Mutations and genetic recombination introduce genetic variation within populations. Natural selection acts on this variation, favoring traits that increase reproductive success. Over generations, this leads to adaptation and evolution of populations as allele frequencies change. Genetic drift also causes random changes in allele frequencies, especially in small, isolated populations. Gene flow moves alleles between populations through migration.

1. Sources of genet ic
variat ions
Ch. 21

3. Mut at ions
1. mutations:
Is a change in the
sequence of nucleotides
in the DNA
May result in new alleles
or new genes
Is usually harmful
Must be in the gametes
to be passed on to
offspring

4.  Mutations are more likely to be
favorable when the environment is
changing

5. Duplicat ions
 Duplication mutations can lead
to an expanded genome with
more loci that can take on new
functions
 Duplication mutations are
thought to be the reason why
certain mammals have more
genes for detecting odors than
others

6. Mut at ion rat es
 Plants and animals
average about 1
mutation for every
100,000 genes per
generation
 Mutations happen more
quickly in organisms that
reproduce quickly, like
bacteria

7. 2. Sexual Recombinat ion of
DNA
 During crossing over
and fertilization
 Produces variations
that make adaptation
possible
 Sexual reproduction
results in unique
combinations of genes
every generation

8. Quick Checks
 What is “sexual recombination”?
 What processes need to occur in cells
for “sexual recombination” to occur?

9. Shif t s in allele f requency

10.  Most evolutionary change the result of…
1. Natural selection
2. Genetic drift
3. Gene flow
 Most evolutionary change the result of…
1. Natural selection
2. Genetic drift
3. Gene flow

11. Nat ural Select ion
 Results in the
differential
success of
organisms in being
able to reproduce
and pass on their
genes to the next
generation

12. Genet ic Drif t
 Fluctuations in allele frequencies in a small
population from one generation to the next
 The smaller the size of the population, the more likely
there is to be a major shift in allele frequencies

13. Genet ic Drif t
Evolution by drift is aimless, not
adaptive, because it is chance
alone
Drift is common in two population
events: Genetic bottlenecks and
Founder events.
3 initial breeding
pairs - shift in
gene pool

14. The Bot t leneck Ef f ect
 when a population
undergoes a drastic
reduction in size as a
result of chance
events (fire, flood,
drought).
 It is completely
arbitrary and
unrelated to
phenotype.

15. The Bot t leneck
Ef f ect
 Lack of variation means less
adaptability
 Some alleles may be
overrepresented in the
survivors, some
underrepresented, and some
not represented at all.
 Humans sometimes create
bottlenecks in other species

16. The
Founder
Ef f ect
 a small group of individuals becomes
separated from the larger population. They
may establish a new population with a gene
pool that is not representative of the larger
population. Helps account for the high
frequency of certain disorders in isolated
populations

17. Gene Flow
 Gene flow - movement of genes between
populations. Gain or loss of alleles from a
population due to migration of fertile individuals,
or from the transfer of gametes.
 Tends to reduce differences between
populations
 Gene flow increases the variability of the gene
pool by adding new alleles.

18. Biof lix direct ions
 Natural selection
 Genetic Drift
 Gene Flow
 Title, explanation,
example
Natural selection
Explanation:
Example:
Genetic Drift
Explanation:
Example:
Gene Flow
Explanation:
Example:

19. Nat ural Select ion in
more det ail
21

20. Variat ion in a
populat ion
 Discrete characters - “either/or”
traits, usually controlled by a single
gene
 E.g. - widow’s peak or no widow’s
peak
 Phenotypic polymorphism - the
population has 2 or more distinct
morphs for a trait that are fairly
common in the population

21.  Genetic polymorphisms - the presence
of 2 or more distinct alleles in the gene
pool of a population
 May result in phenotypes that vary
along a continuum, like height in
humans

22. Geographic variat ion
 Most species
have geographic
variations in their
gene pools
between different
populations
 Due to
environmental
differences

23. Survival of t he
f it t est
 Fitness - the contribution an
individual makes to the gene pool
of the next generation for
advantageous adaptations
 Relative fitness - the contribution
of a genotype to the next
generation. It is dependent upon
the genetic and environmental
context in which it was expressed
(is it still an advantage to have
that gene?)

24. Modes of Select ion
 Directional
 Most common during times of change or migration
 Favors one end of the phenotype range

25. Modes of Select ion
 Disruptive
 Occurs when conditions favor the two extremes in
the phenotype range
 Most likely to result in speciation

26. Modes of Select ion
 Stabilizing -
 Favors the average phenotype
 Reduces variation in a population

28. Why don’t “negat ive” phenot ypes
event ually disappear ?
 Diploids - variation “hides” in
recessive alleles of
heterozygotes
 Balancing selection - natural
selection that favors two or
more phenotypes in a
population, called balanced
polymorphism

29. The Het erozygot e
Advant age
 When individuals that are
heterozygous for a trait have greater
fitness than homozygotes
 Example: sickle cell anemia
 AA - normal RBCs but prone to the
worst effects of malaria
 aa - have sickle cell anemia
 Aa - do not have sickle cell, are
protected against the worst effects of
malaria

30. Frequency Dependent Select ion
 The fitness of any one morph declines if
that morph becomes too common in the
population
 Common in predator-prey relationships

31. Sexual Select ion
 Natural selection for mating success
 Sexual dimorphism - the obvious
differences between males & females in
a population

32.  Intrasexual selection -
members of the same
sex compete with each
other for mates
 Antlers, large size, etc
 Intersexual selection -
mate choice -
individuals of one sex
are choosy in selecting
their mate
 Peacock tails, etc.
 Reflects upon the overall
fitness of the individual
(only healthy males can
grow that big tail)

33. Nat ural Select ion cannot
creat e perf ect organisms
1. New phenotypes are the result of many small
changes to previous phenotypes
2. As the environment changes, previous adaptations
may not be favorable anymore
3. Selection can only act on variations that already
exist in the gene pool