Evolution is likely to occur because of i. the potential for a species' population to increase. ii. genetic differences in offspring. iii. a limited supply of necessary resources. iv. selection of offspring that are best able to survive.

Lll. a limited supply of necessary resources. If there isn't enough resources the species will have to learn to adapt to be able to gather the new food supply they need. Such as birds and there beaks, if the insects population went down and that was the main food supply for that type of bird they would have to be able to adapt in order to survive, so evolution occurs. Over time that bird would change to adapt to its new feeding source.

Hope this helps!