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Sexual selection: Theory and Analysis
Charles Darwin initially suggested that secondary sexual characteristics
exhibited by male organisms such as bright plumage in some birds or mane in an
African lion (Panthera leo), developed since females favored mating with males that
had such characteristics. The article is significant as it highlights sexual selection as
arguably the two forms of natural selection. In the first kind, members of single sex
make a reproductive distinction within the group through mating competition. On the
other type, the female sex selects from prospective mates, this form seems to be
prevalent in birds. This article aims to elaborate on the empirical support for the
theory of sexual selection in different organisms.
According to Ayala, the presence of a certain trait in an organism can to some
extent be an attractive feature to the opposite sex. This element is apparent in many
organisms and has been validated through various experiments. Ayala mentions
organisms such as vinegar flies (Drosophila), pigeons and dogs as notable examples
exhibiting such characteristics. In the case of Drosophila, for instance, females prefer
normal males with yellowish gray pigmentation in comparison to Drosophila with
yellowish bodies arising from spontaneous mutations (n.p). Additionally, many
species exhibit sexual dimorphism (distinction in appearance and size as well as
variation in reproductive organs in males and females of the same species). In diverse
organisms, females can continuously find mates, whereas in males in is not certain.
The male organisms possessing sex appeal or with more attractive characteristics to
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the opposite sex commonly obtain the substantial mating rights leading to sexual
selection.
Experiments put forward to explain sexual selection include the observational
study of the sedge warblers in 1980 by Catchpole. In this study bird species were
observed to ascertain the relationship between male song complexity and mate choice
in sedge warbler species. The experiment aims to illustrate that sexual selection is
responsible for the development and complexity of songs in the European
Acrocephalus species.
The first part of the experiment was an attempt to establish whether male
sedge warblers with complex song attracted females before males with simpler songs.
The second part of the research aimed at learning whether song complexity was an
attractive feature in sedge warblers by the theory of sexual selection and complexity
has developed over time. The six species of Acrocephalus where four are
monogamous while two are polygynous. The results of the first experiment indicated
a significant variation in repertoire size which was plotted against the pairing date.
The results show that males with complex songs and attracted mates before the
competitors with simpler songs (Catchpole 152). In the second experiment, the results
indicate two types of song structure have developed by sexual selection; intrasexual
selection in polygynous species has led to simpler songs whereas intersexual selection
in monogamous species has led to complex songs useful in the attraction (Catchpole,
163).
An experiment with frog species Physalaemus pustulosus and Physalaemus
Coloradorum aimed at determining mate recognition through mating calls. The results
illustrate that in interspecific calls, female species of P. pustulosus and P.
Coloradorum preferred and responded to conspecific calls as compared to calls of up
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to other five heterospecific calls (Ryan and Rand 254). In the intraspecific calls, male
P. pustulosus add chucks to their calls to attract females. And females were
significantly attracted. On the other hand, P. Coloradorum males produced triple or
repeated calls to attract females. The female species P. Coloradorum preferred double
or triple calls to single calls. This element illustrates that the sex appeal in males plays
a significant role in attracting the females and the significance of inherited traits in
mate selection.
Migratory birds like Sedge warbler is a migratory bird are presumed face
enormous sexual selection pressure; this is elaborated by the fact that females are
more attracted to males with more complex songs (Catchpole 163). Nonetheless, this
is because the competition to find mate increases male sexual selection pressure. As a
result of the female choice of complex song, two selection pressures usually, led song
evolution in altered ways.
The results indicate that Acrocephalus species are under intense sexual
selection pressure because of female selection; furthermore, male to male competition
provides intense pressure shown by variation in song physiognomies. The sedge
warblers even after pairing have extended warbles (Catchpole 149). This is in line
with earlier studies indicating that migratory birds seem to possess more complex
songs in comparison to sedentary ones. Amusingly, the variations in song complexity
within the sedge warbler’s population are positively correlated with the limited
available time to breed as well as migration and formation of territories. It supports
the hypotheses that female choice in sexual selection is intense in migratory birds
because of the limitations that arise from movements that affect breeding. Since the
research piece suggests that females fancy a more sophisticated vocalization, sexual
selection is apparent. That is, in migratory birds, the intense pressure to attract a
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female, breed swiftly and utilize the limited time available on each occasion
influences the evolution to a further complex vocalizations in these species.
This behavior can explain various traits in other organisms, as many genes
passed on from parents have effects on the phenotype irrespective of sex, while others
are only sex-specific this leads to sexually dimorphic phenotypic traits. These sexspecific genetic influences on phenotypic traits in various organisms comprise
exceptional selective processes in which manifestation is ideal in one sex only.
However, losses of secondary sexual characteristics, in turn, encompass losses of
these sex-specific genes, through modifications in particular sex in response to the
opposite sex, these factors exhibit sexual selection in diverse organisms.
In conclusion, sedge warblers and species of Physalaemus frogs use songs and
calls respectively to attract mates and compete for mates, and from Darwin, these
complex vocalizations have been broadly approved to advance because of sexual
selection. These empirical studies support the theory of sexual selection through the
revelation of characteristics of calls and song complexity that females prefer. These
features also benefit the male in establishments of territories which in turn influence
competition for females. Sexual dimorphism in Acrocephalus species songs is
generally the product of sexual selection in male species.
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Work Cited
Ayala, R. Sexual Selection. Britannica online encyclopedia.2015
Catchpole, Clive K. "Sexual Selection And The Evolution Of Complex Songs Among
European Warblers Of The Genus Acr Ocephal Us." Behaviour 74.1 (1980):
149-165. Web.
Ryan, Michael J and Rand, S. Phylogenetic patterns of behavioral mate recognition
systems in Physalaemus pustulosus species group (Anura: Leptodactylidae):
the role of ancestral and derived characters and sensory exploitation. ".
Behaviour 74.1” (1981): 251-267