Evidence for Evoluti on

Evolution

“the gradual change in the characteristics of a species over many generations”

Evidence

• People still do not believe it• Evidence must be provided in support• 2 types of evidence– Fossil– Comparative studies

Fossils

A fossil does not have to be a part of an organism. Any preserved trace left by an organism that lived long ago is a fossil. Fossils may therefore include footprints, burrows, faeces or impressions of all or part of an animal or a plant, as well as bones, shells or teeth.

Evolution of the Horse

Fossil Formation• Very rare• may become fossilised when they are buried by drifting sand, mud

deposited by rivers, volcanic ash or by other members of the species.

• soil is very important– In wet, acid soils minerals in the bone dissolved and no fossilisation

occurs. However, if this soil contains no oxygen, as in the case of peat, complete preservation of the soft tissues of the animal as well as the bones may occur.

– Alkaline soils produce best fossils as minerals in bones are not dissolved. New minerals, often lime or iron oxide, are deposited in pores of bone, replacing organic matter that makes up about 35% by weight of the bone. The bone becomes petrified (turned into rock) but the details of structure are still preserved

Dating Fossils

• Absolute- actual age– Potassium Argon Dating– Carbon 14/ Radiocarbon dating– Tree ring dating/ dendrochronology

• Relative- older or younger– Stratigraphy – Fluorine Dating

Potassium Argon Dating

• Based on the decay of radioactive potassium to form calcium and argon.

• Isotope potassium-40 is radioactive and decays to form calcium-40 and argon-40.

• Decay takes place at an extremely slow but constant rate

• has limited usefulness: can only date rocks older than 100 000 to 200 000years.

Carbon 14/ Radiocarbon Dating

• based on the decay of the radioactive isotope of carbon, carbon-14, to nitrogen

• Half life of 5730 -/+ 40 years• After about 70 000 years, quantity of carbon-14 left

is negligible thus radiocarbon dating cannot be used to date back more than about 60 000 years.

• A further limitation is that the material to be dated must be of organic origin,that is, it must contain carbon.

Radioactive Decay of Carbon 14

Half life of 5730 -/+ 40 years

Tree Ring Dating/ Dendrochronology

• Concentric rings on the surface of a cut tree trunk.

• Each ring represents one year’s growth • Rings differ in width according to how

favourable the growing season was. • Marker rings

Tree Ring Dating/ Dendrochronology

The bristle cone pine, which grows in the Sierra Nevada in the United States, has been of great value in tree ring dating. By taking sample cores from the trunks and counting growth rings, some of the living trees have been found to be over 4500 years old. By correlating marker rings in these trees with those of dead pines, accurate dates as far back as 8600 years ago have been established. It has been possible to use wood

Tree Ring Dating/ Dendrochronology

Stratigraphy

• Study of layers or strata• Two ways it can be applied– principle of superposition– correlation of rock strata

Principle of Superpositon

• assumes that layers of sedimentary rock the layers at the top are younger

• than those beneath them. • Thus, any fossils or other material found in the top

layers will be younger than material found lower down (Fig. 17.9).

• This principle must be applied with care- distortions of the earth’s crust occur and it is possible for fossils or artefacts to be buried by animals

Correlation of Rock Strata

• matching layers of rock from different areas.

Fluorine Dating

• When bone is left in soil, fluoride ions, which are present in the water in the soil, replace some of the ions in the bone itself.

• All the fossil bones in a particular deposit should contain the same amount of fluoride, so that fossils that have been displaced can be detected.

• The older the fossil the more fluoride it contains, so that the relative ages can be established.

• It is not possible to decide absolute ages using this method since the concentration of fluoride in ground water varies from place to place and from time to time.

Problems with Fossil Record

• For fossils to be formed, four conditions are usually required:– a quick burial of the material– the presence of hard body parts– an absence of decay organisms– a long period of stability—the organism needs to

be left undisturbed• So fossilisation is usually by chance

Problems with Fossil Record

• Very few fossils have been discovered• Unusual to find fossil of entire organism• Dating techniques not 100% accurate

Comparative Studies

• Homologous features• *Analogous features• Comparative embryology• Vestigial structures• Phylogenic trees• Genetic Comparisons• Molecular Comparisons• DNA sequencing

Homologous Features

• Fundamental similarity or structure– Land plants- seeds of cycads, Ginkgo, conifer trees and flowering

plants show a variety of shapes and sizes but they are the same basic structure

– Mammals- classic examples is the forelimb of vertebrates.• The same bones appear in various forms throughout the vertebrates—

the feet of amphibians and reptiles, the wings of bats and birds, the leg of a horse, the flipper of a whale or seal, and the human hand.

• bones are arranged in a similar way, even though some have developed different functions.

– Organisms possessing organs that are similar in structure are likely to have a common ancestor. Therefore, the arrangement of the bones of the forelimb in such a range of vertebrates is convincing evidence that they have all evolved from a common ancestor.

*Analogous Features

• Serve same function but have evolved separately

• Must distinguish between Analogous and Homologous

Comparative Embryology

• von Baer’s law- features common to all members of a group of animals are developed early in the embryo and that more special features, which distinguish the different members of a group, develop at a later stage of development

Comparative Embryology

• Vertebrates-Dorsal brain and spinal cord, axial skeleton, gill or pharyngeal slits, and aortic arches develop early.

• Special features like fins, fur or feathers, which distinguish classes, develop later

Vestigial Structures

• Reduced structure with no apparent function but is evidence of evolutionary relationships– The pelvic bones of whales are reduced and

functionless– She-oaks have small, brown scales that have no

obvious function but are considered to be reduced leaves

Phylogenic Trees

• Based on the sharing of homologous features, is a branching diagram showing how organisms are related and how they have diverged during evolution

• “phylogeny”- the pattern of how organisms are related through evolutionary descent from a common ancestor