diff --git a/app/seed/__init__.py b/app/seed/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/app/seed/evolution_tracks.json b/app/seed/evolution_tracks.json
new file mode 100644
index 0000000..a4bd1f0
--- /dev/null
+++ b/app/seed/evolution_tracks.json
@@ -0,0 +1,5 @@
+[
+  {"name": "Batter", "card_type": "batter", "formula": "pa+tb*2", "t1": 37, "t2": 149, "t3": 448, "t4": 896},
+  {"name": "Starting Pitcher", "card_type": "sp", "formula": "ip+k", "t1": 10, "t2": 40, "t3": 120, "t4": 240},
+  {"name": "Relief Pitcher", "card_type": "rp", "formula": "ip+k", "t1": 3, "t2": 12, "t3": 35, "t4": 70}
+]
diff --git a/app/seed/evolution_tracks.py b/app/seed/evolution_tracks.py
new file mode 100644
index 0000000..178f68e
--- /dev/null
+++ b/app/seed/evolution_tracks.py
@@ -0,0 +1,41 @@
+"""Seed data fixture for EvolutionTrack.
+
+Inserts the three universal evolution tracks (Batter, Starting Pitcher,
+Relief Pitcher) if they do not already exist.  Safe to call multiple times
+thanks to get_or_create — depends on WP-01 (EvolutionTrack model) to run.
+"""
+
+import json
+import os
+
+_JSON_PATH = os.path.join(os.path.dirname(__file__), "evolution_tracks.json")
+
+
+def load_tracks():
+    """Return the locked list of evolution track dicts from the JSON fixture."""
+    with open(_JSON_PATH) as fh:
+        return json.load(fh)
+
+
+def seed(model_class=None):
+    """Insert evolution tracks that are not yet in the database.
+
+    Args:
+        model_class: Peewee model with get_or_create support.  Defaults to
+            ``app.db_engine.EvolutionTrack`` (imported lazily so this module
+            can be imported before WP-01 lands).
+
+    Returns:
+        List of (instance, created) tuples from get_or_create.
+    """
+    if model_class is None:
+        from app.db_engine import EvolutionTrack as model_class  # noqa: PLC0415
+
+    results = []
+    for track in load_tracks():
+        instance, created = model_class.get_or_create(
+            card_type=track["card_type"],
+            defaults=track,
+        )
+        results.append((instance, created))
+    return results
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/test_evolution_seed.py b/tests/test_evolution_seed.py
new file mode 100644
index 0000000..8aed49c
--- /dev/null
+++ b/tests/test_evolution_seed.py
@@ -0,0 +1,119 @@
+"""Tests for the evolution track seed data fixture (WP-03).
+
+Unit tests verify the JSON fixture is correctly formed without touching any
+database.  The integration test binds a minimal in-memory EvolutionTrack
+model (mirroring the schema WP-01 will add to db_engine) to an in-memory
+SQLite database, calls seed(), and verifies idempotency.
+"""
+
+import pytest
+from peewee import CharField, IntegerField, Model, SqliteDatabase
+
+from app.seed.evolution_tracks import load_tracks, seed
+
+# ---------------------------------------------------------------------------
+# Fixtures
+# ---------------------------------------------------------------------------
+
+_test_db = SqliteDatabase(":memory:")
+
+
+class EvolutionTrackStub(Model):
+    """Minimal EvolutionTrack model for integration tests.
+
+    Mirrors the schema that WP-01 will add to db_engine so the integration
+    test can run without WP-01 being merged.
+    """
+
+    name = CharField()
+    card_type = CharField(unique=True)
+    formula = CharField()
+    t1 = IntegerField()
+    t2 = IntegerField()
+    t3 = IntegerField()
+    t4 = IntegerField()
+
+    class Meta:
+        database = _test_db
+        table_name = "evolution_track"
+
+
+@pytest.fixture(autouse=True)
+def _db():
+    """Bind and create the stub table; drop it after each test."""
+    _test_db.connect(reuse_if_open=True)
+    _test_db.create_tables([EvolutionTrackStub])
+    yield
+    _test_db.drop_tables([EvolutionTrackStub])
+
+
+# ---------------------------------------------------------------------------
+# Unit tests — JSON fixture only, no database
+# ---------------------------------------------------------------------------
+
+
+def test_three_tracks_in_seed_data():
+    """load_tracks() must return exactly 3 evolution tracks."""
+    assert len(load_tracks()) == 3
+
+
+def test_card_types_are_exactly_batter_sp_rp():
+    """The set of card_type values must be exactly {'batter', 'sp', 'rp'}."""
+    types = {t["card_type"] for t in load_tracks()}
+    assert types == {"batter", "sp", "rp"}
+
+
+def test_all_thresholds_positive_and_ascending():
+    """Each track must have t1 < t2 < t3 < t4, all positive."""
+    for track in load_tracks():
+        assert track["t1"] > 0
+        assert track["t1"] < track["t2"] < track["t3"] < track["t4"]
+
+
+def test_all_tracks_have_non_empty_formula():
+    """Every track must have a non-empty formula string."""
+    for track in load_tracks():
+        assert isinstance(track["formula"], str) and track["formula"].strip()
+
+
+def test_tier_thresholds_match_locked_values():
+    """Threshold values must exactly match the locked design spec."""
+    tracks = {t["card_type"]: t for t in load_tracks()}
+
+    assert tracks["batter"]["t1"] == 37
+    assert tracks["batter"]["t2"] == 149
+    assert tracks["batter"]["t3"] == 448
+    assert tracks["batter"]["t4"] == 896
+
+    assert tracks["sp"]["t1"] == 10
+    assert tracks["sp"]["t2"] == 40
+    assert tracks["sp"]["t3"] == 120
+    assert tracks["sp"]["t4"] == 240
+
+    assert tracks["rp"]["t1"] == 3
+    assert tracks["rp"]["t2"] == 12
+    assert tracks["rp"]["t3"] == 35
+    assert tracks["rp"]["t4"] == 70
+
+
+# ---------------------------------------------------------------------------
+# Integration test — uses the stub model + in-memory SQLite
+# ---------------------------------------------------------------------------
+
+
+def test_seed_is_idempotent():
+    """Calling seed() twice must not create duplicate rows (get_or_create).
+
+    First call: all three tracks created (created=True for each).
+    Second call: all three already exist (created=False for each).
+    Both calls succeed without error.
+    """
+    results_first = seed(model_class=EvolutionTrackStub)
+    assert len(results_first) == 3
+    assert all(created for _, created in results_first)
+
+    results_second = seed(model_class=EvolutionTrackStub)
+    assert len(results_second) == 3
+    assert not any(created for _, created in results_second)
+
+    assert EvolutionTrackStub.select().count() == 3