It's similarities. Because you believe in evolution you see it as patterns showing evolution.
No, it's not strictly similarities, it really is patterns.
For example, take fish versus whales. At a superficial level, they might appear similar; similar body types streamlined for moving through water.
Yet, the underlying morphology and genetics are highly different. When you compare things like underlying skeletal structure to functional morphology to genetics to developmental biology, it points to dramatically different origins for each respective group of organisms.
For example, dolphins have olfactory genes yet completely lack an olfactory system. They live in the water, yet still need to breath surface air. They begin to develop hindlimbs during early embryological stages which then get 'switched off' later in development.
It's these types of things that point to an ancestral origins from terrestrial organisms, not special creation for living in the water.
It actually shows common design.
We've been over this already. You haven't been able to actually demonstrate what "common design" is supposed to be, nor construct a framework in which to explain what you observe.
"Common design" is just a bumper sticker slogan.
There are patterns or similarities in all designs in order to make the thing function for best performance. That's exactly what God did in creation.
So why did God create dolphins with olfactory genes but lacking an olfactory system? Why would God create dolphins to be completely aquatic yet lack the ability to breath underwater, but fish have that ability? Why would God create dolphins to start developing hindlimbs as embryos only to have the same development turn off and the hindlimbs re-absorbed?
Can your "common design" explain any of this?