202 research outputs found
Different evolutionary paths to complexity for small and large populations of digital organisms
A major aim of evolutionary biology is to explain the respective roles of
adaptive versus non-adaptive changes in the evolution of complexity. While
selection is certainly responsible for the spread and maintenance of complex
phenotypes, this does not automatically imply that strong selection enhances
the chance for the emergence of novel traits, that is, the origination of
complexity. Population size is one parameter that alters the relative
importance of adaptive and non-adaptive processes: as population size
decreases, selection weakens and genetic drift grows in importance. Because of
this relationship, many theories invoke a role for population size in the
evolution of complexity. Such theories are difficult to test empirically
because of the time required for the evolution of complexity in biological
populations. Here, we used digital experimental evolution to test whether large
or small asexual populations tend to evolve greater complexity. We find that
both small and large---but not intermediate-sized---populations are favored to
evolve larger genomes, which provides the opportunity for subsequent increases
in phenotypic complexity. However, small and large populations followed
different evolutionary paths towards these novel traits. Small populations
evolved larger genomes by fixing slightly deleterious insertions, while large
populations fixed rare beneficial insertions that increased genome size. These
results demonstrate that genetic drift can lead to the evolution of complexity
in small populations and that purifying selection is not powerful enough to
prevent the evolution of complexity in large populations.Comment: 22 pages, 5 figures, 7 Supporting Figures and 1 Supporting Tabl
Does self-replication imply evolvability?
The most prominent property of life on Earth is its ability to evolve. It is
often taken for granted that self-replication--the characteristic that makes
life possible--implies evolvability, but many examples such as the lack of
evolvability in computer viruses seem to challenge this view. Is evolvability
itself a property that needs to evolve, or is it automatically present within
any chemistry that supports sequences that can evolve in principle? Here, we
study evolvability in the digital life system Avida, where self-replicating
sequences written by hand are used to seed evolutionary experiments. We use 170
self-replicators that we found in a search through 3 billion randomly generated
sequences (at three different sequence lengths) to study the evolvability of
generic rather than hand-designed self-replicators. We find that most can
evolve but some are evolutionarily sterile. From this limited data set we are
led to conclude that evolvability is a likely--but not a guaranteed-- property
of random replicators in a digital chemistry.Comment: 8 pages, 5 figures. To appear in "Advances in Artificial Life":
Proceedings of the 13th European Conference on Artificial Life (ECAL 2015
Origin of life in a digital microcosm
While all organisms on Earth descend from a common ancestor, there is no
consensus on whether the origin of this ancestral self-replicator was a one-off
event or whether it was only the final survivor of multiple origins. Here we
use the digital evolution system Avida to study the origin of self-replicating
computer programs. By using a computational system, we avoid many of the
uncertainties inherent in any biochemical system of self-replicators (while
running the risk of ignoring a fundamental aspect of biochemistry). We
generated the exhaustive set of minimal-genome self-replicators and analyzed
the network structure of this fitness landscape. We further examined the
evolvability of these self-replicators and found that the evolvability of a
self-replicator is dependent on its genomic architecture. We studied the
differential ability of replicators to take over the population when competed
against each other (akin to a primordial-soup model of biogenesis) and found
that the probability of a self-replicator out-competing the others is not
uniform. Instead, progenitor (most-recent common ancestor) genotypes are
clustered in a small region of the replicator space. Our results demonstrate
how computational systems can be used as test systems for hypotheses concerning
the origin of life.Comment: 20 pages, 7 figures. To appear in special issue of Philosophical
Transactions of the Royal Society A: Re-Conceptualizing the Origins of Life
from a Physical Sciences Perspectiv
Preoperative amygdala fMRI in temporal lobe epilepsy
Purpose: Anterior temporal lobe resections (ATLR) benefit 70% of patients with refractory mesial temporal lobe epilepsy (TLE), but may be complicated by emotional disturbances. We used functional magnetic resonance imaging (fMRI) to investigate the role of the amygdala in processing emotions in TLE and whether this may be a potential preoperative predictive marker for emotional disturbances following surgery.
Methods: We studied 54 patients with refractory mesial TLE due to hippocampal sclerosis (28 right, 26 left) and 21 healthy controls using a memory encoding fMRI paradigm, which included viewing fearful and neutral faces. Twenty-one TLE patients (10 left, 11 right) subsequently underwent ATLR. Anxiety and depression were assessed preoperatively and 4 months postoperatively using the Hospital Anxiety and Depression Scale.
Results: On viewing fearful faces, healthy controls demonstrated left lateralized, while right TLE patients showed bilateral amygdala activation. Left TLE patients had significantly reduced activation in left and right amygdalae compared to controls and right TLE patients. In right TLE patients, left and right amygdala activation was significantly related to preoperative anxiety and depression levels, and preoperative right amygdala activation correlated significantly with postoperative change of anxiety and depression scores, characterized by greater increases in anxiety and depression in patients with greater preoperative activation. No such correlations were seen for left TLE patients.
Discussion: The fearful face fMRI paradigm is a reliable method for visualizing amygdala activation in controls and patients with mesial TLE. Activation of the right amygdala preoperatively was predictive of emotional disturbances following right ATLR
Individual differences in structural and functional connectivity predict speed of emotion discrimination.
In social interactions, individuals who are slower at differentiating between facial expressions signalling direct and indirect threat might be at a serious disadvantage. However, the neurobiological underpinnings of individual differences in face processing are not yet fully understood. The aim of this study was to use multimodal neuroimaging to investigate how the speed of emotion recognition is related to the structural and functional connectivity underlying the differentiation of direct and indirect threat displays. Our results demonstrate that individuals, who are faster at discriminating angry faces, engaged areas of the extended emotional system more strongly than individuals with slower reaction times, showed higher white matter integrity in the inferior longitudinal fasciculus (ILF), as well as stronger functional connectivity with the right amygdala. In contrast, individuals, who were faster at discriminating fearful faces, engaged visual-attentional regions outside of the face processing network more strongly than individuals with slower reaction times, showed higher white matter integrity in the ILF, as well as reduced functional connectivity with the right amygdala. Our findings suggest that the high survival value of rapid and appropriate responses to threat has defined but separate neurobiological correlates for angry and fearful facial expressions
- β¦