Emerging Diseases

Lecture 3: Important Theories

3.1: Overview3.2: Theory of Evolution

3.3: Cell Theory

3.1Overview: Three Important Theories

The Theory of Evolution (late 1800s)Through preferential reproduction of some

members, species change over time in adaption to external changes such as environmental change.

The Cell Theory (mid 1800s)All living organisms are made up of one or more

cells.

The Germ Theory (late 1800s)See Lecture 1.

3.2: The Theory of Evolution

Charles Darwin credited with the

basic idea and theresearch the research

More individuals are born than can survive.Individuals in a population are different from each

other.These differences are passed down to offspring.

What determines who survives?• The inherited differences that produce some

advantage.• The survivors reproduce-therefore their

offspring have the advantageous characteristics.

• This is called selection.

Natural Selection

• Natural selection is what occurs in the real world

• Modern biology understands

genetic changes such as mutation- though Darwin did not

Darwin realized that generations of natural selection could account for the origin of new

species, biotypes, strains or races.

Artificial Selection

• Caused by humans-usually through selective breeding-for example domestic animals.

• But also in hospital settings

Individuals with maladaptive traits tend not to survive and reproduce

In modern times, this is recognized by the annual “Darwin Awards”

3.3: The Cell Theory

All living organisms are composed of at least one fundamental building block called a cell.

This theory took almost 200 years to develop.

Became fully accepted in the 19th Century.

Modern Understanding of the Cell Theory

All modern day cells are related to one another by descent from a common ancestor (evolution)

There are three types of cells and thus three main groups of organisms. These groups are called

domains.

ArchaeaBacteria (aka Eubacteria)

Eukaryotes

Archaea

• Single-celled• Harsh environments

(extremophiles)• Ancient form of life• No nucleus

(prokaryote) • Not known to cause

disease

Bacteria or Eubacteria

• Single-celled• Many

environments• Very diverse and

“creative”• No nucleus

(prokaryote) • Many are disease

agents

Eukaryotes• Single or multi-

celled• Contain a

nucleus (eukaryote)

• Many are disease agents

• Humans are eukaryotes Considered large and complex

Evidence for Three Domains

• Lots of biochemical evidence• The best evidence comes from studies of

genes. The genes fall into 3 main groups based on the code that the genes contain.

• Evidence first provided by Carl Woese.

“Universal Tree” of Life