# Theodore Jessop MPhys project report January 2017 .pdf

### File information

Original filename:

**Theodore Jessop MPhys project report January 2017.pdf**

This PDF 1.5 document has been generated by TeX / pdfTeX-1.40.16, and has been sent on pdf-archive.com on 08/02/2017 at 13:45, from IP address 130.88.x.x.
The current document download page has been viewed 466 times.

File size: 895 KB (12 pages).

Privacy: public file

## Download original PDF file

Theodore Jessop MPhys project report January 2017.pdf (PDF, 895 KB)

### Share on social networks

### Link to this file download page

### Document preview

UNIVERSITY OF MANCHESTER

1

How long can species co-exist in chaotic flows?

Theodore Jessop

Abstract—We have studied a modified version of the discrete

time voter model with neutral selection wherein the interacting

constituents are moved according to a chaotic flow, corresponding

to a dynamic network. We have developed computer simulations

of these systems in one and two dimensions, demonstrating

that the limits of maximal and minimal flow correspond to the

analytically soluble Moran process and voter process on a square

lattice. We demonstrate that the consensus time of the system

is maximised for intermediate rates of flow. We have studied

evolutionary game theory with a view to extending this model to

more complex games in the second semester.

neither the static graph voter model nor Moran process are

sufficient to capture the relevant dynamics of such systems,

and what is needed is a model which interpolates between

the two such that the flow of the network and evolution of

the voters may be of a similar timescale. Here we develop

such a model, in which the connections of the voter model

are defined by Euclidean distance between voters distributed

spatially and in which the positions of the voters move over

time according to a flow rule. Chaotic flow is studied as it will

have the property that at the limit of large flow, the position of

a voter after an evolutionary time step will be unpredictable.

I. I NTRODUCTION

A. Motivation

B. Structure of this report

Evolutionary dynamics is applied in too many fields to

enumerate. The voter model, explained in detail below in

section II.A., is used to study systems of competition where

connections between the competing actors is of paramount

importance. Examples include ecological competition where

species compete for resources, but only do so directly with

their nearest neighbours. An example from the social sciences

is the study of the diffusion of an idea or opinion through

the population where individuals only communicate with their

friends, family and colleagues.

Both these systems might be well represented by the classic

voter model. However a clear limitation of this model is

that it does not take into account the fact that in practice

these networks are not likely to be static on all timescales

relevant to evolution. Animals will move around to pursue

resources and develop new neighbours as a result. People’s

social connections will change over time as they acquire new

friends and colleagues and lose contact with old ones. If this

changing of the network happens on a timescale much larger

than that over which relevant dynamics in the evolution of

the voters takes place then a static graph may be appropriate.

For example, if one species of animal eliminates all of its

competitors long before the individuals have had time to move

around enough to change their mutual graph then a static voter

model would capture all the needed detail. On the other hand

this movement of individuals may be much faster than the

evolutionary competition such that each individual is exposed

to and competes with every other individual. In this case the

geometry of the graph is irrelevant and the dynamics reduces

to the Moran process wherein every node on the graph is

connected to every other node (detailed below in section II.).

These are what we refer to as the ’static’ and ’well mixed’

limits.

However there is no reason to suppose that most systems

behave in this way where competitive evolution and flow of

the network happen on drastically different timescales. Thus

This report will be presented in three main sections: firstly

an overview of the theory behind the system we have studied,

secondly details of the simulations we have produced this

semester along with their results, and finally a discussion of

how we propose to extend this work into the latter semester.

The first section will explain the principles behind our

model, including the analytically soluble limits of the Moran

process and fixed voter model. The second section details

our simulations and their results, which include numerical

verification of these analytic limits and the principal result

that the consensus time of the system is maximised when

the flow and evolution timescales balance. The final section

will be a brief overview of the elements of evolutionary game

theory that we propose to incorporate into the model to provide

asymmetric selection dynamics.

Supervisor: Dr. Tobias Galla, Project partner: Cory Emmerson

II. T HEORY

A. Discrete time voter model

The voter model is a stochastic system which consists of

a population of constituents, variously called voters, particles,

species, etc, which exist on a the vertices of a graph defines

whether each voter is connected. These voters can be of one of

two types, representing some differing factor such as political

opinion, spin, or biological species. They can be generically

labelled 0 and 1. At each step of time, a single voter is selected

at random from among all voters. Then a voter connected

to the initially chosen voter is also selected. The two then

compete according to some predetermined rule, and the loser

adopts the type of the winner. Thus the two objects which

determine the dynamics of this system are the graph which

determines which voters are connected and the selection rule

that determines how they compete.

