globals [
  energy-threshold    ;; minimum amount of energy needed by creatures to reproduce
  all-tags            ;; sorted list of all possible tags
]

patches-own [
  well      ;; the amount of resources a patch has
  max-well  ;; the max amount of resources for that patch
]

turtles-own [
  energy    ;; current resource level
  offense   ;; offense tag
  defense   ;; defense tag
  mating    ;; mating tag (can be thought of as identifying a species)
]

to setup
  clear-all
  set energy-threshold 100
  setup-all-tags-list
  setup-patches
  setup-creatures
  reset-ticks
end

;; Creates a list of all possible tags (of length 1, 2, or 3)
to setup-all-tags-list
  set all-tags []
  let tag-elements ["a" "b" "c"]
  foreach tag-elements
  [
    let i ?
    set all-tags lput (list ?) all-tags
    foreach tag-elements [
      let j ?
      set all-tags fput (list i ?) all-tags
      foreach tag-elements
        [ set all-tags fput (list i j ?) all-tags ]
    ]
  ]
end

to setup-patches
  ask patches [
    set max-well random 50
    set well max-well
    recolor-patch
  ]
end

to setup-creatures
  create-turtles init-num-creatures
  [
    setxy random-xcor random-ycor
    set offense one-of all-tags
    set defense one-of all-tags
    set mating one-of all-tags
    set size 0.5   ;; since the patch size is so big make the turtles smaller
    set energy random energy-threshold
    recolor-turtle
  ]
end

to recolor-turtle  ;; turtle procedure
  set color tag-color mating
  set label reduce [word ?1 ?2] (sentence offense "." defense)
end

to-report tag-color [tag]
  report 5 + 10 * position tag all-tags
end

to recolor-patch  ;; patch procedure
   set pcolor scale-color green well 0 100
end

to go
  if not any? turtles [ stop ]
  ask patches [ replenish ]
  ask turtles [ grab-resources ]
  ask turtles [ fight ]
  ask turtles [ reproduce ]
  ask turtles [ move ]
  ask turtles [ replicate ]
  ask patches [ recolor-patch ]
  tick
end

to replenish  ;; patch procedure
  if well < max-well [
    set well well + ((max-well - well) * (replenish-speed / 100))
  ]
end

;; Get resources from the environment, if you need them.  You don't fight for these,
;; but instead they are shared with other agents in the same location.
to grab-resources    ;; turtle procedure
  if (energy < energy-threshold) and (well > 0) [
    set energy energy + ( well / count turtles-here )
    set well well - ( well / count turtles-here )
  ]
end

;; Try to get resources from another agent at your location.
;; This is done by comparing tags.
;; We compare id numbers to prevent the same pair from fighting
;; twice during the same tick.
to fight  ;; turtle procedure
  let candidates turtles-here with [self > myself]
  if any? candidates [ match-off-def self one-of candidates ]
end

;; if there are any other agents at your location, reproduce with them
;; We compare id numbers to prevent the same pair from fighting
;; twice during the same tick.
to reproduce   ;; turtle procedure
  let candidates turtles-here with [self > myself]
  if any? candidates [ reproduce-match self one-of candidates ]
end

to move  ;; turtle procedure
  set energy energy - 1
  ifelse energy < 20
    [ die ]
    [ rt random-normal 0 20
      fd random-float 1 ]
end

to replicate   ;; creature procedure
  if energy > energy-threshold and random-float 100 < replicate-chance
  [
    set energy energy / 2  ;; give half of your energy to your offspring
    hatch 1
  ]
end

;; Creates offspring from mating, including selective mating
to reproduce-match [agent1 agent2]
  if ( [energy] of agent1 > energy-threshold ) and
     ( [energy] of agent2 > energy-threshold )
  [
    if (not selective-mating?) or
        ( ( match-score [mating] of agent1 [offense] of agent2 > mating-selectivity ) and
          ( match-score [mating] of agent2 [offense] of agent1 > mating-selectivity ) )
    [
      hatch 1
      [
        set mating mutate cross [mating] of agent1 [mating] of agent2
        set offense mutate cross [offense] of agent1 [offense] of agent2
        set defense mutate cross [defense] of agent1 [defense] of agent2
        setxy random-xcor random-ycor
        set energy random-normal 50 20
        recolor-turtle
      ]
      ask agent1 [ set energy energy / 2 ]
      ask agent2 [ set energy energy / 2 ]
    ]
  ]
end

