Hello there, and welcome to the world of your defeat!
(Oak is a professor in the Pokemon games of Red and Blue, Green, and Yellow as well as their remakes of Leaf Green and Fire Red. In the beginning of Pokemon games, a professor typically introduces players to the world of Pokemon. In this case, however, Oak is introducing Darwin to his defeat in this rap battle)
Here's some trainer tips, I'm the very best. I cannot be beat!
(One very famous line from the original Pokemon theme song is " I want to be the very best, like no one ever was". Oak is claiming he's the very best and one one can defeat him, including Darwin)
You spent years on studying evolution on finches alone
(Darwin traveled to the Galapagos Islands, where, for many years, he studied evolution by studying the different types Finches found throughout the island. A Finch is a type of bird)
While I can top all your research with a single thunder-stone!
(Many Pokemon are capable of evolving to a more powerful form, but some require a evolutionary stone for it to happen. One example is a thunder-stone, which evolves certain electric type Pokemon. In the Pokemon world, evolution using a evolutionary stone happens instantly, unlike in real life, which takes millions of years. Oak says he can outclass Darwin's years of evolution research with a thunder-stone)
It's like I've got the highest speed stat in this rap
(In Pokemon games, the Pokemon with the higher speed stat goes first in a Pokemon battle. Darwin says he's the one with highest speed stat between him and Darwin in the rap battle)
You'll have to wait your turn
(Picking up from the previous line, Oak went first in this battle as a result from his higher speed stat)
Spitting firey rhymes every single time
(Oak states his rhymes will burn Darwin)
You'll need a rawest berry for those burns!
(Pokemon can suffer injuries, like burns, during battle. A Rawst Berry is a berry that heal burns)
You be a well known scientist
Recognised across the world
But I still cannot tell, are you a boy or a girl?
(Oak mocks Darwin in this line. In most of the Pokemon games, a professor, Oak included, asks the players if he/she is a boy or a girl)
I discovered an anomaly, of that no man can think
(Darwin established that all species evolved over time from a common ancestor. This anomaly was established at the time when creation by god was commonly accepted by many, whom never though of evolution)
Man evolved from the apes, and you must be the missing link!
(Missing link is a thing that is needed in order to complete a series, provide continuity, or gain complete knowledge. Darwin theorized that making evovled from apes and the "Chimpanzee–human last common ancestor" is the missing link that explains the last ancestor of humans before mankind finally became humans. As with evolution, when a specie evolved, it is better than its pre-evolutionary predecessors. Darwin looks down upon Oak, as a under classed, inferior specie than humans like himself)
You've had decades of research, and claim to be the very best
(Oak, currently 50, have studied Pokemon as a Pokemon professor for many years. Darwin also reminds Oak the line which Oak said he was the "very best")
Yet all you have to show from your experience is an empty pokedex!
(In the Pokemon games Oak appeared in, he stated he was once a serious Pokemon trainer, who set out to complete the Pokedex. However, he never completed it and, at his old age, asked the player and his/her rival to complete it)
And you're the better professor? HA!
(Darwin don't think Oak is the better professor)
That's just out of question
(Darwin says Oak is obviously not better, picking up from the previous line)
I was chosen to win this battle, but that's just natural selection!
(Darwin saids he's the "natural selection", or the ideal choice, to win the rap. Natural selection is part of Darwin's theory of evolution)
Bring your animal slaves, Oak
(Because Pokémon are kept in small Poké Balls and made to fight each other, Darwin considers them slaves to trainers)
You better reset the game
(Darwin says that Oak has failed and needs to "reset", restart. Resetting, restarting, the game is a common feature among the Pokemon games)
How can you win this battle
(Darwin challenges the idea that Oak will win the rap battle)
If you can't remember your own grandsons name!
(In the Pokemon games, the player can chose his rival's name after the Oak forgot his grandson's name)