Systematics and the Origin of Species On Ernst Mayr's 100th Anniversary (2005) / Chapter Skim
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4 Chromosome Speciation: Humans, Drosophila, and Mosquitoes--FRANCISCO J. AYALA AND MARIO COLUZZI
4 Chromosome Speciation: Humans, Drosophila, and Mosquitoes--FRANCISCO J. AYALA AND MARIO COLUZZI
Pages 46-68
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From page 46... ...
The "suppressed-recombina tion" model of speciation points out that chromosome rearrange ments act as a genetic filter between populations. Mutations as sociated with the rearranged chromosomes cannot flow from one to another population, whereas genetic exchange will freely oc cur between colinear chromosomes.
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From page 47... ...
The biological species concept, as it came to be known, defines species precisely by these two attributes: ability to interbreed within the species and reproductive isolation from other species. The evolutionary process of speciation, by which one species splits into two, is equivalent to the evolutionary emergence of reproductive isolation.
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From page 48... ...
, Dobzhansky writes: "the process of species formation, in contrast to race formation, involves the development of reproductive isolating mechanisms. An ancestral species is transformed into two or more derived species when an array of interbreeding Mendelian populations becomes segregated into two or more reproductively isolated arrays.
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From page 49... ...
commended Dobzhansky for identifying interbreeding and reproductive isolation as the distinguishing features of the species concept and proposed a short definition: "Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." He has used, identically or with some word differences, this definition in later writings. Indeed, Mayr is generally perceived as the leading exponent of the biological species concept and the most successful investigator of the application of this concept to a great variety of species and species groups throughout the animal world, as several papers in this collection bear witness.
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From page 50... ...
We will consider two classes of models, which we will call the "hybrid-dysfunction" and "suppressed-recombination" models of speciation. Hybrid-dysfunction models claim that recombination between rearranged chromosomes generates gametes with some chromosomal segments deleted and others duplicated, thereby creating a partial reproductive barrier because the heterokaryotypic hybrid exhibits reduced reproductive fitness, also called "underdominance." Under these conditions, natural selection will, in both populations, favor mutations that reduce the probability of intercrossing and will eventually lead to complete reproductive isolation.
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From page 51... ...
. If these chromosomal rearrangements occurred early in the divergence between ancestral populations of chimps and humans, they "would facilitate genic divergence during the time when the diverging populations are in parapatry, i.e., have limited gene flow" (Navarro and Barton, 2003b)
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From page 52... ...
have investigated nucleotide sequences that exhibit nucleotide differences between chimps and humans in 115 genes, about evenly distributed between rearranged chromosomes (59 genes) and colinear chromosomes (56 genes)
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From page 53... ...
. (These ratios do not include seven genes for which KS = 0, which would have given a ratio of infinity: five genes in rearranged chromosomes and two in colinear chromosomes.)
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From page 54... ...
Moreover, millions of years have elapsed of separate evolution between the human and chimpanzee lineages, which should have largely erased the signal predicted by the model: namely, the expected greater genic differentiation between the rearranged chromosomes than between the colinear chromosomes, because this differentiation would have occurred so long ago, and other processes would have largely contributed to the current genetic differentiation between the two lineages. One additional difficulty is the implied assumption that the chromosomal rearrangements that differentiate humans from chimps and other apes happened all in the human lineage, early and within a short time.
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From page 55... ...
(2004) have examined nucleotide differentiation between human and chimpanzee DNA sequences amounting to 4,108,833 nucleotides: 1,831,676 nucleotides distributed among seven rearranged chromosomes, and 2,277,157 nucleotides distributed among six colinear chromosomes.
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From page 56... ...
and is inconsistent with their hypothesis of an accelerated rate of evolution in the rearranged chromosomes. An additional analysis involving 304 gene sequences of the ratio of nonsynonymous to synonymous rates of evolution yields a higher proportion of genes under positive selection in colinear chromosomes (29.5% of genes with ratios >1)
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From page 57... ...
It may very well have been the case that gene expression increases in the cerebral cortex were, at least in good part, associated with the increase, by about a factor of 3, that happened in the human lineage during the last 2 million years. Any increased genic differentiation that may have preferentially occurred in rearranged chromosomes during the original divergence of humans and chimps would likely be undetectable after millions of years of further divergent evolution in the two lineages.
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From page 58... ...
SPECIATION IN DROSOPHILA D pseudoobscura is a widely distributed Nearctic species common in temperate forests throughout the western third of North America, extending from British Columbia to Guatemala and from the Pacific to the Great Plains.
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From page 59... ...
. Fixed inversion polymorphisms inhibiting recombination would facilitate genic differentiation along the inverted segments, where genes promoting reproductive isolation between the species would have gradually accumulated.
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From page 60... ...
arabiensis gave rise to Anopheles quadriannulatus, from which it notably differs by three X-chromosome inversions, where factors are located that account for the reproductive isolation between the two species and which has retained the ancestral condition of being zoophilic and exophilic.
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From page 61... ...
A arabiensis, descended from a Pyretophorus species from the Arabian peninsula, is the likely ancestral species of the complex (see Fig.
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From page 62... ...
. These two allopatric species represent relics of the ancestral species, which genically diverged from each other after their geographic distribution became discontinuous.
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From page 63... ...
quadriannulatus by two X-chromosome inversions, where factors responsible for their reproductive isolation are located (as is the case for the three X-chromosome inversions fixed between A arabiensis and A
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From page 64... ...
quadriannulatus B, relics of a widely distributed species that genically diverged allopatrically after their geographic distribution became discontinuous. The suppressed-recombination speciation model also predicts that while gene exchange persists between the diverging populations, genes protected by the rearrangements will accumulate allelic differences faster than genes in the colinear chromosomes, where gene flow occurs between populations.
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From page 65... ...
. Most siblings are outcomes of recent speciation processes detected by paracentric inversions, mostly involving the X chromosome, as well as ribosomal DNA sequences.
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From page 66... ...
(2003) Semipermeable species boundaries between Anopheles gambiae and Anopheles arabiensis: Evidence from multilocus DNA sequence variation.
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From page 67... ...
(2001a) The genetics of reproductive isolation and the potential for gene exchange between Drosophila pseudoobscura and D
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From page 68... ...
(1997) Gene flow and natural selection in the origin of Drosophila pseudoobscura and close relatives.
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