Axel Bender

Degeneracy: A design principle for achieving robustness and evolvability

Robustness, the insensitivity of some of a biological systems functionalities to a set of distinct conditions, is intimately linked to fitness. Recent studies suggest that it may also play a vital role in enabling the evolution of species. Increasing robustness, so is proposed, can lead to the emergence of evolvability if evolution proceeds over a neutral network that extends far throughout the fitness landscape. Here, we show that the design principles used to achieve robustness dramatically influence whether robustness leads to evolvability. In simulation experiments, we find that purely redundant systems have remarkably low evolvability while degenerate, i.e. partially redundant, systems tend to be orders of magnitude more evolvable. Surprisingly, the magnitude of observed variation in evolvability can neither be explained by differences in the size nor the topology of the neutral networks. This suggests that degeneracy, a ubiquitous characteristic in biological systems, may be an important enabler of natural evolution. More generally, our study provides valuable new clues about the origin of innovations in complex adaptive systems.

Networked buffering: a basic mechanism for distributed robustness in complex adaptive systems.

A generic mechanism - networked buffering - is proposed for the generation of robust traits in complex systems. It requires two basic conditions to be satisfied: 1) agents are versatile enough to perform more than one single functional role within a system and 2) agents are degenerate, i.e. there exists partial overlap in the functional capabilities of agents. Given these prerequisites, degenerate systems can readily produce a distributed systemic response to local perturbations. Reciprocally, excess resources related to a single function can indirectly support multiple unrelated functions within a degenerate system. In models of genome:proteome mappings for which localized decision-making and modularity of genetic functions are assumed, we verify that such distributed compensatory effects cause enhanced robustness of system traits. The conditions needed for networked buffering to occur are neither demanding nor rare, supporting the conjecture that degeneracy may fundamentally underpin distributed robustness within several biotic and abiotic systems. For instance, networked buffering offers new insights into systems engineering and planning activities that occur under high uncertainty. It may also help explain recent developments in understanding the origins of resilience within complex ecosystems.

Robustness Against the Decision-Maker's Attitude to Risk in Problems With Conflicting Objectives

In multiobjective optimization problems (MOPs), the Pareto set consists of efficient solutions that represent the best trade-offs between the conflicting objectives. Many forms of uncertainty affect the MOP, including uncertainty in the decision variables, parameters or objectives. A source of uncertainty that is not studied in the evolutionary multiobjective optimization (EMO) literature is the decision-makers attitude to risk (DMAR) even though it has great significance in real-world applications. Often the decision-makers change over the course of the decision-making process and thus, some relevant information about preferences of future decision-makers is unknown at the time a decision is made. This poses a major risk to organizations because a new decision-maker may simply reject a decision that has been made previously. When an EMO technique attempts to generate the set of nondominated solutions for a problem, then DMAR-related uncertainty needs to be reduced. Solutions generated by an EMO technique should be robust against perturbations caused by the DMAR. In this paper, we focus on the DMAR as a source of uncertainty and present two new types of robustness in MOP. In the first type, dominance robustness (DR), the robust Pareto solutions are those which, if perturbed, would have a high chance to move to another Pareto solution. In the second type, preference robustness (PR), the robust Pareto solutions are those that are close to each other in configuration space. Dominance robustness captures the ability of a solution to move along the Pareto optimal front under some perturbative variation in the decision space, while PR captures the ability of a solution to produce a smooth transition (in the decision variable space) to its neighbors (defined in the objective space). We propose methods to quantify these robustness concepts, modify existing EMO techniques to capture robustness against the DMAR, and present test problems to examine both DR and PR.

The role of degenerate robustness in the evolvability of multi-agent systems in dynamic environments

It has been proposed that degeneracy plays a fundamental role in biological evolution by facilitating robustness and adaptation within heterogeneous and time-variant environments. Degeneracy occurs whenever structurally distinct agents display similar functions within some contexts but unique functions in others. In order to test the broader applicability of this hypothesis, especially to the field of evolutionary dynamic optimisation, we evolve multi-agent systems (MAS) in time-variant environments and investigate how degeneracy amongst agents influences the systems robustness and evolvability. We find that degeneracy freely emerges within our framework, leading to MAS architectures that are robust towards a set of similar environments and quickly adaptable to large environmental changes. Detailed supplementary experiments, aimed particularly at the scaling behaviour of these results, demonstrate a broad range of validity for our findings and suggest that important general distinctions may exist between evolution in degenerate and non-degenerate agent-based systems.

Computational scenario-based capability planning

Scenarios are pen-pictures of plausible futures, used for strategic planning. The aim of this investigation is to expand the horizon of scenario-based planning through computational models that are able to aid the analyst in the planning process. The investigation builds upon the advances of Information and Communication Technology (ICT) to create a novel, flexible and customizable computational capability-based planning methodology that is practical and theoretically sound. We will show how evolutionary computation, in particular evolutionary multi-objective optimization, can play a central role - both as an optimizer and as a source for innovation.

