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The fitness landscape of a community of Darwin’s finches
Divergent natural selection should lead to adaptive radiation—that is, the rapid evolution of phenotypic and ecological diversity originating from a single clade. The drivers of adaptive radiation have often been conceptualized through the concept of “adaptive landscapes,” yet formal empirical estimates of adaptive landscapes for natural adaptive radiations have proven elusive. Here, we use a 17-year dataset of Darwin’s ground finches (Geospiza spp.) at an intensively studied site on Santa Cruz (Galápagos) to estimate individual apparent lifespan in relation to beak traits. We use these estimates to model a multi-species fitness landscape, which we also convert to a formal adaptive landscape. We then assess the correspondence between estimated fitness peaks and observed phenotypes for each of five phenotypic modes (G. fuliginosa, G. fortis [small and large morphotypes], G. magnirostris, and G. scandens). The fitness and adaptive landscapes show 5 and 4 peaks, respectively, and, as expected, the adaptive landscape was smoother than the fitness landscape. Each of the five phenotypic modes appeared reasonably close to the corresponding fitness peak, yet interesting deviations were also documented and examined. By estimating adaptive landscapes in an ongoing adaptive radiation, our study demonstrates their utility as a quantitative tool for exploring and predicting adaptive radiation
Designing antibiotic cycling strategies by determining and understanding local adaptive landscapes
The evolution of antibiotic resistance among bacteria threatens our continued
ability to treat infectious diseases. The need for sustainable strategies to
cure bacterial infections has never been greater. So far, all attempts to
restore susceptibility after resistance has arisen have been unsuccessful,
including restrictions on prescribing [1] and antibiotic cycling [2,3]. Part of
the problem may be that those efforts have implemented different classes of
unrelated antibiotics, and relied on removal of resistance by random loss of
resistance genes from bacterial populations (drift). Here, we show that
alternating structurally similar antibiotics can restore susceptibility to
antibiotics after resistance has evolved. We found that the resistance
phenotypes conferred by variant alleles of the resistance gene encoding the TEM
{\beta}-lactamase (blaTEM) varied greatly among 15 different {\beta}-lactam
antibiotics. We captured those differences by characterizing complete adaptive
landscapes for the resistance alleles blaTEM-50 and blaTEM-85, each of which
differs from its ancestor blaTEM-1 by four mutations. We identified pathways
through those landscapes where selection for increased resistance moved in a
repeating cycle among a limited set of alleles as antibiotics were alternated.
Our results showed that susceptibility to antibiotics can be sustainably
renewed by cycling structurally similar antibiotics. We anticipate that these
results may provide a conceptual framework for managing antibiotic resistance.
This approach may also guide sustainable cycling of the drugs used to treat
malaria and HIV
Dynamics of clade diversification on the morphological hypercube
Understanding the relationship between taxonomic and morphological changes is
important in identifying the reasons for accelerated morphological
diversification early in the history of animal phyla. Here, a simple general
model describing the joint dynamics of taxonomic diversity and morphological
disparity is presented and applied to the data on the diversification of
blastozoans. I show that the observed patterns of deceleration in clade
diversification can be explicable in terms of the geometric structure of the
morphospace and the effects of extinction and speciation on morphological
disparity without invoking major declines in the size of morphological
transitions or taxonomic turnover rates. The model allows testing of hypotheses
about patterns of diversification and estimation of rates of morphological
evolution. In the case of blastozoans, I find no evidence that major changes in
evolutionary rates and mechanisms are responsible for the deceleration of
morphological diversification seen during the period of this clade's expansion.
At the same time, there is evidence for a moderate decline in overall rates of
morphological diversification concordant with a major change (from positive to
negative values) in the clade's growth rate.Comment: 8 pages, Latex, 2 postscript figures, submitted to Proc.R.Soc.Lond.
Heterogeneous Adaptive Trajectories of Small Populations on Complex Fitness Landscapes
Background Small populations are thought to be adaptively handicapped, not only because they suffer more from deleterious mutations but also because they have limited access to new beneficial mutations, particularly those conferring large benefits. Methodology/Principal Findings Here, we test this widely held conjecture using both simulations and experiments with small and large bacterial populations evolving in either a simple or a complex nutrient environment. Consistent with expectations, we find that small populations are adaptively constrained in the simple environment; however, in the complex environment small populations not only follow more heterogeneous adaptive trajectories, but can also attain higher fitness than the large populations. Large populations are constrained to near deterministic fixation of rare large-benefit mutations. While such determinism speeds adaptation on the smooth adaptive landscape represented by the simple environment, it can limit the ability of large populations from effectively exploring the underlying topography of rugged adaptive landscapes characterized by complex environments. Conclusions Our results show that adaptive constraints often faced by small populations can be circumvented during evolution on rugged adaptive landscapes
Drift and evolutionary forces: scrutinizing the Newtonian analogy
This article analyzes the view of evolutionary theory as a theory of forces. The analogy with Newtonian mechanics
has been challenged due to the alleged mismatch between drift and the other evolutionary forces. Since
genetic drift has no direction several authors tried to protect its status as a force: denying its lack of directionality,
extending the notion of force and looking for a force in physics which also lacks of direction. I analyse these
approaches, and although this strategy finally succeeds, this discussion overlooks the crucial point on the debate
between causalists and statisticalists: the causal status of evolutionary theory
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