Posted on: 02 March, 2017
Author: Alexander P
Disruptive selection against the pheromone system could accommodate the co- existence of two species which use the same pheromones but occupy different niches. Cross attraction could result in intersp... Disruptive selection against the pheromone system could accommodate the co- existence of two species which use the same pheromones but occupy different niches. Cross attraction could result in interspecic mating or, in the case of bark beetles, increased cohabitation and mutually disadvantageous competition for the same host material. Perhaps pheromone variability between eastern and western 1. pini (Lanier et al. l972) resulted from disruptive selection for pheromone specificity with respect to different sets of sympatric Ip. The existence of sympatric sibling species with different pheromone systems could also be explained by disruptive selection. The sibling species may have pur- sued courses in evolution in the traditional allopatric manner. If the species re- converge, their ecological requirements (different hosts, etc.) may be sufficiently different to permit their coexistence. Not having been under evolutionary pressure, the pheromone systems may have remained unchanged during geographic isolation according to http://sundowndivers.org/pheromones-men-get-results/ Mating between the two entities will result and they may hybridize introgressively. However, if sufficient genetic and/ or chromosomal changes accumulated to cause sub- normal zygote viability or hybrid sterility, or if disruption of the adapted genotype was sufficient to mediate against survival of hybrids, the entities could not exist sympatrically until premating isolating mechanisms developed. Thus, Trogoderma species, may not be able to persist on the same food source because indiscriminate mating resulting from cross attraction produces no brood. The species which is least abundant on the food source probably will disappear (as observed by Nutting 1959) because of failure to reproduce. Lanier (l970a) suggested that the cross attractive- ness of Ips paraconfusus, 1. montanus and 1. corzfusus enforced their contiguously allopatric distribution. Intrusion of individuals of one of these species into the range of another would probably result in inviable interspecic mating and the genetic death of the individuals involved. On the other hand, I. lecontei can coexist with I. confusus owing to differences in pheromone systems (Lanier and Wood 1973) and, probably, stridulating mechanisms (Lanier 1970b). Michael and Rudinsky (1972) found major differences among Dendroctonus species in stridulating organs and their sounds. It is possible that stridulation as well as differences in host preference, etc. (table 9.7) act as premating isolating mechanisms in cases where pheromone systems fail. The response of Ips grandicollis to host terpenes is inhibited by the Dendr0ct0- nus pheromone, frontalin (Werner 1972). This intergeneric inhibition could be mutually advantageous owing to the consequent decrease in competition. However, it seems unlikely that intergeneric inhibition could result in a shift of the pheromones produced by closely related species. A shift in the pheromone system need not require de novo appearance of a new compound or new receptors. Schneider (1969) hypothesized that most insect species possess olfactory receptors which are specialists and others which are generalists. The specialists could be cued to pheromones and occur in great redundancy while the generalists might be of several types and have overlapping response spectra. Pheromone specicity could evolve by promotion of a ‘secondary’ compound to pheromone status and selection for redundancy of receptors already responsive to the compound. For example, both D. brevicomis and D. ponderosae produce brevicomin and transverbenol and each of these compounds is an attractant of the respective species. Source: Free Articles from ArticlesFactory.com Alexander P is a blogger that studies pheromones.