Posts Tagged ‘Sexual Selection’

A lek is an area where many males of a species come together and display.  These displays can include very elaborate vocalizations, plumage, and behaviors such as the complex dances of some birds of paradise.  The displays play a very important role in mate selection in these species because they are the only trait that the females who visit the lek have to judge which male to mate with.  Males of lekking species do not hold territories where the female can raise her young and find food or provide parental aid in any other form.  The females provide all the care for their young, so what are the females choosing in a mate?  They are choosing genes.  Males that have traits that make them attractive have the genes that code for those traits, and so the offspring of these males should also have those traits and be attractive.  So, by choosing attractive mates, the females are maximizing the odds that their own genes will also be passed on to the next generation.  This setting results in some theoretical complications in terms of sexual selection.  One important one is that if females pick traits over and over again for generation after generation all the males will end up having the same traits.  If all the males are the same, there will be no way for the females to tell which males are the fittest and therefore who to pick to mate with and the whole system will collapse.  Yet this does not happen.  So how is variation being maintained in the face of strong selection?  This is the lek paradox.

Several solutions to the lek paradox have been proposed.  One is that females may not choose the same trait every breeding season.  This is often referred to as the fluctuating selection hypothesis (Jia et al. 2000), and it states that during one breeding season large males are favored, but then next breeding season it is males with the longest tails that are preferred, and the breeding season after that it is the loudest males that do the best.  This basically describes fads in fashion.  Also, preferences may not change breeding season to breeding season, but instead the large males may be favored for several seasons and then the shift to a different trait may be more gradual.  Either way, these changing preferences would result in the maintenance of genetic diversity.

Another possible resolution is that females may disagree with one another as to the most important traits.  This has been shown in Guppies by Brooks and Endler (2001) where females were allowed to chose between different males.  Most females seemed to agree that large males were the most fit, but when females were presented with males of equal size, some preferred males with large amounts of orange on their tails, while others preferred males that more black spots on their sides.  This disagreement would also maintain genetic variability.

A third proposed solution is that females are making an overall assessment of male fitness taking into account many traits.  This idea is called the genetic capture hypothesis (Rowe and Houle 1996), and it states that overall fitness is dependent on many morphological and physiological features.  In other words, for a male to be fit he must be able to find lots of food and escape predators and resist parasites etc.  All these abilities are coded for by many many genes, and it is the sum total of these genes that the females are choosing.

These hypotheses are not mutually exclusive, so in all likelihood there is a combination of factors at work.  But determining which is the dominant force is still an interesting goal, and still very much in dispute.

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Vocalizations, like any other phenotypic trait, can change over time.  These changes can have several causes which can be generally divided into drift (the random accumulation of mutations that are generally neutral in terms of fitness) and selection (changes that are under some directional pressure and tend to increase fitness).  The forms of drift and selection fall into five major categories.  In any given population, these categories can act independently of one another, they can act in concert, or they can oppose each other.  The five categories are Cultural Drift, Genetic Drift, Cultural Selection, Natural Selection, and Sexual Selection.

Cultural Drift is the process where changes in vocalizations occur by chance.  These changes can come from imitation errors as young individuals attempt to copy the sounds produced by adults.  They can also arise in the form of innovations where an adult incorporates a new sound element into its vocalization.  The accumulation of these changes can eventually lead to the formation of new vocal types.

Genetic Drift is the random accumulation of mutations at loci that regulate sound production.  Ass these mutations accumulate, the physiological and mechanical abilities of an organism to make sounds may be altered.  Due to this, genetic drift is likely to have a greater effect on the evolution of vocalizations when the mutations happen to effect the limits of performance for an animal.

Cultural Selection occurs when there is differential propagation of vocalizations across generations.  This can occur in the form of vertical transmission from parents to offspring, horizontal transmission between peer groups or siblings, or oblique transmission from adults to unrelated young.  Unlike the following two mechanisms, cultural selection is not directly driven be fitness.  Instead, variations in vocalizations can spread through a population for other reasons.  One example is because of a dominant individual using one particular variation and not others.  Another example is when a particular frequency transmits through a habitat better than others, such as how low frequency sounds travel through dense foliage farther tan high frequency sounds.  This would lead more young individuals to be exposed to low frequency sounds and so learn to imitate them.

Natural Selection can influence vocalizations directly, because of some fitness benefit that a particular vocalizations give the signaler, or indirectly, by altering some physical structure that is used is sound production (changing bill morphology adapting to different seed sizes, for example).  The most commonly discussed role of natural selection in vocal evolution is through the process known as reinforcement.  Reinforcement is where two populations have diverged to the point where hybrids between the populations are less fit than pure bred members of either population.  This might be because the two populations have split to use foods of two different sizes.  A hybrid might not be good and consuming either food size, and so be less fit.  If such hybrid disadvantage exists, natural selection is expected to favor individuals of each population that tend to avoid mating with individuals of the other population.  Vocalizations are frequently the first from of contact that two individuals have, and so they are in a unique position to moderate interactions and will tend to evolve towards greater species-level specificity.

