Re: What triggers species to reproduce?

Dear Vijay,

The beginnings of sex are with the abandonment of the asexual means of reproduction. The reason behind sexual modes of reproduction are of course to produce a variety of offspring, particularly in difficult conditions. Asexual “splitting” can be seen as simply a short term solution to producing new generations of organism.

You mention physical contact, but in the production of gametes, there is always a physical stimulation if you are looking for a drive mechanism. The organism has sex drive in some cases such as vertebrate animals and can regularly produce floods of sperm and egg in the right place. Invertebrates such as corals or many other planktonic reproducers all form gametes at the correct “season” by using physical or chemical stimuli without contact. Witness the romantic moonlight-effect researched here:
http://www.solutions-site.org/artman/publish/article_356.shtml

The instincts you talk of are presumed by us to be human-like sex drives, in both male and female, I’m afraid. All animals, including humans are subject to the drive for reproduction. You don’t have to use relatives such as other Mammalia : the invertebrates will make you realise that they are just as keen to pass their characteristics . Look at the soil nematode (unsegmented worm) in this scientific paper :
http://www.jneurosci.org/cgi/content/short/24/34/7427
These tiny males will seek a mate whenever necessary. Mating SEASONS would enter the equation if the animal were bigger, in a longer lasting environment than the ever-changing soil.

Finally, to contradict your assumption that sex is the (male or female) drive, here is a so-called “cultural” experiment with fruit flies.
http://simon.blanchet1.free.fr/DanchinCIB3-4.pdf
To shorten the experience, the FEMALE chooses an artificially-coloured partner-male depending on her social experience of others mating. I hope you find the answer interesting – the whole plant and animal world is involved in your question. Your answers here are moonlight (quite common), loss of mates and conditioning. Many other events prepare organisms for reproduction, as you probably realise in the simpler idea of making plants flower and fruit! (ie. Temperature or Day-length ).