In the most simple dynamics, this selection rule is simply

that the second chosen voter always loses and adopts the

types of the first. This constitutes neutral selection where the

probability of a voter winning an exchange is constant and

does not depend on its type or situation. More complicated

UNIVERSITY OF MANCHESTER

rules can constitute asymmetric selection and will be discussed

in section III.

2

graph on which the voter model is defined and on the type of

the voter at each vertex of that graph.

The graph defining the network of voters may be determined

in a number of ways. For the case of a static graph we

use a square lattice, where voters are equally spaced on the

interval [0, 1] in one or two dimensions with connections

drawn between nearest neighbours. An illustrative example of

such a lattice network in 2D is shown in figure 2.

Fig. 1. Illustration of a single evolution time step in the voter model. Red

circles are type 1 voters, blue are type 0 and the lines between voters represent

connections, defining the ’network’ or ’graph’. Light grey lines are ’passive’

links between two voters of the same type, heavy black lines are ’active’

links between voters of different types. If the voters across an active link are

chosen for competition then a reaction can occur where one changes type,

whereas should a passive be chosen nothing happens. Here two voters of

different types are selected for competition. The type 0 wins the exchange

and so the losing type 1 becomes a type 0. Note how the reaction reduces the

total number of active links. These reactions continue until every voter is of

one type or the other. This may be thought of as the reactions continuing to

decrease the proportion of active links until it reaches zero in the consensus

state.

Suppose there are N voters, i of which are of type 1

and (N − i) of which are of type 0. During a time step

a reaction between two voters takes place. There are three

possible outcomes:

1) Two voters of the same type compete, in which case

nothing happens as they both remain of the same type.

2) A type 1 meets a type 0 and the type 1 wins the

exchange, replacing the type 0 with a 1, increasing i

by 1.

3) A type 1 meets a type 0 and the type 0 wins the

exchange, replacing the type 1 with a 0, decreasing i

by 1.

An illustration of such an exchange is given in figure 1

Each time step will either change i by 1 in either direction

or do nothing. Of paticular importance are the states with i = 0

and i = N , which correspond to every voter being of type 0

and 1 respectively. If the system is in one of these states then

nothing more can happen as only the null reaction between two

voters of the same type can occur. In this case one species has

gone extinct and the other has reached what we call ’fixation’

or ’consensus’.

This system is stochastic as which pair of voters compete at

each time step is determined at random. Thus each realisation

of the system will take a different number of time steps to

reach consensus. Of interest is the average number of time

steps needed to reach consensus, called the ’mean consensus

time’ and denoted by tc . It is useful to define an alternate

unit of time called ’sweeps’ wherein one sweep is the average

number of time steps taken for a single voter to be chosen

to compete once, given by τ = N t. Accordingly we define

the mean consensus time in sweeps as τc = N tc . The mean

consensus time will obviously depend on the initial state of

the system; i.e. it will vary depending on the nature of the

Fig. 2. Example of voters distributed on a square lattice on [0, 1] in two

dimensions, N = 225, with each voter adopting a random type. Type 0

voters are red, type 1 are blue. The lines represent links in the graph and

are here present between ’nearest neighbours’. Grey lines represent ’passive

links’ where the pair of voters are of the same type and thus only the null

reaction can occur when this pair is selected for competition. Heavy black

lines represent the active links where, should that pair of voters be chosen to

compete, a reaction will take place and one voter will adopt the type of the

other.

We also consider flowing graphs where the positions of the

voters change. For this we define an interaction distance R

whereby voters whose mutual Euclidean is less than R may

compete with one another. I.e., a connection on the graph

exists if and only if the Euclidean distance between two

vertices is less than the interaction distance. An example of this

for randomly distributed voters on [0, 1] in two dimensions is

shown in figure 3. It is useful to define this interaction distance

in units of the average spacing between voters. I.e., if there are

N = nd voters distributed on [0, 1] in d dimensions then we

set R = nδ and use δ as the variable to control the interaction

radius. We do not consider d > 2 here.

