diversification of the Aposematic Signals Assignment PDF

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Running head:APOSEMATISM
Aposematism: The diversification of the Aposematic Signals
[Name of the Student]
[Name of the Institute]
[Date]
Aposematism2
Abstract
The diversification of the aposematic signals in a framework of Müllerian mimicry is an
intriguing phenomenon. While the theory of aposematism and mimicry suggests the evolution
towards a single aposematic signal, impressive variations can be observed between populations,
and this on a small spatial scale. It was assumed that the spatial variation in selection pressures
generated by different predators could be the cause of this phenomenon. In order to test this
hypothesis, we studied the transition between two geographic systems characterized by distinct
aposematic patterns in mimetic and toxic frogs (Dendrobatidae) by combining the tools of
population genetics with ecological tools. In each of these systems, Ranitomeya imitator lives in
sympatry with ventrimaculata or variabilis. This is the main example suggesting that in a
framework of Müllerian mimicry, there is no convergence of the aposematic signals of the two
species, but rather unidirectional convergence where imitator, being polymorphic, imitates
monomorphic species with which it is sympatric.
First, this review refutes the premises that suggest that R. imitator converges to the aposematic
signal of another species. The high genetic similarity between the model species suggests that
they have diverged more recently than the populations of R. imitator or that they are still
connected by gene flow. These reviews indicate that these species have been wrongly identified
as different species. In fact, the identification of the imitator species based on phenotypic
variability is invalidated in this system since R. imitator and R. variabilis / ventrimaculata
demonstrate the same variability. Second, this study demonstrate that predation varies spatially,
both in intensity and in direction, thus creating a heterogeneous landscape of selection pressures.
Thus, strong stabilizing predation pressures allow the geographic organization of different
aposematic signals to be maintained and explain the uniformity ii of these signals as well as
mimetic relationships. On the other hand, the temporary relaxation of predation pressures allows
the appearance of new aposematic phenotypes via neutral evolutionary processes, leading to a
high polymorphism in these populations. The interaction of these selective modes has enabled us
to demonstrate for the first time how Wright's evolutionary theory (shifting balance theory)
allows adaptive diversification in a natural system.
To conclude, this study has made it possible to highlight how dynamic Müllerian mimicry
systems can be. The spatial alternation between neutral evolutionary processes and natural
selection allows the emergence of new aposematic phenotypes on a local scale, as well as the
Aposematism3
appearance of a geographic organization of warning signals and of Mullerian mimicry
relationships.
Keywords:Aposematism, mimicry, polymorphism, spatial structure, predation,
diversification.
Aposematism4
Table of Contents
Abstract............................................................................................................................................2
Introduction......................................................................................................................................5
Discussion........................................................................................................................................6
Effectiveness of Aposematic Signals...........................................................................................6
Phenomena that Arise from Aposematism...................................................................................9
Aposematic Signals and Mimetic Relationships..........................................................................9
Genetic Differentiation of Populations......................................................................................11
Findings......................................................................................................................................12
The Role of Predators in Maintaining Polymorphism............................................................14
The Diversification of Aposematic Signals............................................................................15
Conclusion.....................................................................................................................................17
References......................................................................................................................................18
Aposematism5
Aposematism: The diversification of the Aposematic Signals
Introduction
Aposematism is an adaptive strategy that allows certain organisms to protect themselves
from a warning signal clearly perceptible by potential predators, in order to warn them that they
represent a danger and that they must be avoided (Thomas et al. 2003). This strategy would
therefore be beneficial for both the prey and the predator. Although several types of warning
signals have been documented (i.e. coloring, odor, audible signal; Ruxton et al. 2004), the bright
coloring in relation to the toxicity of certain organisms is a model that has attracted the attention
of scientists. The objective of this study is to highlight the selective processes responsible for the
aposematic polymorphism, by controlling for the other neutral evolutionary processes and
phytogeography (Mappes et al. 2005).
After observing many species of neotropical butterflies with similarities in coloring
patterns, Henry Walter Bates (1862), hypothesized that this was an example of natural selection
where edible species resemble toxic species in order to gain protection against predators. This
situation was later called Batésien mimicry. Bates also documented a number of cases where
several species, all toxic, were similar to each other. Since being toxic confers protection against
predators, he suggested that the convergence of these phenotypes should be caused by a similar
adaptation to the same abiotic environmental conditions.
The naturalist Johannes Friedrich Müller who also worked in Brazil made the same
observations and published 16 years after Bates an explanation for the phenotypic similarity
observed between different toxic species. Müller's (1878) explanation was extremely simple:
"Species with tusks are alike and thus share the costs of educating predators" (Müllerian
mimicry). Although at first glance, the advantages conferred on mimetic species are clear, the
evolutionary mechanisms allowing to reach this situation remain unclear and subject to many
controversies.
In order to understand the theme of this study, the theories associated with the
phenomena of Aposematism and Müllerian mimicry were presented. This allow researcher to
properly target the issues addressed, namely the controversy associated with the direction of
mimetic relationships between species and the theoretical problems arising from the
diversification of aposematic signals (Thomas et al. 2003). The key concepts relating to the
Aposematism6
genetic diversification of populations, as well as the biological model chosen, namely the small
tropical frog Ranitomeya imitator, will finally be presented before approaching the objectives.
Discussion
The Diversification of Aposematic Signals in Müllerian Mimicry Systems is a Puzzling
Phenomenon (Ruxton et al. 2004).The aposematic signals warn predators that their potential
prey has defenses such as toxicity or dangerous weapons.Aposematism explains colors or other
showy signals that are not sexual characters due to selection.This phenomenon is common in
insects andexothermalvertebrates,but rare in mammals and particularly in small herbivores.To
verify the role of showy colors, experimental studies tested the hypothesis that they could allow
them to sometimes blend into vegetation by comparing, in a natural environment, the
detectability of lemmings and that of Sundevall voles.The results show that the lemmings are
more conspicuous than the voles thus confirming that the colors are well aposematic and not
cryptic.(Thomas et al. 2003).
Effectiveness of Aposematic Signals
The effectiveness of an aposematic signal in a habitat depends on two factors: the
cognitive capacity of the predator and the abundance of the signal. Thus, it is essential that the
predator can associate coloring with danger, so that the latter avoids this type of signal (Ruxton
et al. 2004; Mappes et al. 2005). Avoiding a signal is generally an experience-based response,
although in some cases it is an innate response (Pough 1988; Ruxton et al. 2004). In addition, the
effectiveness of an aposematic signal in a habitat is directly proportional to its frequency
(Sherratt 2008). Thus, when the aposematic signal is frequent in a habitat, it is easy to understand
that it represents an advantageous evolutionary strategy: a small proportion of the population will
be killed, and predators will quickly learn to avoid the signal.
Thus, the per capita survival rate of an abundant aposematic signal will be relatively high.
On the other hand, when the signal is scarce, it is difficult to understand how it would be
advantageous. This is mainly due to the fact that a rare aposematic prey should be subject to high
per capita mortality, since a large proportion or even the entire prey population will be killed
during the learning period of predators (Lindstrom et al. 2001; Speed2001). From an
evolutionary point of view, several hypotheses have been proposed in order to explain the
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