diff --git a/src/beast/evolution/speciation/ProbabilitySA.java b/src/beast/evolution/speciation/ProbabilitySA.java
new file mode 100644
index 0000000..1bf49a8
--- /dev/null
+++ b/src/beast/evolution/speciation/ProbabilitySA.java
@@ -0,0 +1,179 @@
+package beast.evolution.speciation;
+
+        import beast.core.Distribution;
+        import beast.core.Input;
+        import beast.core.State;
+        import beast.core.parameter.IntegerParameter;
+        import beast.core.parameter.RealParameter;
+
+        import java.util.List;
+        import java.util.Random;
+
+/**
+ * @author Alexandra Gavryushkina
+ */
+public class ProbabilitySA extends Distribution {
+
+    public final Input<IntegerParameter> SAInput = new Input<IntegerParameter>("SA", "A binary parameter that describe " +
+            "the fact that a lineage has no sampled descendants (zero) or it has sampled descendants (one)", Input.Validate.REQUIRED);
+
+    public Input<RealParameter> timeInput =
+            new Input<RealParameter>("time", "The time of the lineage",Input.Validate.REQUIRED);
+
+    public Input<RealParameter> originInput =
+            new Input<RealParameter>("origin", "The time when the process started",(RealParameter)null);
+
+    //'direct' parameters
+    public Input<RealParameter> birthRateInput =
+            new Input<RealParameter>("birthRate", "Birth rate");
+    public Input<RealParameter> deathRateInput =
+            new Input<RealParameter>("deathRate", "Death rate");
+    public Input<RealParameter> samplingRateInput =
+            new Input<RealParameter>("samplingRate", "Sampling rate per individual");
+
+    //transformed parameters:
+    public Input<RealParameter> diversificationRateInput =
+            new Input<RealParameter>("diversificationRate", "Net diversification rate. Birth rate - death rate", Input.Validate.XOR, birthRateInput);
+    public Input<RealParameter> turnoverInput =
+            new Input<RealParameter>("turnover", "Turnover. Death rate/birth rate", Input.Validate.XOR, deathRateInput);
+    public Input<RealParameter> samplingProportionInput =
+            new Input<RealParameter>("samplingProportion", "The probability of sampling prior to death. Sampling rate/(sampling rate + death rate)", Input.Validate.XOR, samplingRateInput);
+
+
+    // r parameter
+    public Input<RealParameter> removalProbability =
+            new Input<RealParameter>("removalProbability", "The probability that an individual is removed from the process after the sampling", Input.Validate.REQUIRED);
+
+    public Input<RealParameter> rhoProbability =
+            new Input<RealParameter>("rho", "Probability of an individual to be sampled at present", (RealParameter)null);
+
+    // if the tree likelihood is condition on sampling at least one individual then set to true one of the inputs:
+    public Input<Boolean> conditionOnSamplingInput = new Input<Boolean>("conditionOnSampling", "the tree " +
+            "likelihood is conditioned on sampling at least one individual", false);
+    public Input<Boolean> conditionOnRhoSamplingInput = new Input<Boolean>("conditionOnRhoSampling", "the tree " +
+            "likelihood is conditioned on sampling at least one individual in present", false);
+
+    public Input<Boolean> conditionOnRootInput = new Input<Boolean>("conditionOnRoot", "the tree " +
+            "likelihood is conditioned on the root height otherwise on the time of origin", false);
+
+
+    protected double r;
+    protected double lambda;
+    protected double mu;
+    protected double psi;
+    protected double c1;
+    protected double c2;
+    protected double origin;
+    protected double rho;
+    protected boolean transform; //is true if the model is parametrised through transformed parameters
+    private boolean lambdaExceedsMu = false;
+
+    @Override
+    public List<String> getArguments() {
+        return null;
+    }
+
+    @Override
+    public List<String> getConditions() {
+        return null;
+    }
+
+    @Override
+    public void sample(State state, Random random) {
+
+    }
+
+    public void initAndValidate() throws Exception {
+
+        if (originInput.get() == null && !conditionOnRootInput.get()) {
+            throw new RuntimeException("Either specify origin input or set conditionOnRoot input to \"true\"");
+        }
+
+        if (originInput.get() != null && conditionOnRootInput.get()){
+            throw new RuntimeException("Either don't specify origin input or set conditionOnRoot input to \"false\"");
+        }
+
+
+        if (conditionOnSamplingInput.get() && conditionOnRhoSamplingInput.get()){
+            throw new RuntimeException("Either set to \"true\" only one of conditionOnSampling and conditionOnRhoSampling inputs or don't specify both!");
+        }
+
+        if (birthRateInput.get() != null && deathRateInput.get() != null && samplingRateInput.get() != null) {
+
+            transform = false;
+            //mu = deathRateInput.get().getValue();
+            //psi = samplingRateInput.get().getValue();
+            //lambda = birthRateInput.get().getValue();
+
+        } else if (diversificationRateInput.get() != null && turnoverInput.get() != null && samplingProportionInput.get() != null) {
+
+            transform = true;
+
+        } else {
+            throw new RuntimeException("Either specify birthRate, deathRate and samplingRate OR specify diversificationRate, turnover and samplingProportion!");
+        }
+    }
+
+
+    private double oneMinusP0(double t, double c1, double c2) {
+        return 1 - (lambda + mu + psi - c1 * ((1 + c2) - Math.exp(-c1 * t) * (1 - c2)) / ((1 + c2) + Math.exp(-c1 * t) * (1 - c2))) / (2 * lambda);
+    }
+
+
+    private void transformParameters() {
+        double d = diversificationRateInput.get().getValue();
+        double r_turnover = turnoverInput.get().getValue();
+        double s = samplingProportionInput.get().getValue();
+        lambda = d/(1-r_turnover);
+        mu = r_turnover*lambda;
+        psi = mu*s/(1-s);
+    }
+
+    private void updateParameters() {
+
+        if (transform) {
+            transformParameters();
+        } else {
+            lambda = birthRateInput.get().getValue();
+            mu = deathRateInput.get().getValue();
+            psi = samplingRateInput.get().getValue();
+        }
+
+        r = removalProbability.get().getValue();
+        if (rhoProbability.get() != null ) {
+            rho = rhoProbability.get().getValue();
+        } else {
+            rho = 0.;
+        }
+        c1 = Math.sqrt((lambda - mu - psi) * (lambda - mu - psi) + 4 * lambda * psi);
+        c2 = -(lambda - mu - 2*lambda*rho - psi) / c1;
+        if (originInput.get() != null){
+            origin = originInput.get().getValue();
+        }  else {
+            origin = Double.POSITIVE_INFINITY;
+        }
+    }
+
+    @Override
+    public double calculateLogP()
+    {
+        updateParameters();
+
+        if (lambdaExceedsMu && lambda <= mu) {
+            return Double.NEGATIVE_INFINITY;
+        }
+        double t=timeInput.get().getValue();
+        if (SAInput.get().getValue() == 0) {
+            logP = Math.log(1 - oneMinusP0(t, c1, c2));
+        } else {
+            logP = Math.log(oneMinusP0(t, c1, c2));
+        }
+
+        return logP;
+    }
+
+    @Override
+    protected boolean requiresRecalculation() {
+        return true;
+    }
+}