CHANGE OVER TIME

Charles Darwin proposed the principles of natural selection and explained this process's impact on the evolution of species many years ago. There is variation in inheritable characteristics within every species. Some of these characteristics may give groups of individuals within a population an advantage over others in surviving to maturity. These groups of individuals will be more likely to reproduce (because they survive due to the inherited trait) than the group without the inherited advantage. As a result, the inherited advantage is passed onto the next generation. Over several generations, the frequency of individuals that have inherited these favorable adaptive variations in the genes will tend to increase.


Test your Understanding

If giraffes once had short necks, how did they evolve into... long-necked giraffe populations? (Create your own explanation and then select the best explanation)

A The giraffes in the population that could reach the food source in trees survived to reproduce. When these populations reproduced, the next generations inherited the long neck. Over many generations, the giraffes with the longest necks passed on the genes for long necks.

B A giraffe stretched to reach the vegetation in the tall trees because food was needed to survive. This newly-acquired characteristic of a long neck was passed onto his offspring.

C Male giraffes "fight" for the "right" to mate. During these "battles, " the males slam their long necks into each other. Typically, the giraffes with the longest, thickest, strongest neck win the battle and are selected by the females for mating. The genes for a long, thick, strong neck are passed on to the next generation.

D The long neck of the giraffe populations arose from inbreeding with ostriches. Both of these species evolved long necks from a common ancestor population of ostichaffes. Over many periods of environmental isolation, the two separate species of giraffes and ostriches evolved.

click to check your thinking





















CHECK YOUR THINKING
A) Caught you thinking! This explanation is the most popular one found in textbooks on the subject. Scientists believe that genes for neck length came in many variations within the earliest populations of giraffes. Food served as the selection pressure. Those giraffes who could reach the food survived to reproduce and pass on genes for longer necks. The others died of starvation.

B) RETHINKING NEEDED:A popular belief of Lammarck--a scientist who thought that if you acquired a characteristic in your lifetime, you could pass it on to your offspring. For example, Lamark thought if you dyed your hair purple during your lifetime that you could pass this acquired trait onto your offspring. He neglected to think about the genes which determined your "true" hair color. How many people do you know that have purple hair as their "true" hair color?

C) Caught you thinking! As scientists have observed today's giraffe populations, they noticed how the giraffes attracted mates. Additionally, scientists have evidence that male giraffes have longer, thicker, stronger necks than the females. Currently, scientists are rethinking their "food" explanation for neck length in favor of the "fighting for a mate" explanation. Scientists believe that the genes for neck length came in many variations within earliest populations of giraffes. Since the males with the longest, strongest, thickest necks won these contests, their genes for neck length were inherited by the next generation of giraffes.

D) OFF TASK!! Not sure what is being stated here??? Are You???