diff --git a/test/yulPhaser/Common.cpp b/test/yulPhaser/Common.cpp
index 93aa432fa..52cde9590 100644
--- a/test/yulPhaser/Common.cpp
+++ b/test/yulPhaser/Common.cpp
@@ -24,6 +24,12 @@
 using namespace std;
 using namespace solidity;
 using namespace solidity::yul;
+using namespace solidity::phaser;
+
+function<Mutation> phaser::test::wholeChromosomeReplacement(Chromosome _newChromosome)
+{
+	return [_newChromosome = move(_newChromosome)](Chromosome const&) { return _newChromosome; };
+}
 
 vector<size_t> phaser::test::chromosomeLengths(Population const& _population)
 {
@@ -44,6 +50,15 @@ map<string, size_t> phaser::test::enumerateOptmisationSteps()
 	return stepIndices;
 }
 
+size_t phaser::test::countDifferences(Chromosome const& _chromosome1, Chromosome const& _chromosome2)
+{
+	size_t count = 0;
+	for (size_t i = 0; i < min(_chromosome1.length(), _chromosome2.length()); ++i)
+		count += static_cast<int>(_chromosome1.optimisationSteps()[i] != _chromosome2.optimisationSteps()[i]);
+
+	return count + abs(static_cast<int>(_chromosome1.length() - _chromosome2.length()));
+}
+
 string phaser::test::stripWhitespace(string const& input)
 {
 	regex whitespaceRegex("\\s+");
diff --git a/test/yulPhaser/Common.h b/test/yulPhaser/Common.h
index e73260d7a..cbec90f78 100644
--- a/test/yulPhaser/Common.h
+++ b/test/yulPhaser/Common.h
@@ -30,9 +30,11 @@
 
 #include <tools/yulPhaser/Chromosome.h>
 #include <tools/yulPhaser/FitnessMetrics.h>
+#include <tools/yulPhaser/Mutations.h>
 #include <tools/yulPhaser/Population.h>
 
 #include <cassert>
+#include <functional>
 #include <map>
 #include <string>
 #include <vector>
@@ -52,11 +54,21 @@ public:
 	size_t evaluate(Chromosome const& _chromosome) const override { return _chromosome.length(); }
 };
 
+// MUTATIONS
+
+/// Mutation that always replaces the whole chromosome with the one specified in the parameter.
+std::function<Mutation> wholeChromosomeReplacement(Chromosome _newChromosome);
+
 // CHROMOSOME AND POPULATION HELPERS
 
 /// Returns a vector containing lengths of all chromosomes in the population (in the same order).
 std::vector<size_t> chromosomeLengths(Population const& _population);
 
+/// Returns the number of genes that differ between two chromosomes.
+/// If the chromnosomes have different lengths, the positions that are present in only one of them
+/// are counted as mismatches.
+size_t countDifferences(Chromosome const& _chromosome1, Chromosome const& _chromosome2);
+
 /// Assigns indices from 0 to N to all optimisation steps available in the OptimiserSuite.
 /// This is a convenience helper to make it easier to test their distribution with tools made for
 /// integers.
diff --git a/test/yulPhaser/CommonTest.cpp b/test/yulPhaser/CommonTest.cpp
index 9bcab9923..3456a6640 100644
--- a/test/yulPhaser/CommonTest.cpp
+++ b/test/yulPhaser/CommonTest.cpp
@@ -40,6 +40,12 @@ BOOST_AUTO_TEST_CASE(ChromosomeLengthMetric_evaluate_should_return_chromosome_le
 	BOOST_TEST(ChromosomeLengthMetric{}.evaluate(Chromosome("aaaaa")) == 5);
 }
 
+BOOST_AUTO_TEST_CASE(wholeChromosomeReplacement_should_replace_whole_chromosome_with_another)
+{
+	function<Mutation> mutation = wholeChromosomeReplacement(Chromosome("aaa"));
+	BOOST_TEST(mutation(Chromosome("ccc")) == Chromosome("aaa"));
+}
+
 BOOST_AUTO_TEST_CASE(chromosomeLengths_should_return_lengths_of_all_chromosomes_in_a_population)
 {
 	shared_ptr<FitnessMetric> fitnessMetric = make_shared<ChromosomeLengthMetric>();
@@ -51,6 +57,34 @@ BOOST_AUTO_TEST_CASE(chromosomeLengths_should_return_lengths_of_all_chromosomes_
 	BOOST_TEST((chromosomeLengths(population2) == vector<size_t>{}));
 }
 
+BOOST_AUTO_TEST_CASE(countDifferences_should_return_zero_for_identical_chromosomes)
+{
+	BOOST_TEST(countDifferences(Chromosome(), Chromosome()) == 0);
+	BOOST_TEST(countDifferences(Chromosome("a"), Chromosome("a")) == 0);
+	BOOST_TEST(countDifferences(Chromosome("afxT"), Chromosome("afxT")) == 0);
+}
+
+BOOST_AUTO_TEST_CASE(countDifferences_should_count_mismatched_positions_in_chromosomes_of_the_same_length)
+{
+	BOOST_TEST(countDifferences(Chromosome("a"), Chromosome("f")) == 1);
+	BOOST_TEST(countDifferences(Chromosome("aa"), Chromosome("ac")) == 1);
+	BOOST_TEST(countDifferences(Chromosome("ac"), Chromosome("cc")) == 1);
+	BOOST_TEST(countDifferences(Chromosome("aa"), Chromosome("cc")) == 2);
+	BOOST_TEST(countDifferences(Chromosome("afxT"), Chromosome("Txfa")) == 4);
+}
+
+BOOST_AUTO_TEST_CASE(countDifferences_should_count_missing_characters_as_differences)
+{
+	BOOST_TEST(countDifferences(Chromosome(""), Chromosome("a")) == 1);
+	BOOST_TEST(countDifferences(Chromosome("a"), Chromosome("")) == 1);
+	BOOST_TEST(countDifferences(Chromosome("aa"), Chromosome("")) == 2);
+	BOOST_TEST(countDifferences(Chromosome("aaa"), Chromosome("")) == 3);
+
+	BOOST_TEST(countDifferences(Chromosome("aa"), Chromosome("aaaa")) == 2);
+	BOOST_TEST(countDifferences(Chromosome("aa"), Chromosome("aacc")) == 2);
+	BOOST_TEST(countDifferences(Chromosome("aa"), Chromosome("cccc")) == 4);
+}
+
 BOOST_AUTO_TEST_CASE(enumerateOptimisationSteps_should_assing_indices_to_all_available_optimisation_steps)
 {
 	map<string, char> stepsAndAbbreviations = OptimiserSuite::stepNameToAbbreviationMap();