Transcript Document

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Speciation: the origin of species
Species originate by speciation mechanisms
Several different ways
Factors that have accelerated speciation:
In plants: use of different animal pollinators
In animals: modifications in sexual selection
Reinforcement
Speciation mechanisms
1. Polyploidization
A. Autoploidy: doubling of homologous chromosomes
B. Alloploidy: stage in the speciation process
Nicotiana
Plants
Alloploidy
Autoploidy
n = 96
Galeopsis tetrahit: 2n = 32
Suspected of being an allotetraploid
from progenitor species: G. pubescens (2n = 16) and G. speciosa (2n = 16)
Diploid F1 hybrids (2n = 16) crossed and one rare 3n hybrid produced.
The triploid hybrid backcrossed to G. pubescens
One rare 4n hybrid produced.
The recreation of G. tetrahit, with which it was interfertile.
• 1. Polyploidization
• Animals
• Occasionally produces a new
• species
Hybridization = Reticulate evolution
Attempts to recreate an animals species.
Laboratory Hybridization Among North American Whiptail Lizards, Including Aspidoscelis
inornata arizonae X A. tigris marmorata Squamata: Teiidae), Ancestors of Unisexual Clones in
Nature
CHARLES J. COLE, LAURENCE M. HARDY, HERBERT C. DESSAUER, HARRY L. TAYLOR,
AND CAROL R. TOWNSEND
Division of Vertebrate Zoology (Herpetology), American Museum of Natural History
([email protected]).
Division of Vertebrate Zoology (Herpetology), American Museum of Natural History; Museum of
Life Sciences, Louisiana State University in Shreveport, LA 71115-2399 ([email protected]).
Division of Vertebrate Zoology (Herpetology), American Museum of Natural History;
Department of Biochemistry and Molecular Biology, Louisiana State University Health Science
Center, New Orleans, LA 70112.
Division of Vertebrate Zoology (Herpetology), American Museum of Natural History;
Department of Biology, Regis University, Denver, CO ([email protected]).
Division of Vertebrate Zoology (Herpetology), American Museum of Natural History
([email protected]).
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Attempt to create a parthenogenetic species in the lab.
Attempt spanned a period of > 29 years
74 males of four species caged with 156 females of nine species
Each group kept together for at least six months.
A total of only five hybrids from three crosses were obtained over the 29
years.
Hybrids were raised to adulthood to see if they would reproduce, but none
did.
The hybrid status of suspected laboratory hybrids was confirmed by
karyotypic, allozyme, and morphological analyses, and histological studies
were made on reproductive tissues of the hybrids, which were apparently
sterile.
Three laboratory hybrids of two bisexual species, A. inornata arizonae (♀)
x A. tigris marmorata (♂)…..progenitor species of A. neomexicana.
These three individuals from one clutch of eggs were the only hybrids
between two bisexual species that we obtained.
Bisexual parents used in attempt to
recreate the origin of
Aspidoscelis neomexicana
Hybrid intersex
Hybrid male
The attempted recreation,
A. neomexicana
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Speciation Mechanisms
Most animal speciation is visualized as lineage splitting.
typically messy
Y
Basic speciation models require separation of gene pools.
Darwinian idea: slow accumulation of genetic differences.
But there can be large, rapid effects from modest genetic
changes (e.g., in developmental pathways).
How many species of elephants?
Traditional: Loxodonta africana and Elephas maximus
But, African elephants are morphologically different
Savannah and forest populations
Essentially geographically isolated (no BSC)
Phylogenetic species concept applied
Sampled: 195 elephants from 21 populations
4 genes sequenced for each of the 195 individuals
Genetic distances used to construct a phylogeny
Conclusion: two species (L. cyclotis and L. africana)
Sympatric Speciation
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Flies (Rhagoletis pomonella)
(1) Larvae develop in hawthorns (Crataegus)
Native to NE U.S.
(2) Larvae develop in apples (Pyrus)
Apples introduced c. 300 ya
H0: The flies belong to the same species; there is
phenotypic plasticity in use of hosts.
• H1: Speciation has occurred; each species of fly adapted to
one host species.
• Since hawthorns and apples are both within the range of
Rhagoletis, this would represent an example of sympatric
speciation.
• Flies using the two types of fruit cannot be distinguished
morphologically (cryptic).
• But, evidence of lineage splitting:
• 1. Allele frequency differs for 6 different proteins; therefore can be
distinguished genetically.
• 2. Flies imprint on fruit from which they developed.
– Mating takes place on the fruit.
– Provides some degree of physical segregation.
– Only c. 6% of matings are between misimprinted flies.
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Flies are diverging because of natural selection
Selective agent = timing of fruit ripening.
Apples ripen “early.”
Larvae in apples
– Selected to develop slowly.
– Prevents emergence of adults prior to winter.
• Hawthorns ripen ca. 3 weeks later than apples.
• Larvae in hawthorn fruit
– Selected to develop rapidly.
– Enables pupation prior to winter.
• A few mistakes are made: speciation nearing completion.
Speciation by Dispersal
Representative Hawaiian
Drosophila diversity
A speciation problem
Aspidoscelis
carmenensis
A. picta
A. danheimae
A. franciscensis
A. espiritensis
Grismer, L. Lee. 1999. Phylogeny, taxonomy, and biogeography of
Cnemidophorus hyperythrus and C. ceralbensis (Squamata: Teiidae)
In Baja California, Mexico
Unresolved: maybe not species?