Fossils and evolution

Principle of fossil succession (or biologic succession)

William Smith (late 1700's) discovered that certain rock units could be identified by the assemblages of fossils they contained. This knowledge led to the "Principle of biologic succession" (or fossil succession), which states that fossils occur in a consistent vertical order in sedimentary rocks all over the world.

Fossil species appear and disappear throughout the stratigraphic record. The Geologic Time Scale is based on these appearances and disappearances. Each of the Eras ends with a mass extinction. Period boundaries coincide with smaller extinction events, followed by appearances of new species.

Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time.

What could bring about biological changes under natural conditions?

Natural selection
This principle can be succintly stated as "the survival of the fittest". In any population, there will be individuals that are slightly more tolerant of changes in the limiting factors of the environment (temperature, salinity, water depth, amount of food, etc.). When environmental conditions change, bringing individual organisms near their limits of tolerance, there will always be a few which are at least slightly more tolerant or BETTER ADAPTED than the population as a whole.

The better adapted individuals will survive to reproduce. This passes along favorable traits to their offspring.

As the less-well-adapted individuals die off before reproducing, and the more tolerant, better adapted individuals reproduce, favorable traits will become dominant in the population. There will be a shift in the gene pool.

The environment "selects" the best adapted individuals, hence the term "natural selection".

A few basic terms

Population - a group of interbreeding organisms.

Gene pool - the sum of all of the genetic components in a population.

Species - The fundamental unit of biological classification. A group of individuals that are similar in structure, function, and development, with the potential to interbreed and produce fertile offspring.
Reproductive barriers between species prevent interbreeding.

Closely related (but different) species, such as the horse and the donkey CAN interbreed, but do not produce FERTILE offspring (mule).

How do organisms pass along favorable traits?

Genetics is the branch of biology that deals with the study of heridity or inheritance.

Within the nucleus of each of our cells are chromosomes. In a human cell there are 23 pairs of chromosomes. (One of these pairs determines the sex.) Chromosomes consist of long DNA molecules, highly folded and coiled and combined with a variety of protein molecules. DNA stands for deoxyribonucleic acid. The general form of the DNA molecule is described as a "double helix", which resembles a twisted ladder (p. 121). The long part of the ladder is made of phosphate and sugar compounds, and the rungs on the ladder are made of nitrogenous bases (adenine, thimine, guanine, and cytosine).

The structure of the DNA molecule was discovered by Watson and Crick in 1953.

The part of the DNA molecule responsible for the transmission of inheritable traits is called a gene. Chromosomes are composed of genes. There are approximately 100,000 genes in a human cell, only a small number of which have been identified and roughly located on the various chromosomes.

DNA carries chemically coded information from generation to generation, providing instructions for growth, development, and functioning.

Through sexual reproduction, new combinations of chromosomes result. One member of each pair of chromosomes is inherited from each parent. This sexual genetic recombination leads to variability within the species.

Genetic mutations also produce alterations in genes and DNA. Mutations are simply chemical changes to the DNA molecule. Mutations can be caused by a chemical substance, or by exposure to radiation (which includes cosmic radiation and ultraviolet light. Mutations produce much of the variability on which natural selection operates.

Variability is the raw material for natural selection.

Functional morphology

The morphology (shape, body plan) of plants and animals reflects the way that they live. Specialized features of organisms which allow them to perform useful functions are called adaptations. Morphology can be examined in terms of the functions it performs - "functional morphology". When studying the form and function of morphological features, two categories can be considered:

Evolution

Simply stated, evolution = change.

Organic evolution refers to changes in populations.

Phylogeny = the sequence of organisms placed in evolutionary order.

All organisms are controlled by a combination of:

Genotype - the genetic makeup of an organism; the total of all heriditary information carried by an individual.

Phenotype - the appearance of an organism; the observable expression of the genotype. It is influenced by both genetics and environment.

All organisms go through changes in appearance during their lifespan. This is called ontogeny. Examples?

