Adam Bjork

Publications (8)71.52 Total impact

• Article: Evolutionary history of chimpanzees inferred from complete mitochondrial genomes.

  Adam Bjork, Weimin Liu, Joel O Wertheim, Beatrice H Hahn, Michael Worobey
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  ABSTRACT: Investigations into the evolutionary history of the common chimpanzee, Pan troglodytes, have produced inconsistent results due to differences in the types of molecular data considered, the model assumptions employed, and the quantity and geographical range of samples used. We amplified and sequenced 24 complete P. troglodytes mitochondrial genomes from fecal samples collected at multiple study sites throughout sub-Saharan Africa. Using a "relaxed molecular clock," fossil calibrations, and 12 additional complete primate mitochondrial genomes, we analyzed the pattern and timing of primate diversification in a Bayesian framework. Our results support the recognition of four chimpanzee subspecies. Within P. troglodytes, we report a mean (95% highest posterior density [HPD]) time since most recent common ancestor (tMRCA) of 1.026 (0.811-1.263) Ma for the four proposed subspecies, with two major lineages. One of these lineages (tMRCA = 0.510 [0.387-0.650] Ma) contains P. t. verus (tMRCA = 0.155 [0.101-0.213] Ma) and P. t. ellioti (formerly P. t. vellerosus; tMRCA = 0.157 [0.102-0.215] Ma), both of which are monophyletic. The other major lineage contains P. t. schweinfurthii (tMRCA = 0.111 [0.077-0.146] Ma), a monophyletic clade nested within the P. t. troglodytes lineage (tMRCA = 0.380 [0.296-0.476] Ma). We utilized two analysis techniques that may be of widespread interest. First, we implemented a Yule speciation prior across the entire primate tree with separate coalescent priors on each of the chimpanzee subspecies. The validity of this approach was confirmed by estimates based on more traditional techniques. We also suggest that accurate tMRCA estimates from large computationally difficult sequence alignments may be obtained by implementing our novel method of bootstrapping smaller randomly subsampled alignments.
  Molecular Biology and Evolution 01/2011; 28(1):615-23. · 5.55 Impact Factor
• Article: Relaxed molecular clocks, the bias-variance trade-off, and the quality of phylogenetic inference.

  Joel O Wertheim, Michael J Sanderson, Michael Worobey, Adam Bjork
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  ABSTRACT: Because a constant rate of DNA sequence evolution cannot be assumed to be ubiquitous, relaxed molecular clock inference models have proven useful when estimating rates and divergence dates. Furthermore, it has been recently suggested that using relaxed molecular clocks may provide superior accuracy and precision in phylogenetic inference compared with traditional time-free methods that do not incorporate a molecular clock. We perform a simulation study to determine if assuming a relaxed molecular clock does indeed improve the quality of phylogenetic inference. We analyze sequence data simulated under various rate distributions using relaxed-clocks, strict-clocks, and time-free Bayesian phylogenetic inference models. Our results indicate that no difference exists in the quality of phylogenetic inference between assuming a relaxed molecular clock and making no assumption about the clock-likeness of sequence evolution. This pattern is likely due to the bias-variance trade-off inherent in this type of phylogenetic inference. We also compared the quality of inference between Bayesian and maximum likelihood time-free inference models and found them to be qualitatively similar.
  Systematic Biology 01/2010; 59(1):1-8. · 10.23 Impact Factor
• Article: Adaptive modulation of sperm production rate in Drosophila bifurca, a species with giant sperm.

  Adam Bjork, Romano Dallai, Scott Pitnick
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  ABSTRACT: Sperm have traditionally been regarded as energetically cheap and effectively limitless in supply, although evidence conflicting with this view has become increasingly abundant. For instance, males from a variety of taxa have been shown to strategically partition sperm across ejaculates in response to perceived sperm competition risk. It follows that males might also be predicted to adaptively modulate the rate at which sperm are produced. Here we show that, in the giant sperm producing fruitfly Drosophila bifurca, solitary males with infrequent access to females produce sperm at a much lower rate than males raised in association with females and other males. Our results support the prediction that males with little risk of sperm competition risk or few mating opportunities should divert resources away from gamete production.
  Biology letters 11/2007; 3(5):517-9. · 3.76 Impact Factor
• Article: Complex interactions with females and rival males limit the evolution of sperm offence and defence.

  Adam Bjork, William T Starmer, Dawn M Higginson, Christopher J Rhodes, Scott Pitnick
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  ABSTRACT: Postcopulatory sexual selection favours males which are strong offensive and defensive sperm competitors. As a means of identifying component traits comprising each strategy, we used an experimental evolution approach. Separate populations of Drosophila melanogaster were selected for enhanced sperm offence and defence. Despite using a large outbred population and evidence of substantive genetic variation for each strategy, neither trait responded to selection in the two replicates of this experiment. Recent work with fixed chromosome lines of D. melanogaster suggests that complex genotypic interactions between females and competing males contribute to the maintenance of this variation. To determine whether such interactions could explain our lack of response to selection on sperm offence and defence, we quantified sperm precedence across multiple sperm competition bouts using an outbred D. melanogaster population exhibiting continuous genetic variation. Both offensive and defensive sperm competitive abilities were found to be significantly repeatable only across matings involving ejaculates of the same pair of males competing within the same female. These repeatabilities decreased when the rival male stayed the same but the female changed, and they disappeared when both the rival male and the female changed. Our results are discussed with a focus on the complex nature of sperm precedence and the maintenance of genetic variation in ejaculate characteristics.
  Proceedings of the Royal Society B: Biological Sciences 08/2007; 274(1619):1779-88. · 5.41 Impact Factor
• Source

