1. Natural Selection Struggle for Existence Survival of the Fittest
Descent with Modification
Results of Natural Selection

2. Natural Selection
Struggle for Existence – members of each species compete regularly to obtain food, living space, and other necessities of life
From Malthus’ work, Darwin concluded that high birth rates and a shortage of life’s basic needs would force competition for resources.

3. Natural Selection
Survival of the Fittest – some individuals are better suited for their environment than others; those best suited will survive
Fitness – ability of an individual to survive and reproduce in its specific environment
Result of adaptations
Evolutionary Fitness – organism’s success in passing genes to the next generation
Where fitness varies, natural selection can act.
Adaptation – any inherited characteristic that increases an organism’s chance of survival
Could be anatomical, behavioral, physiological
Evolutionary adaptation – any genetically controlled physiological, anatomical, or behavioral trait that increases an individual’s ability to pass along its genes

4. Natural Selection
Descent with Modification – every living species has descended, with changes, from other species over time
Implies that all living organisms are related to one another
Principle of Common Descent – all species (living and extinct) were derived from common ancestors
Allows for a single “tree of life” that links all living things

5. Result of Natural Selection…
…changes in the inherited characteristics of a population
These changes increase a species’ fitness in its environment.
Acts on populations, not individuals. It is populations that evolve over time.

6. Summary of Darwin’s Theory of Evolution
Individual organisms differ (variations exist in a population).
Some of this variation is heritable.
In nature, there is a tendency for overpopulation.
Organisms produce more offspring than can survive. There is a high death rate.
Only the organisms with the variations that are suitable to the environment survive.
Organisms must compete for limited resources.
Each unique organism has unique advantages and disadvantages in the struggle for existence.
The organisms that survive get to reproduce and pass on their genes.
The process of Natural Selection causes species to change over time.
It is the population that changes to match the environment – not individuals.
Species alive today are descended with modification from ancestral species.