People who talk about positive selection annoy me because from my view evolution is never positive it is always winnowing out failure. There is no pat on the back and yes there is a good gene, there is just an indifferent pursuit of any weak members of the population. I need to explain this view and why some might feel it pedantic.
I was reading The Selfish Gene when I came across a description of the progressive nature of evolution. If Dawkins had read the controversies regarding Darwins's theory of natural selection carefully he would know that this idea of progress was the heart of the problem. Darwin denied progress in the creation of random mutations. Any kind of appearance of progress is an accidental gratuity from the survival of the fittest and that only applies to that ecosystem in those particular set of conditions. Popper from a philosophical view was also damning of any view of progressive evolution, which he called the "Poverty of Historicism" and his main argument against Marxism and the view that the Communist Society was going to inevitable triumph. This progress smacks of Teleological thinking and ideas like those of Teilhard de Chardin who tried to reconcile his Catholicism with the evolution of complexity.
The reason for this is that the material that is mutated is a different material to that on which selection applies. Mutations occur in DNA at the genotype level, but selection occurs on the cellular or organism level in the phenotype. Some parts of the genome are more "fixed" than others because changes are likely to lead to an unviable phenotype that most likely will not even be born (sex is useful for filtering out these mistakes without wasting an entire organism). Many proteins have cores or active sites that cannot allow mutations. Often these are hydrophobic regions represented by synonymous codons. Then there are parts of the genome that are largely unregulated and in which a considerable amount of probably neutral mutations can occur. These are often in the surface loops of proteins and these are often hydrophilic amino acids. This bias in the way the codon table has evolved makes it hard to use the simple statistics of looking for sites where there are different numbers of synonymous and non-synonymous mutations complex because it does not mean a site is immutable, it just means that some have a low probability of achieving that change even if it would create an organism that benefits in natural selection.
What they mean by positive selection is a change that makes the organism better than the current one. The way natural selection is working is now the optimum shifts from the organisms of the past to the new variety and so the old variety is less fit compared to the new. Whether this is "progress" or not we cannot know as this might be a step to a dead end. Extinction does not care about progress, only we care about it because we want to make ourselves the culmination of this progressive evolution, but we forget that actually bacteria are more evolved than we are. They have experienced more generations than we have, they have undergone more rounds of natural selection.
So for me "positive selection" is one of these phantom ideas. It exists because of the way we look back in time but it is an illusion, it is an accident. Unlike Popper I do not completely disregard historicism as history plays an important role. Some parts of evolution we can understand as responses to physical and chemical laws (long necks to eat leaves from trees, eyes for seeing)and evolution often converges on these solutions, but some parts of evolution are frozen accidents. Ian Stewart and Jack Cohen coined the terms universals and parochials for the things we can say will happen from the "laws of evolution" and for the serendipitous products of chance. This is the same as the different types of information in the ideas of complexity of Murray Gell-Mann's the Quark and the Jaguar. If we ignore history then we ignore these chance effects that CANNOT be described by any theory. These effects will always remain phenomenological and so the hope of biologists to achieve rigorous laws analogous to those of physics and chemistry is IMPOSSIBLE in the field of evolution. We simply never can know what will happen next, but we can make up some nice "just-so" stories to explain what has happened in the past. That is what Richard Dawkins does best.