Natural versus Sexual Selection: When two forces Collide

Natural Selection is the process by which a slight variation in a trait of an individual increases its likelihood of surviving relative to that of an individual not possessing the trait[i]. Fitness describes an organism’s ability to survive and reproduce viable offspring[ii]. It is this second component of fitness (production of viable offspring) that gives sexual selection its relevance in terms of evolution. In contrast to natural selection, sexual selection is not dependant on a struggle for existence between individuals, but instead on a struggle between individuals of the same species and sex (usually males) for possession of the other sexi.

The Texas field cricket is an abundant species in Texas and the surrounding area[iii]. A file is located on one forewing and a scraper is located on the opposite side of the other forewing. When these two structures are rubbed together the cricket’s wings vibrate producing a call[iv]. This type of call is accomplished via stridulation (rubbing together their modified forewings)[v].

Figure 1: Calling male Acheta Domesticus.  Forewings are at 45 degrees.

Photo Courtesy of Dr. Bertram's Lab at Carleton University

The call of a male Gryllus texensis can be heard here:

http://buzz.ifas.ufl.edu/479a.htm

Calling G.texensis males attract sexually receptive females, but they also attract the parasitoid fly O.ochraceaiii. The fly locates the male cricket by ‘honing in’ on the male’s mating call. Female O.ochracea are larviparous, gravid females lay live larvae on a potential cricket host. The tiny planidia (1st instar ormia larvae) are deposited on or around a cricket host and burrow into the cricket’s body.

Figure 2: Parasitoid fly Ormia ochracea and a singing Gryllus texensis.

 Photo taken by Marie Read

 Once in the cricket the larvae stay in the thorax (usually in the flight muscles) for 3-4 days. On the 3rd or 4th day the larvae migrates towards and attaches itself to the abdominal wall where it completes its development. The larvae cause very little damage to the central nervous system, digestive and reproductive tissue, feeding primarily on fat body, abdominal and thoracic muscle. After 6-10 days, the larvae emerge, purging their guts and making an exit hole in the cricket’s cuticle[vi].

 There have been no cases documented where a cricket survived for more than a couple of hours following the emergence of an O. ochracea larva.

            High-energy calls may be easiest for a fly to locate and that flies can minimize search time by orienting towards these calls[vii].  Both female G.texensis and gravid female O. ochracea exhibit a preference for male calls with an average number of pulses per trill [viii]. The convergence in call preferences between female G. texensis and O. ochracea creates a strong conflict for male G. texensis. Sexual selection will favour the evolution of call characters deemed more attractive to females, while natural selection will direct the male’s call away from the preferences of O.

Figure 3: O. ochracea larva's exit hole in cuticle of G.texensis.

Photo taken by Crystal Vincent

                        ochracea. This evolutionary dilemma has caused much intrigue, prompting many to study the system of G. texensis and O. ochracea.

This conflict between sexual and natural selection is expected to result in an evolutionary arms race between predator and prey. I would like to uncover some of the Behavioural and physiological adaptations an organism may evolve in order to compete in such an arms race. My goal is to investigate what alternate mechanisms or strategies can evolve when male Gryllus texensis are forced to make a decision between surviving and mating.

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Figure4: Emergence of O. ochracea larva.

Photo taken by Marie Read