Question

explain evolution by stages

Answer 1

Evolution by stages is said because the process of evolution had took place over a long period of time. The complex organs are created bit-by bitover generations.

Increasing complexity of organs:

The eye was present in the earliest organisms in the form of a simple patch of photosensitive cells called an eyespot. This was found in lower organisms such as Euglena.

Decreasing complexity of organs:

There are some organs in the human body, which are present in the reduced form and do not perform any function. For example, the nictitating membrane of the eye, third pair of molars, vermiform appendix, body hair, nipples in males etc.

Changed functions of organ:

Some structures during the course of evolution changed their functions. For example, some past reptiles (which later evolved into bird ancestors) would have had feathers, which were not necessarily used for flying, but instead only provided them with protection.

These are certain stages of evolution.

Hope this helps!

Answer 2

Some evolution takes place in various stages due to the complexity of the organ those complex organ cannot be generated by a single dna change so it took evolution by stages.

For e.g. evolution of eye took place in various stages eye provide immense advantage so it can't be generated by a single dna change even an intermediate stage such as a rudimentary eye can be useful. Eyes are present in flat worms, insects, octopus, vertebrates etc and the structure of the eye in each of these organisms is different enough for them to have separate evolutionary origin.