EVOLUTION

Cardinal Scientific Principles

There are broad principles whose influence extends over a whole class of theories and which set down requirements these theories have to meet. They allow scientists to view natural phenomena and theories invented to explain them from a broadly unifying perspective. Here are some examples.

The Principle of Relativity: States that all theories must adhere to the rule that all observers, whether stationary or moving, must see the same laws of physics at work. This then is a sort of "theory about theories".

• Quantum Mechanics: Restricts the precision with which we can measure nature through Heisenberg's uncertainty principle. All theories of matter and energy must ultimately be quantum in nature.

• Gravity: Recognizes an attraction between matter, a force that dominates the universe.

• Evolution: Requires that the universe and objects within it change over time.

Theories Spawned by Cardinal Scientific Principles

A cardinal scientific principle will lead to more specific theories. A good example is gravity. Another is evolution.

Theories of Gravity

• Newton's theory of gravity proposed that all matter attracts all other matter through a force that that falls off as the square of the distance. This theory was extremely successful in computing gravitational effects and is still used today.

• Einstein's theory of general relativity proposed that space and time are warped by the presence of matter and that gravity was the manifestation of objects moving through space and time. This is one of the most precisely verified theories of physics.

• There have been other theories of gravity, such as the Brans-Dicke theory, but none has been as successful as Einstein's theory.

Evolution

Evolution is a far broader concept than just the idea of organic evolution in biology. Evolution can be viewed as a synthesis of the second law of thermodynamics with other laws of physics. According to this synthesis, systems have to evolve; they have no choice. Observation and experiment confirm the entire universe and all its systems are changing and evolving. Here are some examples, which can be thought of as particular evolutionary theories.

• Evolution of the physical Earth: The Earth is still in a process of differentiation as hot material rises and cool material falls. This process is responsible for the layering of the interior of the Earth and, in conjunction with erosive forces, for the evolution of the Earth's landscapes Eventually, the radioactive elements that provide most of the heat will decay, the earth will cool, and alteration of the Earth's interior and surface will slow accordingly.

• Evolution of stars: Studies of the Sun and other stars show they also evolve. They form from cold interstellar clouds, which contract by gravity to the point they are hot and dense enough that fusion of hydrogen into helium begins. At this point they become complex systems - full-fledged stars - often with planetary systems. Eventually they use up their hydrogen to become white dwarfs or supernovas.

• Biological evolution: The fossil record makes it clear that life has "evolved" in the sense that old forms disappear and new forms appear in the geologic record over time. Like gravity, there have been more than one theory to explain this. The currently favored theory is the one of Charles Darwin (natural selection modified and enhanced by new discoveries).

Theories of Biological Evolution

• Lamarck's Theory: Lamarck proposed that organisms acquire characteristics in their lifetime and pass them on. However, modern genetics shows that any acquired characteristics will not be passed along in the genetic code.

• Darwin-Wallace Theory: This theory proposes that every population has a range of characteristics, varying from individual to individual. Characteristics that provide a survival and reproductive advantage are more likely to be passed on than less advantageous ones. This concept is called natural selection and has so far withstood the test of time, observation, and experimentation.

Current Evolutionary Ideas

• In-Species Variation: The basic genetic unit, the gene, can come in many different forms (alleles) in a population. Given the number of genes and their variations, the possible combinations in a population of a species are enormous. Natural selection says that the variations more valuable to survival will tend to emerge over time.

• Mutations: Mutations are changes in genes caused by radiation, chemicals, or other mutagens. Some are apparently spontaneous in that there is no identified cause. Most are harmful or neutral, but occasionally one may be helpful. For example, a small part of a population may have a mutation that currently is not an advantage. However, if the environment changes, this may become an advantage such that they come to dominate the population. Or, an isolated population may have a mutation that helps them in their specific environment. Were this environment to become more widespread, they could expand their range and displace less well-adapted populations.

• Speciation:

  • The term "biospecies" has to do with living organisms. One biospecies does not interbreed with another biospecies in nature. The term morphospecies has to do with fossils. Fossils with certain physical characteristics are lumped together as a morphospecies. There is no guarantee a fossil morphospecies was actually a biospecies.

  • Allopatric speciation: Evolution of an isolated population into a new species due to local requirements for survival. Examples: Darwin's finches, shrimp isolated by Isthmus of Panama.

