One type of evidence for evolution (evidence that organisms are related, descended from a few common ancestors, and change to adapt to their environments) is that organisms are similar to each other, but not exactly the same. Similar organisms have differences that help them adapt to their environments.
Many organisms have similar body plans. Horses', donkeys', and zebras' bodies are set up in pretty much the same way, because they are descended from a common ancestor. As organisms adapt and evolve, not everything about them changes. The differences, such as the zebra's stripes, show that each species adapted to its own environment after branching off from the common ancestor.
The bodies of deer, moose, zebras, and horses are very similar, and these animals are very closely related. One major difference is that deer and moose have antlers and zebras and horses don't. Why is this? Deer and moose live alone or in small groups, while zebras and horses live in large herds. Living in a herd provides its own protection from enemies: it is easier to attack an individual than a huge herd. Therefore, herd-living animals do not need the antlers that their loner relatives need for protection. In addition, running or grazing with large antlers is hard to do in a herd, where it is easy to accidentally stab one's neighbor.
All insects have heads, abdomens, and thoraxes, antennae, six legs, and wings. However, each species is different, and while all insects have wings, some have small, useless wings, because their environments did not force them to evolve useful wings, or because their wings became harmful to survival.
All birds have feathers, beaks, and wings, but are different because they had to adapt to different environments, such as the webbed feet of water birds but not of land birds. On a more distant level, fish and zebras both have eyes, frogs and baboons both have spines. Generally, the longer ago the last common ancestor lived, the less the organisms have in common. Turtles and tortoises share a common ancestor, but began evolving separately a long time ago. The common ancestor of box and painted turtles lived more recently, so the box turtle has more in common with the painted turtle than it does with the tortoise. How similar two organisms are can help people figure out how closely they are related.