pleiotropy

Orr HA 2000 Adaptation and the cost of complexity. Evolution 54:13-20.

  • fitness is determined by n independent (orthogonal) characters
  • at each of our n characters, fitness falls off as a Gaussian function of distance, zi, from the optimum
  • Fisher's model thus allows for a kind of universal pleiotropy
  • we consider a sexual haploid in which evolution is due to new unique mutations
  • the expectation of the product Pa Π Δzfix in eq. (1) equals the product of the individual expectations
  • when using mutations of fixed size r, the speed of adaptation slows with greater organismal complexity, n
  • more complex organisms pay a cost of about n−1/2 in terms of probability of fixation
  • more complex organisms pay a cost of about n−1/2 in terms of the gain in fitness that results when a substitution does occur
  • more complex organisms pay a third cost
  • the probability that a random mutation will be favorable decreases with n
  • random mutations of a given size have a smaller chance of being favorable in complex organisms
  • the probability of fixation for mutations of a given size declines with organismal complexity
  • when favorable mutations do get substituted, a smaller gain in fitness accrues to complex organisms
  • there may well be other, and perhaps more than compensatory, ecological advantages to complexity
  • when using mutations of the same size, complex organisms adapt more slowly than simple ones
  • we are not concerned with the cost of producing a more complex organism
  • in Rechenberg (1984), mutations are built from the "character up"
  • mutations change the value of each character by a random amount
  • these changes are independent and identically distributed across characters
  • the square of the magnitude of mutational effects is chi-square distributed
  • such a distribution appears unbiological as mutational effects are maximized away from zero
  • small mutations are rare or nonexistent, intermediate-sized ones common, and large ones rare or nonexistent
  • this nonmonotonic distribution qualitatively differs from the leptokurtic ones usually assumed in evolutionary biology
  • equation (A6) seem inconsistent with the nearly neutral theory
  • because fitness effects scale with mutation size, exponential or gamma distributions (with small values of the shape parameter) of fitness effects cannot be naturally obtained from equation (A6)
  • in Kimura (1983), Orr and Coyne (1992), and Orr (1998), mutations are made from the "top down"
  • we begin with some biologically plausible distribution of mutational magnitudes (e.g., exponential)
  • any particular mutation is forced to have a random direction
  • each mutation represents a random displacement in n-dimensional space