[IB Ecology]

Natural Selection

Species: One kind of organism. Members of a species generally have the same morphological (body) characteristics. In sexually reproducing species, individuals are able to interbreed under natural conditions to produce fertile offspring.

What determines whether 2 related individuals are members of the same species or not?

a) Historically, body characteristics were used to determine relatedness in regards to species.

b) 2 individuals are able to sexually reproduce and produce living, reproducing young.

c. Nearly identical DNA (DNA tests now used extensively to determine relatedness.)

[An illustrated tour of how natural selection might occur.]

Natural Selection: Process by which organisms best suited to their environmental conditions are most likely to survive and reproduce.

Principles of Natural Selection:

1. Populations produce more offspring than the environment can support.

2. Over production of offspring within a species produces competition for resources or a struggle to survive.

3. All members of a species (except clones) show variation. Variation is a genetic difference between individuals of a species in regards to various traits.

EX.: Brown eyes, blue eyes, green eyes, gray eyes, albino (red) eyes.

4. Individuals with DNA for a particular variation may give the individual a better chance of survival in one set of conditions and may have lesser survival value under other conditions.

Variations which result in new DNA only happen as the result of gene mutations.

Adaptation: An inherited trait that increases an organisms's chance to survive in a particular environment.

"Survival of the fittest" : Those individuals with variations that allow them to survive a particular environmental condition survive to breed. The variation is potentially passed on to the offspring. Those offspring with the same variation then have a better chance to survive if the conditions persist. Over time, genetic change leads to increased fitness or adaptation to the environment.

Genetic variation allows a population to have individuals which are able to survive a wide range of environmental situations. Thus if some members of the species die because they are unable to survive a change, others do survive and produce young that also have a chance to survive the change.

[Multiple Antibiotic Resistance in Bacteria: A current Example of Natural Selection]

[Evidence for Natural Selection]

[An illustrated tour of how natural selection might occur.]