There are in evolution at least, what's called mutations. You're not going to tell me that God causes malformation to occur deliberately. I mean you can tell me, but I won't believe it.
First, there is a difference between genetics (DNA) and development (how an individual organism grows over time). The development is partly determined by genetics, but is also partly determined by environment.
Malformations like having two heads is NOT a genetic mutation. That tends to happen developmentally when something go wrong, for example when two developing embryos merge. One way to tell the difference is that two-headed cats won't give birth to two-headed cats, but regular one-headed cats. The *genetics* doesn't lead to the two heads.
Evolution does NOT happen by 'hopeful monsters' like two-headed animals. It relies on the *genetics*, not the development. Only those changes that can be passed to the next generation are relevant for evolution.
Also, the genetic changes relevant to evolution tend to be small from one generation to the next. If the change (mutation) is beneficial, it tends to get passed on more to the next generations, leading it to be more common in the population. This is why populations evolve, not individuals. Developmental events that are not linked to genetics don't have an evolutionary effect.
Now, all living things have DNA. Plants, mushrooms, animals, bacteria, etc. ALL of them have DNA. This is what determines their *genetics* and what can be passed on to the next generation.
Part of that genetics determines what species you are. All members of the same species will share the DNA that determines those properties of a species. So all humans will share the DNA associated with being human. On top of that, individuals will share *more* of their DNA the closer they are to each other in the line of descent. So, you will share more DNA with your parents than with your grandparents.
But, different species *also* share a great deal of DNA. Simply having two mammals, say cats and dogs, will mean that they share a LOT of DNA. Every genetic aspect that makes them mammals will be shared. Primates (say chimps) will also share the DNA associated with being mammals.
But, dogs and cats will also share the DNA associated with being carnivores (a subtype of mammals) and primates will NOT share the DNA associated with being carnivores. But different primates *will* share the DNA associated with being primates (so chimps and gorillas will share that DNA).
This is how we can determine which species are related to each other: we look at the DNA that they share and compare it.
The remarkable thing is that when this is all cataloged with many different species, we get a branching tree-like structure of relationships. This would not be expected if there was no *inherited* links between the species.
We might think that all ocean species would be similar or all flying things would be similar. But that is NOT the case. Instead, birds that fly share DNA with other birds, not with bats. And dolphins share DNA with other mammals, not the DNA associated with fish.
So, the same comparisons of DNA that show you are related to your parents can show that dolphins are related to land mammals and that, yes, humans are related to primates.
Does that help clarify things?