You are correct- mutations are very random. Some help the organism adapt to its environment, some are neutral, and some can send the organism to an early grave (e.g. poor patterned camouflage). In this case with the Pod Kopiste lizards there was a random variation in head sizes and bite force. The lizards with the mutations for small heads and a weak bite force wouldn't be as well suited to their new environment to the degree of the lizards with the beneficial mutation for masticating the tough, fibrous plants. If an individual organism isn't as well suited then it won't be growing as quickly or reproducing as much as the rest of it's population.
To put it simply- because of the introduction of the new environment the larger heads flourished while the smaller heads died off or didn't reproduce as often, which lead to a gene pool that favored larger heads and increased bite force. That's why the variation doesn't seem random.
this ^^^ is what you said later, which is correct. what i picked up on was the idea that "adaptation" was something which individuals
did in response to an environmental change that requires a shift in their resource utilization in some manner.individuals
don't adapt, only populations
do. they do this by experiencing change in the gene frequencies of the traits undergoing selective pressure. for example, in your lizards, there will be pre-existing range of sizes and jaw structures, which are manifestations of heritable genes. if the distribution of food size or type changes, such that the optimum phenotype drifts away from whatever was stable before, then selection will track that change by favoring survival or fecundity of the organisms that possess the phenotypes better suited to the new distribution. over successive generations, the gene frequencies of the population
will change, and evolution will have occurred. they may stay that way a bit, or continue to change, as they track the moving target of food types.
but no individual organism has adapted to anything, anywhere in the process. the individual organisms merely survive and reproduce at different frequencies, dependent upon their success in processing the change in their adaptive landscape.
now, you can
use the term 'adaptation" in a completely different way by talking about behavioural changes in individuals. for example, if the range food types available to the lizards changes fairly slowly, then individuals can simply modify their choices of what goes into their mouths. this will not be a heritable change, and won't show up as an evolutionary modification. it can result in evolution, but isn't evolution itself, just flexibility. if the resource drifts far enough out of the range of the individual to process or moves too quickly, then the individual will suffer, and if the population phenotypic mean gets left behind by the resource change, then the population will suffer too, and may eventually go extinct.
that's all that i was saying. and i'm not talking about mutation, just frequency changes among alleles that are already in the population. given mutation added to the mix, you can actually introduce things that are new, rather than just changing frequencies of things that are already there.