Transcript Evolution: A change in gene frequency within a population

Evolution: A change
in gene frequency
within a population
Evolution 201
Learning Targets:





I can describe Genetic Drift and the conditions
required for it to happen.
I can give examples of Genetic Drift.
I can define allele.
I can differentiate between the three types of
Modes of Selection
Homework:
study, study, study
Look at this diversity
A change in Gene Frequency




A gene is a section of DNA that codes for a
specific trait.
An Allele is a different version of a gene
We get one copy of a gene (an allele) from
our father and one copy of a gene (an
allele) from our mother.
Evolution looks at what allele is more or
less common.
Alleles:
Background: Darwin’s
Finches as an Example of (A)
Natural Selection
Background on (A) Natural
Selection
From this pattern Darwin recognized that in
nature, organisms struggle for existence and that
more offspring are born than live to reproduce.
 He called the ability of an individual to survive
and reproduce in its specific environment –
fitness.
 This fitness is a result of adaptations and only
those individuals that are the fittest survive.
Background on (A) Natural
Selection

Those organisms with specific characteristics
(adaptations) that allow them to live long
enough to reproduce are considered the fittest.
Background on (A) Natural
Selection

Darwin said that over
long periods of time
organisms that are the
fittest survive passing
on those traits to their
offspring causing a
change in gene
frequency. He called
this a Descent with
Modification.
Background: Evolution 201


We know that in genetic terms, Descent with
Modification results in alleles being passed to
the next generation in proportions that differ
from the present generation.
The three mechanisms that alter allele
frequencies directly and cause most evolutionary
change are



(A) Natural Selection
(B) Genetic Drift
(C) Gene flow
(B) Genetic Drift

Genetic Drift is when chance events can cause
allele frequencies to fluctuate unpredictably
from one generation to the next….

Especially in small populations!!!
What is Genetic Drift?


Since Darwin, one of the most important
advances in Evolution research has been
the recognition of the role of genetic Drift
as an evolutionary force
Genetic drift refers to the power random
events can have in influencing whether
genes increase or decrease in future
populations.
Examples of Genetic Drift:


Certain Circumstances can result in
genetic drift having a significant impact on
a population.
Conditions for Genetic Drift:
Small Populations
 Random Acts
 Variation with the population


Two examples are the founder’s effect and the
Bottleneck effect.
Bottleneck Effect
Bottleneck effect is when there is a sudden
change in the environment, such as a fire or
flood that drastically reduces the size of a
population.
 By chance alone, certain alleles may be
overrepresented by the survivors, others may be
underrepresented and some may be absent. As
generations pass this should lead to substantial
effects to the gene pool.
Bottleneck Effect
Bottleneck Effect
Founder’s Effect
Effects of Genetic Drift: A
summary




Genetic Drift is significant in small
populations
Genetic Drift can cause allele frequencies
to change at random
Genetic drift can lead to a loss of genetic
variation within a population
Genetic drift can cause harmful alleles to
become fixed.
C. Gene Flow


Natural Selection and genetic drift are not
the only phenomena affecting allele
frequency.
Gene Flow is the transfer of alleles into or
out of the population due to the movement
of fertile individuals or their gametes.
Gene Flow
Types of Natural Selection



Directional Selection: when natural
selection favors an extreme phenotype.
Stabilizing Selection: favors the middle
ground – not the extremes.
Disruptive Selection – favors both
extremes not the middle
Types of Natural Selection

Directional Selection: For example, when the
environment selects the fastest deer, most
flexible trees, etc.
Types of Natural Selection

Disruptive Selection: Black-bellied seed cracker
finches (in Cameroon). The smallest finches can
eat soft seeds while the largest finches can crack
the hard seeds. The average size finches can not
do either well.
Types of Natural Selection

Stabilizing Selection: For example, human
baby weight.