Natural Selection

Charles Darwin & Alfred Russell

Wallace

Darwin’s Influences

• Darwin observed such variations in

species on his voyage as a naturalist on

the HMS Beagle

Darwin’s Influences

• Kept vast diaries of the many organisms seen

and their tremendous variations

Ex. Galapagos Islands—finches, tortoises

Ex. Snakes with legs

Ex. Penguins use wings to swim

Ex. Fossils of extinct animals as well

as those that closely resemble

modern animals

Darwin’s Influences

• J. B. de Lamarck

- Claimed that all organisms

had a desire to change

- The more an organism

used a structure, the larger

it became

- These acquired traits could

be inherited by their

offspring

Darwin’s Influences

• Charles Lyell

• “Principles of Geology”

– Geological processes on Earth resulted from

continuous cycling over a long period of

time.

– This allowed Darwin to consider that the

Earth was much older than originally

believed and thus organisms could change

over a very long period of time

Darwin’s Influences

• Artificial Selection

The process in which breeders choose the

variations to be used to produce the

following generations

– Darwin noted that breeders and farmers

create better (stronger/faster) livestock and

crops through selective breeding…

Would make sense that this could

occur in nature too….

Artificial SelectionAll these veggies from wild mustard plants?!

http://evolution.berkeley.edu/evolibrary/article/evo_30

Darwin’s Influences

Thomas Malthus’ theory

• Natural populations do not grow out of

control in nature, even though some

reproduce by the 1000’s

• Populations are kept in check by

predators, diseases, limitations in food,

water, and other resources that are

essential for survival, hence….there is a

“struggle for existence” (Darwin)

Change is slow…

but it does happen!

The following circumstances are needed for

natural selection to occur..

Population Size

• Natural populations do not grow out of

control in nature, even though some

reproduce by the 1000’s

• Populations are kept in check by

predators, diseases, limitations in food,

water, and other resources that are

essential for survival, hence….there is a

“struggle for existence” (Darwin)

Variation

• Variation in genotype and phenotype

exists in populations (from sexual

reproduction)

• Some variations are more suitable for

environmental conditions

Variation in Cuban tree snails…

VARIATION

in flowers…

In lady

bugs…

Fitness of Individuals

• Some individuals have genetic variations

that create more favourable traits in a

particular environment or habitat.

• These favourable traits may allow an

individual to live longer and produce more

offspring.

• This individual is more fit

Natural Selection

• The organisms best suited for the

environment will survive

• The best suited individual will live longer

and leave more offspring

• These offspring will inherit the

advantageous genotypes and phenotypes

of their parents and will also be fit for

their environment.

At the same time….

• Organisms with less favourable traits are

less likely to survive and reproduce

• Thus, they are less likely to pass on these

unfavourable genes to the next generation

• Over time, these genes may be “wiped

out” of a population.

Struggle for Survival

• The fact that there is a constant struggle

for survival among individuals in an

overcrowded population means that some

of the individuals will die before

reproducing.

• The most fit individuals in a population are

the ones that are least likely to die of

starvation etc.

.

NOTE: Natural selection will usually

only cause changes in a population if

the environment the population is in is

changing.

If the environment stays the same, it

will always be the same traits that are

favourable (fit).

A summary of natural selection

1. No two members of a species are identical (there is variation)

2. Variation is inheritable (alleles of genes are passed from parents to offspring)

3. More organisms are born than reach adulthood

4. Organisms compete for resources (struggle to survive)

5. Survival of the fittest

New Species

• Over time, this process can result in

adaptations that specialize populations

for particular ecological niches and may

eventually result in the emergence of new

species.

Pepper Moths & Natural

Selection

In the beginning…

• Trees covered in off-

white lichens…moths

rest on trees

• Light coloured moths

have a survival

advantage

(camouflaged)

Industrial Revolution…

• Soot starts to cover

trees, darkening

them

• Dark coloured moths

have a survival

advantage

(camouflaged)

Discuss with your partner how

you think the peppered moth

example demonstrates all the

conditions needed for natural

selection to occurs

Summary

• Both colour variations exist in the moth

population BEFORE the industrial revol.

• Environmental changes select one colour

over the other as a survival advantage

• More of one colour moth survives longer,

reproduces more…thus, the population of

each colour increases or decreases