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LECTURE 1O
MICROBIAL EVOLUTION &
Classification
Fig. 10.14 Stromatolites
Fossilized, photosynthetic, microbial mats
• The Earth is about 4.5 billion years old.
• Microorganisms have been around for about 4
billion years.
• The early Earth was anoxic and much hotter.
• The evidence suggests that biologically
important molecules could be formed when
gaseous mixtures like the primitive Earth
are irradiated or shocked.
The earliest “organisms” probably did not haveany DNA. They consisted of RNA and just a few,if any proteins.
When self-replicating RNA molecules becameenclosed in membranes, they became the firstcellular life forms.
Soon, proteins become main cellular enzymes.
Using DNA as the genome likely resulted fromthe need to store genetic information in a morestable form.
Evolution of Life - a Time Scale
What is the origin of eukaryotes?
Many theories e.g.:
1) Archaea-like cells evolved a nucleus,
endomembranes, and cytosketeton.
2) Endosymbiotic theory: Archaea like cell
engulfed a Gram - bacterium = mitochondrion
or a cyanobacterium = chloroplast
See next slide and read Perspective 3.1 on
page 76
The Evolution of Endosymbiosis
Classifying Microorganisms
The grouping of organisms according todistinguishing characteristics that theyshare.
Why is taxonomy and systemmatics important?
1) Allows information to be organized so it canbe easily accessed by all in the scientificcommunity.
2) By knowing properties and characteristics, wecan make predictions about related organisms.(“model organisms”)
3) Allows for a common language. Enforces strictrules.
4) Essential to accurately identify organisms.
History of attempts to classify microbes..
A little after the time of Leeuwenhoek (lec. 2),
Linnaeus tried to classify all living things
1759, named all known organisms
using the binomial system (Genus species).
divided the world into
Animal, Vegetable and Mineral
Not much was known about microbes at that
time so Linnaeus gave up in frustration and
put all microscopic
life into one genus, Chaos!
Whittaker (1969) and others
5 kingdom system:
Plants, Animals, Fungi, Protists and Bacteria
3 main modes of nutrition:
photosynthesis, adsorption, and ingestion
show foot tree (next slide)
This tree of life has been widely accepted
but it is not phylogenetically correct.
Archaea??
Not
monophyletic
What is monophyly?
Monophyletic group
Not a monophyletic group
Fig. 10.1. The 3-Domain system used in
modern biology Table 10.01
An aside on taxonomic ranks and the binomialsystem…….
Bacillus anthracis Proteus vulgaris
B. Subtilis
Genus species names should be in italics
or underlined… E. coli or E. coli
The Binomial System SystematicsThe science of characterizing and organizingliving things, hopefully in a phylogenetic(evolutionarily historical) way…
See Figure 10.18
Fig. 10.18Fig. 10.18.Example of aphylogenetictree.Nodes areancesteralbranchpoints orpoints ofcommonancestors.
What are characters?
The information used to compare organisms..
• morphological characters
• metabolic characters
• molecular characters
Molecular Systematics……
Less ambiguous esp. for microbes
G+C content of DNA - useful for grouping Bacteria
Fig. 10.16G + C content may seem like a crude measure,but it’s proven to be quite valuable formicrobial taxonomy.
But, it still doesn’t tell you anything about thesequence of the organisms’ DNA.
Molecular characters - II. DNA-DNA
hybridization
Fig. 10.17
Fig. 10.17. DNA-DNA hybriization
Modern systematics is based on direct examination of the “blue prints” of lifei.e. DNA and RNA sequences…..
The most useful sequences for classifying all forms of life are ribosomal RNA because they are:
1)found in all forms of life2)change slowly (over evolutionary time)
Figure 10.9
Bacteria Archaea Eukaryote
Molecular characters - III. Nucleic Acid Sequencing
SSU Ribosomal RNA
Fig. 10.1