The real resistance to Darwin’s theory focuses on his thesis that the mechanism responsible for small-scale changes within species also gives rise to large-scale changes—namely the origin of new species (speciation).

   Darwin ’s hypothesis for explaining how a change within a species causes the development or emergence of a new species is that small changes in a population accumulate. 

   Given enough little changes, organisms will become very different. Populations within a species that live in very different environmental conditions will become increasingly different. 

   Characteristics that are advantageous in one population will not necessarily be advantageous in another. 

   Over time, populations can become so different from each other that they cannot interbreed.

Although most scientists today accept natural selection as the mechanism for speciation, not all agree. 

For example, there are unanswered questions about why sexual reproduction evolved. Why do some organisms reproduce sexually and others asexually?

But note, there is a big difference between:

1. Having strong evidence that humans descended from ape-like creatures, and... 

2. Having an explanation of exactly why it proceeded in just the way it appears that it did.