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Lecture

Directional selection

2 Pages
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Department
Biology
Course Code
BIOL 1004
Professor
Tamy Superle

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These individuals will then contribute more offspring to the next generation.
Directional selection produced resistance to tetrodotoxin (TTX) in garter snakes.
Disruptive selection favors individuals that vary in both directions from the mean of the population.
Individuals at both extremes of a population are simultaneously favored. Increases variation in the
population
Ex: the bill sizes of black-bellied seedcrackers
Sexual Selection influences reproductive success
Sexual selection was Darwin’s explanation for the evolution of apparently useless but conspicuous traits in males
of many species, such as bright colors, long tails, horns, antlers, and elaborate courtship displays.
He hypothesized that these traits either improved the ability of their bearers to compete for access to members
of the other sex (intrasexual selection) or made them more attractive to the other sex (intersexual selection).
EXPERIMENT:
Hypothesis: having well-developed ornamental traits signals vigor and health
Zebra finch bills are bright red because of carotenoids in their diet.
Carotenoids are antioxidants and part of the immune system. Males in good health will have brighter bills
because they need to allocate fewer carotenoids to immune function.
Zebra finch males were fed diets with and without carotenoids. The diet with carotenoids enhanced
immune function
22.4 How Is Genetic Variation Maintained within Populations?
Genetic drift, stabilizing selection, and directional selection all tend to reduce genetic variation within an animal
population.
However, most species have considerable genetic variation.
Things that help maintain genetic variation:
sexual reproduction
neutral alleles
frequency-dependent selection
populations in different (varying) environments
___________________________________________________________________________________________________
Sexual Production Pros & Cons:
Pros:
Existing genetic variation is amplified.
Sexual recombination generates new
combinations of alleles on which natural
selection can act.
It expands variation in a trait influenced by
alleles at many loci by creating new genotypes.
Sexual reproduction facilitates repair of
damaged DNA. Damage on one chromosome
can be repaired by copying intact sequence on
the other chromosome.
Permits elimination of deleterious mutations
Cons:
Recombination can break up adaptive
combinations of genes.
Reduces rate at which females pass genes to
offspring.
Dividing offspring into genders reduces the
overall reproductive rate.
Ad
Dafda
___________________________________________________________________________________________________
An allele that does not affect the fitness of an organism is called a neutral allele.
Neutral alleles tend to accumulate in a population of organisms over time, increasing genetic variation.
Frequency-dependent selection maintains genetic variation within populations.
A polymorphism is the coexistence of two or more alleles at a locus at frequencies greater than mutations can
produce.
A polymorphism may be maintained when the fitness of a genotype (or phenotype) varies with its frequency
relative to that of other genotypes (or phenotypes).
This process is known as frequency-dependent selection.
Ex: Fish with right- and left-mouthed individuals in Lake Tanganyika
Subpopulations vary genetically because they are subjected to different selective pressures in different
environments.

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Description
 These individuals will then contribute more offspring to the next generation.  Directional selection produced resistance to tetrodotoxin (TTX) in garter snakes. • Disruptive selection favors individuals that vary in both directions from the mean of the population.  Individuals at both extremes of a population are simultaneously favored. Increases variation in the population  Ex: the bill sizes of black-bellied seedcrackers Sexual Selection influences reproductive success • Sexual selection was Darwin’s explanation for the evolution of apparently useless but conspicuous traits in males of many species, such as bright colors, long tails, horns, antlers, and elaborate courtship displays. • He hypothesized that these traits either improved the ability of their bearers to compete for access to members of the other sex (intrasexual selection) or made them more attractive to the other sex (intersexual selection). • EXPERIMENT: • Hypothesis: having well-developed ornamental traits signals vigor and health • Zebra finch bills are bright red because of carotenoids in their diet. • Carotenoids are antioxidants and part of the immune system. Males in good health will have brighter bills because they need to allocate fewer carotenoids to immune function. • Zebra finch males were fed diets with and without carotenoids. The diet with carotenoids enhanced immune function 22.4 – How Is Genetic Variation Maintained within Populations? • Genetic drift, stabilizing selection, and directional selection all tend to reduce genetic variation within an animal population. • However, most species have considerable genetic variation. • Things that help maintain genetic variation:  sexual reproduction  neutral alleles  frequency-dependent selection  populations in different (varying) environments ___________________________________________________________________________________________________ • Sexual Production Pros & Cons: Pros: • Permits elimination of deleterious mutations • Existing genetic variation is amplified. Cons: • Sexual recombination generates new • Recombination can break up adaptive combinations of alleles on which natural combinations of genes. selection can act. • Reduces rate at which females pass genes to • It expands variation in a trait influenced by offspring. alleles at many loci by creating n
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