Finding for evolution, comparing past flower numbers with present
Has there been evolution in this population?

If $p$ differs in the population, then yes. If you $p_1$ and $p_{10}$ (the two different estimates at generation 1 and generation 10) are equal, then any deviation is caused by sampling error. Hence set $P_1 = P_{10}$ as the null hypothesis and performs a chi-square test.

Is natural selection involved?

For this we have to know the generation time and the population size. We can assume the generation to be of one year (10 generations). The population size $N$ could be estimated from genetic data of the sampled butterflies. The expected loss of genetic diversity per generation is $\frac{1}{2N}$ but you will need to consider any possible path by which one can go from $p_1$ to $P_{10}$ using a series of binomial distribution and sum up their probabilities. That being said, it is a little bit computationally expensive if $N$ is large but some Markov Model theory might give you some simplifications.