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MBY365 Theme 1 summary
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Full summary of theme 1 (microbial diversity, species and speciation) made using lecture notes and class notes.
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MICROBIAL DIVERSITY, SPECIES AND SPECIATIONOVERVIEW
- Biologists use species as the unit to describe diversity
- This system was formalised by Carolus Linnaeus around 1735
- Carl Woese helped determine that life is divided into 3 domains - Archaea,
Bacteria and Eukarya
- Microorganisms belong to all 3 domains
TWO DOMAINS OR THREE?
- Recent work shows that Eukarya evolved from within Archaea and are
specifically related to newly discovered archaeal species
- Research shows that the tree of life should consist of only two domains -
bacteria and archaea
- Archaea is paraphyletic because eukaryotes emerged from within it
- Asgard archaea is said to have several genes that encode “eukaryotic
signature proteins”
- Theses are genes that were thought not to have homologs in either Bacteria
or Archaea
- The last Archaeal and Eukaryal common ancestor (LAECA) represents the
branch point that lead to all eukaryotes on one side and all archaea on the
other
- Based on phylogenetic groups, there are only two domains
- Eukaryotes may also be accepted as a third domain due to their differences
from their archaeal ancestors
TAXONOMY
- Monophyly - a group composed of a collection or organisms including the
most recent common ancestor of all the organisms and its descendants
- The hierarchical system of classification is based on the notion that all taxa
should be part of monophyletic groups
- Paraphyly does not include all descendants of a common ancestor
, THE SPECIES CONCEPT
- A species is a habitually interbreeding community
- Reproductive barriers are isolating mechanisms
- They may be a result of incompatibility between parents or hybrid stability
- Species - evolutionary stage at which interbreeding populations become
segregated into separate groups that cannot interbreed
- Speciation is driven by selection and is a random process
- A species is a discrete package of total genetic variability
- Species exist to protect a “harmonious gene pool”
- The term species has two meanings:
- The species taxon
- The species category
- There are currently 3 species concepts used in microbiology:
- Biological species concept
- Genotypic cluster species concept
- Phylogenetic species concept
- BSC - species are groups of interbreeding natural populations that are
reproductively isolated from other such groups
- GCSC - a species is a morphologically or genetically distinguishable group of
individuals that has few or no intermediates when in contact with other such
clusters
- PSC - a species is the smallest exclusive monophyletic group of common
ancestry
- Speciation is a continuous process and not all species have diverged at the
same time
- We have species that have speciated long ago from a common ancestor and
therefore gene flow has been limiter for a long period
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