web design as an analogy for genetics.

The html and css corresponds to the genotype. The final rendered page corresponds to the phenotype.

There are many ways to achieve a similar phenotype. 

Changing just a bit of a genotype can have quite drastic results. The same genotype is not guaranteed to build the same way every single time.

Hedgehogs, spiny tenrecs, porcupines and echidnas are not related (different genotypes). However, they look much the same (similar phenotypes, as a result of similar selection pressures).
There are many ways (some 'better' than others) to make a particular webpage. Divs, tables or iframes might arrange the content the same way, for example. Some ways are 'stupid' but evolution is full of those.

The exact same webpage (same code,  same 'genotype') can render differently across browsers. Building a body from the same genome isn't guaranteed to always have the same result. Cloned cats, for example, can have very different markings from the original cat. The genes basically say 'put markings here' and don't specify it to the hair.

Sometimes it just fucks up (mutations = server errors, connection problems).