Notes: Evolution & Natural Selection

Charles Darwin

His father was a wealthy doctor and wanted Charles to become a doctor.

At age 16, he went to Medical School.In 1827, his father sent him to become a priest.In 1831, a friend asked him to go on a ocean trip as an unpaid naturalist.

Other Scientists and EvolutionHutton- Geologist who studied how geological

forces shaped EarthEarth processes happen very slowlyEarth must be older than a few thousand years

Lyell- Geologist who said that in order to understand the past, we must observe how geological processes shape the earth today. If Earth can change over time, so must life. Examples: volcanoes, earthquakes

Lamark was a French naturalist who…Claimed that organisms tend to be complex and perfect

and are therefore unchanging. Use and disuse- Lamark thought that animals could

change their bodies by using them in new ways. Example: animals could use front limbs as wings and develop flight.

Inheritance of acquired traits- Lamark though that all changes could be inherited. For example, if you lift weights, your children would inherit your muscles.

Lamark’s ideas are not scientifically accepted today!

Darwin’s Voyage on the HMS Beagle

The purpose of Darwin’s trip was to draw accurate maps of the region. Darwin was to collect specimens and document the plant and animal life.

Darwin’s original beliefs

Divine creation - each species was created by God, unchanging and existing as it was originally created.

However, on the voyage, his beliefs began to change when he got to the Galapagos Islands.

Darwin’s Finches

The finches showed convincing evidencethat species evolve.

Darwin was struck by the fact that the animals of the Galapagos Islands resembled those of the nearby coast of South America.

Darwin hypothesized that ancestors of the Galapagos Islands must have migrated to the island and the population slowly changed after they arrived.

Darwin published his book in 1844:

On the Origin of Species by Means of Natural Selection.

Darwin’s Theory of Natural Selection

1. There is variation within any species

2. All organisms tend to over-reproduce

3. There is a struggle for survival

4. The fittest survive

5. The environment determines who is the fittest

1. Variation exists within every population

1. Ladybugs have different spots

2. Zebras all havedifferent stripes

1. Variation exists within every population

2. All species tend to produce more offspring than they can SUPPORT

3. Frog eggs

1. Turtle eggs

2. Insect eggs

4. Baby mice

3. There is a Struggle for Survival

1. To eat

2. To drink

3. Territory

4. To mate

4. The fittest will survive

4. The fittest will survive

Healthy or sick horse?

5. Nature (the environment) will determine which organism is the

fittest to Survive

1. long neck giraffes survive over short neck giraffes.

Evidence of Natural Selection in our lifetime

The Peppered Moth in England

Before the industrial revolution

After the industrialrevolution

Explanation for the peppered moth

There was variation in the moth population to begin with, some were black and some were white.

This variation was due to random mutations.

Before the industrial revolution, the bark of the tree bark was white in color.

The white moths then were easily camouflaged, survived, and reproduced.

Black moths were rare.

After the industrial revolution, the trees became covered in soot/pollution from all of the factories and the trees turned black.

Now the black moth were easily camouflaged, survived, and reproduced.

White moths were rare.

How does the peppered moth illustrate natural selection?

There was variation in the population to begin with (some white, some black).

Moths tend to over-reproduce.

Therefore there is a struggle for survival.

The fittest (those that were camouflaged the best) survived.

The environment determined which moths were the most fit.

How Do New Traits Appear? Recall that all genes have at least 2 alleles

(forms)All organisms have additional “invisible variation”

due to small biochemical differencesMutations- Random mutations also occur as a

result of mistakes in DNA replication or due to environmental factors.

Gene Shuffling- homologous chromosome pairs move independently during meiosis- the 23 pairs in humans can produce 8.4 million different gene combinations!

Natural Selection may lead to Speciation

Speciation - the evolution of a new species

The process of speciationGeographic Isolation - when two

populations become isolated by a geographic barrier such as mountains.

Geographic Isolation

Reproductive Isolation - Sometimes when populations have been separated for a long, long time, they become so different that they can no longer breed and produce fertile offspring.

Same species now, but may become two different ones.

1. Horse and Donkey = MuleMule is infertile (can’t reproduce)

mulehorse donkey

Lion and Tiger = LigerLiger is often infertile (there have

been successful breedings)

2. The Kaibab and Abert SquirrelsBecame reproductively isolated by huge mountains

3. Wood and Leopard Frogs

Leopard frogsWood frogs

Seem to be the same species, but they breed at different times of the year.

Speciation -When two populations become so different that they can no longer breed and produce fertile offspring. They are then considered two separate species.

Speciation in Finches

Types of SelectionDirectional Selection- when individuals at one

end of the curve in a population have higher fitness than in the middle or other end. Example: Supply of small seeds is low, birds with

larger beaks can still feed, birds with smaller beaks cannot. We will see higher numbers of birds with larger beaks.

Disruptive selection- When individuals at the upper and lower ends of the curve have higher fitness than in the middle. Example: If medium seeds are scarce, small and

large seeds are plentiful, birds with small or large beaks will be most fit. Medium beaks will not be as numerous.

Stabilizing selection- when individuals in the center of the curve have higher fitness than those at the ends. Example: babies with very low birth rate may not

survive while very large babies may have trouble being delivered. Average sized babies are most fit.