wew.. and i thought this is a
basic knowledge that all pokemon players should know.. <img src="http://www.pokemon-world-online.net/forum/public/style_emoticons/<#EMO_DIR#>/PikachuExcited.png" class="bbc_emoticon" alt="
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kay, basically there are 4 things that affects pokemon stats :
1. Base Stats
2. IV
3. EV
4. Natures
Base stats are identical for each "species" of pokemon. i.e all alakazam have 135 base spatk, all snorlax have 160 base HP, etc..
IVs are like its DNA, to put it simple, IV is what makes 2 pokemons with the same species and level could have different stats
EV and natures aren't available in PWO yet so i won't say anything abt them atm
and, really these things are mandatory if u've played the handheld version before,,
and the stats of a pokemon is really just a simple math of the factors Lvl, base stats, IVs, EVs, and natures that actually is very easy to figure out (although u dont really put a lot of thought in it since there are already lots of stat calc programs)
so to answer the question : you can improve your poker "stats", just lvl it up (well duh.. )
evolving a pokemon wont change its IVs (or EVs), but it will change the base stats since the evolved form of a poke have better base stats then its pre-evolved form,,