For voters distributed on a square lattice on [0, 1], setting

δ ≈ 1 produces the same graph as in figure 2. This is important

as it represents the static or no-flow limit of the flowing

network model expanded upon below. Crucially this 2D fixed

lattice has the property that the consensus time in sweeps

scales as:

UNIVERSITY OF MANCHESTER

3

Setting δ n means that every voter is connected to every

other voter, a ’complete graph’, an example of which is shown

in figure 4. This means that the geometry of how the voters

are arranged does not affect the dynamics of evolution and is

equivalent to the Moran process, detailed in section II-B.

The voter model may be summarised by the following steps:

1) Select a voter at random from the population.

2) Select a second voter from those that are connected to

the first.

3) Have the two voters compete and determine which one

is victorious.

4) Have the loser adopt the type of the winner.

5) Repeat ad infinitum, unless one type has reached consensus, at which point stop.

The principal addition to this that we make is that the

network changes after a certain number of steps. The details

of this are given below in section II-C.

B. The Moran process

Fig. 3. Example of N = 100 voters arrayed randomly by a uniform

distribution on [0, 1], with δ = 1.8. The point and line colours have the

same meaning as in figure 2

τc ∼ N log N,

d = 2,

(1)

demonstrated in reference [2]. The analogue in 1D is that

of N voters evenly distributed on a line on [0, 1] where each

interacts only with its two neighbours. This has the property

that the consensus time in sweeps scales as:

τc ∼ N 2 ,

d = 1.

(2)

Finally the number or density of active links as a function

of the number of time steps in one dimension is given by:

1

ρ(t) ∼ t− 2 ,

(3)

where ρ is the proportion of links which are active, connecting voters of differing types, and t is the number of time

steps that have elapsed since the system was initialised. This

is demonstrated in [4].

These analytic predictions are strictly only valid in the limit

of large N , and are demonstrated by direct simulation in our

results section III.

Fig. 4. This is a ’complete graph’, where every voter is connected to every

other voter, no matter how the voters are arranged spatially. This produces

the same dynamics as the Moran process detailed in section II-B

The Moran process is equivalent to the voter model on a

complete graph where each voter is connected to every other

voter. Thus the spatial arrangement of the voters is not relevant

to their evolution and we can describe the state of the system

entirely by the number of type 1’s, which we call i. This

process is important because it corresponds to the limit of

maximal flow of voters and is analytically soluble for both the

probability that a given type reaches consensus and the mean

consensus time for given initial conditions. The main quantity

that we study in our simulations is the mean consensus time,

so we present the analytic solution for this, following [1]:

UNIVERSITY OF MANCHESTER

4

Suppose the current state of a system of population size N

is i, i.e. there are i type 1’s and N − i type 0’s. For the system

to move to state i + 1 then a type 1 must be chosen first, a

type 0 second, and the type 1 must win the exchange. These

are independent events so the probability of all happening is

just their product:

Defining γi =

Ti−

Ti+

and yi = ti − ti−1 :

yi+1 = γi yi −

(9)

1

,

Ti+

where y1 = t1 − t0 = t1 . This may be solved to give:

i N − i f1

N N −1 f

Ti+ =

yi = t 1

Qi−1

k=1

γk −

Pi−1

1

j=1 T +

j

Qi−1

k=j+1

γk .

(10)

(4)

To determine t1 we find:

where f1 and f0 are parameters that determine the relative

strengths of the two types, called the fitnesses, and f is the

average fitness:

PN

m=2

ym = t2 − t1 + t3 − t2 ...tN − tN −1 = −t1 ,

(11)

f=

if1 +(N −i)f0

.

N

(5)

and so:

By the same token, the probability for the system to

transition from state i to state i − 1 is

Ti+ =

N −i i f0

N N −1 f

t1 = −t1

(12)

PN

m=2

Qm−1

k=1

γk +

PN

m=2

Pm−1

1

j=1 T +

j

Qm−1

k=j+1

γk ,

which is rearranged to give t1 , the mean consensus time for

a single type 1 invader in a population of N − 1 type 0’s as:

(6)

and Ti0 = 1 − (Ti+ + Ti− ) is the probability that no reaction

happens and the system stays in state i. For neutral selection

where the victor is simply the first voter chosen and so f1 =

f0 = 1.

Of interest is τc (i) = N ti , the expected number of sweeps

needed to reach consensus from the state i. The following

equation for the mean number of time steps to consensus

holds:

ti = 1 + Ti− ti−1 + Ti+ ti+1 + Ti0 ti .

