BIOLOGY Volume 3 - A Guide to General Biology - 2004

26. EVOLUTION, OR THE HISTORY OF LIFE ON EARTH

26.7. Evidence for Evolution

Evidence supporting MODERN CONCEPTS OF evolution comes from various sources, among which the primary ones are data from:

1) paleontology,

2) biogeography,

3) Taxonomy,

4) PLANT AND ANIMAL breeding,

5) comparative anatomy,

6) adaptive radiation,

7) comparative Embryology,

8) comparative biochemistry.

Much of the data presented in this chapter was not available to Darwin and Wallace when they published their papers on THE ORIGIN OF species. However, this did not prevent Darwin from using his intuition, as is characteristic of great scientists. This is evidenced by the following statement of his:

"In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the Struggle for Existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be The formation of new species. Here, then, I had at last got a theory by which to work."

The data presented here largely support The Theory of evolution by natural Selection outlined in section 26.5; it should be remembered, however, that they do not contain any proof that evolution actually occurred in this exact way, nor any proof that no other mechanisms were involved. These data are drawn from various fields of knowledge, and their interpretation largely depends on the assumption that the Concept of Evolution adopted here is correct. Not a few of these pieces of evidence in turn require confirmation, alongside exceptions or information that can be interpreted otherwise. Nevertheless, the broad concept of evolution is based on an overwhelming body of scientific facts. As you read the subsequent sections, try to evaluate the available data for yourself and decide whether the Conclusions drawn from them are justified. Attempt to distinguish between evidence indicating that evolution has occurred and evidence indicating that natural selection is The Mechanism of evolution. The remainder of this chapter is devoted primarily to Evidence for the existence of evolution, while Chapter 27 presents data supporting natural selection.

26.7.1. Paleontology

Paleontology is The Study of fossils, i.e., any preserved remains presumably belonging to living organisms. These can include whole organisms, hard skeletal structures, external and internal Molds, impressions, tracks, and coprolites (fossilized excrement) (Table 26.3).

Class="center">Table 26.3. Types of fossils, their formation, and Examples

Paleontological data alone are insufficient to prove that evolution actually occurred, but they do corroborate the fact of a progressive increase in organism complexity. Fossils were well known long before evolution gained general acceptance. They were considered either the remains of creatures created earlier than others or artifacts placed in rocks by God. Most of the remains discovered so far can be assigned to the same taxonomic groups (phyla and classes) as living species, although the question of whether they are ancestors of modern forms can be debated but not proven.

In the oldest rocks containing fossils, only a few types of organisms are found, all of which have a simple Structure. Younger rocks contain more diverse fossils with increasingly complex structures. Throughout the fossil record, many species that appear at an early stratigraphic level (a level in rock strata) disappear at a later level. In an evolutionary sense, this is interpreted as the emergence and extinction of species in corresponding epochs.

Judging by the available data, geographical regions and climatic conditions have undergone changes throughout Earth's history. Since every organism is adapted to a particular environment, continuously changing conditions may have favored The Emergence of a mechanism for evolutionary change, which helps explain the progressive changes in organism structure reflected in the fossil record. Ecological considerations are also consistent with paleontological data. For example, plants appeared on land before animals, and insects before the plants they pollinate.

One of the main objections to using fossils as evidence for evolution is the lack of continuity in the fossil record. Gaps in the record ("missing links") are considered a strong argument against the theory of the formation of new forms through gradual change. However, several factors help explain the incompleteness of the fossil record:

1) dead organisms decompose rapidly;

2) dead organisms are consumed by scavengers;

3) soft-bodied animals are poorly fossilized;

4) only a small fraction of organisms perished under conditions favorable to fossilization;

5) only a fraction of fossil remains has been discovered.

Evidence supporting the evolutionary process accumulates as a growing number of potential "missing links" are discovered—either fossil remains such as Seymouria (amphibians → reptiles), Archaeopteryx (reptiles → birds), and Cynognathus (reptiles → mammals), or extant organisms such as Peripatus (see Fig. 26.17) and the coelacanth, which closely resemble extinct forms in their structure.

