Fossils and evolution

Pamela J. W. Gore
Georgia Perimeter College

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 (generally) do not produce FERTILE offspring (mule).


Liger. Offspring of a male lion and a female tiger. Wild Animal Safari, Pine Mountain, GA.


Zedonk. Offspring of a zebra and a donkey. Wild Animal Safari, Pine Mountain, GA.

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 (Levin, 8th edition, p. 130). 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:

Example of homology:

The embryos of humans and other nonaquatic organisms have gill slits although they do not breathe through gills. Gill slits are present in the embryos of all vertebrates because their common ancestor is the fish, in which gills first evolved. When the vertebrates invaded fresh water and then colonized the land, there was a shift in method of breathing from gills to lungs. The gills are a vestige of a feeding device found in tunicates and cephalochordates. Early in vertebrate history, a pair of gill arches became modified to form jaws.

Human embryos also have a well-defined tail by the fourth week of development, which reaches maximum length when the embryo is six weeks old. Similar embryonic tails are also found in other mammals, such as dogs, horses, and monkeys. In humans, the tail eventually shortens, persisting only as a vestighial feature in the adult coccyx.

The most familiar vestigial organ in humans is the appendix. This wormlike structure attaches to a short section of intestine called the cecum, which is located at the point where the large and small intestines join. The human appendix is a functionless vestige of a fully developed organ in other mammals, such as the rabbit and other herbivores, where a large cecum and appendix store vegetable cellulose to enable its digestion with the help of bacteria.

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.

There is no change in genes from tadpole (fish-like water dweller) to frog (has legs, breathes air, can live on land). It is reasonable to apply this as a modern analogy to the geologic record to aid in explaining the transition from fish to amphibian. The genotype remains the same but the phenotype changes as a result in a change in a developmental timing gene (either activation of a gene that was always there, or a mutation to a gene). After observing a tadpole "grow up" into a frog, it is not that difficult to understand how a fish in the Devonian could "evolve" into an amphibian.

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.) 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.

The "law of recapitulation" has been discredited since the beginning of the twentieth century. Experimental morphologists and biologists have shown that there is not a one-to-one correspondence between phylogeny and ontogeny. Although a strong form of recapitulation is not correct, phylogeny and ontogeny are intertwined, and many biologists are beginning to both explore and understand the basis for this connection. Ernst Haeckel, U. California Berkeley page.

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. (Levin, 6th edition, p. 522 - 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. (Levin, 6th edition, p. 118)

  5. Similarity of embryos of all vertebrates suggests a common ancestry. (see diagram in class)

  6. 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.

  7. 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.

  8. Changes in domestic animals through selective breeding.

More recent lines of evidence in support of evolution