(7)

This may be justified term by term as being one time step

( 1 ) plus the expected number of steps remaining for the

state adopted after one time step (Ti− ti−1 + Ti+ ti+1 + Ti0 ti ).

Since the consensus states i = 0 and i = N are absorbing,

we impose the boundary conditions t0 = tN = 0. This may

be rearranged:

t1 =

(13)

−

P −11 Qk

1+ N

l=1 γk

k=1

PN −1 Pk

k=1

1

j=1 T −

j

Qk

m=j+1

γm .

In much the same way as before, we find that ti =

P

N

j=i+1 yj , and so general ti is given by:

ti = −t1

(14)

PN −1 Qj

j=i

l=1

γl +

PN −1 Pj

j=i

1

l=1 T +

l

Qj

m=l+1

γm .

For neutral selection this simplifies greatly as f0 = f1 = 1,

−i)

so Ti+ = Ti− = Ni(N

(N −1) , and γi = 1. For large N This may

be approximated as:

ti = −N 2 ( Ni log

i

N

+ (1 −

i

N ) log(1

−

i

N )).

(15)

ti+1 − ti =

(8)

Ti−

(t

Ti+ i

− ti−1 ) −

1

.

Ti+

A special case that we investigate is equal initial proportions

of type 1’s and type 0’s, i.e. with i = N2 . For this we find that

the consensus time is:

UNIVERSITY OF MANCHESTER

5

tc = N 2 log 2,

(16)

i.e., the consensus time in sweeps is a linear function of N :

τc = N log 2.

(17)

This final result is particularly important as it represents

the expected limit of maximal flow in our model. This is

demonstrated numerically in the results section III.

C. Flow of the network

Our principal object of study has been the effect of flow

upon a voter model defined by spatially distributed voters with

an interaction radius. Here we study a discrete time model in

which ’flow events’ occur at particular intervals after given

numbers of voter model evolution time steps have occurred.

A flow event has the effect of moving the positions of the

voters according to some rule, at which point the network is

redrawn according to the interaction radius method.

The relative timescales for evolution and flow are then

controlled by this number of evolution time steps in between

flow events. We define the Damk¨ohler number, Da, as the

number of sweeps of evolution between flow events. Fractional

values of Da correspond to multiple flow events per sweep of

evolution.

The flow rule we use here is the well studied logistic map.

If we write the coordinates of a voter before a flow event as

x and y, with (x, y) ∈ [0, 1], then the new coordinates are:

x0 = rx(1 − x),

y 0 = ry(1 − y),

(18)

where r is some parameter between 0 and 4. For all

simulations we have used r = 3.8. This maps points within the

range [0, 1] to other points within [0, 1]. At each flow event,

the map is applied to the x and y coordinates of every voter at

the same time. After the flow event, the network is redrawn by

the interaction radius rule and the usual voter model evolution

re-ensues.

The logistic map is one of the most comprehensively investigated objects in all of non-linear dynamics, so to expound

on its features here goes beyond the scope of this project.

However, we note that for r > 3.6 the logistic map has the

important property of being chaotic, whereby two points that

are initially very close to one another are likely to diverge

wildly after only a few iterations of the map. This means

that after many such iterations of the map, here realised as

Fig. 5. An N = 9 square lattice with small Gaussian noise after 0, 10 and

30 iterations of the logistic map with r = 3.8 and δ = 1.3. Note how the

near-symmetry of the original structure is eroded after a few iterations.

flow events, the spatial correlation between two voters disap-

UNIVERSITY OF MANCHESTER

6

pears and they have adopted new pseudo-random positions.

Note however that these new pseudo-random positions are

not drawn from a uniform distribution, but rather from a

distribution characterised by the logistic map itself, and by

the choice of parameter r. This distribution for our choice of

r = 3.8 is shown in figure 13.

We also investigate the effect of two other similarly chaotic

maps. Firstly the H´enon Map, defined by:

x0 = 1 − ax2 + y,

y 0 = bx,

(19)

where a and b are parameters to be chosen and where voters

are disributed around [−1, 1] in 2D. Secondly the Chirikov

map, defined by:

x0 = (x + y) mod 2π, y 0 = y + K sin x,

(20)

where K is a parameter to be chosen and voters are

distributed on [0, 2π].

