Natural Selection I:
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Artificial Selection
Darwin and fancy pigeons
•Analogy between artificial and
natural selection central to the
Origin
•If humans can produce such
divergent phenotypes in short
time periods through selection
(as in pigeons) wouldn’t nature
be able to produce the same
over millions of years of natural
selection?
•Darwin discusses products of
domestication: cows, rabbits,
sheep
Secord 1981
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English Carriers
Jacobin
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Fantail
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http://pages.britishlibrary.net
Runt
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Pouter
A common origin?
The Rock Pigeon
Columba livia
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“Great as the differences are between the breeds of pigeons,
I am fully convinced that the common opinion of naturalists is
correct, namely that all have descended from the rockpigeon.”
http://www.samford.edu/schools/artsci/biology/vert-zoo-04s/photos/Columba-livia.jpg
A common origin?
Secord 1981
•Classification methods relied on traditional characters
(mainly the beak)
•Tumblers were grouped together, but the affinities with the
other short-beaked pigeons unknown (third group artificial)
A common origin?
•If fancy types were produced by crossing, the number of wild
progenitors would at least be 7 or 8, most of which would be
extinct
•None of the domesticated forms have returned to their natural
state, as would be expected if close to their wild progenitors
•The ancestral species would have to be capable of
domestication, an unusual property
•If the distinct ancestral species had existed originally, men
would have chosen a very strangely modified set of birds,
different from all existing members of the family
•All the domestic pigeons can be intercrossed, producing fertile
offspring
•The fancy pigeons bear a fundamental similarity to the rock
pigeon-both in habits and general structure-that they share with
no other bird
A common origin?
Secord 1981
•Why was a priority of Darwin’s to establish a common
ancestry for fancy pigeons?
•Is common ancestry necessary to strengthen his
arguments of natural selection?
The process of selection
The fancier was the selecting agent itself, who
could impose two principal types of selection:
1. Methodical selection
2. Unconscious selection
What was Darwin’s
reason for differentiating
the two?
Secord 1981
Artificial selection as an analogy
The argument that artificial selection is analogous
to natural selection is a key component of the
Origin of Species, The Descent of Man, and
Variation of Animals and Plants Under
Domestication
•Darwin’s artificial selection
analogy argued by many
historians and philosophers as
just an analogy to aid in
explaining the parallels with
natural selection
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Artificial selection as an analogy
The argument that artificial selection is analogous
to natural selection is a key component of the
Origin of Species, The Descent of Man, and
Variation of Animals and Plants Under
Domestication
•Wilner (2006) argues artificial
selection should be conceived
as a multifaceted experiment.
•Traditional experiment: lead to
theories of natural selection.
•Non-traditional: disclosed the
nature of hereditary variation
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Darwin’s hopeful monsters
Darwin frequently referred to the results of artificial
selection as “monstrous”
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http://www.dailymail.co.uk
•Many portray this as
an indication of the
irrelevance of artificial
selection to natural
phenomena
Darwin’s hopeful monsters
“The circumstances under which
our domestic productions are
reared are widely different. . . In
conformity with this, all our
domesticated productions, with
the rarest exceptions, vary far
more than natural species.”
-Darwin (1896)
Wilner (2006) views Darwin’s use of “monstrous”
in an experimental sense. Darwin’s “monstrous”
refers to the degree of artificiality in the breeders’
experiment. Experiments uncover the nature of
the elements, often with very artificial treatments.
Is artificial selection analogous to natural
selection?
•Does artificial selection mimic natural selection to
any degree?
•Degree of selection pressure
•Methodical vs unintentional selection
•Convergent evolution
•What would be concrete evidence?
Crop Domestication
Doebley et al. In Press
•Multiple different independent centers of domestication
•Selective breeding of wild plants and animals began 10,000
years ago
Crop Domestication
•Most researchers believe agriculture
began as an attempt to modify the
landscape and encourage growth of
edible wild plants
•Key to domestication would be a switch
from letting wild edible plants to
naturally re-sow themselves in burned
field, to sowing seed from previous
season
•Does this early form of breeding
constitute an unconscious selection
regime?
Doebley et al. In press.
Crop Domestication
•Widely viewed that early artificial
selection of crops would have been
largely unintentional
•Farmers, for instance, would collect
seeds that had not shattered and fallen
to the ground
•The non-shattering allele frequency
would rise in the population
•Similar selection thought to occur for
seed dormancy, synchronous flowering,
increased apical dominance, and larger
seeds
Doebley et al. In press.
Genetic bottlenecks in domestication
•Genetic bottlenecks reduce genetic diversity, which have
implications for further breeding
•Loci that are targets of domestication can have signatures of
selection: nucleotide diversity can be even lower than neutral
genes
Doebley et al. In press.
Do these genetic bottlenecks in domestication reflect natural
systems of selection?
Candidate genes in domestication
•teosinte branched 1
•Teosinte highly branched
•Maize has one dominant
axis of growth, axillary
branches are short and
feminized
•Signature of selection
confined to upstream
promoter region
Hubbard et al. 2002
Candidate genes in domestication
•teosinte glume architecture 1
•SBP-family of transcriptional
regulators
•7 fixed differences within a
1kb region between teosinte
and maize
•1 in coding region, others
potentially affect regulation
Wang et al. 2005
Candidate genes in domestication
•Signatures of positive selection show the a
cauliflower gene carrying a nonsense mutation is
segregating in broccoli, wild cabbage, kale, and
cauliflower. A floral regulatory locus
Many genes implicated in domestication are shown to be
involved in gene regulation or in regulatory regions
themselves. Does this follow a similar pattern to natural
systems?
