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Chapter 7
Selection
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•Natural selection: a culling process, wherein those with advantageous
features survive; those without perish
•Selective agents:
Biotic factor
Abiotic factor
•Herbet Spencer, introduced the phrase survival of the fittest
Fitness: number of off spring an individual contribute to the next generation
•How to measure fitness at the level of genetics?
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Artificial versus natural selection
Artificial selection: the wedding out of organisms by humans for human
purposes
Examples of artificial selection:
1- Plenty of pigeons
FIGURE 7.1 Pigeons
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2-Dogs and cats
FIGURE 7.2 Diversity of Dogs
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3- Agricultural
Selection of corn
Tassels
ears
FIGURE 7.4 Evolution of Corn
Scientists selected for high and low oil contents in corn,
how?.......
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Selection of tomato and roses
FIGURE 7.3 Diversity of Tomatoes
and Roses
Selection of begonias
FIGURE 7.5 Begonias
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•Natural selection: weeding out of organisms by biological processes
without deliberate or directed human interventions
•What happens to organisms that are less suitable to environment?
•The tendency of natural selection to eliminate unfavorable individuals is not
a matter of luck
•Natural selection is not random, but instead a consequence of an
active culling process
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Phenotypes takes a beating
•Natural selection acts on a phenotype & not a genotype
•The intensity of the environmental factors acting upon members of a
population is called selective pressure
•The phenotype is the product of genotype
•It is the phenotype that comes face to face with natural selection
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Examples of natural selection
1-Selection in peppered moth
•Melanic (dark)
•Peppered (light with black flecks)
FIGURE 7.6 Peppered Moth (Biston betularia)
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Prior to industrial revolution
•Light phase was cryptic
•Dark phase was conspicuous
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After industrial revolution
•Dark phase was cryptic
•Light phase was conspicuous
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How to proof the predictions
Kettlewell released known numbers of dark & light phases moth in
unpolluted & polluted woods
•In unpolluted forests, the light phase moth survived more than the dark
phase ones
•In polluted forests, the dark phase moth survived more than the light
phase ones
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2- Selection in snails
FIGURE 7.7 Snail Selection
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•Snail pulls its body inside the shell
•Thrush finds a hard stone “anvil stone” and flings the snail against it
Natural selection on snails
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Cain, Sheppard and associates, examined the prevalence of the
3 color types:
•They collect brown snails from beech woodland forests
•They collect green snails from meadows
•Pink tend to follow brown in both environment
•The snails that possess color similar to that of the habitat tend to escape
predation
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What do you expect to see at the anvil stone?
The shell debris of those that perished
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Cain and Shappard also studied Cepaea in deciduous woodlands in two
seasons: Early spring & Summer
Deciduous woodland
Spring
Common
Common
Rare
Summer
Rare
Rare
Common
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As the habitat changed with season, the selective advantage of the two
color phases of the snails changed as well
Deciduous woodland
Common
Spring
Common
Cryptic
Rare
conspicuous
Rare
Summer
Rare
Conspicuous
Common
Cryptic
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3- Selection in water snake
Water snakes have four different scored color phases:
A (unbanded) to D (banded)
FIGURE 7.8 Water Snake Differential Survival
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So how the differential elimination of C & D color patterns happened?
Against light color background, unbanded snakes are cryptic, while banded
ones are conspicuous
FIGURE 7.8 Water Snake Differential Survival
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Types of natural selection
In groups, there will be individuals with the trait at the extreme, but most
lie somewhere in between
FIGURE 7.9 Bell-Shaped Curve
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•The bell shaped curve represents the distribution of a character in a population
•Shading indicates where in that variation selection acts to eliminate individuals
FIGURE 7.10 Types of Selection-Stabilizing, Directional, and Disruptive
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Stabilizing selection
•The extremes are disadvantageous & eliminated, leaving the intermediate
phenotype favored & preserved
•Variations in the trait is reduced, the ranges narrows & population becomes
more uniform
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Directional selection
•One tail of the bell curve is disadvantageous, and natural selection acts
against it
•The average of population shifts over time, moving away from the
disadvantageous extreme
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Disruptive selection
•Individuals with traits in the middle tend to be eliminated
•The result is to produce two bell-shaped curves at the extreme
•Polymorphism: within the same species, individuals are found with two or
more conspicuous and distinctive forms of the same trait
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Sexual selection
Why do we find major differences between sexes within the same
species?
•Due to differences in copulatory organs
•Due to differences in secondary characteristics
Males & females within the same species differ morphologically & exhibit
different secondary sexual characteristics, a condition termed
sexual dimorphism
FIGURE 7.11 Sexual Dimorphism
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•Sexual dimorphism testifies to a special kind of selection that is unrelated to
adaptation called sexual selection.
•Sexual selection: a type of natural selection wherein individuals of the
same sex compete with each other for success in attracting a mate
•Females choose males with larger size or strength, or those with elaborate
display
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Sexual selection in Barn Swallows
•Barn Swallows are sexually dimorphic
•Males are more colorful than females & carry long tail feathers called
streamers
FIGURE 7.13 Barn Swallows
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•Long tailed males attracted females faster than short tailed males
•Long tailed males had an extra-pair copulation.
•Those extra-pair females that copulated with long tailed males were
more likely to have short tailed partners
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•In sexual selection, secondary sexual characteristics, may attract mates,
but they appear to lack adaptive benefits and may in fact, hinder survival
•Male peacocks have a bright body feather and huge colorful tails.
These features may draw the attention of predators & the tail
may encumber the bird during its flight