;; This crosses tags during reproduction by walking through
;; each position of the string.  When the letters from each tag differ at a position
;; there is an equal chance of it being either one.  When the tags are different lengths,
;; the additional letters are just appended on to the tage of the offspring.
to-report cross [tag1 tag2]
  if length tag1 > length tag2
    [ report sentence cross sublist tag1 0 length tag2 tag2 sublist tag1 length tag2 length tag1 ]
  if length tag2 > length tag1
    [ report sentence cross tag1 sublist tag2 0 length tag1 sublist tag2 length tag1 length tag2 ]
  report (map [one-of (list ?1 ?2)] tag1 tag2)
end

;; Mutates a given string
to-report mutate [tag]
  report map [ifelse-value (random-float 100 >= mutation-rate)
                [?] [one-of remove ? ["a" "b" "c"]]]
             tag
end

;; This procedure determines how resources are transferred between
;; agents.  The offense tag of an agent1 is matched with the defense
;; tag of agent 2, and the offense tag of agent2 is matched with the
;; defense tag of agent1.
to match-off-def [agent1 agent2]

  ;; match respective offense and defense tags
  let a1a2 match-score [offense] of agent1  [defense] of agent2
  let a2a1 match-score [offense] of agent2  [defense] of agent1

  ;; scale resulting scores so that all values are positive
  set a1a2 a1a2 + 6
  set a2a1 a2a1 + 6

  ;; Determine how much resource is transferred between the agents.
  ;; Each agent takes from the other based on the relative magnitude
  ;; of its match score
  set a1a2 (a1a2 / 12) * ([energy] of agent2)
  set a2a1 (a2a1 / 12) * ([energy] of agent1)

  ;; You win some, you lose some.
  ask agent1 [ set energy energy + a1a2 - a2a1 ]
  ask agent2 [ set energy energy + a2a1 - a1a2 ]

end

;; This reports a score that represents how well tag 1 matches tag 2.
;; The match takes places letter by letter -- the first
;; letters of each tag are compared, then the second letters, and so on.
;; An adjustment is made for unequal length tags.The resulting "match-score"
;; is ultimately used to help determine how resources are transferred between agents
;; or the extent to which a potential mate matches an agent's mating conditions
to-report match-score [tag1 tag2]
  if length tag1 > length tag2
    [ report (length tag2 - length tag1) + match-score sublist tag1 0 length tag2 tag2 ]
  if length tag2 > length tag1
    [ report (length tag1 - length tag2) + match-score tag1 sublist tag2 0 length tag1 ]
  report sum (map [ifelse-value (?1 = ?2) [2] [-2]] tag1 tag2)
end


; Copyright 2005 Uri Wilensky.
; See Info tab for full copyright and license.
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@#$#@#$#@
## WHAT IS IT?

Echo is a model about the evolution of fitness, adapted from John Holland's book _Hidden Order_ (1995).  It can be used to facilitate experiments in a variety of domains where an agent's fitness varies with its context.  This particular adaptation of Echo  has a biological flavor and refers to the agents as "creatures", and groups of agents with identical mating preferences as "species."

With Echo, Holland attempted to codify intuitions about complex adaptive systems into a more rigorous and abstract model.  Holland was inspired by notions from ecological systems research, especially with regards to the dynamics of niches.  How does a change in the population dynamics of an ecosystem create new niches and destroy old ones?  Echo was an attempt to create an abstract model that could facilitate understanding of the interplay of evolutionary and ecological processes.  In Echo, different niches are created where creatures can consume, transform, and exchange resources, but this model is not necessarily just a biological model --- it can also be viewed as a model of other phenomena like the emergence of new markets.

It should be noted that Echo as laid out in _Hidden Order_ is not meant to be a specific model, but rather a framework for building models.  There are a number of mechanisms that are not fully specified in _Hidden Order_ and the description includes multiple different mechanisms that can be implemented.  As a result, there is no model which is "the Echo model" there can only be "an Echo model."  This NetLogo Echo model is one particular instantiation of this framework.

