Return to England (1836-1859):

·    Studied all of the data that he collected and realized the animals found on the Galapagos Islands were found no where else in the world.

·    He realized that species from the South American mainland gradually became modified over time.

·    Wrote the essay On the Origin of Species (1859) which proposed the mechanism for evolution he called Natural Selection.

·    Prompted to publish when another scientist, Alfred Russel Wallace, mailed him a manuscript proposing the same theory.



Darwin discovered that variations in organisms were VERY important.


·    Darwin realized that members of each species vary from one another in important ways.

·    Observing breeders, Darwin noted that they would use genetic variation to improve crops or livestock. Darwin called this artificial selection.

·    Artificial selection: nature provided the variation and humans selected those variations that they found useful.

·    Darwin next compared artificial selection to what happens in nature. From this comparison, he developed his theory of natural selection.



5 major points to natural selection:


1.           Genetic variations exist in populations.

·    Some variations are more favorable than others.

·     These variations are inherited.


2. Organisms produce more offspring than can survive.

·    Those that do not survive, do not reproduce.


3. Overproduction of offspring forces competition for resources.

·    Darwin called this the struggle for existence.

·    Not all offspring can possibly survive.


4. Individuals with favorable variations have more “fitness” and are more likely to survive and pass those variations on to their offspring.

·    Darwin called this survival of the fittest.

·    Fitness is the ability of an individual to survive and reproduce.

·    Organisms with low fitness do not survive and therefore, do not reproduce.

·    Populations gradually change or evolve toward the favorable variations over time.


5. Species alive today are descended with modification from ancestral species that lived in the distant past.

·    Darwin called this descent with modification.

·    Common descent means that all living things belong to a single “tree of life”.


Adaptations: any inherited trait that increases the chances of survival and reproduction of an organism.

·     Adaptations increase an organism’s “fitness”.


Examples: Mimicry- one species mimics the appearance of another. 


The viceroy (a non-toxic butterfly) mimics the coloring of the supposedly toxic Monarch butterfly.

             Viceroy                                                Monarch



The bubblebee moth (harmless) mimics the bubblebee.

      Bubblebee Moth                                        Bubblebee


Camouflage- species blend in with surroundings.    

Walking Stick

     Indian Walking Stick

Leopard                                                            Ground Squirrel