Natural SelectionStruggle for ExistenceSurvival of the Fittest

Descent with ModificationResults of Natural Selection

Natural Selection1. 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.

Natural Selection2. 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

Natural Selection3. 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

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.

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.

Evidence of Evolution

Fossil Record Any preserved remains of ancient organisms

Darwin Proposed that the earth was millions of years old and from looking at the fossil records in older rocks and fossils from younger rocks scienists could document that life on earth had changes over time.

Geographic Distribution Different species living in the same

region/environment have similar body structures and behaviors

Homologous Body Structures Structures that have different mature forms

but develop from the same embryonic tissue; look the same but used differently

Strong evidence that all four-limbed vertebrates have descended, with modification from common ancestors.

Evidence (continued)

Vestigial organsOrgans that are so reduced in size that

they are just traces of homologous organs in other species.Example:

Whale hips Human appendix

Evidence (continued)

Comparative GeneticsDNA from different

organisms works the same

Early development is similar

Evidence (continued)