Tempo and Mode in Evolution Genetics and Paleontology 50 Years After Simpson (1995) / Chapter Skim
Currently Skimming:
Molecular Genetics of Speculation and Human Origins
Molecular Genetics of Speculation and Human Origins
Pages 187-212
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 187... ...
Species continuously evolve; after a time, a descendant gene pool, say Z', may be sufficiently different from the ancestral gene pool, Z as to be considered a different species.
|
From page 188... ...
In this paper we explore DNA sequence polymorphism in current human populations in order to shed light on some aspects of human evolution, particularly on the size of human ancestral populations as they evolved from one to another species. Speciation by Founder Effect The theories of "founder effect" speciation propose that speciation often occurs after a founder event or bottleneck; that is, when a new population is established by a pair or very few individuals, as may happen in the colonization of an island, or when an established population declines severely so that extremely few individuals survive, from which a population expands again (Mayr, 1954, 1963, 1982; Carson, 1968, 1975; Templeton, 1980; Carson and Templeton, 1984~.
|
From page 189... ...
As we shall show, DNA polymorphisms in the major histocompatibility complex (MHC) of humans and other primates manifest that no severe population bottleneck has occurred in human evolution.
|
From page 190... ...
190 o ~ In - o a V, — do C ~ .e I_ C,0 .
|
From page 191... ...
This relationship is reflected in a family tree of the four alleles, which shows that the two human alleles diverged from a common ancestral gene before the ancestors of the human and chimpanzee species separated from each other around 6 Myr ago.
|
From page 192... ...
The species and allele trees may not coincide whenever a polymorphism is passed from species Z on to species X and Y; i.e., the root of the alleles may be deeper than species Z Some alleles, such as 0101 and 0201, of the same species are more different from each other than some alleles, such as 0101 and 0103, of different species.
|
From page 193... ...
Evolutionary History of the DRBI Gene Locus Exon 2 of the DRB1 gene consists of 270 nucleotides that specify all the Unchain amino acids involved in peptide binding. No fewer than 58 distinct human alleles have been identified that differ in their exon 2 nucleotide sequences.
|
From page 194... ...
The tree is based on the DNA sequence of exon 2, which consists of 270 nucleotides. Genetic distances are estimated by Kimura's (1983)
|
From page 195... ...
If the generation time is 1 year, the polymorphism will persist for 4 Myr. If a new allele 01 has a positive selective value (it has a selective advantage, s, relative to another allele, 02)
|
From page 196... ...
The coalescence theory was originally developed for neutral genes (Kingman, 1982a, b; Tajima, 1983; Tavare, 1984; and Takahata and Net, 1985) but has been recently extended to allelic genealogies under balancing selection (Takahata, 1990, 1993a, b; Takahata and Net, 1990~.
|
From page 197... ...
On the assumption of overdominance, with a heterozygote advantage on the order of 0.01, and 10-8 selected mutation rate per site per generation, the HLA polymorphism at the DRB1 locus requires a mean effective size of 100,000 individuals over the last 30 million years (Takahata et al., 1992~. Computer simulations lead to estimates also on the order of 105 individuals as the long-term size of the ancestral populations leading to modern humans (Klein et al., 1990; Takahata, 1991, 1993a)
|
From page 198... ...
It has been suggested that a very narrow population bottleneck occurred at the transition from archaic to modern Homo sapiens, some 200,000 years ago (see Cann et al., 1987; Stoneking et al., 1990; Vigilant et al., 1991~. The consequences of a bottleneck depend not only on the size, Nb, of the bottleneck, but also on the number, tb, of bottleneck generations.
|
From page 199... ...
The persistence of HLA polymorphisms over millions of years requires that the size of human ancestral populations be at least Ns = 10 at all times (Takahata, 1990~. If s = 0.01, the minimum population size possible at any time would be Nb = 1000.
|
From page 200... ...
200 / Ayala, Escalante, O'hUigir', and Klein 1.0 0.8 0.6 _` o or ~ 0.4 Q 0.2 0.0 1 .0 0.8 0.6 0.2 0.0 0 400 A s=0 R=1.01 / N=10,000 250 500 750 1000 1250 1500 EFFECTIVE POPUlATION SIZE B s=0.0 1 R=1 .01 N=5,000 or 800 1200 1600 2000 2400 EFFECTIVE POPULATION SIZE FIGURE 6 Probability that a number of alleles will persist after a bottleneck lasting 10 generations, as a function of population size at the bottleneck. The graphs represent averages of 300 computer simulations.
|
From page 201... ...
Theories of Human Origins The origin of anatomically modern humans, Homo sapiens sapiens, occurred around 200,000 years B.P. The transition from H
|
From page 202... ...
The multiregional model proposes regional continuity and local selection pressures in different parts of the world, but with gene flow (indicated by the dashes connecting the vertical lines that represent different regional populations) due to occasional migrations between populations.
|
From page 203... ...
sapiens was associated with a very narrow bottleneck, consisting of only two or very few individuals who are the ancestors of all modern mankind. The Noah's Ark model is supported by an interpretation of mitochondrial DNA analysis showing that the diverse mitochondrial DNA sequences found in modern humans coalesce to one ancestral sequence, the "mitochondrial Eve" or "mother of us all," that existed in Africa about 200,000 years ago (Cane et al., 1987; Stoneking et al., 1990; Vigilant et al., 1991~.
|
From page 204... ...
A variant that might be called the "Levantine hybridization and replacement model" is statistically supported by a correlation analysis of morphological traits that favors the Middle East over Africa as the origin of modern human traits (Waddle, 1994~. At the other extreme, some authors deny a recent African origin for modern humans while emphasizing regional continuity and gene exchange throughout Africa, Europe, and Asia (Wolpo~ et al., 1988; Wolpoff, 1989~.
|
From page 205... ...
and evinces that the ancestral population of modern humans was at no time smaller than several thousand individuals, a result also consistent with the mitochondrial DNA data. Two recent molecular studies favor some degree of regional continuity over complete African replacement.
|
From page 206... ...
We explore the possibility of occasional population bottlenecks and conclude that the ancestral population could not have at any time consisted of fewer than several thousand individuals. The MHC polymorphisms exclude the theory claiming, on the basis of mitochondrial DNA polymorphisms, that a constriction down to one or few women occurred in Africa, at the transition from archaic to anatomically modern humans, some 200,000 years ago.
|
From page 207... ...
(1982) Biochemical comparison of major histocompatibility complex molecules from different subspecies of Mus musculus: Evidence for trans-specific evolution of alleles.
|
From page 208... ...
(1990) Allelic diversification at the class II DQB locus of the mammalian major histocompatibility complex.
|
From page 209... ...
(1990) The major histocompatibility complex and human evolution.
|
From page 210... ...
(1992) Polymorphism and balancing selection at major histocompatibility complex loci.
|
From page 211... ...
(1991) African populations and the evolution of human mitochondr~1 DNA.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.