this is the argument from common similarity. but since common similalrity can poit to a common d esigner it can be used as evidence for a common descent.
I think that you mean that since common similarity can point to a common designer, it
can't be used as evidence for common descent. However, this argument begs the question, since it assumes the existence of a designer; you need to have independent evidence for a designer for the argument to be valid.
Also, there is the well-known fact that some animals have similar anatomy and genes but live in very different environments. For example, bats fly, like birds, but anatomically and genetically they are more similar to mammals; whales, seals, sea-lions, walruses and sirenians swim, like fish, but, again, anatomically and genetically they are more similar to mammals. Chimpanzees, bonobos, gorillas and orang-utans are quadrupeds and forest animals, and they are at least partly arboreal, like monkeys, but anatomically and genetically they are more similar to humans, who are bipeds, do not live in forests, and are very poor at climbing trees.
It does not make sense for a designer to design similar anatomies and genomes for animals that live in such different environments, and these anomalies appear to be better explained as a consequence of descent with modification from a common ancestor.
