Mechanisms of Change

The following four processes are the basic mechanisms by which

evolution occurs.

Its not just one mechanism that Evolution thrives on…..

Natural Selection

Imagine that green beetles are easier for birds to spot (and eat). Brown beetles are a little more likely to survive and produce offspring. They pass their genes for brown coloration to the their offspring. In the next generation, brown beetles are more common than in the previous generation.

Mutation/ Speciation

• A mutation could cause parents with bright green coloration to have offspring with a gene for brown. The genes for BROWN would then become more frequent in the population.

Genetic Drift• Imagine that in one generation, two brown

beetles happened to have four offspring survive to reproduce. Several green beetles were killed when someone stepped on them. The next generation would have a few more brown beetles than green (purely by chance)

• Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

• Small Populations

Gene Flow (Migration)

• Some individuals from a population of brown beetles might have joined the population of green beetles. That would make the gene for brown beetles more frequent in the green beetle population.

Ex of Gene FlowFounder Effect

• Allele frequencies change as a result of the migration of a small subgroup of a population.

Example of Gene FlowBottleneck Effect

• The bottleneck effect is an event in which a population’s size is greatly reduced.

• When this happens, genetic drift may have a substantial effect on the population. In other words, when the population size is radically reduced, gene frequencies in the population are likely to change just by random chance and many genes may be lost from the population, reducing the population’s genetic variation.

Bottleneck Effect

Term to know Gradualism

•Changes in species occurring slowly and incrementally.

Ex Gradualism

Term to know: Punctuated Equilibrium

•Long periods of little or no change interrupted by rapid and dramatic changes.

Punctuated Equilibrium

Non Evolution: Artificial Selection

• Long before Darwin and Wallace, farmers and breeders were using the idea of selection to cause major changes in the features of their plants and animals over the course of decades. Farmers and breeders allowed only the plants and animals with desirable characteristics to reproduce, causing the evolution of farm stock. This process is called artificial selection because people (instead of nature) select which organisms get to reproduce.

• Dogs are artificially selected all the time;

• Five conditions are required to maintain genetic equilibrium from generation to generation:

• There must be random mating

• The population must be very large

• There can be no movement into and out of the population

• No mutations

• And no natural selection

Evolution Vs. Genetic Equilibrium

Genetic equilibrium: the situation in which allele frequencies remain constant.