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Natural Selection I: Artificial Selection. Darwin and fancy pigeons Secord 1981 Analogy between artificial and natural selection central to the Origin.
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Natural Selection I:
Artificial Selection
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Darwin and fancy pigeons
Secord 1981
•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
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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 rock-pigeon.”
A common origin?
•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)
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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
•The fancy pigeons bear a fundamental similarity to the rock pigeon-both in habits and general structure-that they share with no other bird
•All the domestic pigeons can be intercrossed, producing fertile offspring
•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
•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
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•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?
A common origin?
The process of selection
Secord 1981
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?
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
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•Darwin’s artificial selection analogy argued by many historians and philosophers as just an analogy to aid in explaining the parallels with natural selection
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
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•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
Darwin’s hopeful monsters
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Darwin frequently referred to the results of artificial selection as “monstrous”
•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
•Multiple different independent centers of domestication
•Selective breeding of wild plants and animals began 10,000 years ago
Doebley et al. In P
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Crop Domestication
Doebley et al. In press.
•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?
Crop Domestication
Doebley et al. In press.
•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
Genetic bottlenecks in domestication
Doebley et al. In press.
•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
Do these genetic bottlenecks in domestication reflect natural systems of selection?
Candidate genes in domestication
Hubbard et al. 2002
•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
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
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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?
Vollbrecht et al. 2005
•Inflorescences of maize, unlike those of related grasses lack long branches
Is this a traditional or non-traditional experiment?
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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 monophyletic. Fitness experiments indicate both lineages continued to evolve which contributed to their general frequencies over time. 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.
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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
•Each pop. Experienced 2 X 109 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?
ReferencesDoebley, 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 populations. Proc. Natl. Acad. Sci USA. 91: 6808-6814.
Parker, H.G. and Ostrander, E.A. 2005. Canine genomics and genetics: running with the pack. PLOS. 1: 507-513.
Purugganan, M.D., Boyles, A.L., Suddith, J.I. 2000. Variation and selection at the cauliflower florwal homeotic gene accompanying the evolution of domesticated Brassica oleracea. Genetcs. 155:855-862.
Ruse, M. 1975. Charles darwin and artificial selection. J. Hist. Ideas. 36: 339-350.
Secord, J.A. 1981. Nature’s fancy: Charles Darwin and the breeding of pigeons. ISIS. 72: 163-186.
Vollbrecht, E. and Sigmon, B. 2005. Amazing grass: developmental genetics of maize domestication. Biochemical Society Transactions. 33: 1502-1506.
Vollbrecht, E., Springer, P.S., Goh, L., Buckler, E.S. IV, Martienssen, R. 2005. Architecture of floral branch systems in maize and related grasses. Nature. 436: 1119-1126.
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.
Wilner, E. 2006. Darwin’s artificial selection as an experiment. Stud Hist Philos Biol Biomed Sci. 37: 26-40.
Wright, S.I., Bi, I.V., Schroeder, S.G., Yamasaki, M., Doebley, J.F., McMullen, M.D., Gaut, B.S. 2005. The effects of artificial selection on the maize genome. Science. 308: 1310-1314.