The problem here is the question. Evolution has two meanings in our current society: Evolution natural selection, and Evolution theory of “molecules to man” (to steal a phrase from Ken Ham). People use both interchangeably without really understanding the fundamental difference between them.
Natural selection is not an improvement in the genetic code, or an evolution into a more complex form. In fact it is the other way around! What actually happens in natural selection is that animals “evolve” into LESS complex forms, their DNA structure is reduced not increased.
So: Do organisms ALWAYS evolve into more COMPLEX forms?
No. And in fact, I would argue that organisms as we see them today NEVER evolve into more complex forms.
The difference is hard to explain – however, bear with me.
As an example:
Can poodles breed with poodles and produce a wolf? NO.
Can wolves be bred over a succession of time into poodles? Quite possibly.
Some would say the DNA in a wolf is less “evolved”; I would argue instead that it is less specialised.
The difference is in how we are using the term “evolution”.
Some would call evolution “an improvement of a species”, in that the species adapts to new circumstances to help it survive. That IS evolution (natural selection) but it is NOT “theory of evolution” (animals evolving into complex forms).
The animals in question have not had bits added to their DNA (or in other words evolved into more complex life forms), instead they have lost an ability, for the most part, to go back to what they were before the change.
So the question: do organisms evolve into more complex life forms? is actually a badly worded question.
Organisms progress through natural selection into seemingly more complicated forms in terms of their biology, but in terms of DNA they have actually de-evolved into less complex forms.
Answers in Genesis is a creationist website that excels in ironing out these differences between the two different uses of “Evolution” and is well worth checking out as they explain the whole difference between evolution: natural selection and evolution: origin of a species so much clearer.
These days we see only minor changes in complexity, white moths changing to black moths, new breeds of dog. The genetic codes have been gradually reduced over time. Biologically the animals seem to become more complex, but genetically? Definitely not.
Which means: Do organisms evolve into more complex life forms? A resounding NO!