Speciation and tree thinking
1. Mechanisms of divergence in marine species
• Allopatric speciation
•
Cosmopolitan and sympatric
Pseudo-nitzschia pungens
Geographic distribution of P. pungens. Morphology-based records are indicated
by open squares and verified isolates based on DNA sequencing are
shown as filled squares.
Casteleyn et al. 2012
• Evidence for sympatric specation?
ML tree inferred from rbcL (A), LSU (B), ITS-1 (C), and ITS-2 (D) sequences. Boldface numbers in
each node indicate the bootstrap values (1000 replicates). (A) Numbers in the first row
represent the number of strains analysed. Horizontal bars indicate the clustering patterns
recognized by gross (LM and ultrastructural (TEM) morphology, molecular markers (listed on
the right side), and sexual compatibility (Amato et al. 2007)
Allopatric speciation
•
Allopatric speciation in Pseudo-nitzschia pungens
Allopatric speciation
• Processes in Pseudo-nitzschia pungens :
Principal component analysis of pairwise FST values among the seven
predefined populations of P. pungens clade I.
• Also – when were the last common ancestors?
Mechanisms of divergence in marine species
• Sympatric speciation
Hexagrammos
otokii
Sympatric speciation
Number Larvae
H.oc ⁄ H.ag
Allopatric
H.oc ⁄ H.ot
Allopatric
Comparison of survivorship
between replicates of pure and hybrid
crosses. X-axis is number of days and
Y-axis is number of larvae surviving.
pure H. agrammus
H.ot / H.ag
sympatric
Pure H. octogrammus
H. otakii
Hybrids in nature
Days
Hybrids never occur in nature
Gene frequencies
High
High* Medium
Low
Pop
1
Speciation in marine
prokaryotes
•
•
Pop
2
Homologous
recombination
Gene
flow
Gene loss
High rates of
genetic turnover
Vertical
inheritance
Ecology
Common functions
independent of
habitat
Habitatspecific
functions
Biological interactions
•
Ocean water is highly
structured at small
(millimetre to micrometre)
scales
“Interpretation of genecontent diversity requires
the delineation of
populations according to
cohesive gene flow and
ecology, as microevolutionary changes arise in
response to local selection
pressures and population
dynamics”
Cordero and Polz 2014
Dispersal in marine bacteria
Simulated biogeographic pattern
(nucleotide divergence) in global
surface ocean microbes predicted
by a neutral model and quantified
by genome alignment
•
Models suggest microbes evolve faster than oceans currents
can disperse them (Hellweger et al. 2014, Science)
Tree thinking
• Definition of syn apo morphy:
• Synapomorphy = a trait that certain species have that no
other species has
• Synapomorphies identify monophyletic groups
• Shared
• Derived
• Character