Download Evolutionary Change Without Selection File

EVOLUTIONARY CHANGE
WITHOUT SELECTION
-describe evolutionary mechanisms & how they affect the
development & extinction of various
species
-genetic drift
-genetic bottlenecks
-founder effect
- the Hardy-Weinberg Principle
Overview
• Not all evolutionary changes are the result of
natural selection.
• Some changes such as genetic drift, bottlenecks
and the founder effect are not influenced by the
traits of individuals.
• Each of these changes tends to reduce genetic
diversity within a population
Genetic Drift
Genetic Drift:
• is the random shifting of the genetic makeup of the
next generation.
• is changes to allele frequency within populations and
species as a result of chance; such changes are much
more pronounced in small populations.
• allows for evolution to occur by helping to increase
and change genes that are present in the gene pool.
Genetic Drift and Population Size
• The larger the size of a population, the smaller the
effect of genetic drift.
• Genetic drift is much more obvious in small
populations than in larger ones.
E.g., if you toss a coin 5 times there is a possibility
that all 5 times heads may appear. However, if you
toss the coin 50 times, it is more likely that heads will
appear 50% of the time and tails will appear 50% of
the time.
BOTTLENECKS
Genetic bottlenecks:
• dramatic, often temporary, reductions in
population size.
• result in a loss in genetic diversity.
• have unfavorable consequences for populations.
• eventually, the population may recover and grow
to their original size again. However, the allele
frequencies that were previously present may have
been altered.
Figure 1
• Severe bottleneck
may decrease the
chance of evolving
because of a decrease
in genetic variation.
Figure 2
• An example of this type of genetic drift
is cheetahs; they have very little genetic
variation because their population was
subject to a genetic bottleneck.
FOUNDER EFFECT
• occurs when a small number of individuals
establish a new population.
• the allele frequencies of the new smaller
population will be different from those of the
original larger population.
E.g., About 30 Amish people in Pennsylvania left a large
population and one of the Amish had a recessive trait for
shorter limbs and six fingers. The frequency of the short
limbs and 6 fingers increased
because of the founder effect.
The Hardy-Weinberg Principle
Relative frequencies of genotypes in a
population from generation remain constant.
That means, the dominant gene does not become
prevalent and the recessive gene does not
disappear.
E.g. among 1000 fruit flies: 910 are red-eyed, 90
are green-eyed; after mating in the next generation
the ratio remained the same.
5 Conditions Required for
Hardy-Weinberg Equilibrium:
1. No mutation
2. Random mating
3. Very large population size
4. Isolated from other population (no immigrations
or emigrations)
5. No natural selection.
SUMMARY
• Genetic drift produces evolutionary changes independently
of natural selection.
• Bottlenecks and the founder effect enhance the influence of
genetic drift.
• Genetic drift, bottlenecks and the founder effect increase
genetic variation in species and populations.
• The Hardy-Weinberg principle can be used to identify
factors that will result in evolutionary change.