Darwin’s Theory of Evolutionas a Mechanistic Process

• Darwin’s Idea of Common Descent

• Darwin’s Idea of Gradualism

• Darwin’s Idea of Multiplication of

Species

• Darwin’s Idea of Natural Selection

Darwin’s Idea of

COMMON DESCENT

• evolution = descent with modification

• All organisms are related through descent from some unknown

ancestor that lived in the distant past.

• As the descendants spilled into various habitats over time, they

accumulated diverse modifications (adaptations) that fit them to

specific ways of life.

• The history of life is like a tree.

• The Linnean classification scheme reflected the branching

genealogy of the tree of life, with organisms at the different

levels related through descent from common ancestors.

The evolutionary history of

organisms can be portrayed

as a tree growing through

time.

Genealogy of the primates

Darwin’s Idea of

GRADUALISM

• The origin of new species and adaptation are closely related

processes.

• A new species would arise from an ancestral form by the

gradual accumulation of adaptations to a different

environment.

• e.g. Darwin’s finches ADAPTIVE RADIATION

large ground finch small tree finch woodpecker finch

Darwin’s Idea of

MULTIPLICATION of SPECIES

The existence of an enormous number of species

some species are very similar (not as distinct from

each other!)

gradual changes in various characteristics as

organisms became modified according to the

conditions in which they lived

Darwin’s Idea of NATURAL SELECTION

as the Mechanism for Evolution

• Overproduction

- All species have a tendency and the potential to

increase at a geometric rate.

2. Competition

- The conditions supporting life are limited.

- Only a fraction of the offspring in a population will

live to produce offspring, so that the number of

individuals in a population remains fairly constant.

The environments of most organisms have been

in constant change throughout geologic time.

3. Variation

- Individuals in a population vary greatly in their

characteristics.

4. Adaptation

- Some variations enable individuals to produce more

offspring than other individuals.

5. Natural Selection

- Individuals having favorable traits will produce more

offspring, and those with unfavorable traits will produce

fewer offspring.

• Speciation- Given time, natural selection leads to the accumulation of

changes that differentiate groups from one another, such

that a new species may arise.

Industrial Melanism:

The Peppered Moth (Biston betularia)

Natural Selection Survival of the Fittest

Other examples:

1. Insecticide resistance

2. Drug resistance in bacteria

A population is the smallest unit that can

evolve.

Natural selection acts on individuals, but

individuals do not evolve.

Natural vs. Artificial Selection

Camouflage as an example of

evolutionary adaptation

Patterns of Evolution

Divergent evolution – from one species to

several different forms; adaptive radiation

Convergent evolution – results in

increased resemblance between

unrelated species

Coevolution – occurs when two or more

species evolve in response to each other

Darwin’s Theory of Evolution in a nutshell

Biological diversity is the product

of evolution.

The mechanism of modification

has been natural selection

working continuously over long

periods of time.

At the time, Darwin did not understand the genetic basis for

evolution.

Variations arise from mutation and genetic recombination.

Much of the variation observed in the individuals of a

population is heritable.

The Synthetic Theory of Evolution

(Neo-Darwinism)

Variation mostly occurs as a result of gene mutations

and genetic recombination.

Evolution is the change in allele frequency within a

population over time.

gene

allele

frequency

gene pool

Ernst Mayr