Sexual Selection can take the form of intersexual selection or intrasexual selection.  Intersexual selection can drive the evolution of vocalizations by the preferences of one sex (usually the female) for particular vocalizations of the other sex (usually the male), by sensory bias where one sex (usually the male) uses a vocalization that the other sex (usually the female) is predisposed to respond to, when a display can only be produced by individuals of high fitness, when the production of a display carries some fitness cost such as increased risk of predation, or when a vocal display can inform the receiver as to their likely genetic compatibility with the sender.  Intrasexual selection on vocalizations come ins the form of members of same sex (usually males) using vocalizations to compete with one another.  Here, the evolution of vocalizations can occur when vocalizations contain information about the sender.  This information can be in the form of the senders size, strength, willingness to fit, social status, etc.  Facets of vocalizations that are often favored include increased vocal complexity, high amplitude, low frequency, and high calling rate.

These mechanism for the evolution of vocalizations are most thoroughly studied in bird songs.  However, bird calls may be susceptible to all of these types of evolution as well.  This would be particularly true of calls that are learned, as opposed to innate, for which more and more examples are being discovered.

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Mate choice, or intersexual competition, is the non-random selection of sexual partners of one sex by the other sex.  In most vertebrates, this is expressed as females choosing which males to mate with.  Many mechanisms have been proposed for how and why mate choice might evolve.  These mechanisms can generally divided into five categories, which are direct phenotypic effects, sensory bias, Fisherian, indicator mechanisms, and genetic compatibility mechanisms.  Some of these mechanisms give direct fitness benefits to the chooser (direct phenotypic effects and sensory bias), others give indirect benefits to the chooser by benefiting their young (Fisherian and indicator mechanisms), and others may give other genetic benefits (genetic compatibility mechanisms).  These mechanisms are not mutually exclusive.  In fact, it is likely that several operate simultaneously in any give system.  Each mechanism is described in more detail below.

Direct Phenotypic Effects are when females display a preference for a male ornament that gives the female some benefit.  This benefit can be in the form of the male holding a superior territory, providing more or better food, increased protection from predators, or higher contribution to parental care of offspring.  All these benefits are good for the female herself, directly.  As such, signals that males can display that indicate that the male will provide these benefits can be preferred and selected for.

Sensory Bias is when mate choices are made on the basis of a preference that evolved for some other reason.  For example, many animals have been shown to prefer specific colors.  These colors are often associated with common or preferred food types.  If a male develops the same color on his skin, fur, feathers, etc. females may tend to prefer him over other males that have not developed that color display.  Biases could exist for other reasons (associated with habitat selection, predator avoidance, etc.), and also using other senses (scent, sound, etc.).  The point is that the females sensory system already existed and was set up with a bias for certain displays that the males happened on and it then proved successful.

Fisherian (or Fisharian Runaway or Sexy Sons) is a process by which there is a genetic link between the display of a male and the preference for that display in the female.  Once this link occurs (and it could occur through selection or genetic drift), females will be more likely to choose males that have that trait.  They will then have sons that posses that trait and daughters that prefer it.  In this way, the mate choice process becomes self reinforcing.  This positive feedback between preferences and traits can drive the male trait to become quite extreme.  The tail of a male Peacock is thought to be an example of Fisherian selection.  This explanation of mate choice evolution was first developed by R. A. Fisher, a very famous evolutionary biologist, hence the name.

Indicator Mechanisms (or Good Genes or the Handicap Principle) is when a display trait in one sex has a selective disadvantage to the individual who has that trait.  In this way, having that trait is a cost, and males who possesses such a trait are indicating that they are able to survive despite the added cost of the display trait.  In other words, that they can succeed even with a handicap.  These traits are also condition-dependent, meaning that the condition of the display trait is dependent on the condition of male himself.  Females use these traits to determine which males have broadly high genetic quality.  An important aspect of this mechanism of  mate choice evolution is that it can favor males with a variety of genes because different males will be able to succeed in different way (some will be fast, some will strong, etc.).  This idea of maintaining genetic variation is sometimes referred to as the genetic capture hypothesis.

Genetic Compatibility Mechanisms are ways in which one individual may choose a mate based on how the genes of that mate will interact with their own.  For example, an individuals’ immune systems has been shown to mount a better defense against infections when the individual has a large amount of genetic diversity.  This diversity generally comes in the form of being heterozygotic (having two different versions, or alleles, of a gene on each chromosome) at many loci.  Females have been shown to prefer to mate with males who have fewer similar immune systems genes.  Since the female and male immune system genes are so dissimilar they will tend to compliment each other, and this will increase the heterozygosity of their offspring.

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