Charles darwin observed a unique beak size and shape in the finch population of each of the galapagos islands that he visited. mc003-1.jpg which is the most likely cause of the observed variation?

The most likely cause of the observation that Charles Darwin made about the unique beak size and shape of the finch population on each of the Galapagos Islands was the fact that each of them were subject to different selective pressures which made them develop different beaks better suited to the environment through time. 

Each parent had at least one allele for white petals.

Explanation:

According to the law of inheritance, it is necessary that each offspring must inherit one allele from each of its parents for a given gene.

Since in the given question, a rose plant inherits a white flower petal allele, it is necessary that the plant must have an inherited one allele from each parent, therefore, the two parents must have had at least one allele for white petal.

The cross is shown below ,

W---> red petal

w > white petal

Atleast one allele => Ww X Ww ---> WW Ww Ww ww

More than one allele => ww X ww > ww  

Here ww is the offpsring with white petals.