THE EVIDENCE OF EVOLUTION

By Edward L. Crisp, Ph.D.

Professor of Geology

Organic Evolution

In order to understand the history of life, we have to understand the patterns of organic evolution. Darwinian evolution is the most accepted theory of organic evolution today. First proposed by Charles Darwin and Alfred Russell Wallace in 1858 and then expounded upon by Charles Darwin (1859) in On the Origin of Species by Means of Natural Selection. This concept is sometimes expressed as "Survival of the Fittest", however, this expression is often misunderstood. Survival of the Fittest really means that the organisms that survive to reproduce, or reproduce more offspring than other members of their species, will selectively pass on more of their traits to their offspring (thus a change will occur in the gene frequencies of the gene pool, thus evolution will take place). For Darwinian evolution we use phylogeny to show relationships of ancestors to descendants.

Evolution means descent with modification. We use phylogeny (Greek: phylum = tribe, genos = birth or origin) to show relationships of ancestors to descendants, therefore, phylogeny explains the history of descent of organisms.

In modern phylogenetic methods, we use cladograms to show monophyletic groups (natural groups that descended from a common ancestor). Polyphyletic groups are groups that do not share a closest common ancestor, and thus are not of value in determining phylogeny. Paraphyletic groups are groups that do not include all the descendants of a common ancestor.

Figure 1. From: http://www.ucmp.berkeley.edu/glossary/gloss1/phyly.html

If we, as scientists and students of science, are capable of understanding the world around us and the ways of science, then organisms have changed over time and have descended from a common ancestor. Therefore, Organic Evolution is a Fact. Overwhelming evidence supports the Darwin-Wallace Theory of Evolution by Natural Selection. Thus, natural selection is the means by which evolutionary change takes place. [Note: Creationist jump on the debate about evolution by scientists, but scientists argue the mechanisms and rates of evolution, not whether evolution has occurred]. The bringing together of evidence from many different fields of study in biology, paleontology and geology converges on a conclusion that is almost indisputable, organisms have evolved over time.

The biota has evolved!!! As Darwin said, descent with modification. The mechanism of evolution, as first proposed by Charles Darwin and Alfred Russell Wallace in a joint presentation to the Linnaean Society of London in 1858, is natural selection.

EVIDENCE FOR EVOLUTION

We have already discussed much of the evidence for evolution in previous lectures, however, let's summarize the evidence here and discuss some aspects in a little more detail.

Fossils (The Paleontological or Rock Record)

The rock record shows that organisms have changed over time. Fossils groups from the past show differences as compared to modern organisms. Also, species of organisms occur in a particular order and are not random in their appearance. "Each species or group is preceded by a logical and related ancestor: descent with modification." (Kardong, 2005). However, this does not mean that evolution is progressive in a goal oriented sense, from simple forms to more complex. Natural selection does not guide evolution progressively, but instead allows organisms to evolve to be best suited (based on their inherent constraints of body plan, molecular chemistry, etc.) for their environment. However, as time passes diversity will have a tendency to increase. Organisms will find new ways to exploit their habitats.

The farther back in time the more simple, less diverse, and different are the fossils as compared to modern organisms.
The youngest rocks contain the fossils that are most similar to modern organisms and contain more complex and diverse forms.
Mosaic Evolution and "Missing Links:
- Example: Archaeopteryx from the Jurassic Solnhofen Limestone - Link between dinosaurs and birds. Has both reptilian and bird characteristics. Archaeopteryx had the characteristics of a small theropod dinosaur, but had feathers, so is considered the first bird. However, the fossil record is not perfect; there are gaps in the fossil record. We do not find transitional fossils for every major transition of past organisms. However, we have found many transitional fossils (missing links) in the fossil record. There are also examples of sequential transitions in the fossil record, from ancestors to descendents (see the example for the evolution of the modern horse in your textbook, Figure 6.5, p. 87, Kardong, 2005).

Figure 2. Archaeopteryx skeleton with feather impressions and reconstruction of a Jurassic scene with the crow sized Archaeopteryx capturing a meal (from: Evolutionary Genetics at http://www.zoology.ubc.ca/~bio336/Bio336/Lectures/Lecture5/Overheads.html)

The logical inductive conclusion from the evidence of the fossil record is that species and groups of organisms succeed themselves in a logical and predictable manner as we move from older rocks to younger rocks and ultimately to the present. This natural order would not be expected if species were created separately, one at a time, by an intelligent designer. If this were the case we would expect a haphazard order to the fossil record, with perhaps a fossilized skeleton of a wolf occurring in older rocks than a trilobite, or a eukaryotic cell being found in older rocks than the first prokaryotic cells. This is definitely not the case.

Comparative Anatomy

Evolution (in morphology, genetic make-up, behavior, etc.) by natural selection involves modification such that ancestral (primitive) features (characters) are retained and new (derived) features are evolved.

