Why does growth happen?

Question

Why does growth happen?

Answer 1

There are two answers to your question -- one relates to physical constraints, and the other to fitness (from an evolutionary biology perspective).

First, growth happens because organisms are unable to produce exact copies of themselves. Think about a hypothetical organism that reproduces via fission: splitting the individual equally, you now have two individuals, each 50% as large as the original. Without growth, these would then eventually divide into four individuals, each 25% of the original. And so on... after a few generations, the offspring would be nowhere near as large as the original parent. A few more, and they would effectively cease to exist. Therefore, organisms grow to "regain" the size of the adult stage.

Second (and related), selection tends not to favor individuals that reproduce only once, allocating all of their resources to producing just one clone of themselves (but see other problems with that strategy above). Instead, fitness will be higher if organisms allocate sufficient resources to each offspring to give them a decent chance of making it past the early stages of development, while saving resources for making multiple such offspring. Because there is always a size-number trade-off, selection will then favor that combination of size and number that tends to result in the highest fitness (aka most descendants in future generations). Again, growth in offspring after they are produced reduces the amount of energy that each parent must allocate for successful reproduction.

Answer 2

the cells in the organism continuously divide, building up volume and mass