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 Variation The 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 Variation Hi. 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!

Adaptation A trait that helps an organism survive in its environment Adaptations can be physical (something an organism has) or behavioral (something an organism does)

Fitness The 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 Selection Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species’ fitness in its environment

Natural Selection Over 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 Selection Over 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 Selection Even though these giraffes look similar. They still have differences between them. Some are bigger or faster. Others may be smarter, hardier, hairier, stronger, meaner, or have better stamina or camouflage. What are these differences between individuals of the same species called?

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

Common Descent Each 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