The Daleks are the oldest and most popular enemies in Doctor Who. Their evolution began in December 1963 with the episode, "The Dead Planet" (which was later renamed to episode one of "The Daleks"). In that episode the only view of a dalek was it's menacing sucker arm creeping toward the helpless, screaming Barbara Wright (played by Jacqueline Hill - imdb). Even though the full view of the dalek was never shown in "The Dead Planet" they had already paralyzed British audiences with fear.

In the second episode, "The Survivors" the full dalek is revealed. A sort-of 'pepper pot' shaped robotic creature that moved around effortlessly on some unseen method of propulsion. They capture Barbara and keep her prisoner.

Overall, in this episode we learn the true nature of the Dalek. A malevolent and hateful creature that wishes to destroy anything unlike itself - especially Thals (the former co-inhabitants of their planet of Skaro). Also introduced is the concept that the original daleks were dependent on static electricity for their propulsion. A limitation which the Doctor eventually learns to exploit.

The daleks later appear in the episode, "Dalek Invasion of Earth" where the daleks are no longer dependent on static electricity but are able to move about on dry land (and even in water) with the aid of a radar dish attachment on their dalekanium exterior. With this, they cause all manner of death and destruction on Earth before being eventually defeated by the Doctor.

In later episodes the daleks are no longer limited by the need for static electricity or radar dishes but are able to move freely (if not awkwardly) about. They eventually develop a primitive means of time travel and begin destroying and enslaving races throughout time and space while their foremost goal was to destroy their arch nemesis, the Doctor.

One particular oddity regarding the evolution of the dalek race comes in the Tom Baker episode "Genesis of the Daleks" (1975) where the Doctor travels back in time to the very beginnings of dalek evolution and is confronted with the task of hindering the evolution of, or completely destroying the daleks. In this episode the daleks are not dependent on static electricity for movement as in their very first appearance. Also, another discrepancy with earlier dalek lore is that there is mentioned that the creature that exists within the dalekanium shell is known as a Kaled (mutant) instead of a dalek as was explained in the first episode (1963). Perhaps Terry Nation (creator of the daleks and author of many of their episodes - imdb) had forgotten what he had written earlier or was expecting the television audience to not remember the episodes from 1963 - '64.

Through the years we encounter Davros (creator of the daleks), emperor daleks, dalek primes, dalek enemies (such as movellans) and major evolutional changes to the daleks, who's mainstay remained the same: total extermination of anything non-dalek. Eventually the daleks mastered temporal engineering, hovering, special-weapons daleks, etc.

Eventually the daleks destroy the Time Lords and their home planet of Gallifrey leaving the Doctor to be the only survivor of his race.

[ SPOILER ]

The daleks end up back on Earth in the form of 'the cult of skaro' - the last few remaining daleks in existance. There they are yet again defeated, leaving but one dalek surviving. Who knows what terrors it may dream up.