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