Figure 2.25: These leaves in the top and middle photographs are from two different species
trees in the Acer, or maple, genus. One of the characteristics of the maple genus is winged
seeds (bottom). ( 28 )
species. For example, tigers and lions can mate in zoos and produce kittens that are half
tiger and half lion. But we still consider tigers and lions separate species. The two cats look
and behave differently and are not known to interbreed in the wild, even though they can.
Groups of lions and tigers do not interbreed.
Domains of Life
All life can be divided into 3 domains: Bacteria, Archaea, and Eukarya (Figures2.27,2.28
and2.29). This is the largest and least specific classification, so the organisms might not
look much alike, but they do have some very important traits in common. For example, you
might be surprised that mushrooms, plants, and people are all in the same domain. But
when you look at the cells of mushrooms, plants, and people, you will see that they do have
some similar features. They are all eukaryotic organisms, or in the domain Eukarya. The
other two domains are composed of prokaryotic organisms. Prokaryotic and eukaryotic cells
will be discussed in the chapter titledCells and Their Functions.
All the cells in the domainEukaryakeep their DNA inside a membrane, a structure called
the nucleus. The cells of other domains have DNA, but it is not inside a nucleus. The
domain Eukarya is made up of four diverse kingdoms: plants, fungi, animals, and protists.
Plants, such as trees and grasses, survive by capturing energy from the sun, a process called
photosynthesis. Animalssurvivebyeatingotherorganismsortheremainsofotherorganisms.
Animalsrangefromtinywormstoinsects, dogs, andthelargestdinosaursandwhales. Fungi,
such as mushrooms and molds, also survive by eating other organisms or the remains of other
organisms. The last group listed here are theprotists. Protists are not all descended from
a single common ancestor in the way that plants, animals, and fungi are. Protists are a sort
of miscellaneous group; they are all the organisms that are not something else. Protists are
a diverse group of organisms that include many kinds of microscopic one-celled organisms,