Environmental factors are an obvious push but does a less complex organism have a better chance of adapting than a more complex one?
The short answer is a qualified â€˜yesâ€™. The primary reason is that complex organisms tend to have longer generation times and generation time is one of the key factors limiting the rate of evolution. Shorter generation time means that faster evolution is possible (not inevitable). Another factor is that higher organismal complexity tend to be associated with more genetic constraints. The easiest anology here is the â€˜58 Chevy compared to the modern computerized version. You could tinker around with the old one a lot more easily and take liberties modifying it without risking fatal damage to some critical system. Complexity always comes with a cost, e.g. a longer, more vulnerable developmental period.
Is biological complexity itself an advantage or disadvantage?
This is entirely context-dependent. Obviously, complexity can confer many specific advantages in various contexts, but it carries with it specific constraints that tend to delimit or prevent alternative types of â€˜complexityâ€™. In other words, complexity can be a kind of an evolutionary rut. Especially when there is ecological co-evolution. Like the complexity of the orchid flower that perfectly mimics a female Euglossine bee in appearance, color and smell in order to get pollinated by the male bee. A highly efficient system to enlist specialized pollinators with a high degree of flower fidelity, but if that particular species of bee should go extinct, all the â€˜complexityâ€™ in the flower will be of no advantage whatsoever.
Sexual reproduction spreads genes around significantly, so sexual reproduction might be a characterstic for a fast evolving organism.
You are careful with your choice of words (as always), so I can't really nail you here :) 'Might be'...in certain cases.
Sexual reproduction creates a 'gene pool' as opposed to a population comprised of many clonal lineages that never converge. However, it is a common misconception that sexual populations evolve faster than asexual populations as a rule. This is not so. While sexuality creates opportunities for new gene combinations that may facilitate evolutionary processes in some situations, asexual populations can also evolve at alarmingly rapid rates through simple process of mutation and clonal replacement.
Yes, many possible advantages have been suggested. There are at least two whole books on the topic.
G.C. Williams "Sex and Evolution" and G. Bell, "The Masterpiece of Nature".
Old Graham Bell was once my undergrad thesis supervisor at McGill U.
There are at least four or five plausible arguments put forward in these books (advantages in heterogenous or unpredicatable environments, the Red Queen model, the lottery model, etc.) to which another, more recent one can be added.
It has now been suggested that the greater cross-generational genetic diversity created by sexual reproduction would have signficantly reduced parasite loads from parents to offspring, at least initially. This is because of the high degree of physiological orchestration of parasites with their hosts. A species evolving sexuality would immediately put a lot of their parasite populations into severe difficulties, giveing them a big survival advantage over their non-sexual counterparts.