Say you have a population of mosquitos. Each mosquito has a needle-like mouth, enabling them to puncture the skin of bigger animals and suck their blood. Some mosquitos have average size needles, some have slightly smaller needles, and some have needles that are just that little bit longer. 

The longer ones enable the mosquitos to suck more blood and to penetrate tougher hide. These mosquitos have a slight advantage compared to their peers. The next generation have longer needles too, so this small benefit tends to spread among later and later generations. 

Over time, all the mosquitos in the population have larger needles. Other mosquitos, in other populations, develop special characteristics to fit their circumstances, so that after a long period of time they look, behave and interact differently to each other.

That’s evolution. Fairly logical, wouldn’t you say?