It is also possible that new species arose abruptly and that intermediate forms did not exist. Eldredge and Gould described a process termed "punctuated equilibrium," which helps explain the sudden appearance of new species in the fossil record. In their view, species remain unchanged over long periods of time and then give rise to new species over relatively short intervals. This is hypothesized to result from relatively sudden and substantial environmental changes. It should also be borne in mind that evolutionary rates can vary, allowing new species to appear rapidly, thereby creating the impression of incomplete fossil data. Such apparent "jumps" in the evolutionary sequence have given rise to the term "saltational evolution." Darwin himself discussed this possibility, writing in On the Origin of Species:

“I do not believe that this process [speciation]... occurs continuously; it is far more probable that each form remains unchanged for a long period, and then once again undergoes modification.”

Evolution, therefore, need not always be gradual.

Fig. 26.3. Photograph of fossil trilobites from Cambrian deposits.

Evolutionary METABOLISM/13.html">History of the Horse

One of the best examples of phylogeny, based on the study of a nearly complete paleontological record from North American sedimentary deposits dating from the early Eocene to the present, is the Evolution of the horse.

The earliest perissodactyls appeared 54 million years ago; modern perissodactyls include horses, tapirs, and rhinoceroses. The earliest known animal in the fossil record belonged to the genus Hyracotherium, which was widespread throughout North America and Europe in the early Eocene. By the beginning of the Oligocene, it had become extinct everywhere except North America. It was a small, slender, cursorial animal. Its short, slender legs terminated in elongated feet, and its digits were positioned nearly vertically. The forelimbs bore four digits, while the hindlimbs had three. The incisors were small, and the molars had low crowns with rounded, enamel-coated cusps.

The proposed evolutionary Lineage of horses from Hyracotherium to Equus comprises at least 12 genera and several hundred species. The Main Trends in horse evolution concerned locomotion and feeding. They reflected adaptation to changing environmental conditions and included the following tendencies:

1) increase in body size;

2) elongation of LEGS AND FEET;

3) reduction of lateral digits;

4) elongation and thickening of the third digit;

5) straightening and increased rigidity of the back;

6) refinement of Sense Organs;

7) enlargement and complexification of the Brain associated with The Development of sense organs;

8) broadening of the incisors;

9) replacement of premolars by molars;

10) lengthening of the teeth;

11) increase in molar crown height;

12) reinforcement of teeth through the development of cement between the folds;

13) increase in tooth surface area resulting from the development of enamel folds.

Fig. 26.4 illustrates the progressive evolution of the horse during the Cenozoic era, featuring genera that dominated this geological epoch as examples. However, we lack evidence to claim a direct ancestral relationship among these forms.

Fig. 26.4. Evolution of the modern horse's ancestors.

The Significance of the evolutionary sequence of forms shown in Fig. 26.4 is that it Supports the theory of the Progressive development of homologous structures, in this case, limbs and teeth. Homologous structures are similar structures found in different species that share a common evolutionary origin. Each of the species represented in Fig. 26.4 corresponds to a stage of development that flourished for several million years (as evidenced by the Abundance of fossil remains) before becoming extinct. The extinction of one species, however, did not mean the disappearance of the entire family. As the fossil record shows, the niche of an extinct species was invariably taken over by another closely related species. Since all species in this sequence share structural and ecological similarities, this reinforces the theory of evolution by descent with modification. Other fossils found in the same strata indicate changing climatic conditions; this, along with other evidence, suggests that each species was adapted to the prevailing conditions of its time.

The evolutionary lineage of the horse does not show a gradual transition uniformly distributed across time and space, nor can the paleontological record be considered complete. Apparently, several side branches diverged from the lineage shown in Fig. 26.4, but all of them became extinct. Modern horses clearly descend from Pliohippus. The extant genus Equus originated in North America during the Pleistocene and migrated to Eurasia and Africa, giving rise to zebras, donkeys, and the modern horse. Remarkably, after surviving in North America for millions of years, horses became extinct there a few thousand years ago, coinciding with the arrival of humans. Cave paintings preserved in other PARTS OF THE world indicate that early humans likely used horses as a food source. Horses were reintroduced to North America nearly 500 years ago.