Smells (1)

 

Smells are volatile chemicals. For example, phenyl ethyl alcohol (PEA) shown below smells like ‘rose’ to anyone that inhales it.

Rose smell (Phenyl ethyl alcohol)

PEA enters the nose along with the air one breathes in. As it rushes into the lungs along with air, it flows past specialized neurons present in the roof of the nasal cavity called olfactory sensory neurons, OSNs for short. OSNs bear odorant receptors (ORs) or proteins capable of binding PEA on hairlike cilia. The location of OSNs in the nose is shown below. Below that is a blowup of OSNs bearing the hair like cilia.

olfactory neurons in nose

 

Blowup (electron micrograph) of OSNs bearing cilia

(Image source: http://darwin.iz.uj.edu.pl/iz/anatomy/jakub/grafika/o2e.html)

Once PEA binds to ORs on the OSN cilium, it triggers a ‘cascade’ of events in the OSN that ultimately lead the OSN to produce an electrical signal. The OSN passes this electrical signal to the brain and thus ‘informs’ the brain that a ‘rose’ is nearby.

the orchid and the wasp

This Australian orchid synthesizes a chemical called chiloglottone, which deceives the wasp into ‘thinking’ that the orchid is a female wasp. (clicking on the picture will take you to the website of a scientist involved in this research)

structure of chiloglottone

This chemical is synthesized by the wingless female wasp and serves to attract males to her. The orchid makes the exact same chemical in larger quantities than the female wasp!

In addition, a part of the orchid flower also somewhat resembles the female wasp (observe the picture carefully to spot the shiny black labellum of the flower underneath the wasp.)

The male attempts to copulate with the flower and receives a dusting of its pollen. This pollen covered male then attempts to copulate with at least another orchid specimen thereby cross-pollinating the second plant. What reward does the wasp get for its exertions? None at all!

At first glance the male wasps seem to be quite easily fooled by the orchid. Wasps are not stupid, in fact, they are capable learners. Yet the male wasp is apparently fooled by some pretty obvious visual effects and a very potent chemical, a pheromone. Does the pheromone drive the wasp to ‘programmed’ behavior?

If the wasp does not receive any rewards for its exertions, then why has this behavior survived? It violates evolutionary theory since any behavior that does not confer adaptive advantage should be eliminated by natural selection.

How is this violation of natural selection to be solved? Come back later for the current explanation.