  • Phyletic gradualism: A term introduced by Niles Eldridge and Stephen J. Gould referring to the gradual and continuous evolution of one species into another. They attributed the idea to Charles Darwin, but that is disputed by many scientists, who think phyletic gradualism was not an idea proposed by Darwin. It appears that gradualism is most commonly expressed in the change in size of organisms over time. However, there are examples of apparent gradual change in the fossil record, such as the diatom, Rhizosolen, and the North American primate, Pelycodus.

  • Punctuated equilibrium: The theory proposed by Eldridge and Gould. Long periods of stasis (no significant change in a species) are "punctuated" by times of rapid evolution, possibly as fringe members of a population become rapidly dominant. An example is the Caribbean bryozoan, Metrarabdotos.

  • Punctuated gradualism: Basically the same as punctuated equilibrium, but not involving a branching into several new species.

  • The fossil record is incomplete because the sedimentary record is incomplete. For example, the rock layers in the Grand Canyon - even between recognized unconformities - cannot contain the complete depositional record. This can account for some, but probably not all, of the gaps in the fossil record. Most of the apparent gaps probably indicate punctuated equilibrium, or something similar. It has been joked that gradualism is evolution by "creeps", whereas punctuation is evolution by "jerks". Overall, evolution looks to have occurred as an erratic, "jerky" process.

• Patterns of Evolution

  • Divergent evolution: Ancestral species give rise to widely differing descendants. Example: Darwin's finches.

  • Convergent evolution: Development of similar characteristics in unrelated organisms. Example: Placental vs Marsupial evolution.

  • Parallel evolution: Development of similar characteristics in closely related organisms. Example: Orioles.

• Cladistics: Grouping together organisms according to derived, rather than primitive, characteristics. What is derived vs primitive depends on the situation. For example, the presence of a backbone does not help distinguish fish from mammals, but it does help distinguish fish from segmented worms. Cladogram: A "family tree" of related organisms.

  • Molecular systematics: Comparison of the DNA between organisms to determine relationships. This has shown some cladistic associations based upon characteristics to be in error, but has generally supported traditional cladistic analysis.

  • Molecular clock: Closely related to molecular systematics. Mutation rates are used to estimate when organisms may have diverged along different evolutionary lines in the past. Generally needs to be "calibrated" by using the fossil record. Example: Adaptive radiation of bats in the Eocene.

• Extinctions: Two general types of extinctions are recognized.

  • Background extinction: The continual extinction of species that find themselves "deselected" by natural selection. Estimates range from one to two million years for the average time span of a species.

  • Mass extinctions: Scientists are realizing that random violent events have had a huge impact on evolution. There has been a lot of resistance to this idea in the scientific community because it wasn't considered to be compatible with uniformitarianism. Some resistance is still seen in attributing the extinction of the dinosaurs and other creatures at the end of the Cretaceous period to an asteroid strike on the earth, in spite of the fact the crater has been located. However, most scientists now realize that evolutionary history has been radically changed by random events - some violent, some more gradual but still devastating.

Evidence of Evolution

Fossil succession: The observation that a similar progression of fossils from older to younger strata occurs worldwide. Evidence of biological change and speciation in fossils deposited during continuous sedimentation.

• Homologous structures: Similar structures adapted to different uses in related organisms. Example: Digestive systems follow "tube in a tube" pattern but have adapted to widely different sources of food.

• Vestigial structures: Non-functioning or partly functioning structures. Example: Vestigial hind legs in some snakes.

• Atavisms: Reappearance of a structure previously lost in evolution. Example: Humans have genes for a tail, but they are nearly always suppressed. Occasionally, humans are born with tails, however.

Fossils

• Molds and casts: A mold is a space left after the organism decays. The space is often filled with material, and this is called a cast.

• Body fossils: Actual remains of the organism's body, including shells and teeth.

  • Unaltered body fossils: Ice age mammals frozen in permafrost, insects preserved in amber (tree resin), La Brea tar pits in Los Angeles, humans preserved in peat bogs, mummification.

  • Altered body fossils: Permineralization (addition of minerals, as in petrified wood), replacement (for example, of wood by silica in petrified wood), recrystallization (aragonite in shells converted to calcite), carbonization (plant fossils).

• Trace fossils: Indications of organism activity, such as footprints, trails, burrows.