Review of "The evolutionary origin of complex features" Harsha V. Madhyastha This paper aims to study the intricacies of how Darwin's theory of evolution by natural selection works. This study is carried out in a test environment wherein each organism is a self-replicating computer program. Each program comprises of a sequence of instructions, each of which is one of 26 chosen instructions. The replication is subject to errors so as to simulate mutations. The "health" of each organism is evaluated on the basis of how many logical operations it can perform, from those possible with the available instruction set. The merit derived from any logical operation is determined by the minimum number of NAND gates required to implement that logical operation. The authors then study the ancestral hierarchy of the organisms produced and make an attempt to pinpoint the reasons for development of the best possible organism, which in the given test setting is a program that can perform an EQUALS operation. The major strength of the paper is clearly the elaborate test environment that the authors setup for this study. Being able to simulate the process of replication and natural selection helps the authors to get a complete trace of the sequence of generations produced. This is the key strength of this study as being able to look at the exact sequence of organisms produced helps the authors reason out the how and why the evolution process proceeded the way it did. In the real-world, one can only make guesses about how evolution might have proceeded due to the lack of complete information. The other main strength of this paper is the depth to which the authors have explored the reasons behind evolution of the "best" organism. The thorough study performed addresses almost all questions that might arise in a reader's mind and leaves hardly anything to speculation. The main bone of contention in this paper is its representativeness. Though the environment used for testing is pretty elaborate, the authors had to make various choices such as the choice of instructions, the possible ways in which mutations can occur and the means of evaluating the health of any organism. Whether these choices are in anyway representative of evolution as it occurs in the real-world is anybody's guess. Though the conclusions drawn in this paper appear possible explanations to evolution in the real-world, the question remains as to whether there could more intricacies involved which were lost in the test environment chosen. One question that I felt the authors could have addressed more is study the genealogical sequence of organisms with moderate health that lingered on for several generations. The authors restricted much of their study to the evolution of the best possible organisms and though they did a pretty thorough job of it, it would have been interesting to see whether there are some kinds of features which linger on for several generations even though they might not necessarily be the best and look at the reasons behind such evolution. The process of evolution can be modeled as a search problem but it would extremely complex. Each state would be representative of a generation, which is a set of organisms. However, unlike in a normal search, where one can define either probabilistically or deterministically the successor(s) to each state, in this case, the successor to any state also depends on the interactions between the organisms in any state. The new state produced is dependent on the competition between the organisms in a given state for the available resources. This also cannot be accounted for by factoring the state into a set of variables because the number of variables in each state is not fixed. On the whole, it does not look like evolution can be modeled as a search problem to which any normal search algorithm can be applied.