Evolution and Complex Structures:

1
Evolution and Complex Structures

• Simulated Evolution Hints at Features?
• Eric Duchon
• March 17, 2008

2
Complex Structures
Darwin
To suppose that the eye with all its inimitable
contrivances for adjusting the focus to different
distances, for admitting different amounts of
light, and for the correction of spherical and
chromatic aberration, could have been formed by
natural selection, seems, I freely confess,
absurd in the highest degree.
Even today, it is not clear how many of the
complex structures in Nature evolved.
3
The Eye
How do genetic mutations create more complex eyes
without intermediate steps destroying their
advantages?
4
Arguments For Simulation

• Fossil records not complete enough to track
  emergence of complexity
• Lab experiments limited by number of generations
  and by ability to track mutations through
  generations
• Computer simulations allows exact tracking of
  mutations
• Limited by computer resources and a simplified
  model

5
Computer Models

• Evolutionary simulations are usually modified
  cellular automata. Although not useful for
  directly modeling biological systems, they can
  offer support for suspicions and theories. In
  particular, work with Avida has elucidated how
  complexity can arise.

6
Digital Organisms

• The genome is a circular sequence of instructions
  (26 possible)
• Energy received single instruction processing
  units (SIPs) relative to the rest of the
  organisms
• Rate of errors when replicating the genome
• 0.175 an instruction to be copied is switched
  for another
• 0.05 single instruction is deleted or added
• Environment determined by what merited additional
  SIPs

7
Competition and Fitness

• Competition was introduced by assigning
  additional computational time to organisms which
  demonstrated logical functions
• The SIPs an organism received was proportional to
  the product of genome length and computational
  merit.

8
Reading a Digital Genome
9
Locating Complexity

• Computational merit was assigned on the basis of
  complexity of the genome required to produce the
  logic function.
• With the possible instructions, NOT and NAND were
  the easiest to create while EQU was the most
  difficult (it required at least 19 instructions).
  So to investigate complexity, the emergence of
  the EQU operation was tracked.

10
Case Study A genotype with all operations

• This genotype achieved all logical operations.
  Not all the mutations were advantageous, as seen
  on top right. However, even the deleterious
  mutation that knocked out the NAND function was
  essential for forming EQU in the next
  replication.

11
Conclusions

• Support for Darwins general idea that complex
  structures evolve from simpler ones.
• A reasonable demonstration of the usefulness of
  cellular automata?

12
More Generally,

• Out of 50 populations, 23 gained EQU.
• The final genomes ranged from 49 to 356
  instructions, so tendency to larger genomes.
• Median of seven of eight simpler functions
  already apparent before EQU.
• The mutation to EQU caused 20 of 23 genotypes to
  lose at least one simpler operation.
• But when only EQU was rewarded, no populations
  evolved that trait.