Spring School in Complexity Science

Introduction toComplexity Science

Adaptation

Seth Bullock, 2006

Adaptive Complexity

Seth Bullock, 2006

Terms

Adaptive1. Flexible, plastic: capable of

changing2. Fit for purpose: evolutionarily

adaptiveAdaptation

1. An evolved trait:2. The process that generates

adaptations3. Also process of change in, e.g., the

eye Adapt, Adaption

To change purposefully: cf chameleonAdapted

Altered in order to response

Seth Bullock, 2006

Complex Adaptive Systems

Complex adaptive systems come in many forms:

Evolutionary, Neural, Cultural, Linguistic, etc.Markets, Languages, Brains, etc.

Their hall-mark:without global co-ordinationorganisation comes to reflect environment

Seth Bullock, 2006

Gaia

Gaia is not an organism, but “an emergent property of interaction among organisms”.

What is the claim here? That genes bring about and maintain global homeostasis?

H: “our planet functions as a single organism that maintains conditions necessary for its survival” “Life, or the biosphere,

regulates or maintains the climate and the atmospheric composition at an optimum

for itself”.

Seth Bullock, 2006

Fallacies, Misconceptions, etc.

Consequently, adaptive systems are extremely subtle, and at times, counter-intuitive.But they are:

the most potent source of biological inspirationextremely important in their own right

Since adaptive systems almost always combine…scaleconnectivitynon-linear interactions

…they are often complex adaptive systems.

Seth Bullock, 2006

Units of Selection

Why do we serve genes and not vice versa?

Because genes are replicators “longevity, fecundity and copying-fidelity” they, not us, persist across generations changes to genes, not bodies, are copied

e.g., amputees do not give birth to amputees

“We are survival machines—robot vehicles blindly programmed to preserve the selfish molecules known as genes. This is a truth which still fills me with astonishment. Though I have known it for years, I never seem to get fully used to it” – Richard Dawkins.

Seth Bullock, 2006

Group Selectionist Thought

Ageing: good for the herd, weeds out stragglersSex ratio: efficient for pairing males & femalesCommunication: the exchange of information

“Groups”:speciesecosystemsbiosphereorganism!

Dawkins’ work contributed to the debunking of evolutionary explanations cast at the group level.

It can be seductive to imagine a kind of “selective hierarchy”.

But “higher” selective pressures are undercut by those below…

…so genes hold the trump card.

Seth Bullock, 2006

Game Theory

Tragedy of the Commonsa little restraint ensures a common resource for all

Prisoners Dilemmaif both prisoners resist temptation, both go free

Hawk-Dove gameif all play Dove, resources are shared non-violently

In each game, short-termist, individualist, gene-eyed behaviour destroys fragile mutualism.

Unfortunately such games are everywhere.

Game theory suggests that global altruism is unlikely to survive individual selfish interests:

Seth Bullock, 2006

Progress

Short-termist, individualistic, gene-eyed evolution seems progressive: a march from monad to man.

Seth Bullock, 2006

Evolution ≠ Optimization

But, why would a blind process of random change + selection inexorably drive towards… anything? “our theory of evolution does

not predict an increase in anything”

Progress: change that is directed and in some sense positive.

Could any apparent direction be an artefact?

Seth Bullock, 2006

Arms Races

Dawkins & Krebs suggested that “arms races” between coevolving species might drive progress.

More significant, are major innovations: cells, sex, multi-cellularity, sociality, even language…

Might evolution “never be the same again” after each “major transition”.

During an arms race, “the equipment for survival, on both sides, is improving”. But arms races do not spiral upwards. And surely this is short-term

stuff? Is it, in fact, just adaptation?

Seth Bullock, 2006

Major Transitions

For Dawkins, major transitions are “watershed events” in the history of life. They bring about new ways of being adaptive, new opportunities...“boosting evolution itself in ways that seem entitled to the label progressive”

Once brought about, such transitions may be difficult (though not impossible) to reverse, conferring a “one-way ratchet of of progressive innovation”.

Seth Bullock, 2006

Fitness Landscapes

Central to much reasoning about adaptation is an iconic visual metaphor: the fitness landscape.

dimensionalityneutralitylocalitypassivityobjectivity

Seth Bullock, 2006

“Open-Ended Evolution”

Since adaptation is a response to environmental pressures, some authors suggest that an ever-changing environment may be such a feature.

E.g., Co-evolution amongst species…Additionally, the replicators involved will necessarily need to be able to build more complicated (complex?) replicators.

However, still an open(-ended?) question…

“What features must be present in a system if it is to lead to indefinitely continuing evolutionary change?” – Maynard Smith.

Seth Bullock, 2006

Apparent Design ≠ Evolution

In On Growth and Form D’Arcy Thompson argues that honeycomb structure arises for the same reasons as spherical bubbles.

“Natural selection is the only known mechanism capable of effecting the appearance of design.”

What other structures, behaviours, organisations can be explained in this way?

Seth Bullock, 2006

Constraints vs. Opportunities

Often thought of as “constraints” on evolution.But it is as accurate to use “enabling” language.

Physics and chemistry, then, define, populate and structure the space of possible adaptations.

The “morphospace” of available forms seeds natural selection with self-organising structures.

Is morphospace sparse or densely populated?

Your answer to this question fixes where you sit on the “self-organisation vs. selection” debate.

Seth Bullock, 2006

Self-Organization vs. Selection

“Ultra-Darwinists” believe that it is practical to neglect morphospace considerations.

An opposite position holds that understanding natural complexity is largely a matter of grasping the nature of self-organisation.

“We stand in the need of a new conceptual framework that allow us

to understand an evolutionary process in which self-organization, selection and historical accident find their natural places with one

another.” – Kauffman.

Seth Bullock, 2006

Evolution To The “Edge of Chaos”

A system cannot exhibit adaptation (via natural selection) if it tends to either fixity or disorder.Stasis/Fixity Disorder/FluxComplexity

If possible, systems will evolve such that they sit between fixity and flux: at the “edge of chaos”.This regime has some interesting

properties:complex patterns, robustness, efficiencyscale-invariant behaviourbut not just adaptive systems

Seth Bullock, 2006

Power Laws

Is this due to self-organised criticality?Observed in distributions of earthquakes, firm & GDP growth, evolutionary extinction, internet traffic & structure, epidemics, heart arrhythmias…How long will it be before the power law has as central a role in science as the normal curve?

solar flare size is not

oak leaf size is normal

What unites those systems that exhibit “power laws”?