## HOW IT WORKS

Each agent, or creature, has two components -- a reservoir to keep resources it collects, and a chromosome that contains its genetic material and defines its capabilities.

A creature expends resources moving around the world looking for resources.  A creature can gain resources either directly from its environment, or from another creature at its current location.

The amount of resources gained from its environment depends on the amount of available resources and the number of other creatures at its current location.

The amount of resources gained from another creature depends on the match between the creatures' chromosomes.  More precisely, a creature's chromosome is compromised of three "tags" --- an offense tag, a defense tag, and a mating condition tag -- which are represented by strings of "a's", "b's" and "c's".  The amount transferred from one creature to another depends on how closely the first creature's offense tag matches the second creature's defense tag.  However, all interactions, or "fights", between creatures are two-way exchanges --- i.e., both agents match their offense tags to the other's defense tag.  Consequently, resources can be either gained or lost on any given interaction with another agent.  For example, offense tag "aaa" of creature 1 to defense tag "aaa" of creature 2 would be a perfect match. It would entitle creature 1 to all of creature 2 resources, but the second creature could possibly get some, or even all of the resources back based on the match of his offense tag to the other agent's defense tag.

If an agent's resource level drops below a minimum threshold, it dies.

An agent can reproduce only if it has acquired a minimum level of resources.  Reproduction occurs in two ways:  1) simple replication, as determined by the REPLICATE-CHANCE slider, in which case the replicated agent is identical to the parent, and 2) mating with other agents.  By default, an agent can mate with any other agent, resulting in an offspring that contains a mixed set of elements, or a "cross", of both parent's chromosomes.  If the SELECTIVE-MATING? switch is enabled, an agent will not mate with just anyone, but instead only mate with agents whose offense tags match its own mating condition tag.  The similarity between the mating tag and offense tag necessary for a match to occur is determined by the MATING-SELECTIVITY slider.  With either process, it is possible that the characteristics of offspring are not an exact copy or cross of the parent's chromosomes.  Mutations can occur at a rate determined by the MUTATION-RATE slider.

## HOW TO USE IT

Click the SETUP button to setup the world, then click the GO button.  The agents will begin to move, exchange resources, and reproduce.  The sliders allow you to change each of the parameters of the model as described below.  The plots provide an update on the composition of the populations of agents.

The agents are colored by mating tags.  Identical mating tags share the same color; mating tags that are close are similar in color; and so on.  Each creature has its offense and defense tags as its label in the format "offense-tag.defense-tag".  For example, a label of "aab.bcc" represents a creature with an offense tag of "aab" and defense tag of "bcc."

The model includes the following parameters:

NUM-CREATURES --- The number of agents with which to start the simulation.

REPLENISH-SPEED --- The speed at which the resources in the environment replenish.

REPLICATE-CHANCE --- The chance that a creature with sufficient energy will replicate.

SELECTIVE-MATING? --- If 'Off' it is possible for agents to mate with any other agent that has enough energy to reproduce.  If 'On' agents will only mate with other agents whose offense tag matches their own mating tag.

MATING-SELECTIVITY? --- Determines how similar a mating tag and offense tag must be in order for two agents to mate.  Higher, positive numbers are more selective.  '0' is the midpoint.

MUTATION-RATE --- The rate at which a letter in the offense or defense tags of offspring may change during replication.

Here are descriptions of all the plots in the model:

"Offense Tags" --- A histogram of the number of each offense tag currently in existence.

"Defense Tags" --- A histogram of the number of each defense tag currently in existence.

"Species Counts" --- A histogram of the number of each species currently alive.  A species is defined as a group of agents that share an identical mating tag.

"Populations" --- This shows the number of creatures in the two largest species currently in the world.

"Speciation" --- This shows the current total number of different species, the total number of different offense tags, and the total number of different defense tags in the world.

Some other options are described below in THINGS TO TRY.

## THINGS TO NOTICE

Populations often become dynamically stable with several species at oscillating population levels.