These maps both have many fascinating properties which

go beyond the scope of this project. The key fact is that they

are all chaotic.

The key effect is that a single flow event will change the

landscape of whatever network we have, breaking links and

forming new ones, but spatial correlations will remain intact.

This means that a link that existed before the flow event will

be more likely than not still to exist after the flow event. I.e.,

few flow events perceptibly change the system, but do not

scramble it entirely such that any voter has an equal chance

of being connected to any other. However, many flow events

will have this property as the effects of chaos dominate and

the voters adopt pseudo-random positions. An illustration of

this for the logistic map is shown in figure 5. The limit of

Da → 0 produces this effect, as it corresponds to many flow

events per step of evolution, or the flow timescale being much

larger than the evolution timescale. Intuitively, this limit should

be equivalent to the Moran process as it means that any voter

is equally likely to compete with any other voter during any

step of evolution such that the effective graph is complete. We

call this the ’well mixed’ or ’maximal flow’ limit.

The other limit is Da → ∞, where the flow timescale is

vastly slower than the evolution timescale. This corresponds

to vanishingly few flow events per step of evolution. For

large enough Da the system will have reached consensus long

before the first flow event, and so the system is functionally

static. We call this the minimal flow or static limit.

III. S IMULATION AND R ESULTS

We developed computer simulations of the systems described above. Programming these simulations has taken up

the majority of time spent on this project throughout this

semester. Particular attention had to be paid to optimising

the code as most algorithms we used became much slower

to compute results with larger population sizes. Performing

voter model simulations to consensus for population sizes

greater than N ≈ 1500 proved prohibitively computationally

expensive. A single realisation could take up to an hour to

compute, whilst measurements of all interesting quantities

required taking averages over hundreds or even thousands of

realisations. It seemed an injudicious use of time to determine

exactly how these computation times scale with N , but, based

on experience of how sensitive we found it to be, we would

conjecture that it is a fairly high-degree polynomial. This

seems reasonable as there are processes that the algorithms

must perform that become more complex with increasing

N , such as choosing voters for competition, which requires

measuring relative positions to all other voters. So each time

step becomes more computationally expensive with increasing

N , and this, combined with the fact that the overall number

of steps needed scales polynomially with N , means that the

overall computation time should increase very fast with N .

Much of what we wanted to measure took the form of

scaling laws with N which are based on approximations

valid for large N . Ideally we would want to verify these by

measuring them over a wide number of values of N which

extends to very large values. However, since we found N

greater than around 1500 totally infeasible to work with, we

opted to measure lower values more accurately by increasing

the number of realisations averaged over. We would then have

very reliable values for, for example, the mean consensus time

at these values and could then check the scaling laws. This is

feasible because by construction the overall computation time

scales linearly with this number of repeats, and so where we

found that more precision was needed it was easy to increase

the accuracy without wildly escalating the amount of time such

measurements would take.

Initially we developed in C++, but found the environment

needlessly cumbersome when performing analysis of the data

obtained and when optimising the algorithms. We then decided

to convert the program into R, and found that this improved

work-flow drastically for a number of reasons. The many

graphing functions in R meant that it was relatively easy to

create visualisations of the processes being simulated which

were impractical with C++, making it possible to get a grasp of

exactly how the system evolves. We also found it easier in R to

have simulations run in parallel on multiple cores, effectively

octupling the processing power available to us. We hope to

modify this process next semester for use on the University’s

Condor system.

A. Static results

We first developed simulations of the simple static system

without any flow events at all, using voters arranged on a line

for one dimension and a square lattice for two. The evolution

of this type of system is fairly intuitive. Each step of evolution

reduces the number of active links, or interfaces, by one and

so regions with a high density of interfaces rapidly condense

into blocs of one species type or another, called domains.

UNIVERSITY OF MANCHESTER

7

Fig. 7. The number of active links, or interfaces, in an N = 300, 1D

nearest neighbour voter model as a function of the number of time steps

since initialisation. Produced by averaging over the results of 1000 realisations

of such systems. The straight line of gradient − 12 part of this log-log plot

Fig. 6. Visualisation of the evolution of a static nearest neighbour voter model

in 1D with N = 625. Time is plotted logarithmically on the vertical axis,

with each vertical line representing the history of a single voter. Red and blue

correspond to type 1 and 0 voters respectively, and the horizontal position of

a line corresponds to its position on the interval [0, 1]. The voters are initially

assigned random types with equal proportions. The initially heterogeneous

configuration with a high density of active links rapidly evolves into one

with large domains of a single type. These domains then slowly merge into

each other until one type has reached consensus. This final process takes a

comparatively long time as the low density of active links means that the vast

majority of evolution exchanges take place between two voters of the same

type where no change of the configuration occurs. This is in agreement with

the long tail of the active link density as a function of time, shown in equation

3 and figure 7.

