1. CHAPTER 24
THE ORIGIN OF SPECIES

2. I. WHAT IS A SPECIES?
THE BIOLOGICAL SPECIES CONCEPT EMPHASIZES REPRODUCTIVE ISOLATION
THE BIOLOGICAL SPECIES CONCEPT DEFINES A SPECIES AS A GROUP OF POPULATIONS WHOSE INDIVIDUALS HAVE THE POTENTIAL TO INTERBREED AND PRODUCE FERTILE OFFSPRING WITH EACH OTHER BUT NOT WITH MEMBERS OF OTHER SPECIES.

3. B. PREZYGOTIC AND POSTZYGOTIC BARRIERS ISOLATE THE GENE POOLS OF BIOLOGICAL SPECIES.
PREZYGOTIC BARRIERS PREVENT MATING OR FERTILIZATION BETWEEN SPECIES.
SPECIES THAT OCCUPY THE SAME GEOGRAPHIC AREA OFTEN
LIVE IN SEPARATE HABITATS (HABITAT ISOLATION)
POSSESS UNIQUE, EXCLUSIVE MATING SIGNALS AND COURTSHIP BEHAVIORS (BEHAVIORAL ISOLATION)
BREED AT DIFFERENT TIMES (TEMPORAL ISOLATION)
HAVE ANATOMICALLY INCOMPATIBLE REPRODUCTIVE ORGANS (MECHANICAL ISOLATION)
HAVE INCOMPATIBLE SEX CELLS (GAMETIC ISOLATION)
EVEN IF TWO DIFFERENT SPECIES MANAGE TO MATE, POSTZYGOTIC BARRIERS USUALLY PREVENT THE INTERSPECIFIC HYBRIDS FROM DEVELOPING INTO ADULTS, BREEDING WITH EITHER PARENT SPECIES, OR PRODUCING VIABLE, FERTILE OFFSPRING.

5. C. THE BIOLOGICAL SPECIES CONCEPT DOES NOT WORK IN ALL SITUATIONS
FOR INSTANCE, IT IS NOT APPLICABLE TO FOSSILS OR TO ORGANISMS THAT REPRODUCE ONLY ASEXUALLY.

6. ALTERNATIVE CONCEPTS INCLUDE
D. OTHER SPECIES CONCEPTS EMPHASIZE FEATURES AND PROCESSES THAT IDENTIFY AND UNITE SPECIES MEMBERS
ALTERNATIVE CONCEPTS INCLUDE
MORPHOLOGICAL SPECIES CONCEPT: DEFINES SPECIES BY PHENOTYPE
THE COHESION CONCEPT: DEFINES SPECIES AS POPULATIONS WITH DISCRETE CLUSTERS OF GENETIC TRAITS
RECOGNITION SPECIES CONCEPT: EMPHASIZING MATING ADAPTATIONS THAT ARE FIXED IN A POPULATION
ECOLOGICAL SPECIES CONCEPT: STRESSES THE ROLE OF SPECIES IN THE ENVIRONMENT
EVOLUTIONARY SPECIES CONCEPT: RECOGNIZES SPECIES AS INDEPENDENTLY EVOLVING UNITS PERFORMING UNIQUE ECOLOGICAL ROLES.

7. II. MODES OF SPECIATION
GEOGRAPHICAL ISOLATION CAN LEAD TO THE ORIGIN OF SPECIES: ALLOPATRIC SPECIATION
ALLOPATRIC SPECIATION OCCURS WHEN A SPLINTER POPULATION DIVERGES FROM ITS PARENT POPULATION AFTER BECOMING GEOGRAPHICALLY ISOLATED.
SMALL SPLINTER POPULATIONS ARE BETTER CANDIDATES FOR ALLOPATRIC SPECIATION THAN LARGER ONES BECAUSE GENETIC DRIFT AND NATURAL SELECTION CAN CHANGE A SMALL GENE POOL FASTER.
ADAPTIVE RADIATION IS THE EVOLUTION OF NUMEROUS SPECIES WITH DIVERSE ADAPTATIONS FROM A COMMON ANCESTOR.

8. A MODEL FOR ADAPTIVE RADIATION ON ISLAND CHAINS

9. B. A NEW SPECIES CAN ORIGINATE IN THE GEOGRAPHICAL MIDST OF THE PARENT SPECIES: SYMPATRIC SPECIATION
SYMPATRIC SPECIATION: THE FORMATION OF NEW SPECIES WITHOUT THE PRESENCE OF GEOGRAPHIC BARRIERS.
POLYMORPHISM: THE EXISTENCE OF TWO OR MORE DISTINCT FORMS OF INDIVIDUALS IN THE SAME POPULATION.
BALANCED POLYMORPHISM: A TYPE OF POLYMORPHISM IN WHICH THE FREQUENCIES OF THE COEXISTING FORMS DO NOT CHANGE NOTICEABLY OVER MANY GENERATIONS.
EX. DARK AND LIGHT COLORED PEPPERED MOTHS
POLYPLOIDY: IS THE POSSESION OF MORE THAN THE NORMAL TWO SETS OF CHROMOSOMES FOUND IN DIPLOID (2N) CELLS.
HYBRIDIZATION: OCCURS WHEN TWO DISTINCTLY DIFFERENT FORMS OF A SPECIES MATE AND PRODUCE PROGENY ALONG A GEOGRAPHIC BOUNDARY CALLED A HYBRID ZONE.

10. POLYPLOIDY IN PLANTS

11. C. GENETIC CHANGE IN POPULATIONS CAN ACCOUNT FOR SPECIATION
REPRODUCTIVE BARRIERS MAY ARISE AS SIDE EFFECTS OF SPECIATION AS TWO POPULATIONS GENETICALLY DIVERGE WHILE ADAPTING TO DIFFERENT ENVIRONMENTS.
SEXUAL SELECTION MAY LEAD DIRECTLY TO REPRODUCTIVE BARRIERS.
ACCORDING TO THE RECOGNITION SPECIES CONCEPT, NATURAL SELECTION WOULD AMPLIFY ADAPTATIONS THAT ENHANCE REPRODUCTIVE SUCCESS WITH MEMBERS OF THE SAME SPECIES.
CONSISTENT WITH THE COHESION SPECIES CONCEPT, TWO SPECIES MAY INTERBREED FREELY IN A HYBRID ZONE WITHOUT LOSING THEIR DISTINCTIVENESS AWAY FROM THE ZONE.
SPECIATION MAY BE ASSOCIATED WITH CHANGE AT JUST A FEW GENE LOCI OR WITH THE CUMULATIVE DIVERGENCE OF MANY LOCI.

12. D. THE PUNCTUATED EQUILIBRIUM MODEL HAS STIMULATED RESEARCH ON THE TEMPO OF SPECIATION
ONE VIEW OF SPECIATION IS THAT IT USUALLY OCCURS GRADUALLY BY AN ACCUMULATION OF MICROEVOLUTIONARY CHANGES IN GENE POOLS.
IN CONTRAST, THE PUNCTUATED EQUILLIBRIUM MODEL VIEWS SPECIES AS CHANGING MOST WHEN THEY BUD FROM AN ANCESTRAL SPECIES AND THEN UNDERGOING RELATIVELY LITTLE CHANGE FOR THE REST OF THEIR EXISTENCE.

13. TWO MODELS FOR THE TEMPO OF SPECIATION