Robustness (evolution)

[17][18] The cell population level log-normal distribution of mRNA content[19] follows directly from the application of the Central Limit Theorem to the multi-step nature of gene expression regulation.

Variable environment can therefore select for environmental robustness where organisms can function across a wide range of conditions with little change in phenotype or fitness (biology).

Plants, in particular, are unable to move when the environment changes and so show a range of mechanisms for achieving environmental robustness.

[21] Additionally the flux through a metabolic pathway is typically limited by only a few of the steps, meaning that changes in function of many of the enzymes have little effect on fitness.

[32] There is evidence that proteins show negative design features to reduce the exposure of aggregation-prone beta-sheet motifs in their structures.

[33] Additionally, there is some evidence that the genetic code itself may be optimised such that most point mutations lead to similar amino acids (conservative).

[12] During embryonic development, gene expression must be tightly controlled in time and space in order to give rise to fully functional organs.

Robustness against this noise and genetic perturbation is therefore necessary to ensure proper that cells measure accurately positional information.

[43][44][45] Additionally, the structure (or topology) of signaling pathways has been demonstrated to play an important role in robustness to genetic perturbations.

[47] Similarly, robustness of dorsoventral patterning in many species emerges from the balanced shuttling-degradation mechanisms involved in BMP signaling.

[48][49][50] Since organisms are constantly exposed to genetic and non-genetic perturbations, robustness is important to ensure the stability of phenotypes.

Being robust may even be a favoured at the expense of total fitness as an evolutionarily stable strategy (also called survival of the flattest).

[64][65][66][67] One hypothesis for how robustness promotes evolvability in asexual populations is that connected networks of fitness-neutral genotypes result in mutational robustness which, while reducing accessibility of new heritable phenotypes over short timescales, over longer time periods, neutral mutation and genetic drift cause the population to spread out over a larger neutral network in genotype space.

[68] This genetic diversity gives the population mutational access to a greater number of distinct heritable phenotypes that can be reached from different points of the neutral network.

In sexual populations, robustness leads to the accumulation of cryptic genetic variation with high evolutionary potential.

Experimental systems for individual genes include enzyme activity of cytochrome P450,[57] B-lactamase,[58] RNA polymerase,[13] and LacI[13] have all been used.

A network of genotypes linked by mutations. Each genotype is made up of 3 genes : a, b & c. Each gene can be one of two alleles . Lines link different phenotypes by mutation . The phenotype is indicated by colour. Genotypes abc, Abc, aBc and abC lie on a neutral network since all have the same, dark phenotype. Genotype abc is robust since any single mutation retains the same phenotype. Other genotypes are less robust as mutations change the phenotype (e.g. ABc).
Core eukaryotic metabolic network . Circles indicate metabolites and lines indicate conversions by enzymes . Many metabolites can be produced via more than one route, therefore the organism is robust to the loss of some metabolic enzymes
Each circle represents a functional gene variant and lines represent point mutations between them. Light grid-regions have low fitness , dark regions have high fitness. ( a ) White circles have few neutral neighbours, black circles have many. Light grid-regions contain no circles because those sequences have low fitness. ( b ) Within a neutral network, the population is predicted to evolve towards the centre and away from 'fitness cliffs' (dark arrows).