Science 20 Unit D: Living Systems
Evidence for Evolution
Evolution in Action
http://www.pbs.org/wgbh/nova/link/evolution.html
• It is impossible to directly observe what the Earth was like millions of years ago.
• This is why we refer to evolution as a theory, it can not be 100% confirmed, like a law.
A well-supported theory
• Evidence for evolution comes from many different areas of science:
• Some evidence comes from direct observation and experimentation.
• Some evidence is indirect, we only study the effects.
Evidence from Fossils• Direct Evidence• Scientists have discovered 250 000 fossil
species.• Bones, shells, footprints, burrows, even
chemical remains may form fossils.
The fossil record provides the following evidence:
(1) The complexity of organisms increases from past to present.
(2) Fossils appear in chronological order. Fossils found in young layers of rock are much more similar to species alive today than fossils found in deeper, older layers (Law of Superposition).
(3) Not all organisms appear in the fossil record at the same time. Very few of today’s species were alive even 1 million years ago. Most of the species that have lived are now extinct.
(4) Similar fossils are grouped geographically.
The Burgess Shale• One of the most important geological finds in
the world• Discovered in 1909 in British Columbia,
Canada• Marine fossils from 500 million years ago
during the “Cambrian Explosion” (a time of huge increase in Earth’s biodiversity)
• The fossils are very high quality - some showing micro-organisms and soft-bodied organisms
Transitional Fossils• Critics of evolutionary theory
question the “gaps” in the fossil record - for a long time, the fossil record was only “snapshots” of ancestral forms.
• Now… transitional fossils are continually being discovered - fossils that show intermediary links between groups of organisms.
(eg) present day whales and their terrestrial ancestors
• Ambulocetus natans - lived both on land and in water
• Walked like a sea lion, swam like an otter
• Dorudon sp. lived only in water
• Had a tiny pelvis and 10 cm legs
• Paleontologists are still working to “fill in the gaps”
• Many transitional species are predicted, but not yet found
Archaeopteryx
Characteristics of both reptiles and birds: feathers, teeth, claws, and a bony tail
http://www.pbs.org/wgbh/nova/microraptor/program.html
Evidence from Biogeography
• Biogeography is the study of the past and present geographical distribution of organisms.
• Many of the observations that Darwin used to develop his theory were based on biogeography.
Darwin hypothesized that species evolve in one location and then
spread out to other regions.
• Biogeography supports this hypothesis in the following ways:
(1) Geographical location has more influence on the relatedness of species than does environment.
• (eg) Desert and forest habitats of South America will have more closely related species than a desert in Africa and a desert in Australia.
• (eg) Cacti are native only to North, Central, and South American deserts.
(2) Animals found on islands closely resemble animals found on the nearest continent
(3) Fossils 150 million years and older are found evenly distributed over all the continents. “Younger” fossils are restricted to separate continents
(4) Closely related species are almost never found in exactly the same location or habitat.
• New species would only appear once the ancestor reached an environment that provided enough selective pressure for it to change significantly.
Evidence from Anatomy
• Vertebrate forelimbs have various functions, but all contain the same number and type of bones, organized in similar ways
• How is this possible? The basic vertebrate forelimb must have originated from a common ancestor
Homologous Structures• Homologous structures are those that have
similar structural origin (due to a common ancestor) but have evolved to fulfill different functions.
Analogous Structures
• Analogous structures perform similar functions but do not have a common evolutionary origin.
Vestigial Features
• Vestigial features are rudimentary structures that serve no useful function in the organism in which they are found.
• The features likely served some function in an ancient ancestor.
• Humans have vestigial features - the appendix, and wisdom teeth
Evidence from Biochemistry
• More closely related species have more similar protein structure.
• Geneticist have found large numbers of both homologous and vestigial genes in the DNA of all species.
• More closely related species have more similar sequences of DNA.
Evidence from Embryology
• The embryos of different organisms exhibit similar stages of embryonic development.
Evidence from Artificial Selection
• The fact that humans can use artificial selection to produce changes in a species over relatively short periods of time provides compelling evidence that similar (and even more dramatic) changes could occur in nature over millions of years.
http://www.pbs.org/wgbh/nova/programs/07.html
