Preliminary Examination of Sexual Selection

Many male organisms exhibit traits that, intuitively, should be disfavored by selection. From the massive peacock tail fan to guppies with brightly colored tails, it would seem that selection should have weeded out traits that make the organism more vulnerable to predators. However, selection has preserved these traits instead, through a special type of selection called sexual selection. At least this is the standard theory presented by the Darwinists. Parts of it may be accurate, parts may be inaccurate. We’re going to spend some time in 2021 looking at sexual selection, pulling out the meat, and spitting out the bones.   

Male traits are most often targets of sexual selection because, in most species, males have to either attract a mate with some kind of display or fight for access to a mate against other males.  Females get the choice of the strongest, best looking, or best-sounding males.  Thus the traits, whatever they are, are reproductively beneficial to the males, allowing them to be maintained and even favored in the population.  This is the standard model and is frequently used to account for the existence of wildly extravagant

However, sometimes sexual selection acts more strongly on females than males. Usually, when this occurs, it is because the male performs most of the parental care, effectively reversing the roles normally assumed. One such case involves the pipefish species Syngnathus typhle, which undergoes strong sexual selection on females (Jones et al, 2000). In this species, females compete for males. They do so by displaying a zig-zag pattern on their bodies (Berglund et al, 1998). These patterns are used to compete with other females for the attention of the males, who will provide parental care. 

Because males are the choosy members of the species, they get to decide what females they will mate with (Berglund et al, 1989). Berglund et al’s (1998) experiment tested whether males preferred females that were camouflaged against their natural environment, females that had enhanced coloration and contrast, and natural females.  Males did not prefer the ones that matched the background, nor did they exhibit a strong preference for the females exhibiting enhanced contrast. Ornamented females are preferred (Berglund and Rosenqvist, 2001). Further, in another test, females with sexual ornamentation were the most successful reproductively. In fact, in the presence of predators, only the females with the ornamentation were able to reproduce. Those without were unable to induce the males to take the risk of mating when a predator was present. 

Clearly, in this instance, sexual selection is acting strongly on females. Males simply refuse to mate with females that do not show ornamentation in the most realistic scenarios. Even when no predator is present to simulate wild conditions, males still exhibit a strong preference for females with the ornamentation. That this trait is under strong sexual selection in females is made even more apparent when it is considered that males prefer larger-bodied female pipefish but will mate with any nearby female if a predator is nearby(Berglund, 1993). Yet, when presented with a predator in Berglund et al (1998), the males would only mate with females displaying sexual ornamentation.  Sexual selection is thus strongly influencing the behavior of the female pipefish.

This strong selection on female traits is only possible because of the reversed sexual roles. Normally, when females provide parental care, the females are the ones making the choices and thus providing the sexual selection pressure.  However, since the roles are reversed, the males are the ones picking mates. This places the selection pressure on the females, causing them to compete for mates, and express sexually selected traits.

Sexual selection, while usually associated with male traits, can act on females as well. In this species of pipefish, females express additional sexual ornamentation that is absent in males. It is strongly reproductively beneficial to females to maintain their sexual ornamentation to ensure maximum reproductive success, given the strong preference males have for ornamented females. Therefore, this trait is under strong sexual selection.

Obviously there is a lot more we could get into here. We didn’t even mention Bateman’s gradients, or other aspects of sexual selection. However, from a very cursory overview as is presented here, it seems like sexual selection might account, in some cases, for sex related traits being emphasized. However, it does not explain their origin. That must be explained at the genetic level, and genetics cannot account for the origin of new morphology. We will be digging more into sexual selection as we go forward in 2021 so any atheist trolls who see this and want to object, hold your horses. This in no way represents our final opinion on the topic. If you know of papers pro or con on sexual selection you think we should read, send us links.

References:

Berglund A, Rosenqvist G, Svensson I. 1989.  Reproductive success of females limited by males in two pipefish species. Am Nat. 133(4).

Berglund A. 1993. Risky sex: male pipefishes mate at random in the presence of a predator.

Anim Behav. 46(1):169-175.

Berglund A, Rosenqvist G, Bernet P. 1998. Female-female competition affects female ornamentation in the sex-role reversed pipefish Sygnathus typhle. Behaviour. 135(5):535550.

Berglund A, Rosenqvist G. 2001. Male pipefish prefer ornamented females. Anim Behav.

61(2):345-350.

Jones AG, Rosenqvist G, Berglund A, Arnold SJ, Avise JC. 2000. The Bateman gradient and the cause of sexual selection in a sex-role-reversed pipefish. Proc R Soc Lond B Biol Sci.

267(1444):677-680.

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