Part of the discussion boils down to a debate about the proper "null hypothesis" in evolutionary theory.
Here are some explanations from the textbooks that may help explain the "null hypothesis."
The most widely used methods for measuring selection are based on comparisons with the neutral theory, in which variation is shaped by the interaction between mutation and random genetic drift (Chapter 15). The neutral theory serves as a well-understood null hypothesis, and deviations from it may be caused by various kinds of selection. In the following sections, we examine ways of detecting and measuring selection by comparison with neutral theory.
EVOLUTION by Nicholas H. Barton, Derek E.G. Briggs, Jonathan A. Eisen, David B. Goldstein, and Nipam H. Patel, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, 2007 (p. 530)
The first step in a statistical test is to specify the null hypothesis. This is the hypothesis that there is actually no difference between the groups. In our example, the null hypothesis is that the presence or absence of wing markings does not effect the way jumping spiders respond to flies. According to this hypothesis, the true frequency of attack is the same for flies with markings on their wings as for flies without markings on their wings.
The second step is to calculate a value called a test statistic....
The third step is to determine the probability that chance alone could have made the test statistic as large as it is. In other words, if the null hypothesis were true, and we did the same experiment many times, how often would we get a value for the test statistic that is larger than the one we actually got?
EVOLUTIONARY ANALYSIS by Scott Freeman and Jon C. Herron, Prentice Hall, Upper Saddle River, New York 1998 (p. 73)
Genetic drift and natural selection are the two most important causes of allele substitution—that is, of evolutionary change—in populations. Genetic drift occurs in all natural populations because, unlike ideal populations at Hardy Weinberg equilibrium, natural populations are finite in size. Random fluctuations in allele frequencies can result in the replacement of old alleles by new ones, resulting in non-adaptive evolution. That is, while natural selection results in adaptation, genetic drift does not—so this process is not responsible for those anatomical, physiological, and behavioral features of organisms that equip them for survival and reproduction. Genetic drift nevertheless has many important consequences, especially at the molecular genetic level: it appears to account for much of the differences in DNA sequences among species.Here are some papers from the scientific literature that illustrate how one goes about using the null hypothesis to ask questions about evolution.
Because all populations are finite, alleles at all loci are potentially subject to random genetic drift—but all are not necessarily subject to natural selection. For this reason, and because the expected effects of genetic drift can be mathematically described with some precision, some evolutionary geneticists hold the opinion that genetic drift should be the "null hypothesis" used to explain an evolutionary observation unless there is positive evidence of natural selection or some other factor. This perspective is analogous to the "null hypothesis" in statistics: the hypothesis that the data does not depart from those expected on the basis of chance alone. According to this view, we should not assume that a characteristic, or a difference between populations or species, is adaptive or has evolved by natural selection unless there is evidence for this conclusion.
EVOLUTION by Douglas Futuyma, Sinauer Associates Inc., Sunderland, MA, USA 2009 (p. 256)
Duret, L. and Galtier, N. (2007) Adaptation or biased gene conversion? Extending the null hypothesis of molecular evolution. Trends in Genetics 23:273-27 [doi:10.1016/j.tig.2007.03.011]
Orr, H.A. (1998) Testing Natural Selection vs. Genetic Drift in Phenotypic Evolution Using Quantitative Trait Locus Data. Genetics 149:2099-2104. [Abstract]
Brown, G.B. and Silk, J.B. (2002) Reconsidering the null hypothesis: Is maternal rank associated with birth sex ratios in primate groups? Proc. Natl. Acd. Sci. (USA) 99:11252-11255. [doi: 10.1073/pnas.162360599]
Nachman, M.W., Boyer, S.N., and Aquadro, C.F. (1994) Nonneutral evolution at the mitochondrial NADH dehydrogenase subunit 3 gene in mice. Proc. Natl. Acd. Sci. (USA) 91:6364-6368. [Abstract]
Fincke, O.M. (1994) Female colour polymorphism in damselflies: failure to reject the null hypothesis. Anìm. Behav. 47:1249-1266. [PDF]
Roff, D. (2000) The evolution of the G matrix: selection or drift? Heredity 84:135–142. [doi:10.1046/j.1365-2540.2000.00695.x]