Kingdoms=Group... Species=IndividualsScientists have all these complex ways of organizing living things. They look at your physical traits, how you develop as a fetus, or in an egg. They look to see if you are a plant.
Must Make MoreBut there is one big thing that helps them determine whether you are in the same species as another animal or plant. You are the same species as another being if you can do a really important thing. You can reproduce and have babies. Also, those babies have to be able to have more babies. They have to be fertile.
If you are a type of fern, scientists would know that another fern is the probably the same species if the two ferns can create a similar fern that is fertile. The same rules apply if you are a mouse. Not all mice can have fertile babies together. They can't have babies because there are different types of mice. And of course there is you. You can't have babies with a dog. You can't have babies with a horse. You can't have babies with a monkey that looks a lot like your brother or sister. You can only have offspring (babies) with other humans. You two would be the same species.
No Beginning and EndingSo remember, everything we talked about here describes a living system. Scientists are always re-evaluating and developing the classification system. Science will always change and evolve as new facts are discovered. Even the idea of a species is difficult.
There is no sure fire way to say, "Here is where one species ends and another begins." Some species are so closely related that they can have fertile offspring. Species are defined by man, and it is often difficult to make clear definitions when two organisms are incredibly similar. If those organisms reproduce and create a completely new line of creatures, those parents actually were not that different when it came time to mix up the genes.
It's Never Black and WhiteIn all of biology and the world, there is overlap. Coming up with definite black and white answers that always work rarely happens. You can get definite answers in math. You can even get definite answers in physics. Biology has many gray areas. Scientists will always debate and try to improve ideas and explanations. You may learn about ligers. Ligers are the offspring of lions and tigers but they are not fertile offspring. Those two organisms are incredibly similar, genetically, but their differences are enough to make them separate species.
Life on Other Worlds (NASA eClips Video)