DMEA: a direction-based multiobjective evolutionary algorithm

A novel direction-based multi-objective evolutionary algorithm (DMEA) is proposed, in which a population evolves over time along some directions of improvement. We distinguish two types of direction: (1) the convergence direction between a non-dominated solution (stored in an archive) and a dominated solution from the current population; and, (2) the spread direction between two non-dominated solutions in the archive. At each generation, these directions are used to perturb the current parental population from which offspring are produced. The combined population of offspring and archived solutions forms the basis for the creation of both the next-generation archive and parental pools. The rule governing the formation of the next-generation parental pool is as follows: the first half is populated by non-dominated solutions whose spread is aided by a niching criterion applied in the decision space. The second half is filled with both non-dominated and dominated solutions from the sorted remainder of the combined population. The selection of non-dominated solutions for the next-generation archive is also assisted by a mechanism, in which neighborhoods of rays in objective space serve as niches. These rays originate from the current estimate of the Pareto optimal front’s (POF’s) ideal point and emit randomly into the hyperquadrant that contains the current POF estimate. Experiments on two well-known benchmark sets, namely ZDT and DTLZ have been carried out to investigate the performance and the behavior of the DMEA. We validated its performance by comparing it with four well-known existing algorithms. With respect to convergence and spread performance, DMEA turns out to be very competitive.

Evolving stories: Grammar evolution for automatic plot generation

In this paper, we propose a computational framework for automated story-based scenario generation. Under this framework, a regular grammar is developed to model various causal relationships inside a given story world. The grammar is then evolved using evolutionary computation techniques to generate novel story plots, i.e. story-based scenarios. To evaluate these newly generated scenarios, a human-in-the-loop model is used. An experimental study was carried out, in which the proposed framework was used to create novel plots based on the famous Little Red Riding Hood fairy tale. The experimental study demonstrated that evolutionary computation can potentially contribute significantly to story generations. Some challenges were identified including the difficulty to quantify such subjective measures as plot interestingness and creativity.

A dominance-based stability measure for multi-objective evolutionary algorithms

Over the years, we have been applying multi-objective evolutionary algorithms (MOEAs) to a number of real-world problems. solving multi-objective optimization problems (MOPs) in the real world faces a number of challenges including when to terminate the algorithm. This paper addresses this challenge by introducing what we call a “stability measure”. We use this measure to estimate when to stop the multi-objective evolutionary search.

Evolutionary mechanics: new engineering principles for the emergence of flexibility in a dynamic and uncertain world

Engineered systems are designed to deftly operate under predetermined conditions yet are notoriously fragile when unexpected perturbations arise. In contrast, biological systems operate in a highly flexible manner; learn quickly adequate responses to novel conditions, and evolve new routines and traits to remain competitive under persistent environmental change. A recent theory on the origins of biological flexibility has proposed that degeneracy—the existence of multi-functional components with partially overlapping functions—is a primary determinant of the robustness and adaptability found in evolved systems. While degeneracy’s contribution to biological flexibility is well documented, there has been little investigation of degeneracy design principles for achieving flexibility in systems engineering. Actually, the conditions that can lead to degeneracy are routinely eliminated in engineering design. With the planning of transportation vehicle fleets taken as a case study, this article reports evidence that degeneracy improves the robustness and adaptability of a simulated fleet towards unpredicted changes in task requirements without incurring costs to fleet efficiency. We find that degeneracy supports faster rates of design adaptation and ultimately leads to better fleet designs. In investigating the limitations of degeneracy as a design principle, we consider decision-making difficulties that arise from degeneracy’s influence on fleet complexity. While global decision-making becomes more challenging, we also find degeneracy accommodates rapid distributed decision-making leading to (near-optimal) robust system performance. Given the range of conditions where favorable short-term and long-term performance outcomes are observed, we propose that degeneracy may fundamentally alter the propensity for adaptation and is useful within different engineering and planning contexts.

Modelling defence logistics networks

Military logistics concerns the activities required to support operational forces. It encompasses the storage and distribution of materiel, management of personnel and the provision of facilities and services. A desire to improve the efficiency and effectiveness of the Australian Defence Force logistics process has led to the investigation of rigorous military logistics models suitable for analysis and experimentation. Logistics networks can be viewed as distributed discrete event systems, and hence can be formalised with discrete event techniques which support concurrency. This paper presents a Coloured Petri Net (CPN) model of a military logistics system and discusses some of our experience in developing an initial model. Interesting modelling problems encountered, and their solutions and impact on CPN support tools, are discussed.