Population change over time

“Evolution” means change over time.

The “Theory of Evolution” says:

– Living things on Earth have changed over time.

– Many species that once existed are now extinct. – Many species alive today were not here when life on Earth first formed.

First thought of by the ancient Greeks (in 600 BC ) but they did not try to explain HOW it happened.

First scientific attempt to explain how evolution happened came 2400 years later! In 1802 Jean LAMARCK Jean LAMARCK proposed model:proposed model:

Evolution by Acquired CharacteristicsEvolution by Acquired Characteristics 1. More individuals are born than the

environment can support - there is a struggle to survive.

2. Some individuals respond by developing traits that give them a better chance to survive.

3. When these individuals reproduce their offspring inherit their newly acquired, improved traits and develop them even further.

4. Eventually ALL members of the species have the favorable trait. Jean Baptiste Lamarck

1744-1829

Lamarck’s explanation of how the giraffe got its long neck:Lamarck’s explanation of how the giraffe got its long neck:

1. ALL early giraffes had the same (short ) necks.

1 2 3

2. As giraffe population increased there wasn’t enough foodwithin reach for all so some began stretching their necks to reach food higher in the trees. – The more they stretched, the longer their necks grew.

3. When they reproduced they passed their longer necks on to their offspring who stretched their necks even more. – Eventually all giraffes had long necks.

Lamarck’s model was quickly discarded. Scientists tried but could not find evidence to support his main ideas.1. All members of a species are NOT alike as

Lamarck said. Great variation normally and naturally exists within a species.

3. Organisms can change and improve some things during their lifetimes (lifting weights to increase strength etc.) but these traits CANNOT be

passed to offspring. If this man wants his future children to have small noses will it help to get a nose job??

2. Organisms cannot change most of their basic physical traits at will, even if survival depends on It. They cannot “adapt” because they need to. For example giraffes cannot make their necks grow longer by stretching them!

Is there anything he could do that might help?

In 1831, Charles Darwin (failed medical student, failed divinity student, avid beetle

collector) began a 5 year journey on a small ship, the H.M.S. Beagle.

Charles Darwin1809-1882

90’ long

75 people

5 years

Aroundthe world!

Purpose: survey coast of South AmericaDarwin’s job: observe geology & biology of the area.

– Did NOT set out to develop a model, but couldn’t ignore what he saw. Hiscuriosity drove him to try to explain what he had observed.

– Kept meticulous records of observations. His observations from the

Galapagos IslandsGalapagos Islands led to his model of how evolution occurred.

Galapagos IslandsGalapagos Islands are 600 miles off west coast of Ecuador (South America).

Volcanic -

first rose above the

ocean surface less than 15 million years ago.

Why is this special?Why is this special?1) Life has been there a relatively short time (compared to billions of years on the mainland!). 2) It developed in isolation from life on the mainland.

High lush island

Low, desert island

Darwin was surprised to find species that clearly were closely related but that had distinct differences on different islands.

Darwin’s explanation of how evolution occurred:

2. Within every species there is naturally variation.

1. More individuals areborn in a species than the environment cansupport. There is a struggle to survive.

3. Variations of some give them anadvantage over others in the struggle tosurvive.

4. More of these individuals will surviveand reproduce than will those with lessfavorable traits.

6. More individuals with favorable traits reproduce, so the # of individuals with favorable traits increases in each generation.The # with less favorable traits decreases.

7. Eventually the favorable trait becomes the most common form and over many generations (and usually millions of years) such differences accumulate until a new species results.

5. Many traits are passedfrom parents to offspring.

Does it sound familiar???

Darwin called his modelDarwin called his model

““Natural Selection”Natural Selection”

1. Ancestral giraffes had necks of various lengths.

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How Darwin explained the giraffe’s How Darwin explained the giraffe’s long neck:long neck:

2. When food became scarce, longer-necked individuals could reach more food sources so they survived in greater numbers than those with shorter necks, many of whom starved and died.

– more long necks survived, so more reproduced and passed the long-necked trait to their offspring.– each generation had more long and fewer short necksthan the one before.

3. Eventually after many generations and millions of years ALL giraffes had long necks.

CHECK YOUR UNDERSTANDING:CHECK YOUR UNDERSTANDING:

1. Would you expect individual organisms to “evolve” in their lifetime?

2. What does evolve over time?3. What “force” shapes the characteristics of species - in

other words, what “selects” traits?

4. What might cause evolution to“speed up” - that is, cause species to change more rapidly?

5. What was the original cause of variation within species?

NO!POPULATIONS EVOLVE!

THE ENVIRONMENT

DRASTIC CHANGE IN ENVIRONMENT

MUTATION