The document discusses Hardy-Weinberg equilibrium, which describes populations where genotype frequencies remain stable between generations because five conditions are met: the population is very large to prevent genetic drift, there is no migration or gene flow, no new mutations, random mating occurs, and no natural selection influences survival. Deviations from Hardy-Weinberg equilibrium indicate evolution is occurring due to factors like natural selection, genetic drift, or gene flow changing allele frequencies across generations.

1. Hardy-Weinberg Equilibrium
 If there are two alleles for a particular gene
 Then
 dominant alleles + recessive alleles = 100%
 100% can also also be represented as 1.0
 The proportion of each allele is also called its
FREQUENCY
 % = proportion = frequency

2. Figure 23.9-1
Generation 1
p (frequency of CR
) = 0.7
q (frequency of CW
) = 0.3
CR
CR CR
CR
CR
CW
CW
CW CR
CR
CR
CW
CR
CR
CR
CW
CR
CR
CR
CW
frequency of CR
= 0.7
q (frequency of CW
) = 0.3

3. Hardy-Weinberg Equilibrium
 With two alleles, there are
three possible genotypes:
 XX
 Xx
 xx

4. Hardy-Weinberg Equlibrium
If a population is not evolving, then you should
have the same number of
XX , Xx, and xx individuals in every generation.
But if the proportions of XX, Xx, and xx change
from one generation to the next, then the
population is EVOLVING.

5. Hardy-Weinberg Equlibrium
 Let’s call the frequency of the dominant allele
(X)… p.
 Let’s call the frequency of the recessive allele
(x)… q.
 If only X and x alleles exist, then p + q = 1.0
 If you know q, you can figure out p.

6. Hardy-Weinberg Equilibrium
 If the relative frequencies of X and x change
from one generation to the next, then the
population is evolving.
 If the proportion of XX, Xx and xx individuals
in a population changes from one generation
to the next, then the population is evolving.

7. Hardy-Weinberg Equilibrium
 A population that is NOT
EVOLVING is said to be in
Hardy-Weinberg equilibrium.
 We can use HW calculations to
measure microevolution in
populations.

8.  Three major factors alter allele frequencies and bring
about most evolutionary change:
 Natural selection
 Genetic drift
 Gene flow
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9. Natural Selection
 Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions
 For example, an allele that confers resistance to
DDT increased in frequency after DDT was used
widely in agriculture
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10. Components of Natural
selectionOVERPRODUCTION

11. HERITABLE VARIABILITY

12. COMPETITION

13. I. Natural Selection
Green mamba is arboreal
Its venom is most effective
against birds.
Black mamba is terrestrial
Its venom is most
effective against
mammals.

14. Genetic drift
 Genetic drift also called as
 allelic drift or the Sewall Wright effect
 is the change in the frequency of
a gene variant (allele) in a population due to
random sampling of organisms.

15. Genetic drift
 One of the requirements for the maintenance
of allele frequencies in populations is a very
large population size.
 Genetic drift is the consequence of small
population size.

16. GENETIC DRIFT
 The smaller the population, the less genetic
variety it has.
 In a very small population, alleles can be lost
from one generation to the next, simply by
random chance.
 When a population evolves only because of
this type of random sampling error,
GENETIC DRIFT is taking place.

17. Genetic Drift
 The smaller a sample, the greater the chance of
deviation from a predicted result
 Genetic drift tends to reduce genetic variation
through losses of alleles
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18. Effects of Genetic Drift
1. significant in small populations
2. causes allele frequencies to change at
random
3. can lead to a loss of genetic variation within
populations
4. can cause harmful alleles to become fixed
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19. The Founder Effect
 The founder effect occurs when a few individuals
become isolated from a larger population
 Allele frequencies in the small founder population
can be different from those in the larger parent
population
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20. FOUNDER EFFECT

21. The Bottleneck Effect
 sudden reduction in population size due to a change
in the environment
 New gene pool may not reflect original
 If the population remains small, it may be further
affected by genetic drift
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22. Genetic Bottlenecks
• Populations may suffer a dramatic
drop in numbers.
• This could be due to a chance
event, such as a volcanic eruption,
earthquake, flood, fires etc
• The few survivors will contain a
much smaller variety of alleles than
the original population.
• The genetic diversity of the new
population will remain restricted.
• E.g. Cheetahs

23. Gene Flow
 Gene flow consists of the movement of alleles among
populations
 Alleles can be transferred through the movement of
fertile individuals or gametes (for example, pollen)
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24. BOTTLENECK EFFECT

25.  Gene flow can increase the fitness of a population
 Consider, for example, the spread of alleles for
resistance to insecticides
 Insecticides have been used to target mosquitoes that
carry West Nile virus and malaria
 Alleles have evolved in some populations that confer
insecticide resistance to these mosquitoes
 The flow of insecticide resistance alleles into a
population can cause an increase in fitness
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26. Hardy-Weinberg equilibrium
describes populations that are
not evolving.
 Genotype frequencies stay the same if five
conditions are met.
 very large population: no genetic drift
 no emigration or immigration: no gene flow
 no mutations: no new alleles added to gene pool
 random mating:
no sexual selection
 no natural selection:
all traits aid equally
in survival