Here we use neutral selection and initial conditions with equal

proportions of each type so there is no preference for which

type these domains adopt. The static system is also highly

symmetric, so there is no preference for where these domains

appear. The evolution of the system progresses by these

domains slowly merging into one another until one covers the

entire network and the system has reached consensus, in this

case with type 1 dominating. The dynamics here are similar

to the well known Ising model of ferromagnetism at low

temperature, with voter types standing in for particle spins. In

one dimension, this process is readily visualised, an example

of which is shown in figure 6.

The fraction of active links, which in 1D may be thought

of as the number of pairs of neighbours on a line which have

1

differing types, is predicted by [4] to scale with time as t− 2 .

We have verified this prediction, with the results shown in

figure 7.

Equation 2 predicts that the mean consensus time for this 1D

system should scale as N 2 . This was measured, with results

shown in figure 8.

Similarly, for a 2D lattice, equation 1 predicts that the mean

consensus time should scale as N log N . Similar measurements confirming this are shown in figure 9.

The consensus time itself is a random variable. Whilst the

mean consensus time is the main object of interest, we have

measured the distribution for which this variable is drawn,

with the results shown in figures 10 and 11 for 1D and 2D

respectively.

Finally, we simulated the Moran process directly. Results

confirming the scaling law of equation 17 and the dependence

1

confirms the t− 2 scaling predicted from equation 3. At large times this scaling

breaks down as the number of voters of the type not destined for consensus

becomes small enough that the assumptions used in the derivation of 3 cease

to be valid, and the log-log plot ceases to be a straight line. The shape of this

function makes a certain amount of intuitive sense. Initially ever voter has a

random type, and so there are many pairs of neighbours with differing types

and so a high interface density. However, because this density of active links is

so high, many evolution steps will choose these active links for competition.

These exchanges can only ever move interfaces or destroy them, so their

density goes down rapidly. Once the density is so reduced, more and more

evolution steps will choose passive links for competition where no change to

the system occurs. This reduces the rate of decrease of the interface density,

1

and corresponds to the long tail of a t− 2 distribution.

Fig. 8. The scaling of the mean consensus time, τc , with N for a 1D nearest

neighbour interaction voter model. Produced by averaging over the results of

1000 realisations. Above is shown τc as a function of N 2 , demonstrating

τc

the prediction of equation 2. For confirmation, below is shown N

2 as a

function of N. As expected, there is insignificant variation in this ratio. These

measurements suggest that the proportionality constant of equation 2 is around

0.1.

UNIVERSITY OF MANCHESTER

8

Fig. 11. A histogram of the consensus time of 10,000 realisations of a 2D

square lattice voter model with N = 100.

Fig. 9. The scaling of mean consensus time, τc , with N for a 2D square lattice

voter model. As in figure 8, above is shown τc as a function of N log N and

τc

below is the ratio N log

as a function of N . Both reproduce the predictions

N

of equation 1. Here the proportionality constant is around 0.2.

Fig. 12. Results of simulations of the Moran process. Mean consensus time

is on the vertical axis, whilst the horizontal is the initial proportion of type 1

voters in the simulation. The solid lines show the exact solutions of equation

15, whilst the crosses show the results of the simulations, averaged over 100

realisations and in good agreement with the model. Red is for N = 50, green

for N = 150 and blue for N = 250. The symmetry of these results reflects

the fact that this uses neutral selection, so neither type of voter is preferred.

Fig. 10. A histogram of the consensus time of 10,000 realisations of a 1D

nearest neighbour voter model with N = 100.

of the consensus time on the initial population from equation

15 are shown in figure 12.

B. Behaviour of the logistic map

The logistic map is the main flow rule we use in the

simulation to move voters. We have made simulations to

determine two relevant properties of this logistic flow: the

probability that a flow event breaks a connection between

voters, and the clustering behaviour of the logistic flow.

