gametes die before waiting or fail to unite
Postzygotic Barriers: Prevent the Development of Fertile Adults
Hybrid Inviability
Hybrid zygotes fail to develop or to reach sexual maturity
Hybrid Sterility
Hybrids fail to produce functional gametes
Hybrid Breakdown
Offspring of hybrids are weak or infertile
THE ORIGIN OF SPECIES
• WHY DO THESE REPRODUCTIVE BARRIERS
ARISE?
– GEOGRAPHIC ISOLATION CAN LEAD TO
SPECIATION
• ALLOPATRIC SPECIATION
 GENE FLOW IS
BLOCKED BY THE
EXISTENCE OF A
GEOGRAPHIC BARRIER
THE ORIGIN OF SPECIES
• ALLOPATRIC SPECIATION EXPERIMENT
THE ORIGIN OF SPECIES
• ISLANDS ARE LIVING LABORATORIES FOR
SPECIATION
– ADAPTIVE
RADIATION 
THE EMERGENCE
OF NUMEROUS
SPECIES FROM A
COMMON
ANCENSTOR
INTRODUCTED
TO NEW AND
DIVERSE
ENVIRONMENTS
THE ORIGIN OF SPECIES
• WHY DO THESE REPRODUCTIVE BARRIERS
ARISE?
– REPRODUCTIVE ISOLATION CAN LEAD TO
SPECIATION (OVERLAPPING GEOGRAPHY)
• SYMPATRIC SPECIATION  REPRODUCTIVE ISOLATION
DEVELOPS AND NEW SPECIES ARISE WITHOUT
GEOGRAPHIC SEPARATION
THE ORIGIN OF SPECIES
• SYMPATRIC SPECIATION
– TYPICALLY THE RESULT OF SOME GENETIC
CHANGE (I.E. POLYPLOIDY)
THE ORIGIN OF SPECIES
• AS ORGANISMS BECOME MORE ISOLATED
(GEOGRAPHICALLY OR REPRODUCTIVELY) THE
BARRIERS BETWEEN THEM INTENSIFY
• THIS EVENTUALLY IN RESULTS IN ORGANISMS
THAT ARE SEPARATE SPECIES, AND CONTINUE
ON SEPARATE EVOLUTIONARY PATHS
• HOWEVER; THE RATE OF THIS CHANGE IS NOT
CONSISTENT!!
THE ORIGIN OF SPECIES
• RATE OF SPECIATION
– GRADUALISM VSPUNCTUATED EQUILIBRIUM
• GRADUALISM 
POPULATIONS EVOLVE
DIFFERENCES SLOWLY
(GRADUALLY) AS THEY
BECOME ADAPTED TO THEIR
LOCAL ENVIRONMENT, NEW
SPECIES EVOLVE GRADUALLY
FROM ANCESTRAL SPECIES
• PUNCTUATED EQUILIBRIUM 
EVOLUTION OCCURS IN
SPURTS (SHORT, QUICK, LARGE)
THE ORIGIN OF SPECIES
• WITH ALL THIS SPECIATION OCCURING, HOW
DO WE KEEP TRACK
OF ALL THESE
DIFFERENT
ORGANISMS?
• THAT’S WHAT YOU
GET TO FIND OUT IF
YOU COME BACK TO CLASS!!!