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• Chapter 20: Genes
within population
•
i)
ii)
iii)
iv)
v)
Lamarck’s theory of
evolution
New needs are generated
by environmental changes
These changes determine
use or disuse of organs
Use of organs determines
their development and
disuse determines
diminish of organs
Acquired characters are
hereditary or inheritance
of acquired characters
Functions create organs
•
Darwin’s theory of
evolution
i) Differences are displayed
by different individuals
ii) Individuals with superior
differences (natural
selection) are able to
reach adulthood,
reproduce and transmit
their traits to their off
springs.
iii) Individuals with inferior
qualities cannot survive
or lose
Polymorphic variation
• When loci have more than
one allele occurring at a
frequency greater than 5%
• Vertebrates are less
polymorphic (5-8%)
• Drosophila and
invertebrates (15%)
• Plants (8%) Ex: Loosestrife
– Lythrum salicaria
exhibits variation in flower
color
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1.
2.
3.
4.
5.
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Hardy-Weinberg principle
Proposed by Godfrey H. Hardy and Wilhelm Weinberg in
1908
Explains stability of population for reasons:
No mutation takes place
No genes are transferred to or from other sources (no
immigration or emigration takes place)
Random mating is occurring
Size of population is very large
No selection occurs
Since no change in genotype proportions therefore also
known as Hardy-Weinberg equilibrium
• p and q designates the frequency of B and b alleles, respectively
• Sum of three genotypes BB + Bb + bb = 1
• For homozygous BB and bb – probability to receive B and b alleles is
possible from both the parents (p2 or q2)
• For heterozygous Bb – probability to receive B allele from father and
b allele from mother or vice versa (pxq and qxp, respectively)
• P2+2pq+q2 = 1
•
Five agents of
Evolutionary change
1. Mutation changes alleles
- Mutation in any allele can
change proportions of
particular alleles in
population
- Occurrence is 1/10,000
cell division
- Not affected by natural
selection
• Five agents of Evolutionary
change
2. Gene flow occurs when
individuals or gametes
move between populations
- Drifting of gametes or
immature stages of plants
or marine animals
- Wind dispersal of pollen or
seeds or carried by animals
- Mating of individuals of
adjacent populations
• Five agents of Evolutionary
change
• Nonrandom mating
- Assortative mating happens
when phenotypically
similar individuals mate
and result in more
homozygotes.
- Disassortative mating
happens when
phenotypically different
individuals mate and result
in more heterozyotes
• Five agents of Evolutionary
change
• Genetic drift
- Some descendants of a
generation/population
survive
- Does not lead to
adaptations
- Is not a result of natural
selection
- Can happen because of a
catastrophe
The Founder effect – One or few individuals disperse and become founders of a new
isolated population away from their place of origin.
The Bottleneck effect – Organisms do not move from place to place and their
population decreases due to various natural forces.
• Five agents of Evolutionary
change
• Selection- Natural and Artificial
selection
• Based on phenotype and behavior, an
individual is able to produce more
progeny.
• Natural selection can lead to evolution
due to three reasons
- Variation must exist among
individuals in a population
- Variation must result in differences in
the number of offspring surviving in
the next generation
- Variation must be genetically inherited
• Selection to avoid
predators:
- Selection leading to
adaptative
evolutionary changes
- To avoid predation
population is evolving
via natural selection
and adapting to
genetic changes.
• Selection to match climatic conditions
• - Enzyme allele frequencies vary with latitude
- Mummichog (Fundulus heteroclitus) has enzyme
lactate dehydrogenase.
- Lactate dehydrogenase is a better catalyst at low
temperature for fishes of north side than south side
- Selection for pesticide resistance
- Houseflies have genes pen, kdr and dld-r which act as
resistance genes that decrease uptake of insecticide
Fig. 20.8
• Selection occurs when individual of one phenotype produce
more surviving offspring than its counter phenotype
• Reproducing ability can be related to fitness of an organism
• Fitness is a combination of several factors like survival,
mating success and number of offspring per mating
• Gene flow can be constructive as it can spread evolutionary
change carrying alleles
• Gene flow can also constrain evolution or natural selection
Example of gene flow opposing natural selection is occurring
on abandoned mine sites in Great Britain
• Frequency-dependent selection: Fitness of an individual
depends upon its frequency within population
• Negative frequency-dependent selection: Search image
formed by predators makes latter prey upon common types
• Positive frequency-dependent selection: Search image
formed by predators makes latter prey upon uncommon
types
Fig. 20.12
Heterozygote exhibit greater fitness than homozygote
Resistance to malaria and
increased fertility in
heterozygotes
How disruptive selection removes intermediates?
Disruptive selection eliminates finch birds with intermediate beaks which are
neither able to feed on large nor small seeds
How directional selection eliminates phenotypes on one end of a range?
Individuals concentrated towards one extreme of the array of
phenotypes are favored
How stabilizing selection favors individuals with intermediate phenotypes?
Both smaller and larger weight babies have a greater tendency to die than
those of intermediate weight.
Fig. 20.17
Field and
laboratory
experiments
proved that
selection can
lead to rapid
evolutionary
changes
Fig. 20.19
Limitations of selection
Genetic variation can get removed by intense selection pressure
Fig. 20.20
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