Biology 13.2 Evidence of Evolution

Biology 13.2 Evidence of Evolution Fossil Records Evolution Have you ever looked at a series of maps that show how a city has developed over time? Buildings and streets are added over time, changed, or destroyed. In the same way, fossils of animals show a pattern of development from early ancestors to modern day organisms.

Biology 13.2 Evidence of Evolution Fossil Records Evolution Fossils provide a record of past life forms and their stages of evolution. The age of the fossil shows where that organism was at that particular stage of it’s development to what it now is today.

Biology 13.2 Evidence of Evolution Fossil Records Evolution Fossils of animals show a pattern of development from early ancestors to modern descendents. Fossils are the most direct evidence that evolution takes place. Recall that a fossil is the preserved or mineralized remains or imprint of an organism that lived long ago. Fossils therefore provide an actual record of Earth’s past life-forms. Change over time can be found in fossilized records; species found in older rocks are different than species found in newer age rocks.

Biology 13.2 Evidence of Evolution Fossil Records Evolution After observing differences, Darwin predicted that intermediate forms between the groups of organisms would eventually be found. Since Darwin’s time, some of the intermediate steps have been found; some have not. For example, intermediate steps have been found between fishes and amphibians, between reptiles and birds, and between reptiles and mammals.

Biology 13.2 Evidence of Evolution Today, scientists agree that Darwin’s theory is the best available explanation for he biological diversity on Earth. Based on a large body of supporting evidence, most scientists agree on the following three major points Earth is about 4.5 billion years old Organisms have inhabited Earth for most of it’s history All organisms living today share common ancestry with earlier simpler life-forms

Biology 13.2 Evidence of Evolution Formation of fossils: The fossil record, and thus the record of life on Earth, is not complete. Many species have lived in environments where fossils do not form. Most fossils form when organisms are buried in fine sediments deposited by wind, water, or volcanic eruptions.

Biology 13.2 Evidence of Evolution Formation of fossils: The environments that are most likely to cause fossil formation are wet lowlands, slow-moving streams, lakes, shallow seas, and areas near volcanic eruptions. The chances that organisms living in upland forests, mountains, grasslands, or deserts will die in the right spot to be fossilized is very low.

Biology 13.2 Evidence of Evolution Even if an organism lives in a environment where fossils can form, the chances are slim that it’s dead body will be buried in sediment before it decays. It is very likely to be eaten and scattered by scavenges. The bodies of some organisms decay faster than others do. For example, a creature with a hard exoskeleton, like a crab, would have a better chance of becoming fossilized than a soft-bodied organism like an earthworm.

Biology 13.2 Evidence of Evolution Because of these reasons, fossil records will never be entirely complete. Fossils still present us with the best evidence that evolution has taken place however. When a fossil is discovered, paleontologists (scientists who study fossils) analyze the sediments around it.

Biology 13.2 Evidence of Evolution By radiometric dating techniques (measuring half lives of surrounding rocks), paleontologists can arrange the fossils in order from oldest to youngest. When this is done, orderly patterns of change through evolution can be seen clearly.

Biology 13.2 Evidence of Evolution At right we see the proposed evolution of the modern whale. Scientists have pieced this evolution together by looking at series of fossils and trying to place them in an order that shows the development.

Biology 13.2 Evidence of Evolution Scientists still may disagree on whether or not a particular evolutionary path is correct, but as new fossils are discovered and fill in gaps, theories are adjusted to reflect new evidence. Sometimes theories are proved, some disproved, by new evidence.

Biology 13.2 Evidence of Evolution Anatomy and development: Comparisons of the anatomy of different types of organisms often reveal basic similarities in body structures even though the structures functions may differ between organisms. Vestigial hip bones in modern whales show that whales descended from creatures that once walked on land

Biology 13.2 Evidence of Evolution Anatomy and development: Sometimes bones are present in an organism but are reduced in size and either have no use or a less important use than is found in other related organisms. These structures are considered to be evidence of an organism’s evolutionary past, and are called vestigial structures. Vestigial hip bones in modern whales show that whales descended from creatures that once walked on land

Biology 13.2 Evidence of Evolution Homologous Structures: As different groups of vertebrates evolved, their bodies evolved differently. But similarities in bone structure can still be seen, suggesting all vertebrates share a common ancestor.

Biology 13.2 Evidence of Evolution Homologous Structures: In the diagrams at right, you can see that the forelimbs of vertebrates are composed of the same basic groups of bones. We call these structures homologous. Homologous structures are structures that share a common ancestry.

Biology 13.2 Evidence of Evolution Most scientists believe that the evolutionary history of organisms is also seen in the development of the embryos. At some time in their development, all vertebrate embryos have a tail, buds that become limbs, and pharyngeal pouches. The tail remains in most adult vertebrates.

Biology 13.2 Evidence of Evolution Biological molecules The picture of change seen in fossil records allows scientists to make predictions that can be tested. If species have changed over time, than the genes that determine the specie’s characteristics should also have changed by mutation and selection. As species evolved, one change after another should have become part of their genetic instructions. Therefore, changes in a gene’s nucleotide sequence (DNA) should build up over time.

Biology 13.2 Evidence of Evolution This prediction was first tested by analyzing amino acid sequences of similar proteins found in several species. If evolution had taken place, than species that are more recent ancestors should have more amino acids in common than ancestors that are more distant in time on the evolution scale. For example; you would have more genes in common with your grandparents than with your great-great-great grandparents. You are more closely related to them.