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Candidate genes in domestication
•Inflorescences of maize,
unlike those of related
grasses lack long branches
•Maize ramosa1 gene
controls inflorescence
architecture
•ra1 has a signature of
positive directional selection
•Similar patterns in natural
species, Micanthus sirensis
and Sorghum bicolor
Vollbrecht et al. 2005
Candidate genes in domestication
•Inflorescences of maize,
unlike those of related
grasses lack long branches
•Maize ramosa1 gene
controls inflorescence
architecture
•ra1 has a signature of
positive directional selection
•Similar patterns in natural
species, Micanthus sirensis
and Sorghum bicolor
Vollbrecht et al. 2005
Developmental constraint in dog domestication
•Multiple origins of dogs from wolves greater than 14,000 years ago
with repeated genetic exchange between dog and wolf populations
•Metrics describing
aspects of canine
skeletal morphology
extracted from X rays
and DNA
•Principal component
analysis
•Skull and limb
lengths inversely
correlated with the
strength of the limb
and axial skeletons
Parker and Ostrander 2005
Convergent evolution in dog domestication
•Dogs can read human
communicative gestures
(ie.pointing) better in comparison
than wolves
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•Unclear if this ability is a result of
direct selection or a by-product of
selection against fear and
aggression towards humans
•Experimental population of fox kits
bred over 45 years to approach
humans fearlessly and nonaggressively
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•As skillful as dog puppies in
human gestures and more skillful
than a second control population of
fox kits
Experimental evolution in E. coli
•>30,000 generations starting
from a clonal line
•Serial transfer regime,
populations diluted 1:100 each
day into 10ml
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•6.6 generations per day
•Homogeneous environment
•Replicate populations
•Samples from each generation
stored for further genetic
analysis
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Experimental evolution in E. coli
“We emphasize that our
experiments employ natural
selection, and not artificial
selection as practiced by
breeders and many
experimentalists”
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True?
Experimental evolution in E. coli
Cell Volume
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Lenski and Travisano 1994
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Experimental evolution in E. coli
Fitness
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Lenski and Travisano 1994
Is this a traditional or non-traditional experiment?
Experimental evolution in E. coli
Other findings:
•Pleiotropic effects of fitness mutations. Both postitive and negative.
•Two ecologically and morphologically distinct types evolved: L and S
by generation 6,000 and persisted for more than 12,000 generations
after. Phylogenetic analysis of over 200 clones indicates that S was
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and a
monophyletic. Fitness experiments
both lineages continued
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to evolve which contributed
to their
general
Different species.
•Parallel changes in DNA topology between populations. Mutations in
topA and fis, which control DNA supercoiling.
•Parallel changes in gene expression from growth in a glucose-limited
medium. Expression of 59 genes had changed significantly in the
same direction in two selected populations. A mutation in a regulator
spoT produced many of these expression differences.
Experimental evolution in E. coli
Some mutation facts about E. coli:
•After 10,000 generations, each pop. Underwent 7.5 X 1011
replications
•Mutation rate: 2.5 X 10-3 mutations
per genome replication
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•Each pop. Experienced 2 X 10 mutations
•With 5 X106 bp per genome and three alternative point mutations at
each bp, this translates to >100 occurrences of every point mutation
in the whole genome
Conclusions
Is artificial selection analogous to natural selection?
Can artificial selection be used as evidence for evolution?
References
Doebley, J.F., Gaut, B.S., Smith, B.D. In press. The molecular genetics of crop domestication.
Evans, L.T. 1984. Darwin’s use of the analogy between artificial and natural selection. Journal of the History of Biology. 17:
113-140.
Gould, S.J. 1991. What the immaculate pigeon teaches the burdened mind. Natural History. 100: 12-21.
Hare, B., Plyusnina, I., Ignacio, N., Schepina, O., Stepika, A., Wrangham, R., Trut, L. 2005. Social cognitive evolution in
captive foxes is a correlated by-product of experimental domestication. Curr Biol. 15: 226-230.
Hubbard, L., McSteen, jP., Doebley, J., and Hake, S. 2002. Expression patterns and mutant phenotype of teosinte branched1
correlate with growth suppression in maize and tewosinte. Genetics. 162: 1927-1935.
Lenski, R. and Travisano, M. 1994. Dyanmics of adaptation and diversification: A 10,000 generation experiment with bacterial
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Parker, H.G. and Ostrander, E.A. 2005. Canine genomics and genetics: running with the pack. PLOS. 1: 507-513.
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Secord, J.A. 1981. Nature’s fancy: Charles Darwin and the breeding of pigeons. ISIS. 72: 163-186.
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Vollbrecht, E. and Sigmon, B. 2005. Amazing grass: developmental genetics of maize domestication. Biochemical Society
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Vollbrecht, E., Springer, P.S., Goh, L., Buckler, E.S. IV, Martienssen, R. 2005. Architecture of floral branch systems in maize
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Wang, H., Nussbaum-Wagler, T., Li, B., Zhao, Q., Vigourous, Y., Faller, M., Bomblies, K., Lukens, L., Doelbey, J. 2005. The
origin of naked grans of maize. Nature. 436: 714-719.
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