Evidence for Evolution Subtitle

The Fossil Record Fossils are preserved remains of ancient organisms and are usually found in sedimentary rocks. The layers of rock follow a geological time scale where the lower layers of rock are older than the upper layers of rock, as long as they haven’t been disturbed through earthquakes or tectonic plate movement. Since the lower layers of rock are older, scientist can determine the sequence of changes in an organism based on the fossils found in different layers

This picture shows the changes in a single species of snail This picture shows the changes in a single species of snail. (Many of the forms are now extinct.)

DNA & Embryology Similarities in the structure and development of embryos provide evidence of descent from a common ancestor.

During the embryonic development stage very different species show similarities. These similarities during certain embryological stages indicate that similar genes are at work. The genes that are active during early development of fish, birds, humans and related animals all shared a common ancestor whose particular sequence of genes controlled its early development. Much like the development of today’s organisms mentioned above. This particular sequence of genes has thus been passed on to the species that descended from it.

As the embryos grow and develop the embryos become more and more different. These differences are caused by genes that have changed over the course of evolution (through mutation). If the organism survives then the DNA, or genetic code, is passed along to surviving offspring. All organisms share many biochemical compounds and they all use DNA and or/RNA as the genetic code to pass on genetic information from on generation to the next and to control growth and development. Therefore the more closely related two species are, the more closely their chemical compounds, such as DNA, resemble each other.

Comparative Anatomy– Homologous Structures The evolutionary relationships between groups of organisms are determined mainly by structural similarities called homologous structures. These homologous structures suggest that these organisms all descended from a common ancestor. For example the bones of the forelimb of air-breathing vertebrates are composed of similar bone arrangements in a comparable pattern.

Analogous Structures Some organisms have structures that are similar in appearance and function, but they have different origins. They usually have very different internal structures. These structures are called analogous structures. For example, birds, butterflies and bats all have the ability to fly using wings. However when look closer at the structures of these wings you will see that the internal structures are completely different. A butterfly’s wing is made of thin nonliving membranes, a birds wing is made of skin, muscles and bones; and a bats wing is made of skin stretched between elongated finger bones.

Vestigial Organs Some organisms have organs or appendages that don’t have any apparent purpose. These are called vestigial organs. Many of these organs or structures are “leftover” from previous ancestors. For example, humans have an appendix that serves no purpose in digestion today. However, a large functioning appendix is found in animals that primarily eat plant materials, such as the koala.