diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5f5877a..0270901 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.25)
-project(lilfbtf5 VERSION 0.1.35)
+project(lilfbtf5 VERSION 0.1.36)
 
 option(ENABLE_ADDRSAN "Enable the address sanitizer" OFF)
 option(ENABLE_UBSAN "Enable the ub sanitizer" OFF)
diff --git a/include/lilfbtf/fwddecl.h b/include/lilfbtf/fwddecl.h
index 3aa17c8..a23600d 100644
--- a/include/lilfbtf/fwddecl.h
+++ b/include/lilfbtf/fwddecl.h
@@ -29,13 +29,22 @@ namespace fb
     namespace detail
     {
         class node_t;
+        
+        struct fitness_results
+        {
+            double fitness;
+            blt::size_t hits;
+        };
     }
     
     class func_t;
+    
     class tree_t;
     
     class type_engine_t;
+    
     class gp_system_t;
+    
     class gp_population_t;
     
     // no way we are going to have more than 4billion types or functions.
@@ -44,7 +53,7 @@ namespace fb
     using arg_c_t = blt::size_t;
     using func_t_call_t = std::function<void(func_t&, blt::span<detail::node_t*>)>;
     using func_t_init_t = std::function<void(func_t&)>;
-    using fitness_eval_func_t = std::function<void(detail::node_t*)>;
+    using fitness_eval_func_t = std::function<detail::fitness_results(detail::node_t*)>;
     using function_name = const std::string&;
     using type_name = const std::string&;
 }
diff --git a/include/lilfbtf/system.h b/include/lilfbtf/system.h
index f0f7aea..90061e4 100644
--- a/include/lilfbtf/system.h
+++ b/include/lilfbtf/system.h
@@ -21,6 +21,7 @@
 
 #include <lilfbtf/fwddecl.h>
 #include <lilfbtf/tree.h>
+#include <blt/std/thread.h>
 #include <vector>
 
 namespace fb
@@ -28,7 +29,22 @@ namespace fb
     
     class gp_population_t
     {
-    
+        private:
+            blt::thread_pool<true>& pool;
+            std::vector<tree_t> population;
+            
+            void crossover();
+            
+            void mutate();
+        public:
+            explicit gp_population_t(blt::thread_pool<true>& pool): pool(pool)
+            {}
+            
+            void init_pop();
+            
+            void execute(const fitness_eval_func_t& fitnessEvalFunc);
+            
+            void breed_new_pop();
     };
     
     class gp_system_t
@@ -36,7 +52,7 @@ namespace fb
         private:
         
         public:
-            gp_system_t(type_engine_t& types): types(types)
+            explicit gp_system_t(type_engine_t& types): types(types)
             {}
         
         private:
diff --git a/include/lilfbtf/tree.h b/include/lilfbtf/tree.h
index 1c4facc..3fcc798 100644
--- a/include/lilfbtf/tree.h
+++ b/include/lilfbtf/tree.h
@@ -173,7 +173,7 @@ namespace fb
             static tree_t make_tree(detail::tree_construction_info_t tree_info, blt::size_t min_depth, blt::size_t max_depth,
                                     std::optional<type_id> starting_type = {});
             
-            detail::tree_eval_t evaluate();
+            detail::tree_eval_t evaluate(const fitness_eval_func_t& fitnessEvalFunc);
             
             blt::size_t depth();
             
@@ -191,6 +191,7 @@ namespace fb
             {
                 blt::size_t depth = 0;
                 blt::size_t node_count = 0;
+                detail::fitness_results fitness;
                 bool dirty = true;
             } cache;
     };
diff --git a/libs/BLT b/libs/BLT
index 9bba525..16641a2 160000
--- a/libs/BLT
+++ b/libs/BLT
@@ -1 +1 @@
-Subproject commit 9bba525b1f33495a68c43fa1f66c0a239220e68c
+Subproject commit 16641a27cbe6fccb8f6a6518d903432311cbeb81
diff --git a/src/system.cpp b/src/system.cpp
index 1341cd7..87a96a5 100644
--- a/src/system.cpp
+++ b/src/system.cpp
@@ -19,5 +19,28 @@
 
 namespace fb
 {
-
+    void fb::gp_population_t::crossover()
+    {
+    
+    }
+    
+    void fb::gp_population_t::mutate()
+    {
+    
+    }
+    
+    void fb::gp_population_t::init_pop()
+    {
+    
+    }
+    
+    void fb::gp_population_t::execute(const fitness_eval_func_t& fitnessEvalFunc)
+    {
+    
+    }
+    
+    void fb::gp_population_t::breed_new_pop()
+    {
+    
+    }
 }
\ No newline at end of file
diff --git a/src/tree.cpp b/src/tree.cpp
index 06d7158..b683811 100644
--- a/src/tree.cpp
+++ b/src/tree.cpp
@@ -63,7 +63,7 @@ namespace fb
         return tree;
     }
     
-    detail::tree_eval_t tree_t::evaluate()
+    detail::tree_eval_t tree_t::evaluate(const fitness_eval_func_t& fitnessEvalFunc)
     {
         using detail::node_t;
         std::stack<node_t*> nodes;
@@ -87,6 +87,8 @@ namespace fb
             node_stack.pop();
         }
         
+        cache.fitness = fitnessEvalFunc(root);
+        
         return {root->type.getValue(), root->type.getType()};
     }