  Available from: arizona.edu

  Article: Point, counterpoint: the evolution of pathogenic viruses and their human hosts

  Michael Worobey, Adam Bjork, Joel O Wertheim
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  ABSTRACT: Viral pathogens play a prominent role in human health owing to their ability to rapidly evolve creative new ways to exploit their hosts. As elegant and deceptive as many viral adaptations are, humans and their ancestors have repeatedly answered their call with equally im-pressive adaptations. Here we argue that the coevolutionary arms race between humans and their viral pathogens is one of the most important forces in human molecular evolution, past and present. With a focus on HIV-1 and other RNA viruses, we highlight re-cent developments in our understanding of the human innate and adaptive immune systems and how the selective pressures exerted by viruses have shaped the human genome. We also discuss how the antiviral function of cellular machinery like RNAi and APOBEC3G blur the lines between innate and adaptive immunity. The remark-able power of natural selection is revealed in each host-pathogen arms race examined.
  Annu. Rev. Ecol. Evol. Syst. 01/2007; 38:515-40.
• Source

  Available from: syr.edu

  Article: Mechanisms underlying the sperm quality advantage in Drosophila melanogaster.

  James M Pattarini, William T Starmer, Adam Bjork, Scott Pitnick
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  ABSTRACT: Contrary to early predictions of sperm competition theory, postcopulatory sexual selection favoring increased investment per sperm (e.g., sperm size, sperm quality) has been demonstrated in numerous organisms. We empirically demonstrate for Drosophila melanogaster that both sperm quality and sperm quantity independently contribute to competitive male fertilization success. In addition to these independent effects, there was a significant interaction between sperm quality and quantity that suggests an internal positive reinforcement on selection for sperm quality, with selection predicted to intensify as investment per sperm increases and the number of sperm competing declines. The mechanism underlying the sperm quality advantage is elucidated through examination of the relationship between female sperm-storage organ morphology and the differential organization of different length sperm within the organ. Our results exemplify that primary sex cells can bear secondary sexual straits.
  Evolution 11/2006; 60(10):2064-80. · 5.15 Impact Factor
• Source

  Available from: syr.edu

  Article: Intensity of sexual selection along the anisogamy-isogamy continuum.

  Adam Bjork, Scott Pitnick
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  ABSTRACT: Research into the evolution of giant sperm has uncovered a paradox within the foundations of sexual selection theory. Postcopulatory sexual selection on males (that is, sperm competition and cryptic female choice) can lead to decreased sperm numbers by favouring the production of larger sperm. However, a decline in sperm numbers is predicted to weaken selection on males and increase selection on females. As isogamy is approached (that is, as investment per gamete by males approaches that by females), sperm become less abundant, ova become relatively less rare, and competition between males for fertilization success is predicted to weaken. Sexual selection for longer sperm, therefore, is expected to be self limiting. Here we examine this paradox in Drosophila along the anisogamy-isogamy continuum using intraspecific experimental evolution techniques and interspecific comparative techniques. Our results confirm the big-sperm paradox by showing that the sex difference in sexual selection gradients decreases as sperm size increases. However, a resolution to the paradox is provided when this finding is interpreted in concert with the 'opportunity for selection' and the 'opportunity for sexual selection'. Furthermore, we show that most of the variation in measures of selection intensity is explained by sperm length and relative investment in sperm production.
  Nature 07/2006; 441(7094):742-5. · 36.28 Impact Factor
• Source

  Available from: William D Brown

  Article: No evidence that polyandry benefits females in Drosophila melanogaster.

  William D Brown, Adam Bjork, Karin Schneider, Scott Pitnick
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  ABSTRACT: Understanding the evolution of polyandry (mating with multiple males) is a major issue in the study of animal breeding systems. We examined the adaptive significance of polyandry in Drosophila melanogaster, a species with well-documented costs of mating in which males generally cannot force copulations. We found no direct fitness advantages of polyandry. Females that mated with multiple males had no greater mean fitness and no different variance in fitness than females that mated repeatedly with the same male. Subcomponents of reproductive success, including fecundity, egg hatch rate, larval viability, and larval development time, also did not differ between polyandrous and monogamous females. Polyandry had no affect on progeny sex ratios, suggesting that polyandry does not function against costly sex-ratio distorters. We also found no evidence that polyandry functions to favor the paternity of males successful in precopulatory sexual selection. Experimentally controlled opportunities for precopulatory sexual selection had no effect on postcopulatory sperm precedence. Although these results were generally negative, they are supported with substantial statistical power and they help narrow the list of evolutionary explanations for polyandry in an important model species.
  Evolution 07/2004; 58(6):1242-50. · 5.15 Impact Factor