Increasing the initial NUM-CREATURES slider does not necessarily result in a larger stable population, indeed the opposite may occur as creatures compete for limited resources.

## THINGS TO TRY

Change REPLENISH-SPEED to examine the impact of making resources more scarce.

See what impact increasing or lowering the MUTATION-RATE and/or switching SELECTIVE-MATING? 'On' has on the number of species that ultimately survive in situation with dynamically stable populations.

## EXTENDING THE MODEL

Add a mechanism that allows selective interaction for the exchange of resources, in addition to selective mating.

Resources could be more nuanced than the simple "energy" variable.  Certain locations in the environment could give off certain resources.  For example, every creature might need at least one "c" resource to live, but only certain wells might give resources of type "c".  Furthermore, resources could be transformed.  For example, one "a" and two "b's" could be turned into a "c" for some cost.

Currently the exchange of resources between two agents is a fight --- a zero-sum exchange.  However, this interaction could be changed to an interaction where both parties can become better off --- a situation that resembles a trade more than a fight.

Holland describes several additional extensions in some detail in Chapter 3 of _Hidden Order_.

## EXTENDING THE MODEL (CATERPILLAR-FLY-ANT)

Holland discusses an interesting example from nature that can be imitated in Echo -- the Caterpillar-Fly-Ant triangle.  The triangle refers to the stable, triangular relation between the three different species: Species 1 --  Flies lay eggs on caterpillars and become prey through their larva.  Species 2 -- Ants are aggressive predators on the flies, but not of caterpillars.  Species 3 -- Caterpillars produce nectar on their skin which ants find very attractive.  The more ants around a caterpillar the less likely it is to be attacked by a predatory fly.  This can be represented in Echo with the following tags:

     ant - offense: aaa, defense: cb
     fly - offense: aab, defense: aaa
     caterpillar - offense: c, defense: aab

Notice that the ants' offense tag matches perfectly with the fly's defense tag, making it a perfect predatory on the fly.  Likewise, the fly's offense tag matches perfectly on the caterpillar making it a perfect predator on the caterpillar.  The ant consumes the nectar of the caterpillar but does not prey on the caterpillar; thus, the ant and caterpillar have tags that result in an exchange of resources.

Try replicating this scenario.  You'll probably want to set SELECTIVE-MATING? to on and MATING-SELECTIVITY to a high value such as 5.  Is it possible to achieve a stable relationship between the three species?  (Are changes to the parameters and/or the rules of the model necessary...?)

To get you started, here's some code for creating a roughly equal number of the three species.  (You may want to experiment with different initial proportions.)  The mating tags are completely different, to discourage interspecies mating.

     create-turtles init-num-creatures
     [
       let choice random 3
       if choice = 0  ; ant
       [
         set offense ["a" "a" "a"]
         set defense ["c" "b"]
         set mating  ["a" "a" "a"]
       ]
       if choice = 1  ; fly
       [
         set offense ["a" "a" "b"]
         set defense ["a" "a" "a"]
         set mating  ["b" "b" "b"]
       ]
       if choice = 2  ; caterpillar
       [
         set offense ["c"]
         set defense ["a" "a" "b"]
         set mating  ["c" "c" "c"]
       ]
     ]


## NETLOGO FEATURES

Tags are represented by lists of strings, so the code is heavily dependent on lists. `map`, `reduce`, and `foreach` are used to do list processing.

## RELATED MODELS

Sample Models > Biology > Evolution > (entire section)  
Sample Models > Biology > Rabbits Grass Weeds  
Sample Models > Biology > Wolf Sheep Predation

## CREDITS AND REFERENCES

Holland, J. (1995). _Hidden Order: How Adaptation Builds Complexity_. Addison-Wesley, Reading, Massachusetts.


## HOW TO CITE

If you mention this model in a publication, we ask that you include these citations for the model itself and for the NetLogo software:

* Wilensky, U. (2005).  NetLogo Echo model.  http://ccl.northwestern.edu/netlogo/models/Echo.  Center for Connected Learning and Computer-Based Modeling, Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL.
* Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL.

## COPYRIGHT AND LICENSE

Copyright 2005 Uri Wilensky.

