Mechanisms of Change

What is a population?

A population is a group of individuals of the same species that can interbreed

A population shares a common group of genes

Gene Pool – all of the genes, including different alleles, that exist in a population

Mutation�A mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. That would make genes for brown coloration more frequent in the population than they were before the mutation.

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

Gene flow

• Gene flow — also called migration — is any movement of genes from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries.

​

​

​

​

​

• If genes are carried to a population where those genes previously did not exist, gene flow can be a very important source of genetic variation. �

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 and had no offspring. The next generation would have a few more brown beetles than the previous generation — but just by chance. These chance changes from generation to generation are known as genetic drift.

Genetic Drift

• In each generation, some individuals may, just by chance, leave behind a few more descendants (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the "lucky" individuals, not necessarily the healthier or "better" individuals. That, in a nutshell, is genetic drift. It happens to ALL populations — there's no avoiding the results of chance.

​

Bottleneck Effect

• This occurs when a random catastrophic event reduces the population of an organism by a large number.
• The remaining individuals repopulate the area after the event but the genetic variation of the population is greatly reduced.

Founders Effect

• This occurs when a group of individuals are separated from the main population and establish a new population.
• This population’s genetic variation is also greatly reduced.

Defining a Species

• A species is a group of individuals that actually or potentially interbreed in nature.
• For example, these happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator.

​

​

Speciation

• Speciation is a lineage-splitting event that produces two or more separate species. 

Causes of speciation

• Allopatric –Split by a geographic barrier (organisms can’t get to each other)
• Sympatric Speciation – in same geographic region so there is no geographic isolation but there may be

behavioral isolation

​

​

​

​

​

A.) Gradualism – Organisms change slowly over long periods of time and at a steady pace.

Paces of Evolution

Modern Biologists propose two possible models of how organisms appear to diverge over time by natural selection.

B.) Punctuated Equilibrium –

organisms go through periods of rapid change followed by long periods of stability.

Example of Punctuated Equilibrium

In Lake Victoria, an isolated lake which formed recently in the African rift valley, over 300 species of cichlid fish diverged (developed) from one parent species in just 15,000 years.

​

​

Patterns of Evolution

Modern Biologists also describe two patterns of evolution that can be observed in living organisms and the fossil record.

1.) Convergent Evolution – unrelated species may independently evolve phylogenic similarities due to adaptations to similar environments.

Example: Dolphins and whales (which are mammals) have dorsal fins and tails

which function the same as a fish’s.

​

Divergent Evolution – branching out of a population into new environments resulting in new species.

For example, Darwin thought that Galapagos Islands finches were descendants of South American finches, brought there by a hurricane or other large storm.

DNA studies have shown that Darwin was correct. All the finches on the Galapagos Islands were related to a species of finch in South America.

​

Traits which helped the finches survive in their new environment were passed on to offspring. These adaptations helped the finches survive in their new environment.

​

After arriving in the Galapagos Islands, the finches reproduced and spread from island to island.

(geographic isolation)

Natural selection can affect the distributions of phenotypes in any of three ways:

• directional selection
• (The population moves in direction.)
• stabilizing selection
• (The population stabilizes.)
• disruptive selection
• (The population is disrupted.)

Directional Selection

• When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place.

Stabilizing Selection

• When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.

Disruptive Selection

• When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.

What is being described?

What is being described?

What is being described?

What is being described?

What is being described?

What is being described?

• Geographical Isolation – when organisms can’t get to each other to mate due to a geographical barrier, causes speciation
• Genetic Drift – when, by chance, part of a population passes its genes to future populations