In each of these examples, no genetic change is involved. The genotype remains the same, but the phenotype varies dramatically. It has been stated by Ernst Haeckel (1834-1919) that as a general rule "ontogeny recapitulates phylogeny". In other words, the series of changes that an individual goes through during the course of its lifetime mimics the stages that the organism has passed through during evolution. The early stages of descendants resemble the adult stages of the ancestors.

In each case, rapid and dramatic shifts in appearance have occurred. Indeed, the casual observer might conclude (erroneously) that a caterpillar and a butterfly represented completely different species. The cause of these variations relates to developmental timing. (Note that a mutation causing a slight shift in developmental timing can lead to mature individuals which retain juvenile forms, or juveniles which take on some of the characteristics of adults - such as accelerated sexual development. This is known as paedomorphosis.)

Can you cite any examples of changes within populations going on today, or in recent history?

These changes which we know so well are examples of microevolution or evolution at or below the species level.

Changes or evolution at or above the species level are termed macroevolution.

Lines of evidence which led Charles Darwin to propose his theory of natural selection:

  1. South America is the home of unique animals not found on other continents, AND extinct giant relatives of the living forms are found there as fossils. (p. 530 - glyptodonts)

  2. Species of marine organisms on either side of Panama (in the Atlantic and Pacific Oceans) are very different, although they are only a few miles apart. (Barrier to dispersal).

  3. Tortoises of the Galapagos Islands are different on each island. Suggests a common ancestry. They differentiated in form as a result of living apart in different environments.

  4. Finches in the Galapagos Islands are adapted to exploit a variety of different niches. Beaks show a lot of variation and specialization for different functions. Similar types of changes seen th the Hawaiian honeycreepers. (p. 125)

  5. Similarity of embryos of all vertebrates suggests a common ancestry. (see overhead)

  6. Homologous organs and bone configurations have a common ancestry (toes of land-dwelling mammals vs. bat wings). Evidence of this abounds in animal and plant kingdoms.

  7. Vestigial organs suggest a common ancestry. (Serve no apparent purpose, but resemble functioning organs in other animals.) Whales have useless pelvic bones (and occasionally rear feet) resembling those in other mammals. Why develop a useless structure? All mammals have similar structures whether they are used or not.

  8. Changes in domestic animals through selective breeding.

    More recent lines of evidence in support of evolution

    All forms of life are made up of 20 amino acids.

    All forms of life are based on DNA (similar chemistry).

    Careful analysis of DNA sequences shows that chimp and gorilla sequences are 97.9% similar.

    Human and chimp DNA sequences are 98.4% similar.

    Sources: (1) Discover magazine, March 1988, and (2) National Geographic magazine November 1985.

    How do new species originate?

    How does macroevolution occur?

    The question is not whether evolution occurs, but rather, exactly how it occurs. What is the mechanism?

    Several theories about how evolution occurs:

    1. Phyletic gradualism - gradual progressive change (old idea)
    2. Punctuated equilibrium - sudden changes punctuating long periods of little change. (Gould and Eldridge).

    In general, a stimulus like an environmental change causes the evolution into a new species. The organism must either:

    1. adapt to changing conditions if possible
    2. survive by changing habits or migrating into another area
    3. go extinct

    Allopatric speciation

    When a population (a small population of a larger species) becomes geographically isolated, it contains a small gene pool. It will be reproductively isolated from the remaining populations of the species. The population experiences genetic drift - it will include a limited range of variability, and will experience unique mutations. The differing environmental conditions in that area will guide natural selection.

    When the barriers are removed, the isolated population may have changes so much that it is no longer able to interbreed with the remaining populations of the species. Changes may be in morphology, in function, in developmental timing, or in behavior.

    The removal of the barriers and the migration of the isolated population to the larger area will look like the sudden appearance of a new species.

    Contributions of paleontology to evolution

    Hypothesis: If evolution is true, we should see evidence of it in the fossil record. If it is false, we should see no changes.

    Fossil taxa appear and disappear in succession

    The pattern provides strong evidence for evolution .

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    This page created by Pamela J. W. Gore
    pgore@gpc.edu
    DeKalb College, Georgia
    Created Fall 1995
    Last modified November 6, 1996