V. SPECIATION 

A. Allopatric Speciation  

B. Parapatric Speciation (aka Local or Progenitor - Derivative)

 C. Adaptive Radiation

 D. Sympatric Speciation [Polyploidy]

A. Allopatric Speciation  “different homes”  

1. subdivision  

a. geographic isolation -- non-biological 

b. extinction of intermediate pops. 

c. result: NO GENE FLOW

2. gradual accumulation of mutations

 

3. genetic divergence over time

 

4. reproductive isolation [follows divergence]

BUT… 

5. intercontinental disjunct congeners in plants are fertile!

e.g. Datisca, Platanus, Magnolia, Liriodendron, etc.

P. occidentalisSE USA

Platanus

P. × acerifolia

P. orientalisSW Asia

Datisca cannabinaSW Asia

Datisca glomerataCalifornia

Nei’s genetic identity = 0.142

Molecular Clocks

I.Molecular divergence is positively correlated with time (Zuckerkandl & Pauling, 1965)

A. difficult with protein data – not neutral

B. today there is abundant DNA data, but the

“accuracy” of molecular clocks is questionablee.g. Hillis et al. 1996, Molecular Systematics p. 531-541

r = K / 2Tr = rate for neutral mutationsT = divergence timeK = number of substitutions per site

II. Clock Calibrations “the Achilles heal”

A. Estimates of T are never precise, subject to under and overestimates

1.volcanic islandse.g. Hawaii, Canary Islands

2.biogeographic reconstruction

a. Gondwanan and Laurasian distributions

b. 2-25 mya estimates for 12 E. Asian- E. N. Am. disjuncts see Wen ARES 30:421-55, 1999

c. long distance dispersal is always a possibility

3. Fossilsa. relationship to extant taxa uncertain

b. no unequivocal fossil DNA

c. DNA degradation confounds mutation rate estimates

III. Model-based approaches

1. see Sanderson, 1998 (Mol. Syst. Plants 2) for an introduction

2. take into account the stochasticity of divergence estimates, and imprecision of

time estimates

B. Local Speciation (Progenitor - Derivative)Parapatric Speciation

 1. isolation

 a. migration

 b. long distance dispersal

 c. peripheral population

2. genetic bottlenecks 

a. population reduction  

b. increased inbreeding & genetic drift 

c. adaptation ?? maybe, maybe not i.e. selection pressure could cause the fixation of genetic differences, but so might random events

3. examples of adaptation:

a. edaphic endemics [serpentine, limestone, heavy metals]

b. pollinators

4. fixation of mutations between populations

a. with or without reproductive isolation 

b. faster than allopatry 

c. reduced genetic diversity in derivative 

d. relatively high genetic identity betw. progenitor & derivative

 

5. Chromosomal Rearrangements 

a. rearrangement established[e.g. translocation]

 b. hybrid sterility

 ex. Clarkia species (H. Lewis; L. Gottlieb)

 

6. Mating System Change a. self-compatibility arising from self-incompatibility 

e.g. Stephanomeria malheurensis

Oregon endemic, described in 1975

 

C. Adaptive Radiation   

1. open habitats  

2. little competition 

3. radiation into new ecological niches -  

4. often w/o genetic reproductive isolation  

5. generally w/o much genetic divergence 

6. can result in a “star phylogeny”

Hawaiian tarweedadaptive radiation

Rapid diversification, inferred from short branches & unresolved polytomy