31,284 research outputs found
Simple model for the Darwinian transition in early evolution
It has been hypothesized that in the era just before the last universal
common ancestor emerged, life on earth was fundamentally collective. Ancient
life forms shared their genetic material freely through massive horizontal gene
transfer (HGT). At a certain point, however, life made a transition to the
modern era of individuality and vertical descent. Here we present a minimal
model for this hypothesized "Darwinian transition." The model suggests that
HGT-dominated dynamics may have been intermittently interrupted by
selection-driven processes during which genotypes became fitter and decreased
their inclination toward HGT. Stochastic switching in the population dynamics
with three-point (hypernetwork) interactions may have destabilized the
HGT-dominated collective state and led to the emergence of vertical descent and
the first well-defined species in early evolution. A nonlinear analysis of a
stochastic model dynamics covering key features of evolutionary processes (such
as selection, mutation, drift and HGT) supports this view. Our findings thus
suggest a viable route from early collective evolution to the start of
individuality and vertical Darwinian evolution, enabling the emergence of the
first species.Comment: 9 pages, 5 figures, under review at Physical Review
Session 4: Evolutionary Indeterminism
Proceedings of the Pittsburgh Workshop in History and Philosophy of Biology, Center for Philosophy of Science, University of Pittsburgh, March 23-24 2001 Session 4: Evolutionary Indeterminis
Genetic draft, selective interference, and population genetics of rapid adaptation
To learn about the past from a sample of genomic sequences, one needs to
understand how evolutionary processes shape genetic diversity. Most population
genetic inference is based on frameworks assuming adaptive evolution is rare.
But if positive selection operates on many loci simultaneously, as has recently
been suggested for many species including animals such as flies, a different
approach is necessary. In this review, I discuss recent progress in
characterizing and understanding evolution in rapidly adapting populations
where random associations of mutations with genetic backgrounds of different
fitness, i.e., genetic draft, dominate over genetic drift. As a result, neutral
genetic diversity depends weakly on population size, but strongly on the rate
of adaptation or more generally the variance in fitness. Coalescent processes
with multiple mergers, rather than Kingman's coalescent, are appropriate
genealogical models for rapidly adapting populations with important
implications for population genetic inference.Comment: supplementary illustrations and scripts are available at
http://webdav.tuebingen.mpg.de/interference
Human bony labyrinth is an indicator of population history and dispersal from Africa.
The dispersal of modern humans from Africa is now well documented with genetic data that track population history, as well as gene flow between populations. Phenetic skeletal data, such as cranial and pelvic morphologies, also exhibit a dispersal-from-Africa signal, which, however, tends to be blurred by the effects of local adaptation and in vivo phenotypic plasticity, and that is often deteriorated by postmortem damage to skeletal remains. These complexities raise the question of which skeletal structures most effectively track neutral population history. The cavity system of the inner ear (the so-called bony labyrinth) is a good candidate structure for such analyses. It is already fully formed by birth, which minimizes postnatal phenotypic plasticity, and it is generally well preserved in archaeological samples. Here we use morphometric data of the bony labyrinth to show that it is a surprisingly good marker of the global dispersal of modern humans from Africa. Labyrinthine morphology tracks genetic distances and geography in accordance with an isolation-by-distance model with dispersal from Africa. Our data further indicate that the neutral-like pattern of variation is compatible with stabilizing selection on labyrinth morphology. Given the increasingly important role of the petrous bone for ancient DNA recovery from archaeological specimens, we encourage researchers to acquire 3D morphological data of the inner ear structures before any invasive sampling. Such data will constitute an important archive of phenotypic variation in present and past populations, and will permit individual-based genotype-phenotype comparisons
Evolution on a smooth landscape
We study in detail a recently proposed simple discrete model for evolution on
smooth landscapes. An asymptotic solution of this model for long times is
constructed. We find that the dynamics of the population are governed by
correlation functions that although being formally down by powers of (the
