Re: why do organisms have life cycles

Hi Leow,

We have a fine question here that typifies exactly what you should be thinking at your level in Biology.

It would seem sensible to evolve a suitable body for your purpose and then carry out your life-style in the way that suits you, wouldn't it? Throughout evolution though, living things have been faced with predator-prey relationships and frequent extinctions that present us with a set of present-day survivors.

Some have extreme specialities that fit them for a "job" or niche that they really compete well for. Others are generalists, like some of your school-friends who are good at most things. When food is short, just think how useful it would be if some of your population could eat something else. Remember living is all about getting energy and then spending it wisely so that your offspring will appear as a successful next generation (ie. feeding and reproduction are major life processes).

Well the metamorphosis of insects, frogs or many other creatures shows us that it's possible to eat two different things in your life--- the tadpole or caterpillar eats plants (herbivorous mode), then the adult frog or insect can eat something else (often a carnivorous mode), or in the case of mayflies and several butterflies, nothing at all. That removes some competition with the smaller larva, the adult being obviously a larger competitor.

There are of course other advantages to a cycle in which you start protected and then complete your existence as a different kind of organism. Ants use their metamorphosis to produce short-lived males who don't have to be fed. The numerous large queens they can produce of course are the colonists who can find new worlds to conquer (ant species being very aggressive in many cases).

Think also about yourself. There are no human larva as such, but mammals almost always have a stage in which they can't feed themselves, even after the placental stage. Without teenagers, too, there would be very little learning at the cutting edge of human technologies. They do record data on mathematicians which seems to indicate a remarkably able period around the age of 20!

If you need evidence beyond that, plant species always have life cycles in order to progress from their small embryonic stage to a full grown organism. Probably the immobility is responsible for such distinctive stages, but their cycles even involve two generations to help get over the lack of movement. Animals of course have to be sedentary on say, coral reefs, so they all possess larval forms that distribute the species. While butterflies are a mobile phase in insects, it's often the larvae that have to swim for their lives, literally!