Species and Speciation 1. Species concepts
Species Concept Criteria for definition Advantages and Disadvantages Morphological
Biological
Phylogenetic
General lineage
Species concepts in prokaryotes
Species Concept Criteria for definition Support for concept Morphological
Biological
Phylogenetic
Ecologically distinct populations (Ecotypes) Phylogenetic species concept in prokaryotes
• The marine cyanobacterium Prochlorococcus is the smallest and most abundant photosynthetic organism on Earth.
The phylogenetic distribution of Prochlorococcus clades as determined by rRNA internal transcribed spacer (ITS) sequence diversity.
Biological species concept in prokaryotes
• Horizontal gene transfer occurs more frequently within recognized species (Dykhuizen and Green, 1991)
Distance trees for alleles ofgnd and trp from the same set of strains. All alleles are from E. coli except the one from S. typhimurium, LT2. Prokaryote Ecotype “species”
The Prochlorococcus federation Prokaryote ecotypes
• Prochlorococcus
Unshared genes Unshared
rRNA ITS identity % Prokaryote Ecotypes
• Challenges: • Hierarchies within clusters • Size of a cluster that corresponds with an ecotype • Ecotypes don’t stay fixed Phylogeny of the Bacillus licheniformis subclade of the B. subtilis–B. licheniformis clade based on the neighbor joining analysis of sequence diversity at rpoB. Question
• Why do we need species concepts? Species concepts in Conservation
• Evolutionarily Significant Units
Criteria for ESU determinations:
1. Substantial reproductive isolation
2. Importance to evolutionary legacy of the species as a whole
2. Barriers to gene flow
Barrier Process Example mechanisms Pre-mating Geographic
Reproductive
Prezygotic Reproductive: After Mating
Postzygotic Reproductive: After fertilization
3. 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 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
H.oc ⁄ H.ag Allopatric
Comparison of survivorship between replicates of pure and hybrid crosses. X-axis is number of days and H.oc ⁄ H.ot Y-axis is number of larvae surviving. Allopatric Number Larvae pure H. agrammus H.ot / H.ag Allopatric Pure H. octogrammus H. otakii Hybrids in nature
Days Hybrids never occur in nature Gene frequencies High High* Medium Low Speciation in marine Pop 1 prokaryotes
• Ocean water is highly structured at small (millimetre to micrometre) scales Pop • “Interpretation of gene- 2 content diversity requires High rates of the delineation of Homologous Gene loss genetic turnover recombination populations according to Gene cohesive gene flow and flow ecology, as micro- Vertical evolutionary changes arise in inheritance response to local selection Ecology pressures and population dynamics”
Common functions Habitat- Biological interactions independent of specific • Cordero and Polz 2014 habitat functions 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)