Natural Selection

Lab 14

Evolution - Defined  

Evolution

composition of a population over time. – a change in the genetic  A change in the frequency of certain alleles.

On a larger scale, evolution can be used to refer to the gradual appearance of all biological diversity.

A Mechanism for Evolution 

Darwin

presented a mechanism for evolution –

natural selection

.

 Organisms that are in some way more successful at reproduction will pass on more of their genes.

  Over time the traits responsible for that success will become widespread in the population.

This theory holds up very well!!

Natural Selection  Natural selection occurs when organisms with particular heritable traits have more offspring that survive & reproduce.

Natural Selection  Natural selection can increase the adaptation of an organism to its environment.

Natural Selection  When an environment changes, or when individuals move to a new environment, natural selection may result in adaptation to the new conditions.

 Sometimes this results in a new species.

Natural Selection   Individuals do not evolve; populations evolve.

Evolution is measured as changes in relative proportions of heritable variations in a population over several generations.

Natural Selection  Natural selection can only work on heritable traits.

 Acquired traits are not heritable and are not subject to natural selection.

Natural Selection  Environmental factors are variable.

 A trait that is beneficial in one place or time may be detrimental in another place or time.

Natural Selection  When natural selection is occurring, some individuals are having better reproductive success than others.

 Alleles are being passed to the next generation in frequencies that are different from the current generation.

Genetic Drift  The smaller the sample, the greater the chance of deviation from expected results.

 These random deviations from expected frequencies are called

genetic drift

.

 Allele frequencies are more likely to deviate from the expected in small populations.

Genetic Drift   Which allele was lost is due to random chance.

Over time, drift tends to reduce genetic variation through random loss of alleles.

Natural Selection & Adaptation   Natural selection is a way of altering the gene pool that results in adaptation.

 Genetic drift alters the gene pool but does not result in adaptation.

Selection depends on variation.