An Introduction to the Theory of Evolution

By: C. Chary

Learning Target

• LT BI8.a*: I can explain how “natural selection” determines the differential survival of groups of organisms.

Natural VariationThe differences between individuals of the same

species.

All three of these giraffes are the same species, but they have some differences too. These differences are called natural

variation.

If there was no natural variation, all organisms of any species would look exactly alike!

Natural VariationHi. I’m a giraffe. I have

a long neck so I can easily eat leaves on tall trees. I also have long legs and can run fast.

Hi, I’m a giraffe too. I have a pretty long neck so I can eat

many of the leaves on tall trees. I can also eat the tougher

plants near the ground. I have short legs, so I can’t run too

fast.

Hi. I’m a giraffe too. I can’t reach the leaves up high since I’m too

short, but I have strong jaws and can chew the tougher plants nearer to the ground. I have shorter legs

and a smaller body than my friends, but I can run really fast!

AdaptationA trait that helps an organism

survive in its environmentAdaptations can be physical (something an organism has) or behavioral (something an organism does)

FitnessThe ability of an organism to survive and reproduce in its

environment.Let’s take a look at the giraffes’ environment

Giraffes eat plants…Some plants are tall, and their

leaves are hard to reach Some are short

and easier to reach, but the

leaves are tougher to eat.

Giraffes also have predators in their

environment.Lions can run pretty fast

and hunt in groups

I can find food and outrun most of my predators. I have a high

level of fitness.

I can find food easily. Even though I’m

slow, I can still outrun some

lions. I have an average level of

fitness.

I can find food easily and can outrun almost any lion alive!

I have an extremely

high level of fitness.

The Struggle for Existence

All of the giraffes compete with each other and other species to

survive in this environment.

The competition within and between species to survive and reproduce.

One thing is for sure…

The environment will always change.

What would happen if this environment changed and there

were no more short trees?

I can still find food and outrun

most of my predators. I still

have a high level of fitness.

I can still find food easily. Even though I’m slow, I can still outrun some lions. I still have an average level of fitness.

I can’t find enough to eat.

I still can outrun almost any lion alive, but that won’t matter if I starve.I have a very low level

of fitness.

Since I have a low level of fitness, I probably

won’t survive. My adaptations are not

good in this new environment.

I won’t reproduce and pass on my “short”

genes to future giraffes.

Even though I can find food, I will probably get caught by a lion sooner or

later.Because of my low level of fitness, I

won’t survive and pass my “slow” genes to future giraffes.

Survival of the Fittest

Organisms with the highest level

of fitness will survive and

reproduce more successfully.

Natural SelectionOver time, natural selection results in

changes in the inherited characteristics of a

population.These changes increase a species’ fitness in its

environment

Natural SelectionOver time, natural selection results in

changes in the inherited characteristics of a

population.These changes increase a species’ fitness in its

environment

Let me translate this for you…

I had the highest level of fitness.

I have a better chance to survive and

reproduce.I will pass on my

adaptations to my offspring,

Over time, all giraffes will have longer necks

and faster speed.

Natural SelectionOver time, the average giraffe has a longer neck than before. This change in a species over

time is called evolution.

It is important to understand that these changes take

millions of years to happen… evolution is a very slow

process.

Natural SelectionEven though these giraffes look similar. They still have differences between them.

Some are bigger or faster. Others may besmarter, hardier, hairier, stronger,

meaner, or have better stamina or camouflage.

What are these differences between individuals of the

same species called?

Descent with ModificationEach living species has descended, with changes, from other species over time.

Common DescentEach living species came

from common ancestors. If we look far enough back, we can find the common

ancestor for all organisms.

An Introduction to the Theory of Evolution

THE END