Table 1 Glossary of terms

Evolutionary constraint

[1]: the phenomenon of evolution producing a finite number of genomic and associated phenotypic outcomes from a near infinite number of possible genetic variants.

Genetic constraint

The portion of evolutionary constraint, which is determined at the level of genes or their gene products, for example codon constraint or developmental genetic pathways.

Functional network constraint

The portion of network constraint attributed to the structure or architecture of gene interactions that can be expressed in the form of a network. Networks consist of nodes (genes) and edges (functional interactions between these genes).

Genic evolution

The phenomenon of different evolutionary outcomes being the outcome of independent mutation and selection events in different genes. For example, the occurrence of convergent evolution in diverging populations, both of which are caused by evolution in distinct genes.

Rapid adaptation

The phenomenon of adaptive change in allele frequencies of a population to natural selection, taking place within just a few generations.

Convergent evolution / convergence

Traditionally defined as similar phenotypes evolving from similar selective pressure in response to similar environments [2]. May be caused at the genomic level through genomic re-use of the same genes or alleles, which is also called parallel genetic evolution or genomic re-use.

Gene dispensability

A variable to estimate gene essentiality. The less dispensable a gene is for organismal growth and function, the more essential it is. An estimator for the mean fitness effect of all possible mutations of a gene across environments the cell is likely to encounter. In yeast, this is experimentally determined through knockouts.

Pleiotropy and cost of complexity

Traditionally defined as one gene influencing more than one trait. In the papers cited in this study, has been defined as gene products with more than one functional interactions with other gene products, with the link to pleiotropy of phenotypic traits being implied. It is therefore called “gene pleiotropy”.

Gene expression level CAI

The amount of mRNA produced by each gene in regular somatic cells. CAI (Codon Adaptation Index) is used as a substitute variable in this paper, and is derived from codon use bias in yeast that correlates with mRNA levels.

Omega ω

The ratio of nonsynonymous to synonymous substitutions dN/dS. It is assumed that dS remains constant, and dN is used here as an estimator for directional evolution.

Gamma γ

A score developed for estimating events of rewiring functional connections between network nodes over the course of evolution. Developed on the example of five species of yeasts.

Neighborhood connectivity

A network statistic used to describe the structure of a functional genetic network. Describes the number of connections of all neighbors of each node. Highest values are expected in intermediately located nodes within a network.

Betweenness centrality

A network statistic used to describe the structure of a functional genetic network, describing where a node lies within paths between other nodes. Nodes with many paths progressing through them may be important in transmitting information. Highest values are expected in nodes central to a network.

Average shortest path length

A network statistic used to describe the structure of a functional genetic network. Shortest distance between a node and other nodes. Highest values are expected in peripheral nodes of a network.