Evolution is certainly local. The evidence from Earth is overwhelming. Most species have specific locations, likely the ones they evolved in, and they stay there. They evolved characteristics which gave them advantages in that particular locality, and these characteristics imprisoned them as well. If they left that locality, they would be facing better competitors who evolved to match the adjacent locality, and they wouldn't be the fitness champions and would be eliminated. This explains the incredible diversity of species on Earth, as we have hundreds of localities, each with a full range of species adapted to it.
The same should hold on alien planets. There is no reason to think that an alien planet wouldn't have climate differences, tectonic differences, weather differences and so on, all of which affect the fitness competitions. So, an alien planet visited before their species have been affected by the dominant intelligent alien species would be diverse. For one thing, this assists the aliens in making their technology step up in genetics, as there are countless examples of the expression of genetic codes around.
With all these localities, like little mini-worlds, why would intelligence be arising all over the place, with different creatures getting smarter in deserts, on seacosts, near rivers, in wetlands, in forests, in savannahs, in areas with droughts or floodings or monsoons or what-have-you? Why didn't it happen on Earth, with bright species all coming to flower everywhere and maybe even contacting one another?
This has implications for the presence of alien life on origin planets, where life gets started despite all odds, and generates myriad species. The life getting started stuff is what is hard, we think, but once it does, evolution is like a mighty machine that simply will not stop until all the niches where life can survive have been filled.
In each location, for each species, we have mutations and fitness competitions. But the fitness competitions are specific in nature. Consider large animals on Earth: they often compete in mating selection. Large antelope bucks fight with each other, and the winner is the one who mates the most. The ones which survive are those which can outrun the predators in the area, typically felines of some sort. So, evolution is choosing fighting ability and running ability, not intelligence. The variance of traits such as these, which are affected by many genes, is large, and the key point is that lesser traits, for other functions such as intelligence, are lost in the noise. A buck antelope might have a gene which makes it smarter, but what is competed for are fighting attributes, such as horn size and shape, strength, size, aggressiveness, and many other attributes which contribute to a complex skill such as combat for mating preference. These large numbers of genes are all being competed for, and it takes a long time for some improvements to evolve to being common, as there are so many other genes which produce some changes in the metric for fitness, combat ability. A gene which bumps up intelligence doesn't even register, and therefore is lost in the shuffle of those genes which are involved in the few competitive contests that evolution holds. If running ability conflicts with fighting ability, the fitness competition is even more complicated, and it is even less likely that intelligence would be selected for.
Something should be said about what is meant by intelligence here. It is not neural processing. If you look at predators, they have a great deal of processing ability for hunting ability, for example, for carefully examining visual fields for signs of prey. This involves huge numbers of neurons, and could be thought of as intelligence in a field we do not measure. There is no hunting IQ component, nor one which involves noticing something in a high-definition visual field that has certain characteristics. Nor is there a motion-connected IQ component, although efficient chasing and catching of prey certainly involves, in large creatures, a huge amount of processing of muscle commands and recognition of state of the body, as well as recognition of terrain and adaptation to it. So while neurons can evolve and become complicated, what an animal does with them is subject to the fitness competitions, and they simply blur over intelligence of the kind we emphasize, the kind that creates cities, and stress the kind that solves problems of survival and reproduction. Not as much neural processing is needed here, but some certainly is and it certainly is evolved for.
So if we are trying to figure out if evolution on an alien planet would necessarily produce intelligence, of the kind we like to think about, the city-building kind, we cannot rely on the huge diversity of locations on the typical alien planet to invariably produce it somewhere. It is simply not typically selected as an important variable, and may even have side effects which interfere with the important variables that are being selected for. Largely, it would be in the noise of fitness competitions, and not selected for. We need to figure out when it would be.
Down this path, there is another aspect of evolution that makes a large difference in what gets selected. Consider the speed of evolution. If a species has 100 mutations and is competing them for optimal, it will be evolving faster than one which has 20. In other words, evolution is proportional in some vague sense to the number of creatures. A species which occupies a large habitat, and numbers in the millions, will be evolving much faster than one which occupies a smaller habitat, and numbers in the tens of thousands. The speed of evolution will be tremendously greater, meaning that the homogeneity of the alien planet plays a large role in the evolution of intelligence. If intelligence is way down in the noise of fitness competitions, at some period of evolution, but is not overwhelmed by side effects from the important attributes, such as combat or prey-hunting, so that it has a very slow evolutionary rate, it will not evolve in a habitat that is small and doesn't last too many generations. The species will simply migrate or become extinct, meaning that any small gains in intelligence would be lost.
The evolution of intelligence needs a large habitat, so minor changes in unimportant aspects, such as intelligence of the kind we care about, can evolve and stick in the organism when it finally migrates. The large habitat needs to support large numbers of creatures, not simply a sparse collection. So finally, a little glimmer of understanding seems to be peeking out. Homogeneity of a planet is important. That is something a very large telescope might even see.
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