![CC BY-NC-SA 3.0](http://i.creativecommons.org/l/by-nc-sa/3.0/88x31.png)

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.  To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

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Polygon -7500403 true true 105 90 120 195 90 285 105 300 135 300 150 225 165 300 195 300 210 285 180 195 195 90
Rectangle -7500403 true true 127 79 172 94
Polygon -7500403 true true 195 90 240 150 225 180 165 105
Polygon -7500403 true true 105 90 60 150 75 180 135 105

plant
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Rectangle -7500403 true true 135 90 165 300
Polygon -7500403 true true 135 255 90 210 45 195 75 255 135 285
Polygon -7500403 true true 165 255 210 210 255 195 225 255 165 285
Polygon -7500403 true true 135 180 90 135 45 120 75 180 135 210
Polygon -7500403 true true 165 180 165 210 225 180 255 120 210 135
Polygon -7500403 true true 135 105 90 60 45 45 75 105 135 135
Polygon -7500403 true true 165 105 165 135 225 105 255 45 210 60
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Rectangle -16777216 true false 60 60 240 240

star
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Polygon -7500403 true true 151 1 185 108 298 108 207 175 242 282 151 216 59 282 94 175 3 108 116 108

target
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Circle -7500403 true true 0 0 300
Circle -16777216 true false 30 30 240
Circle -7500403 true true 60 60 180
Circle -16777216 true false 90 90 120
Circle -7500403 true true 120 120 60

tree
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Circle -7500403 true true 118 3 94
Rectangle -6459832 true false 120 195 180 300
Circle -7500403 true true 65 21 108
Circle -7500403 true true 116 41 127
Circle -7500403 true true 45 90 120
Circle -7500403 true true 104 74 152

triangle
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Polygon -7500403 true true 150 30 15 255 285 255

triangle 2
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Polygon -7500403 true true 150 30 15 255 285 255
Polygon -16777216 true false 151 99 225 223 75 224

truck
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Rectangle -7500403 true true 4 45 195 187
Polygon -7500403 true true 296 193 296 150 259 134 244 104 208 104 207 194
Rectangle -1 true false 195 60 195 105
Polygon -16777216 true false 238 112 252 141 219 141 218 112
Circle -16777216 true false 234 174 42
Rectangle -7500403 true true 181 185 214 194
Circle -16777216 true false 144 174 42
Circle -16777216 true false 24 174 42
Circle -7500403 false true 24 174 42
Circle -7500403 false true 144 174 42
Circle -7500403 false true 234 174 42

turtle
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Polygon -10899396 true false 215 204 240 233 246 254 228 266 215 252 193 210
Polygon -10899396 true false 195 90 225 75 245 75 260 89 269 108 261 124 240 105 225 105 210 105
Polygon -10899396 true false 105 90 75 75 55 75 40 89 31 108 39 124 60 105 75 105 90 105
Polygon -10899396 true false 132 85 134 64 107 51 108 17 150 2 192 18 192 52 169 65 172 87
Polygon -10899396 true false 85 204 60 233 54 254 72 266 85 252 107 210
Polygon -7500403 true true 119 75 179 75 209 101 224 135 220 225 175 261 128 261 81 224 74 135 88 99

wheel
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Circle -7500403 true true 3 3 294
Circle -16777216 true false 30 30 240
Line -7500403 true 150 285 150 15
Line -7500403 true 15 150 285 150
Circle -7500403 true true 120 120 60
Line -7500403 true 216 40 79 269
Line -7500403 true 40 84 269 221
Line -7500403 true 40 216 269 79
Line -7500403 true 84 40 221 269

x
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Polygon -7500403 true true 270 75 225 30 30 225 75 270
Polygon -7500403 true true 30 75 75 30 270 225 225 270

@#$#@#$#@
NetLogo 5.0.4
@#$#@#$#@
@#$#@#$#@
@#$#@#$#@
@#$#@#$#@
@#$#@#$#@
default
0.0
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0.0 1 1.0 0.0
0.2 0 0.0 1.0
link direction
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Line -7500403 true 150 150 90 180
Line -7500403 true 150 150 210 180

@#$#@#$#@
0
@#$#@#$#@
