[podcastmaker (OP)]

Why is evolution considered a theory?

[Bored chemist]

Because that's what it is.
https://en.wikipedia.org/wiki/Scientific_theory

[wolfekeeper]

Because it's only a theory. Like gravity.

[alancalverd]

It isn't. It is an everyday observation.

You don't (can't!) look like both of your parents. Your children will probably look even less like them.Your family has evolved. It is an inescapable consequence of sexual reproduction. The mechanism is all bound up with the plasticity of DNA. Living things that reproduce asexually still have some inherent DNA plasticity but with fewer input variables.

Evolution to fill an environmental niche has been observed where a species has changed color, size or shape over a few generations when an environment has changed. Evolution by selective predation has also been observed, "samurai crabs" being a particularly fine example. Obviously these observations tend to be confined to species with short life cycles but there is no reason to suspect that the capability does not extend to trees and large mammals. Interestingly, humans may not be subject to further significant evolution as we tend to normalise our environment (through agriculture and architecture) rather than adapt to it. We also have an exceptional tendency and ability to maintain the weak, eliminate endemic disease,  and grant reproductive rights to all, so "survival of the fittest" does not apply and the species will become more diverse over time.   

"Evolution of species" doesn't quite fit into the category of theory or observation as the term "species" is not consistently defined, and seems if anything to be a consequence of environment-driven selection.

[Bored chemist]

You don't (can't!) look like both of your parents. Your children will probably look even less like them.Your family has evolved.

I look quite a bit like my parents.
Indeed, if offspring didn't inherit traits from them evolution wouldn't work.
If the Samurai crabs didn't have children that looked "like them", the trait would be lost.

[alancalverd]

The missing word is "exactly". And even without it, you can't possibly look like both of them unless they are identical twin hermaphrodites. Perhaps incestuous earthworms don't evolve, but this thread isn't about diehard Republicans.