We have chosen r = 3.8 in the logistic map for our

simulations. This is well into the map’s region of chaotic

behaviour. Of importance is how voters will cluster under these

logistic mappings; these locations will constitute localised

groups of nodes which are all connected to one another. This

is equivalent to determining the pseudo-random distribution

referred to in section II-C. This can be determined by considering how much of its time a single voter spends in different

locations over many iterations of the logistic map. I.e., the

distribution will be equivalent to a normalised histogram of

all the values of a long sequence of iterates of the map. The

result of an implementation of this is shown in figure 13.

This distribution is very far from a uniform distribution,

so it is important not to confuse a network generated by the

maximal flow limit with a uniform distribution on [0, 1] as

shown in figure 3. Under maximal logistic flow, voters will

UNIVERSITY OF MANCHESTER

Fig. 13. The probability density function from which is drawn the pseudorandom number of the result of many iterations of the logistic map with

r = 3.8. Generated by a histogram of the result of 3162278 iterates of the

logistic map, starting at 0.1. This may be thought of as illustrating where

voters will cluster after many flow events. Note that this is not the same thing

as the logistic distribution, which is an entirely different object.

cluster at points with x and y coordinates around the peaks of

this distribution. This clustering is important as connections

are defined by an interaction distance and so voters tightly

clustered together will likely all be connected to each other. A

typical example of such a network with this logistic clustering

is shown in figure 14.

We have measured the rate at which the logistic flow breaks

connections between voters by considering the following.

Generate coordinates of two connected voters in the range

[0, 1] by drawing the first, x1 from a uniform distribution

on [0, 1] and the second, x2 , from a uniform distribution

on [x1 − R, x1 + R], redrawing x2 if it falls outside [0, 1]

and where R = nδ is the interaction radius. Then perform

j iterations of the logistic map upon both coordinates and

check if they are still connected, i.e. if |x1 − x2 | < R. If one

performs this process many times, then the proportion of such

realisations which satisfy the inequality is a measurement of

pj , the probability that j iterations of the logistic map does

not break a connection. The results of such measurements are

shown in figure 15.

The fact that p1, the probability of a single flow event

maintaining a given link between voters, is about 0.6 means

that after a flow event only around 40% of connections

will be broken, and so the network as a whole will remain

similar. Thus the mean consensus time should not change

very much if only a single flow event occurs between the

system’s initiation and its reaching consensus as the network

changes only slightly, so a species close to reaching consensus

9

Fig. 14. A typical arrangement of voters after many (100) logistic flow events.

Here N = 100 and δ = 1.6. Note how the density of voters is much higher in

regions who’s x and y coordinates correspond to peaks in the density function

in figure 13, such as the region around the point (0.9, 0.9).

before the flow event will likely still be close to reaching

consensus after the flow event. However, for larger values

of j, pj decreases rapidly and reaches its value for random

distribution with that value of δ at around j = 10. Thus for

1

Da ≈ 10N

, i.e. when there are approximately 10 flow events

per step of evolution, all spatial information between voters

has been eroded by the flow. This means that voters will have

approximately equal chances of connecting to any other voter

during a given evolution exchange. This should have the same

dynamics as the Moran process, and so we conclude that the

system will reach the behaviour of its well mixed limit at least

1

by Da = 10N

, if not sooner.

C. Effect of flow

We investigated the effect of chaotic discrete maps upon

the dynamics the voter model. We have mostly investigated

using the logistic map described above, but have also obtained

similar key results using other chaotic maps. This involves

applying the map to the positions of all voters at regular intervals of evolution time steps as determined by the Damk¨ohler

number. Visualisations of how this affects the 1D nearest

neighbour voter model is shown in figure 16.

The main effect we investigate is the effect of varying

the Damk¨ohler number, Da, on the mean consensus time.

I.e., what effect does the relative size of the evolution and

flow timescales have upon how long heterogeneity in the

voter model can survive? Our simulations of these systems

reproduce the results of the known limits of minimal flow,

equations 1 and 2, and maximal flow, equation 17. Of interest

### Link to this page

#### Permanent link

Use the permanent link to the download page to share your document on Facebook, Twitter, LinkedIn, or directly with a contact by e-Mail, Messenger, Whatsapp, Line..

#### Short link

Use the short link to share your document on Twitter or by text message (SMS)

#### HTML Code

Copy the following HTML code to share your document on a Website or Blog