Evolution ❮ 151
❯ Rapid Review
There are four modes of evolution:
- Genetic drift:change in allele frequencies because of chance events (in small populations).
- Gene flow:change in allele frequencies as genes move from one population to another.
- Mutation:change in allele frequencies due to a random genetic changein an allele.
- Natural selection:process by which characters or traits are maintained or eliminated in
a population based on their contribution to the differential survival and reproductive
success of their “host” organisms.
There are three requirements for natural selectionto occur:
- Variation:differences must exist between individuals.
- Heritability:the traits to be selected for must be able to be passed along to offspring.
Traits that are not inherited cannot be selected against.
- Differential reproductive success:there must be variation among parents in how many
offspring they produce as a result of the different traits that the parents have.
Adaptationis a trait that, if altered, affects the fitness of an organism. Includes physical or
intangible traits.
Selection typesare as follows:
- Directional:members at one end of a spectrum are selected against, and population
shifts toward that end.
- Stabilizing:selection for the mean of a population; reduces variation in a population.
- Disruptive (diversifying ):selects for the two extremesof a population; selects against the
middle.
- Sexual:certain characters are selected for because they aid in mate acquisition.
- Artificial:human intervention in the form of selective breeding (cattle).
- E—Mutations in and of themselves are not evi-
dence for evolution, although they are necessary
if evolution is going to occur.
- D—Inorganic molecules could not have formed
in the presence of oxygen because oxygen would
have taken the place of other elements in every
chemical reaction (because it is such a highly
reactive element).
- D—This is the only answer that shows evidence of
natural selection, which is the nonrandomprocess
by which evolution occurs. The two elk species
splitting (answer A) is an example of allopatric
speciation caused by a random factor (a geologic
event). A mutation is also a random event (answer
B); for example, if we had said that the new variety
became the dominant allele in a population
because it had an advantage over other variants,
then that wouldbe natural selection. A home range
shift (answer C) is not evolution, but rather a
behavioral change within an organism’s lifetime.
Finally, a spider species declining in an area
because individuals are slowly changing territory is
an example of gene flow, which we know to be a
random process of evolution.
http://www.ebook3000.com
