Directions: After reading the passage below, fill in each blank with a proper word given in the box. Each word can only be used once. Note that there is one word more than you need.

A. eventually        B. certain        C. evolution        D. particular        E. passed        F. likely G. change             H. fitted        I. happened          J. controlled          K. occasionally

Today we understand much more about how evolution works because we know about genes and DNA. Darwin didn’t know about genes and DNA. Gregor Mendel was the first person to work out how 【小题1】 characteristics such as colour (of eyes, hair, skin, flowers, leaves or bark) were 【小题2】 from one generation to the next.

Characteristics such as eye colour are 【小题3】 by genes. They are like a recipe written into the DNA of each cell, telling the cell what it should be and do. These “recipes” are passed on from parents to offspring through the DNA in egg and sperm cells.

Every now and then something goes wrong in the recipe, and this 【小题4】 is called a mutation. It means that the offspring will be different in some way from the parents. Often mutations are fatal and the offspring don’t survive, but 【小题5】 a mutation gives the offspring an advantage. It helps them fit into their environment better than others who lack the mutation. It allows them to compete successfully for scarce water, food, light, space or mates.

Darwin realized that those individuals who suited their environment best would be more 【小题6】 to survive and have lots of offspring. Most of the offspring would have the same mutation, making them fit in well with their environment, too. They in turn would have lots of offspring, and 【小题7】 , over a very long time, the offspring with the mutation would outnumber or replace those without the mutation. If enough of these changes or mutations occur, a new kind (or species) of animal or plant could eventually be created. This process is called 【小题8】.

When some living things survive whilst others do not, we call this process natural selection. It’s as if nature selects the things that best suit the world at that 【小题9】   time and place. For example, in a very cold climate, creatures with an extra thick layer of fat to keep them warm would have a better chance of surviving than thin creatures. Consequently, fat creatures would be “fitter” than thin creatures because they 【小题10】 into their environment better. This process is also called the “survival of the fittest”.