Why does convergent evolution happen?

Answer 1

Distantly related organisms can find themselves experiencing similar environmental circumstances. For example, ducks rummage for food in the mud of ponds and lakes, and the shapes of their bills reflect that adaptation. There is quite a bit of variability in bird bill shape, and for ducks the varaition which works best for their habitats is a wide shape so that food can be gathered up with mud and water then strained. In Australia, the platypus--a mammal-- feeds upon similar organisms and in a similar way. It also has a bill shaped much like a duck's, but it is not made up of similar tissue. A duck's bill is made up of a hard, horny material, similar to fingernails, while the platypus bill is made up of soft tissue. The bills of these two organsisms, while completely different structurally, converged on a similar shape due to similar dietary habits. One can find lots of other examples of this phenomenon. Consider organisms that swim in water. The optimal shape for that is elongated and smooth. Fish certainly possess that shape, but so do whales, dolphins and porpoises. Yet both groups have very different evolutionary histories, and their shapes, while similar, betray their evolutionary origins with significant differences. Fish swim with a side-to-side motion, and their tail fins are vertical. Whales, on the other hand, being descended from land-dwelling organisms, do not have backbones which bend easlily from side to side, so they swim with an up-and-down motion of the tail, and their tail fins, or flukes, are aligned horizontally.

Answer 2

Adaptive radiation may result in convergent evolution, if two species occupy the same ecological niche in different locations (such as a hedgehog and porcupine) or when separated by time (such as dolphins and sharks).