TOPIC 8
SPECIES & SPECIATION
I. SPECIES CONCEPTS
A. MORPHOLOGICAL SPECIES
B. BIOLOGICAL SPECIES
II. PATTERNS OF SPECIATION
A. ANAGENESIS
B. CLADOGENESIS
III. MECHANISMS OF SPECIATION
A. ALLOPATRIC SPECIATION
1. GEOGRAPHICAL PATTERNS OF VARIATION
2. VICARIANT VS. PERIPATRIC SPECIATION
3. ROLES OF RANDOM PROCESSES
B. SYMPATRIC SPECIATION
IV. REPRODUCTIVE ISOLATION
A. PREZYGOTIC BARRIERS
B. POSTZYGOTIC BARRIERS
V. TEMPO OF EVOLUTIONARY CHANGE
SPECIES CONCEPTS
MORPHOLOGICAL SPECIES CONCEPT
Species are defined based on measurable physical features
Original concept used by Linnaeus
BIOLOGICAL SPECIES CONCEPT
Focuses on reproductive isolation
"Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups" (Mayr, 1942)
PATTERNS OF SPECIATION
1. GEOLOGICAL VIEW OF SPECIATION = ANAGENESIS
= SPECIES 1 EXPERIENCES SELECTION, CHANGES & EVOLVES THRU TIME INTO SPECIES 1A.
2. GEOGRAPHICAL VIEW OF SPECIATION = CLADOGENESIS
= DISTINCT POPULATIONS OF SPECIES 1 EXPERIENCE DIFFERENT SELECTION PRESSURES.
SOME POPULATIONS OF SPECIES 1 DISPLAY MINOR CHANGES. SPECIES 1 PERSISTS.
OTHER POPULATIONS CHANGE SIGNIFICANTLY => NEW, SPECIES 2.
GEOGRAPHICAL PATTERNS OF VARIATION
POPULATION = LOCALIZED GROUPOF INDIVIDUALS BELONGING TO THE SAME SPECIES.
LOCAL ADAPTATION LEADS TO DIFFERENCES AMONG POPULATIONS OF A SPECIES
HOW CAN SELECTION ACT ON A LOCAL POPULATION TO ALTER THE AVERAGE PHENOTYPE & THE RANGE OF PHENOTYPES OBSERVED?
3 MODES OF SELECTION
ACT ON CONTINUOUS (QUANTITATIVE) CHARACTERS [VS. DISCRETE CHARACTERS]
1. STABLIZING SELECTION: DECREASES VARIANCE, PRESERVES THE MEAN. ELIMINATES EXTREMES, FAVORS INTERMEDIATE INDIVIDUALS.
2. DIRECTIONAL SELECTION: ALTERS THE MEAN, MAINTAINS THE VARIANCE. FAVORS ONE OF THE ORIGINAL EXTREMES
3. DIVERSIFYING (DISRUPTIVE) SELECTION: SELECTION AGAINST THE MEAN, FAVORS BOTH EXTREMES = BIMODAL DISTRIBUTION.
LEADS TO POLYMORPHISM
ENHANCED BY ASSORTATIVE MATING
GEOGRAPHICAL PATTERNS (cont.)
CLINAL VARIATION = GRADED CHANGE IN ONE OR MORE CHARACTERS THAT PARALLELS A GRADIENT IN THE ENVIRONMENT
ECOTYPES = RACES WITHIN A SPECIES ADAPTED TO PARTICULAR ECOLOGICAL CONDITIONS
THE DEGREE OF GEOGRAPHICAL VARIATION REFLECTS THE AMOUNT OF GENE FLOW
= LOSS OR GAIN OF ALLELES TO A POPULATION’S GENE POOL DUE TO THE MOVEMENT OF INDIVIDUALS OR GAMETES
ALLOPATRIC SPECIATION
AN INITIALLY CONTINUOUS SERIESOF INTERBREEDING POPULATIONS IS FRAGMENTEDBY A GEOGRAPHIC BARRIER
VICARIANT MODEL
AN EXTRINSIC FACTOR LEADS TO GEOGRAPHIC SEPARATION
PERIPATRIC MODEL
PERIPHERAL POPULATIONS EXPERIENCE DISTINCTIVE CONDITIONS
THEY ARE SMALL & INHERENTLY ISOLATED (e.g., on islands)
RANDOM FACTORS
1. GENETIC DRIFT = CHANGES IN THE GENE POOL OF A (SMALL) POPULATION DUE TO CHANCE ALONE
2. FOUNDER EFFECT = COLONIZATION OF A HABITAT BY A LIMITED NUMBER OF INDIVIDUALS = COLONISTS DO NOT GENETICALLY REPRESENT THE SOURCE POPULATION
3. BOTTLENECK EFFECT = CATASTROPHIC REDUCTION OF A POPULATION = SURVIVORS DO NOT GENETICALLY REPRESENT THE INITIAL POPULATION
ALLOPATRIC SPECIATION (cont.)
