Why evolution involves sex?

the author of <<chance and chaos>> David Ruelle says: "because adds is significant". interesting viewpoint! the difference between sexual reproduction and asexual reproduction is the process of DNA does crossover or not. DNA determines a life's adaption for a specified environment. if we view the process of DNA changing as a search in a space that relates environment, the explanation could be easy. for asexual reproduction: the chance of DNA changing only is mutation. and this effect is rather small because mutation probability is small, and it operates on rather smal section of DNA. so the range of search is narrow. unlike asexual reproduction, crossover is more improtant than mutation, because evolution can be viewed as a process of error trying,crossover has ability to making more different DNA to be tried. so the search range has been enlarged respectively. it has larger probability to discovery the peak in "environment map".

naturally, there are 2 questions:

1. crossover is a method to enlarging the range of search, why there is only 2-crossover(2 parents)? this anwer may be concerned with the process of evolution. all lifes has limited or correspond complexity which reflected in the length of DNA related to environment they living in. then given limited length and characteristic of schema(viewed as element of utility in DNA, represent a concrete function in this life), existing a optimal proportion between them, or else the probability to breaking up schema will result in random search or slow convergence in evolution

2. why complex creatrue has ability to selecting its partner to reproduction rather than no selection such as bacteria? Holland explains this in his book <<hidden order>> like this: the occasional combination of reproduction advantadge and tagging(tagging is a kind of identity), this advantadge will be consolidated by the recogintion of that tagging. and this selection is exclusive: rejecting others who have not tagging.so selcection has been formed. for human: the "first-sight" tagging may be good-looking, in average, good-looking people has higher treatment in our society, it is a culture outcome. the "hardware" of our boby became unimportant due to our medicine tech.