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 POPULATIONíS 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.