Darwinian Natural Selection

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Darwinian Natural Selection Pt. 1

• Artificial selection domestic plants and animals
• cabbage, domestic dog
• Natural selection
• the argument
• flower color in snap dragons

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Artificial Selection 1

• An analogy to natural selection
• Chapter 1 of The Origin of Species Variation
  under Domestication

3
Artificial Selection 2

• Variation is the rule No case is on record of
  a variable being ceasing to be variable under
  cultivation. Our oldest cultivated plants, such
  as wheat, still often yield new varieties our
  oldest domesticated animals are still capable of
  rapid improvement or modification.
• Inheritance is common the number and
  diversity of inheritable deviations of structure,
  both those of slight and those of considerable
  physiological importance, is endless. No
  breeder doubts how strong is the tendency to
  inheritance like produces like is his
  fundamental belief.

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Artificial Selection 3

• Breeders choose individuals that have desirable
  characteristics to be the parents of the
  following generations (i.e., breeders are the
  selective agent and the characters that they use
  to chose parents are the targets of selection)
• If the selected trait is heritable then the breed
  or variety will come to express the selected
  trait to a greater degree, or completely new
  breeds and varieties can be created

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Varieties of Cabbage (Brassica oleracea)
Wild cabbage
Cabbage
Kohlrabi
Brussels sprouts
Cauliflower
Broccoli
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Fig 3.2 Cauli-flower pheno-type in Arabi-dopsis
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Dog breeds A
http//www.akc.org/breeds/breeds_a.cfm
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Dog breeds C
http//www.akc.org/breeds/breeds_c.cfm
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Domestic Dogs

• gt 400 described breeds
• 152 breeds recognized by the American Kennel Club
• Most modern breeds lt 400 years old
• Closest wild relative of domestic dogs is the
  gray wolf

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Genetic Structure of the Purebred Domestic Dog.
H. G. Parker, et al. 2004. Science 3041160-1164

• 414 individuals from 85 breeds
• 96 microsatellite loci
• 27 of total genetic variation is among breeds
  (510 among human populations)
• Strong genetic isolation among breeds
• Only 4 of 414 individuals assigned to wrong breed
  on basis of genotype
• Possible to assign breeds to groups which often
  correlate with morphological similarity and
  shared geographic origin

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Fig. 2. Consensus neighbor-joining tree of 85 dog
breeds and the gray wolf
Fig. 2. Consensus neighbor-joining tree of 85 dog
breeds and the gray wolf. Nine breeds that form
branches with statistical support are shown. The
remaining 76 breeds show little phylogenetic
structure and have been combined into one branch
labeled "All other breeds" for simplification.
The entire tree is shown in fig. S1. The trees
that formed the consensus are based on the chord
distance measure. Five hundred bootstrap
replicates of the data were carried out, and the
fraction of bootstraps supporting each branch is
indicated at the corresponding node as a
percentage for those branches supported in more
than 50 of the replicates. The wolf population
at the root of the tree consists of eight
individuals, one from each of the following
countries China, Oman, Iran, Sweden, Italy,
Mexico, Canada, and the United States. Branch
lengths are proportional to bootstrap values.
Published by AAAS
H. G. Parker et al., Science 304, 1160 -1164
(2004)
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Chinese shar-pei
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Fig. 3. (A) Population structure of 85 domestic
dog breeds
Fig. 3. (A) Population structure of 85 domestic
dog breeds. Each individual dog is represented by
a single vertical line divided into K colors,
where K is the number of clusters assumed. Each
color represents one cluster, and the length of
the colored segment shows the individual's
estimated proportion of membership in that
cluster. Black lines separate the breeds that are
labeled below the figure. Representative breeds
pictured above the graph from left to right
Akita, Pekingese, Belgian Sheepdog, Collie,
Doberman Pinscher, Basset Hound, American Cocker
Spaniel, Bedlington Terrier, Flat-Coated
Retriever, Newfoundland, and Mastiff. Results
shown are averages over 15 structure runs at each
value of K.
Published by AAAS
H. G. Parker et al., Science 304, 1160 -1164
(2004)
14
Evolution and Adaptation by Natural Selection
The Darwin-Wallace Argument

• Postulate 1 There is variation among
  individuals
• Postulate 2 Some of the variation is heritable
• Postulate 3 In every generation some
  individuals are more successful at surviving and
  reproducing than others the struggle for
  existence (see Chapter 3 of the Origin)
• Postulate 4 The survival and reproduction of
  individuals are not random. Individuals with the
  most favorable variations, those who are better
  at surviving and reproducing, are naturally
  selected.

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Evolution and Adaptation by Natural Selection
The Darwin-Wallace Argument - 2

• If these 4 postulates are true (and if favorable
  variations are heritable), then a population
  changes from one generation to the next as
  favorable variations spread through the
  population.

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Evolution and Adaptation by Natural Selection
The Darwin-Wallace Argument - 3

• The selective agent is the environment (
  nature), or something in the environment, in the
  broadest sense.
• The targets of selection are any traits or
  characteristics that influence the chance that an
  individual will survive and reproduce
• An adaptation is a phenotype that increases the
  likelihood that an individual will survive and
  reproduce
• Adaptations spread through populations under the
  influence of natural selection provided that they
  are heritable
• Natural selection ( heritability) is a process
  that produces descent with modification (
  evolution)

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The Evolution of Flower Color in an Experimental
Snapdragon Population 1

• Jones, K.N. J. Reithel. 2001. Am. J. Botany
  88447-454
• Flower color
• all yellow vs. white (with yellow spots)
  (Postulate 1)
• monhybrid inheritance, white dominant (Postulate
  2)
• Experimental garden
• genotypes planted in F2 proportions
• Plants pollinated by free-living bumblebees

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The Evolution of Flower Color in an Experimental
Snapdragon Population 2

• Observations
• Number of bee visits to each flower
• Number of seeds produced

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The Evolution of Flower Color in an Experimental
Snapdragon Population 3

• Results
• Individual plants showed considerable variation
  in reproductive success both as pollen donors and
  as seed mothers (Postulate 3)
• White flowers attract twice as many bees as
  yellow flowers ( non-random variation in
  reproductive success via pollen flow and seed
  set?) (Postulate 4)
• White flowers had slightly higher seed production
  ( non-random reproductive success as seed
  mothers) (Postulate 4)

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The Evolution of Flower Color in an Experimental
Snapdragon Population 4

• Did the population evolve?
• Proportion of yellow-flowered plants in the
  offspring declined to 23

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Fig. 3.3 Experimental evolution in snapdragons