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Evolution of Populations
In genetic terms, evolution is any change in the relative ________________________ of alleles in a population.
Evolution as Genetic Change
How does natural selection affect polygenic traits? Natural selection can affect the distributions of phenotypes in any of
three ways:

_____________________ - Selection eliminates one extreme
from a phenotypic array.

_____________________ - Selection acts to eliminate both
extremes from an array of pheynotypes..

_____________________ - Selection eliminates intermediate
types.
Genetic Drift: A _____________________ in allele frequency. Can cause
a loss of genetic diversity in a population. Bottleneck Effect & Founder Effect

Genetic drift may occur when a small group of individuals colonizes a new habitat.

Individuals may carry alleles in different relative frequencies than did the larger population from which they came.
Bottleneck Effect: This occurs after an __________ greatly _____________________________ of a population.

Over hunting of northern elephants seal during the
1800’s. By the 1890’s, the population was reduced to
about 20 individuals. These seals did not represent
the genetic diversity of the original population.
Now the population had grown to over 100,000
individuals. However it has very little genetic
variation.
Founder Effect: Occurs when a small number of individuals
_______________________________ a new area. The gene pools of these populations are often very different from those of the larger
populations.

In small populations, individuals that carry a particular
allele may have more descendants than other individuals.
Over time, a series of chance occurrences of this type can
cause an allele to become more ______________ in a
population. This model demonstrates how two small
groups from a large, diverse population could produce
new populations that differ from the original group.
When allele frequencies change due to migration of a small subgroup of a population it is known as the founder
effect.
The Process of Speciation

Natural selection and chance events can change the relative frequencies of alleles in a population and lead to
speciation. Speciation is the formation of new ___________________. A species is a group of organisms that breed with
one another and produce fertile offspring.

Isolating Mechanism: What factors are involved in the formation of new species? The gene pools of two populations
must become separated for them to become new species. As new species evolve, populations become
____________________ isolated from each other. When the members of two populations cannot interbreed and produce
fertile offspring, reproductive isolation has occurred.

Behavioral Isolation: Behavioral isolation occurs when two populations are capable of interbreeding but have
differences in _____________ rituals or other reproductive strategies that involve behavior.

Geographic Isolation: Geographic isolation occurs when two populations are separated by geographic
______________________ such as rivers or mountains.

Temporal Isolation: Temporal isolation occurs when two or more species reproduce at different ______________.
Patterns in Evolution

Adaptive Radiation: Adaptive radiation is one example of _________________evolution. Divergent evolution is the
process of two or more related species becoming more and more dissimilar.

Adaptive radiation is the process by which a single species or a small group of species evolves into __________________
forms. For example, in the adaptive radiation of Darwin's finches, more than a dozen species evolved from a single
species.

Rapid diversification of a species or a few species as they adapt to many environmental niches.
o
Birds with long beaks could use them to penetrate the soil to capture insects.
o
Large, claw-shaped beaks could crush nuts.

Convergent Evolution: Different organisms undergo adaptive radiation in different places or at different times but in
________________________ environments. The process by which unrelated organisms come to resemble one another is
called convergent evolution. Results in ________________________ structures.

Coevolution: Sometimes organisms that are closely connected to one another by ecological interactions evolve
____________________________. The process by which two species evolve in response to changes in each other over time is
called coevolution.