1. Deleterious Mutations in Novel Environments
Jesse Thaden, Art Covert
Sign epistasis causes deleterious mutations to become
beneficial, changing their fitness impact from negative to
positive.
Genotype Space
Fitness
ab
Ab
aB
AB
Avida is a program that simulates ordinary evolutionary
interactions by modeling self-replicating organisms. We use this
program to test the importance of sign-epistasis in a novel
environment, defined as an environment that has different
evolutionary peaks than the previous environment.
Change in
environment
Arthur Covert has demonstrated in multiple papers that deleterious
mutations have the potential to become beneficial through sign epistasis. But
how does the positive effect of deleterious mutations change with the
evolutionary opportunities provided by the environment?
Reverse Deleterious (A), Replace Deleterious (B), Replace Deleterious and Lethal (C), and Control (D) environments
respectively; the control environment, which allows for deleterious mutations, ultimately yields the highest fitness.
I sought out to replicate these results by subjecting XOR-
performing organisms to a XOR/EQU-rewarding environment, a
more complex environment. First, organisms were primed to
perform only the XOR function, which is of above-average
complexity. However, it is not the most complex function.
Organismsperformingfunction
NOT NAND AND ORN OR ANDN NOR XOR EQU
Functions (traits) evolved
Art initially found that deleterious mutations were important for
evolution primarily in environments that were more complex than
the previous environment. This is approximately demonstrated
above, where the environment which rewards the more complex
function (EQU) yields a higher fitness in the treatment which
allows deleterious mutations than the treatment that doesn’t. The
other environments which are not more complicated than the
original environment do not benefit from deleterious mutations.
Control RpD Control RpD
Fitness
Control RpD
NOR NOR/XOR XOR/EQU
Control treatments allow for deleterious mutations; replace deleterious (RpD) treatments do
not.
25 genomes that only performed XOR were pulled from these priming runs.
20 replicates of each genome then filled a more complex environment that
rewarded XOR and EQU and were allowed to evolve for 100,000 updates.
25 unique genomes performing only XOR
Tasks
rewarded:
XOR/EQU
Tasks
rewarded:
XOR/EQU
20
replicates
each
Control RpD
In my results, the x axis represents the average final dominant fitness
per genome (out of 20 replicates) of the replace deleterious treatment
(which disallowed deleterious mutations), and the y axis represents
the average final dominant fitness of the control treatment (which
allowed deleterious mutations) per genome.
20
replicates
each
My results conclude the expected: the control treatment results in
a higher average final dominant genome fitness than the replace
deleterious treatment per genome, though more slightly than
expected; 13 out of the 25 genomes have a higher fitness in the
control treatment. This is to be expected, as deleterious
mutations allow for the traversal of fitness valleys, or areas of
lower relative fitness, which are often required for evolution to a
more complex fitness peak.
I’d like to develop more significant results for this conclusion, but
my results support the theory that evolution in more complex
novel environments is supported by deleterious mutations.
Preventing deleterious mutations from occurring ultimately
lowered final dominant genotype fitness. I plan to continue to
improve this conclusion by eliminating errors in my experimental
process, and would also like to test this theory in an environment
that alternates between rewarding XOR and EQU.
Citations:
• Covert, A. W., R. E. Lenski, C. O. Wilke, and C. Ofria. "Experiments on the
Role of Deleterious Mutations as Stepping Stones in Adaptive
Evolution."Proceedings of the National Academy of Sciences 110.34
(2013): E3171-3178. Web.
• Whitlock, Michael C. "Founder Effects and Peak Shifts Without Genetic
Drift: Adaptive Peak Shifts Occur Easily When Environments Fluctuate
Slightly."Evolution 51.4 (1997): 1044-048. JSTOR. Web.
<http://www.jstor.org.ezproxy.lib.utexas.edu/stable/2411033?seq=1&>.
• Covert, Arthur W., III. "The Role of Deleterious Mutations in Adaptation
to a Novel Environment." (n.d.): n. pag. Web.