Darwin and the Theory of Natural Selection

Chapter 10

Hypothesis and TheoryA hypothesis is a possible explanation for a

set of observations or a possible answer to a scientific question.

A theory is a well-tested explanation that is based on observations, experimentation, and reasoning. It has been tested and confirmed as a general principle helping to explain and predict natural phenomena.

Hypothesis and TheoryThere are several

theories of evolution today.

Was Darwin the first person to develop a theory of evolution?

What theory did Darwin develop that is still widely-accepted in the scientific community today?

Darwin and the Voyage of the BeagleIn his early 20’s, Darwin

set sail on the H.M.S. Beagle. The voyage to the Galapagos Island was one of the most important voyages in science.

Darwin made numerous observations and collected evidence that led him to propose a revolutionary hypothesis about the way life changes over time.

Darwin and the Voyage of the BeagleOver many years of

study and experimentation supported by a huge body of evidence, his hypothesis became a theory.

His theory of evolution is known as the theory of natural selection.

Darwin and the Voyage of the Beagle

Darwin and the Voyage of the BeagleGalapagos IslandsHe noted that

although the islands were all close together, they had very different climates.

The lowest islands were hot, dry, and almost barren. They had sparse vegetation.

The higher islands had greater rainfall and, thus, rich vegetation. They had a greater diversity of plants and animals.

Darwin observed that similar species had traits adapted to the different environments.

Galapagos IslandsHood Island Hood Island

Galapagos Islands

Hood Island tortoise

Isabella Island tortoise

Galapagos IslandsGalapagos TortoisesShape of shell

corresponds to different habitats.

Hood tortoise shell is curved and open around neck and legs, and a longer neck. Why?

Isabela tortoise shell has a “domed” shell and shorter neck. Why?

Galapagos IslandsGalapagos FinchesDarwin wondered about the

changes in shape of bird beaks from island to island. So-called cactus finches have longer, more pointed beaks than their relatives the ground finches. Beaks of warbler finches are thinner and more pointed than both. These adaptations make them more fit to survive on available food where they live.

Scientists Who Influenced Darwin HuttonGeologistProposed that layers of

rock form slowly; some are forced up, some buried, others pushed up from sea floor. These rocks, valleys, and mountains are shaped by natural forces (rain, wind, heat/cold). These processes occur extremely slowly.

LyellGeologistExplained how

geological features could be built up or torn down over extremely long periods of time.

Ex. Forces raising the bottom of a sea floor to the top of a mountain.

Scientists Who Influenced DarwinBoth Hutton and

LyellThey both

provided evidence for a very old Earth. At the time, people thought the Earth was thousands of years old. Hutton and Lyell argued it must be at least millions of years old.

How old do scientists estimate the Earth to be today?

Darwin asked himself, if the Earth changes over time, couldn’t life change, too? That would only be possible if the Earth were extremely old.

Scientists Who Influenced Darwin

Scientists Who Influenced Darwin

Scientists Who Influenced DarwinLaMarckRecognized before Darwin that living things changed

(evolved) over time.According to Lamarck, organisms altered their

behavior in response to environmental change. Their changed behavior, in turn, modified their organs, and their offspring inherited those "improved" structures.

3 main hypotheses (Q- Why aren’t they theories?)Tendency toward perfection Use and DisuseInheritance of Acquired Characteristics

Scientists Who Influenced DarwinTendency Toward

PerfectionLaMarck believed all

organisms had a natural tendency toward perfection. They “strived” to become better-adapted to their environment through their behavior.

Scientists Who Influenced DarwinUse and DisuseLaMarck believed

organisms can alter the size/shape of organs (or make a new one appear) by using them in new ways. Conversely, an organ could decrease in size or go away if it was not used.

Scientists Who Influenced DarwinLaMarck believed that

organisms “needed” to become more

perfect, and out of this will and their

behavior, developed new adaptations.

Scientists Who Influenced DarwinInheritance of

Acquired Characteristics

LaMarck believed acquired characteristics were passed to offspring.

Ex: giraffes that stretched their necks would have babies with long necks

What did LaMarck have no knowledge of which could explain why he believed this? DNA!

Did Darwin have knowledge of DNA?

NO

Scientists Who Influenced DarwinThomas MalthusNoticed babies were

being born faster than people were dying

Reasoned that if the human population continued to grow unchecked, there would not be enough food and living space

Forces he thought controlled growth were war, famine, and disease.

When Darwin read Malthus’s work, he reasoned it applied even more strongly to plants and animals b/c they produce far more offspring than humans.

Scientists Who Influenced DarwinThis is him, the real

Thomas Malthis! He is a real

“looker”!

Carolus LinnaeusDeveloped a system

for classification of organisms.