Other Causes of Variation

14.3, 14.4, 14.5Starting page 299

1. Chromosomal Mutations:

Random variation is due to mutations in genes. When variations in genes occur, natural selection selects the successful ones.

Gene pool: the group of individuals in a population share a group of genes. Different forms of the same genes are called alleles. The number of times the form of an allele occurs is called the relative frequency of the allele.

2. Genetic Drift:

Geneticists have learned that an allele can become common in a population by chance.

A) Animals with a specific allele may produce more offspring by chance. 

B) Environmental events could wipe out individuals who carry an allele. Ex: The eruption of Mt. St. Helens wiped out most of the Mountain Goats in the area.

Genetic drift usually occurs in small populations as natural disasters are less likely to affect all members of a large population.

3. Gene Flow:

Movement of individuals into and out of the population alters the genetic equilibrium.

Immigration: addition of genes

Emigration: removal of genes.

14-4 The Development of a New Species

 New species arise from pre-existing species through a process called speciation.

The combination of an organism’s habitat and its role in that habitat is its niche. No two species can occupy the same niche in the same location. The more efficient species will survive, reproduce and drive the less efficient species to extinction.

Cause of Speciation: Reproductive Isolation

  If a population of the same species is separated,

natural selection can operate differently on each group. The gene flow between the isolated groups stop. Eventually the groups become so different that they can’t interbreed and two new species form.

Reproductive Isolation can occur from geographical barriers (mountains, rivers, roads), differences in courtship behavior or fertile periods.

Example: Darwin’s Finches on Galapagos Islands:

13 bird species evolved from a single ancestral species. Each species exhibits body structures and behaviors that allow it to inhabit a specific niche.

1. Somehow they arrived at Island A and where able to survive and reproduce.

 2. Somehow, some birds got from Island A to Island B. The oceans between them prevented them from reproducing.

 3. Overtime the birds from the two islands adapted to the needs of their environments and their beak size changed according to these conditions.

 4. Overtime the birds on each island become different species and can no longer interbreed.

Example: Darwin’s Finches on Galapagos Islands:

Divergent evolution (adaptive radiation):

when a number of different species diverge or move away from a common ancestor.

Example: Darwin’s finches, dinosaurs, Hawaiian honeycreepers.

Convergent evolution: when evolution of different

organisms produce species that are similar in appearance and behavior.

Example: wings of

butterfly, bird and bat are very different internally and come from different origins but have similar appearances.

Types of Evolutionary Change Gradualism: the theory that evolutionary change

occurs very slowly. (Evidence from fossil record). Most of the time plants and animals are in a state of equilibrium.

Rapid Evolutionary Change:-this happens when the state of equilibrium is disrupted. -caused when a small part of a population becomes isolated and evolves more quickly because of smaller gene pool.

Punctuated Equilibrium: when long stage periods are interrupted by brief periods of change.

Role of Extinction in Evolution

When a species of plant and animal become so unfit for their environment, they become extinct.

Plants and animals that are so unfit for their environment are endangered species.

Rapid changes in the environment result in mass extinctions. This leaves empty niches for remaining species.