goodei, Bluefin Killifish, are
fundulid fish native to southeastern U.S. They display various color
polymorphisms on their dorsal, anal, pelvic, and caudal fins. Females display
promiscuous behavior and often will mate with many males 1. There is an
interesting interplay between male competition, female choice, as well as
genetic and environmental conditions that influence sexual selection. In this
paper, I will examine the topic of behavioral asymmetry in sexual selection and
how this also influences color variation. I hypothesize that Lucania goodei have evolved to display
distinct coloration and behavioral characteristics, such as aggression, that
are also affected by the environment and genetics as a result of sexual
selection. My hypothesis relates to evolutionary biology because it delves into
sexual selection, a form of natural selection, and how it influences mate
choice in Bluefin killifish. The coloration of the males is also a form of
sexual selection. Aggressiveness and different morphologies have evolved for
reproductive success.

goodei are one of the most abundant fish species in Florida and
quantitative data on sexual selection can be examined in field or lab studies.
In one study, male fish were observed for 30 minutes once every 2 hours for 10
days outside. Aggressive male behaviors, number of courting females, number of
spawning with females, and number of female visitation to the males were recorded.
It was found that spawning success and the frequency of male-male aggressive
displays and courtship with females is positively correlated. An open
environment such as this allows us to discern the relative importance of
male-male aggression and female choice. Not only was their male-male
aggression, but males also displayed aggression against females and
heterospecifics 2. It was initially thought that males were either guarding
their eggs from predators or using as a food source but recent research shows
no evidence of male parental care nor differential egg cannibalism 2,3.

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            In a later experiment, Lucania goodei were collected from a
river and maintained in a laboratory before one male was assigned to one of the
two chambers of the tank with a single female isolated in the middle. There
were three stages of choice, competition, and interaction. It was determined
that male dominance is essential to mating success. This evidence from a formal
experiment is consistent with that of the field study. While female preference
did not correlate with male dominance, females did spawn more quickly when their
chosen male was dominant. Due to their ability to spawn in succession and
release a variable amount of eggs, the length of time to spawn is important and
therefore influences the relative fitness of the eggs and sexual selection 1.

            In a third study, Lucania goodei were collected and 8
males were randomly selected to interact and determine a relative male
dominance rank. Males were then combined with a female to see if a correlation
between rankings and reproductive success existed. They found that body size
and body condition do not determine dominance rank, but rather these behavioral
types are strongly correlated with relative dominance rank and this is stable.
It is not known whether these behavioral types arise from genetic or social
determinants, and this is an area of future research. Male behavior and
dominance rank correlate with attaining females and reproductive success,
therefore showing that these characteristics are sexually selected for 4.

            The color patterns and polymorphisms
within Lucania goodei is poorly
understood, but sexual selection, predation, lighting, and/or carotenoid
presence could all explain the matter 2. In one study, adult male fish were
collected from various rivers and fins were analyzed spectrophotometrically to
determine pigment types. They found carotenoid, pterin, and melanin-derived
pigments are uncoupled and allow for an analysis of potential functions and
their evolution. Melanin was associated with dominance and obtaining a greater
proportion of spawns in males; the melanic border may signal to others when
establishing this dominance 5. Carotenoid levels were associated with
parasitism levels as well as body condition, even though body condition does
not correlate with dominance rank and therefore male-male interactions, this
pattern might affect female choice 4,5. Males advertise themselves and
females might use caudal carotenoid to evaluate the male’s condition. This is
further evidenced by an increase of carotenoids in groups and decrease in
isolation. Pterin was also positively associated with spawning success and may
signal immunocompetence to females, however, this needs further research. Each
pigmented ornament signals differently and evolves independently to shape its
function 5.

            Another study looks more
specifically at how these color polymorphisms are maintained by the sensory
environment creating selection pressures. These microhabitats and sexual
selection alone can maintain the seen color polymorphism even when female preferences
are weak 6. Along with these microhabitats, heterogeneous conditions can
further be stimulated with different environmental conditions, such as lighting.
Due to developmental plasticity, developing under different conditions can
change the vision and therefore female preference, as well as changing color
perception 7. Environmental conditions can also affect opsin expression,
light-sensitive proteins involved in vision. For example, fish raised in murky
conditions had higher levels RH2 and LWS opsins and fish raised in clear
conditions had higher levels of SWS1 and SWS2B opsins 8. It was determined
that females reared in clear springs preferred red males while males reared in
swamps preferred blue males showing genetic differentiation. However, there is
also a complex interaction between genetics, developmental plasticity and
lighting environment as preferences change when the fish are moved to different
environments. There is no current explanation why females have these
preferences in terms of fitness, but this could be further explored 7.

            In conclusion, Lucania goodei provide an excellent model for delving into
evolutionary biology and sexual selection. Male-male competition and dominance
correlate with reproductive success as well as female choice to an extent. As
well, behavioral types are strongly correlated the relative dominance rank
between males. In addition, there are polymorphisms that exist between males
within the fins. Different pigment levels indicated different signals to
females. Lastly, the environment that the fish developed in and mated in
affected mate choice. In all, Lucania
goodei have evolved to display distinct coloration and behavioral
characteristics such as aggression that are also affected by the environment
and genetics as a result of sexual selection.

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