Question
i know that the carrot was developed from a common european weed. how can one change a plant to be different. im talking about changing variety,...

Answer
Hi Mishael,
Thanx for your question. I'm not a biologist, geneticist, botanist and so my knowledge is not as scientifically comprehensive as an expert's in those fields.  I am a horticultural hobbyist.

Basically, here's the way it goes as I understand it.  Plants have a sexual process for reproduction just has humans do.  There is an egg (seed# and there is sperm #pollen#.  Plants are fertilized by insects, animals, wind, by themselves, seeds develop embryos and a plant germinates and grows to maturity and blooms and is pollinated and produces seed and therefore reproduces itself.

Mankind has been taking the best plants and breeding them for thousands of years resulting in better plants.  Plants that have more production, larger kernels, better adapted to heat or cold, or wetness or drought, plants that are easier to harvest, harvest earlier or later, sweeter, less sour, you get the picture.

Basically, what modern day scientists do is pick out plants with that have the most desirable characteristics and breed those plants with each other to get closer to the desired end.  

Over the millennia, Mexicans developed corn from a grass that grew in the mountains of Mexico.  Middle Easterners developed wheat from grass growing in the Middle  East.  Peaches, nectarines and plums are thought to have been developed from coin-sized wild fruit in China and Persia.  Tulips came from turkey.  Chiles whether from New Mexico, Peru, Mexico, India or Cameroon, all started as a small pea sized fruit in Latin America.  Potatoes originated in the mountains of the Andes.  People took these and developed them into new and improved vegetables that could feed them more effectively.  Corn, an American original, as saved the world many times over.  Peppers, another plant native to the Americas is used now, by just about every culture in the world, so are tomatoes.  Potatoes are now a staple in Ireland, Britain, Scandinavia, Germany, Poland and Russia.

Here is a good link to what a hybrid is.

http://en.wikipedia.org/wiki/Hybrid_#biology)

Here's one on plant hybridization.  I forgot...Gregor Mendel, look him up.  He did a lot of research using peas in his study of genetics.

http://en.wikipedia.org/wiki/Experiments_on_Plant_Hybridization

Mishael, you will find that independent research of the Internet can be quite lucrative and satisfying AND, you don't have to rely or wait on someone else for the answer.  If the answers you find appear to be too complicated, consult with a teacher.  If the teacher is unwilling to help or you still have trouble, please let your parents or a school counselor or another teacher know.  I remember school days and I remember there were some teachers more helpful than others.  Your education is important and should be affirmed and supported by your family and your school authorities.

Good luck to you and happy researching!

I hope this helps.

Tom