NATURAL SELECTION ACTS ON ISOLATES. IF GEOGRAPHICAL BARRIERS ARE REMOVED, 3 POSSIBLE RESULTS OBSERVED:
1. POPULATIONS INTERBREED FREELY (NO SPECIATION)
2. POPULATIONS REMAIN SEPARATE (SPECIATION OCCURS)
LIMITED SUCCESSFUL INTERPOPULATION REPRODUCTION
INTIAL POST-ZYGOTIC ISOLATING MECHANISMS LEAD TO EVOLUTION OF PRE-ZYGOTIC MECHANISMS
3. SOME LEVEL OF HYBRIDIZATION PERSISTS (INTROGRESSION),
BUT POPULATIONS REMAIN DISTINCT (SPECIATION OR NO??)
SYMPATRIC SPECIATION
A SUBPOPULATION BECOMES REPRODUCTIVELY ISOLATED IN THE MIDST OF ITS PARENT POPULATION
NO PHYSICAL BARRIERS TO REPRODUCTION
REPRODUCTIVE ISOLATING MECHANISMS CRITICAL
RARE IN ANIMALS, COMMON IN PLANTS
CAN ARISE VIA FAULTY CELL DIVISION DURING GAMETE FORMATION
AUTOPOLYPLOIDY = 1 SPECIES ONLY
ALLOPOLYPLOIDY = HYBRIDIZATION BETWEEN 2 SPECIES INVOLVED
REPRODUCTIVE ISOLATION
A. PREZYGOTIC BARRIERS
1. PREMATING
a. HABITAT ISOLATION
b. TEMPORAL ISOLATION
c. BEHAVIOURAL ISOLATION
2. POST-MATING
(PRE-FERTILIZATION)
a. MECHANICAL ISOLATION
b. GAMETIC ISOLATION
A. PREZYGOTIC BARRIERS DEFINED
1. PREMATING
HABITAT ISOLATION = OCCUR IN DIFFRENT HABITATS
TEMPORAL ISOLATION = REPRODUCE AT DIFFERENT TIMES OF DAY OR YEAR
BEHAVIOURAL ISOLATION = LACK OF ATTRACTION OR CO-ORDINATION
PREMATING - SEXUAL SELECTION
SELECTION FOR TRAITS THAT ENHANCE REPRODUCTIVE SUCCESS ALONE = SECONDARY SEX CHARACTERISTICS
MALE - MALE COMPETITION
(INTRA-SEXUAL SELECTION)
FEMALE CHOICE
(INTER-SEXUAL SELECTION)
ASSORTATIVE MATING
A. PREZYGOTIC BARRIERS (cont.)
2. POST-MATING (PRE-FERTILIZATION) DEFINED
MECHANICAL ISOLATION = STRUCTURAL DIFFERENCES PREVENT GAMETE TRANSFERS
GAMETIC ISOLATION = GAMETES FAIL TO SURVIVE OR FUSE
B. POSTZYGOTIC BARRIERS
1. POST-FERTILIZATION
a. REDUCED HYBRID VIABILITY
b. REDUCED HYBRID FERTILITY
c. HYBRID BREAKDOWN
1. POST-FERTILIZATION REPRODUCTIVE BARRIERS DEFINED
REDUCED HYBRID VIABILITY
= HYBRID ZYGOTES DO NOT DEVELOP OR HYBIRDS DO NOT SURVIVE TO MATURITY
REDUCED HYBRID FERTILITY
= HYBRIDS FAIL TO PRODUCE FUNCTIONAL GAMETES
HYBRID BREAKDOWN
= OFFSPRING OF HYBRIDS EXPERIENCE REDUCED SURVIVAL OR FERTILITY
TEMPO OF EVOLUTIONARY CHANGE
GRADUALISM
SLOW ACCUMULATION OF MANY SMALL GENETIC CHANGES LEADS TO THE APPEARANCE OF NEW SPECIES
MICROEVOLUTION =>=>=>=>=>=>=> MACROEVOLUTION
PUNCTUATED EQUILIBRIUM
FOR LONG PERIODS OF TIME NO CHANGE OCCURS = STASIS ( = EQUILIBRIUM PHASE IN FOSSIL RECORD)
NEW SPECIES ARISE RAPIDLY, WITHOUT TRANSITIONAL FORMS ( = PUNCTUATION PHASE IN FOSSIL RECORD)
MICROEVOLUTION DOES NOT RESULT IN MACROEVOLUTION
MECHANISMS BEHIND MACROEVOLUTION
CHANGES IN GENES CONTROLLING DEVELOPMENTAL PATTERNS OR KEY PROCESSES
EXPLOITATION OF EXAPTATIONS (PRE-ADAPTATIONS)
SPECIES & SPECIATION
TEXT :
Pages: 428 - 429; 432 - 437; 441 - 442; 445 - 460
FIGURES: 23.4; 23.5; 23.8; 24.1; 24.5; 24.6; 24.9; 24.10; 24.11; 24.13; 24.16
GLOSSARY - SPECIES & SPECIATION
adaptive radiation, allele, allopatric speciation, allopolyploidy, anagenesis, assortative mating, autopolyploidy, biological species, bottleneck effect, cladogenesis, cline, directional selection, diversifying selection, ecotype, exaptation, founder effect, genetic drift, gradualism, hybrid, introgression, macroevolution, microevolution, modes of selection, peripatric speciation, peripheral isolate, postzygotic isolating mechanism, morphological species, preadaptation, prezygotic isolating mechanism, punctuated equilibrium, reproductive isolation, sexual selection, stablizing selection, sympatric speciation, vicariant speciation.