Transcript Mechanisms of Change

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 Flow
Founder Effect
• Allele frequencies change as a result of
the migration of a small subgroup of a
population.
Example of Gene Flow
Bottleneck 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;
Evolution Vs. Genetic Equilibrium
Genetic equilibrium: the situation in which allele frequencies remain
constant.
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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