Evolution

Evolution…the definition Biological evolution… is descent with modification helps us understand the history of life It is the change in the inherited traits of a population from generation to generation.

The Explanation Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren't examples of biological evolution because they don't involve descent through genetic inheritance.

Acquired vs. Inherited Traits 1. Make two columns and title the first one Inherited Characteristics. Title the second column Acquired Characteristics. In the first column, list the characteristics that you believe you have always had. 2. In the second column, list your acquired characteristics. 3. Which of the items in your lists do you think you might pass on to your children? Explain your answer.

Lines of Evidence Fossil Evidence The fossil record provides snapshots of the past that, when assembled, illustrate a panorama of evolutionary change over the past four billion years. The picture may be smudged in places and may have bits missing, but fossil evidence clearly shows that life is old and has changed over time.

Transitional Forms Fossils or organisms that show the intermediate states between an ancestral form and that of its descendants are referred to as transitional forms. There are numerous examples of transitional forms in the fossil record, providing an abundance of evidence for change over time.

Pakicetus (below left), is described as an early ancestor to modern whales. Although pakicetids were land mammals, it is clear that they are related to whales and dolphins based on a number of specializations of the ear, relating to hearing. The skull shown here displays nostrils at the front of the skull. A skull of the beluga whale that roams the seas today (below right) has its nostrils placed at the top of its skull. It would appear from these two specimens that the position of the nostril has changed over time and thus we would expect to see intermediate forms.

Comparative Anatomy Organisms that are closely related to one another share many anatomical similarities. Sometimes the similarities are conspicuous, as between crocodiles and alligators, but in other cases considerable study is needed for a full appreciation of relationships.

Homologies Evolutionary theory predicts that related organisms will share similarities that are derived from common ancestors. Similar characteristics due to relatedness are known as homologies. Homologies can be revealed by comparing the anatomies of different living things, looking at cellular similarities and differences, studying embryological development, and studying vestigial structures within individual organisms.

Anatomy Vestigial Structures Features that apparently serve no function in an organism and are allegedly holdovers from an evolutionary past. Such features, though no longer useful (as far as we can tell), are presumed to have been useful in ancestral species. Some examples of vestigial structures include the whale pelvis, tiny snake pelvic and limb bones, and the eyes in cave-dwelling salamanders and fish that are completely blind.

Homologous Structures (plants) In the following photos of plants, the leaves are quite different from the “normal” leaves we envision. Each leaf has a very different shape and function, yet all are homologous structures, derived from a common ancestral form. The pitcher plant and Venus’ flytrap use leaves to trap and digest insects. The bright red leaves of the poinsettia look like flower petals. The cactus leaves are modified into small spines which reduce water loss and can protect the cactus from herbivory.

Homologous Structures (animals) Another example of homology is the forelimb of tetrapods (vertebrates with legs). Turtles, birds, alligators and mammals all have different forelimbs, reflecting their different lifestyles. But those different forelimbs all share the same set of bones - the humerus, the radius, and the ulna. These are the same bones seen in fossils of the extinct transitional animal, ancient lobed-finned fish, which demonstrates their common ancestry. Turtle Alligator Bird Mammal Ancient lobe-finned fish

Modification of the tetrapod skeleton Whales and hummingbirds have tetrapod skeletons inherited from a common ancestor. Their bodies have been modified and parts have been lost through natural selection, resulting in adaptation to their respective lifestyles over millions of years. On the surface, these animals look very different, but the relationship between them is easy to demonstrate. Except for those bones that have been lost over time, nearly every bone in each corresponds to an equivalent bone in the other. Compare anatomy of whale and humming bird

Developmental Biology Studying the embryological development of living things provides clues to the evolution of present-day organisms. During some stages of development, organisms exhibit ancestral features in whole or incomplete form.

Cellular & Genetic Evidence All organisms are made of cells, which consist of membranes filled with water containing genetic material, proteins, lipids, carbohydrates, salts and other substances. The cells of most living things use sugar for fuel while producing proteins as building blocks and messengers. Notice the similarity between the typical animal and plant cells pictured below — only three structures are unique to one or the other.

Genetic Evidence Different species share genetic homologies as well as anatomical ones. Roundworms, for example, share 25% of their genes with humans. These genes are slightly different in each species, but their striking similarities nevertheless reveal their common ancestry. In fact, the DNA code itself is a homology that links all life on Earth to a common ancestor. In some cases, if we were to transfer genetic material from the cell of one living thing to the cell of another, the recipient would follow the new instructions as if they were its own.