population size) nonetheless control the evolution process after a very short
transient. The long-time behavior can be found analytically since only one of
these higher-order correlators (the two-point function) is relevant. We compare
and contrast the exact findings derived herein with a previously proposed
phenomenological treatment employing mean field theory supplemented with a
cutoff at small population density. Finally, we relate our results to the
recently studied case of mutation on a totally flat landscape.Comment: Revtex, 15 pages, + 4 embedded PS figure
Muller's ratchet and mutational meltdowns
We extend our earlier work on the role of deleterious mutations in the extinction of obligately asexual populations. First, we develop analytical models for mutation accumulation that obviate the need for time-consuming computer simulations in certain ranges of the parameter space. When the number of mutations entering the population each generation is fairly high, the number of mutations per individual and the mean time to extinction can be predicted using classical approaches in quantitative genetics. However, when the mutation rate is very low, a fixation-probability approach is quite effective. Second, we show that an intermediate selection coefficient (s) minimizes the time to extinction. The critical value of s can be quite low, and we discuss the evolutionary implications of this, showing that increased sensitivity to mutation and loss of capacity for DNA repair can be selectively advantageous in asexual organisms. Finally, we consider the consequences of the mutational meltdown for the extinction of mitochondrial lineages in sexual species
Red Giants in the Small Magellanic Cloud. I. Disk and Tidal Stream Kinematics
We present results from an extensive spectroscopic survey of field stars in
the Small Magellanic Cloud (SMC). 3037 sources, predominantly first-ascent red
giants, spread across roughly 37.5 sq. deg, are analysed. The line of sight
velocity field is dominated by the projection of the orbital motion of the SMC
around the LMC/Milky Way. The residuals are inconsistent with both a
non-rotating spheroid and a nearly face on disk system. The current sample and
previous stellar and HI kinematics can be reconciled by rotating disk models
with line of nodes position angle, theta, ~ 120-130 deg., moderate inclination
(i ~ 25-70 deg.), and rotation curves rising at 20-40 km/s/kpc. The metal-poor
stars exhibit a lower velocity gradient and higher velocity dispersion than the
metal-rich stars. If our interpretation of the velocity patterns as bulk
rotation is appropriate, then some revision to simulations of the SMC orbit is
required since these are generally tuned to the SMC disk line-of-nodes lying in
a NE-SW direction. Residuals show strong spatial structure indicative of
non-circular motions that increase in importance with increasing distance from
the SMC centre. Kinematic substructure in the north-west part of our survey
area is associated with the tidal tail or Counter-Bridge predicted by
simulations. Lower line-of-sight velocities towards the Wing and the larger
velocities just beyond the SW end of the SMC Bar are probably associated with
stellar components of the Magellanic Bridge and Counter-Bridge, respectively.
Our results reinforce the notion that the intermediate-age stellar population
of the SMC is subject to substantial stripping by external forces.Comment: To appear in MNRA
The fixation probability of rare mutators in finite asexual populations
A mutator is an allele that increases the mutation rate throughout the genome
by disrupting some aspect of DNA replication or repair. Mutators that increase
the mutation rate by the order of 100 fold have been observed to spontaneously
emerge and achieve high frequencies in natural populations and in long-term
laboratory evolution experiments with \textit{E. coli}. In principle, the
fixation of mutator alleles is limited by (i) competition with mutations in
wild-type backgrounds, (ii) additional deleterious mutational load, and (iii)
random genetic drift. Using a multiple locus model and employing both
simulation and analytic methods, we investigate the effects of these three
factors on the fixation probability of an initially rare mutator as a
function of population size , beneficial and deleterious mutation rates, and
the strength of mutations . Our diffusion based approximation for
successfully captures effects (ii) and (iii) when selection is fast compared to
mutation (). This enables us to predict the conditions under which
mutators will be evolutionarily favored. Surprisingly, our simulations show
that effect (i) is typically small for strong-effect mutators. Our results
agree semi-quantitatively with existing laboratory evolution experiments and
suggest future experimental directions.Comment: 46 pages, 8 figure
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