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If you download the NetLogo application, this model is included. You can also Try running it in NetLogo Web 
This model investigates emergent patterns in a demographic property of a population: the adult sex ratio (ASR). ASR is defined as the ratio of males to females in the adult population. Most sexually reproducing organisms have an ASR of 1:1. Fisher (1930) explains a rationale for this phenomenon based on natural selection, irrespective of a particular mechanism of sexdetermination, which is now known as Fisher’s principle (Hamilton, 1967).
This model is based on Fisher's assumptions of biparentism and Mendelian inheritance. It allows user to explore the emergence of the Fisherian Sex Ratio (1:1) across a wide range of initial sexratios and lifehistory traits.
This is a population dynamics model of sexually reproducing organisms. The demographic changes in the population as well as the adult sex ratio are plotted as the time progresses.
There are two types of individuals in the model:  Males (Indicated by green color)  Females (Indicated by blue color if not carrying a child and indicated by orange color if carrying a child)
At every timestep the individuals:  Age and check if dead – a random value of longevity is assigned at birth if an agent’s age is more than that value it dies  Move randomly  Search for an adult partner if male. This procedure involves mating, which is dependent on the assigned value of the mating chance. If agents mate, the female carries. (For simplicity, the mating chance in this model incorporates conceptionchance as well.)  Reproduce if female
The genetic mechanism of influencing the proportion of male children in the litter is abstracted as a parameter, called MALECHILDCHANCE. Each male and female carry a trait called MALECHILDCHANCE. This determines the probability of a child being male. In this basic model, the sex of the child is equally influenced by the MALECHILDCHANCE values of both the parents.
A child inherits this trait from its parents with a mutation. The inherited value for the trait is drawn from a random normal distribution with a mean equal to the average of MALECHILDCHANCE of the parents.
Using the sliders, set your desired initial population and individual values (see the Sliders section below for more details). Click the SETUP button to setup the population. To run the model, click the GO button.
The plots and monitors allow users to observe populationlevel changes and see the emergence of the Fisherian sexratio equilibrium.
INITIALPOPULATIONSIZE: the initial number of individuals in the population INITIALADULTSEXRATIO: the initial adultsexratio (number of males/number individual * 100) in the population INITIALAVERAGEMALECHILDCHANCE: the initial average malechildchance of all the individuals in the population MEANLONGEVITY: the average longevity (number of ‘ticks’ an individual lives) GESTATIONPERIOD: the number of 'ticks' a female is going to carry a child MAXIMUMLITTERSIZE: the maximum number of children a female would give birth to after she completes gestation MATINGCHANCE: the probability that a female mates with a male when a male finds a female partner
ADULT SEX RATIO: shows the percentage of male agents POPULATION AVERAGE MALECHILDCHANCE: shows how the population's malechildchance changes over time GENDER DISTRIBUTION: shows numbers of agents of both sexes MATING SUCCESS RATIO (MALE TO FEMALE): shows how the ratio of male mating success to female mating success changes over time
ADULT SEX RATIO (ASR): the proportion of male agents, at the moment RUNNING MEAN OF ASR: the average of adult sex ratio in the past 1000 ticks RUNNING STD DEV OF ASR: standard deviation of adult sex ratio in the past 1000 ticks
When you run the model, notice how the number of males and females vary in the GENDER DISTRIBUTION plot. Also, notice that for a wide range of parameter values, if you run the simulation long enough (4000 to 5000 ticks), the ASR converges to 50% as seen in the ADULTSEXRATIO plot.
Notice that the average value of the trait malechildchance across all agents in the AVERAGE MALECHILDCHANCE plot also converges to 0.5 for a wide range of parameters.
Also notice that once the ASR reaches 50%, it is stable around the 50% point. The AVERAGE MALECHILDCHANCE is also stable around the value of 0.5.
Vary the INITIALSEXRATIO and INITIALAVERAGEMALECHILDCHANCE, which are the two variables that affect the sex ratio the most in this model. How do these variables relate to the mating success of each sex?
Try setting different values of INITIALADULTSEXRATIO and INITIALAVERAGEMALECHILDCHANCE to see how that affects the emergence of the adult sex ratio equilibrium.
See if you can change the parameters and see if you can get to a different stable ASR.
This model implements searchforapartner
by a male. See if you can change it and make it a female action.
You can also add a variety of features related to evolutionary population genetics and sex ratio equilibrium studies, such as different mortality rates for males and females, sexual selection by females.
Fisher, R. A. (1930). The genetical theory of natural selection: a complete variorum edition. Oxford University Press.
Hamilton, W. D. (1967). Extraordinary sex ratios. Science, 156(3774), 477488
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Copyright 2020 Uri Wilensky.
This work is licensed under the Creative Commons AttributionNonCommercialShareAlike 3.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/byncsa/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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