{
  "displayName": "Example Species",
  "speciesDescription": "A versatile custom lineage created via JSON. Grants speed and basic survival gear upon selection.",
  "speciesID": "example_species",
  "order": 10,
  "hasSubspecies": false,
  "isSubspeciesOf": "human",
  "locksOnSelect": true,
  "artDirectory": "textures/example_species_art.png",
  "onSelect": {
    "type": "command",
    "command": [
      "/give @s minecraft:iron_sword 1",
      "/effect give @s minecraft:speed 9999 0 true"
    ]
  }
}

Java Code Example:
NEOFORGE BELOW

//How a Human is registered in NeoForge 1.21.1
public static final DeferredHolder<Species, Species> HUMAN = SPECIES.register(
            "human",
            () -> new Species(
                    "Human",
                    "All-Rounder Species, your Classic Minecraft Experience.",
                    "human",
                    true,
                    player -> {},
                    null,
                    Integer.MIN_VALUE
            )
    );
//How a Normal Human is registered in NeoForge 1.21.1
    public static final DeferredHolder<Species, Species> NORMAL_HUMAN = SPECIES.register(
            "normal_human",
            () -> new Species(
                    "Normal Human",
                    "All-Rounder Species, your Classic Minecraft Experience.",
                    "normal_human_sub",
                    false,
                    player -> {},
                    null,
                    Integer.MIN_VALUE
            )
    );
// Brand New LOCKED Species feature, where if it gets selected it gets locked for everyone else until they are forced to choose another species
    public static final DeferredHolder<Species, Species> TEMP_LINEAGE = SPECIES.register(
            "temp_lineage",
            () -> new Species(
                    "Temp Lineage",
                    "A temporary species designed for testing lineage selection and locking mechanics.",
                    "temp_lineage",
                    false,
                    player -> {
                        player.sendSystemMessage(Component.literal("§aYou have selected the Temp Lineage!"));
                        player.addEffect(new MobEffectInstance(MobEffects.NIGHT_VISION, 6000, 0, false, false));
                    },
                    null,
                    1,
                    true
            )
    );

FORGE BELOW

public static final RegistryObject<Species> ELF = SPECIES.register(
        "elf",
        () -> new Species(
                "Elf",
                "Agile archers attuned with nature.",
                "elf",
                true,
                player -> {
                    // Example 1: Add potion effects directly
                    player.addEffect(new MobEffectInstance(MobEffects.MOVEMENT_SPEED, 999999, 0, false, false));
                    // Example 2: Give an item directly
                    player.getInventory().add(new ItemStack(Items.BOW));
                    // Example 3: Send a message or play a sound
                    player.sendSystemMessage(Component.literal("You have attuned with nature!"));
                },
                null,
                -1
        )
);

Made by Hivemere, aka me.

[FORGE - 1.20.1 / NEOFORGE - 1.21.1]