Relationships of anatomical, morphologic, or biochemical features (i.e. comparative anatomy) is one line of evidence for the evolutionary relationship of organisms. When two anatomical structures (or other features) can be traced back to a single structure (or feature) in a common ancestor, we say that the two structures (or features) are homologous. Thus homologous structures are called homologues (or homologies). Thus, homology refers to two or more features that share a common ancestry.

Biochemistry: The similarity of DNA, blood proteins, and other organic molecules among organisms must be related to organisms that share a common ancestor.
- DNA Molecule:
- From: DNA Molecule - Two Views at http://www.accessexcellence.org/AB/GG/dna_molecule.html
- The four nucleotide bases in DNA.
- From: BIOL 1400 -- Lecture Outline 21 at http://www.accessexcellence.org/AB/GG/dna_molecule.html
- The similarity of DNA among organisms is considered by many as the strongest line of evidence in favor of evolution.
Homologous Anatomical Features

Our hands (as with all mammals) are homologous to the digits on dinosaur forelimbs and the common ancestor to both mammals and dinosaurs had digits on the forelimb. Examples of a morphological series within a group in the fossil record are important lines of evidence for evolution because they show the sequential change of homologous structures, from ancestor to descendant, to perform different functions (examples: evolution of the horse hoof, amphibian legs from the fins of lobe-finned fish, etc.).

Figure 3. Example of homologous structures (bones) in the forelimbs of several animals. From: Evolutionary Genetics at http://www.zoology.ubc.ca/~bio336/Bio336/Lectures/Lecture5/Overheads.html

Obvious (but often ignored) evidence of evolution is the hierarchical distribution of homologous characters in nature. Some homologous characters are present in all organisms (such as DNA and/or RNA and cell membranes). Some homologous characters are present in smaller groups. And some homologous characters are very restricted to small groups.

Classification of Organisms: classifying organisms into groups based on their similarities. A formal classification of organisms into major groups was devised by the Swedish naturalist Carolus Linnaeus (1707-1798) during the late 1700s. The Linnaeus system of classification is a hierarchical scheme, as one proceeds up the classification ladder the categories become more inclusive. Obviosuly Linnaeus used similarities of shared characters to relate organisms into groups, however, he believed in the immutability of species and thought his classification reflected God's plan, from simple primitive organisms to complex organisms with man at the apex. However, the fact that organisms share many characters in common is a very good line of evidence for evolution. Why would organisms share so many homologous characters if they were not related to a common ancestor? The probablity is very low that such similar characters would evolve by chance and if by special creation, why would such similar body plans be present in so many organisms (particularly since some characters and body plans could have been made better for the niche that they occupy if they were not the result of modification of an ancestral body plan)?

The Linnaean Classification Scheme:

Major Subdivisions Example

Kingdom                                        Animalia
Phylum                                           Chordata
     Subphylum                                      Vertebrata
        Class                                                Reptilia
            Order                                               Theropoda
                 Family                                             Tyrannosauridae
                     Genus                                             Tyrannosaurus
                         Species                                            Tyrannosaurus rex

Analogues (Analogous Structures) perform a similar function in two different organisms, but may or may not trace back to a common ancestor. For example, the wings of an insect and the wings of a bird are not homologues, but are analogues; they also cannot be traced back to a single structure in a common ancestor (thus, they have a different embryological origin). Homoplastic structures look similar, but may or may not be analogous or homologous. Sometimes organisms evolve structures that look similar to structures in other organisms, but these structures cannot be traced back to a similar structure in the ancestors of two different organisms; the structures may also not perform the same function in two different organisms, although they may look superficially similar. One example of homoplasy is when organisms evolve structures that mimic the structures on other organisms (like large spots on the wings of a moth that resemble eyes, perhaps to fool a potential predator).

Understanding evolution requires the recognition of homologous structures (including homologous molecular structures). Obviously this also means recognizing that some structures in organisms may be analogous and/or homoplastic due to convergent evolution. So, we often must look at the pattern of evolution to determine and try to define which anatomical structures are homologous and which are due to convergent evolution.

EVOUTIONARY PATTERNS

Divergent, Convergent, and Parallel Evolution

Divergence occurs when one species evolves into two (or more) species, which continue to change over time and become less and less alike. As time goes on and more divergence occurs of these related forms (with a common ancestory), we say that adaptive radiation has occurred. Adaptive radiation involves numerous divergences of an ancestral stock (common ancestor) into a number of descendants that have exploited different niches in the environment. For example, when the dinosaurs became extinct at the end of the Mesozoic Era, the ancestral mammal stock (which was present when the dinosaurs were alive) diverged and adaptively radiated into many niches that had been occupied by dinosaurs.
Adaptive radiation of the ancestral mammal stock into many different forms of mammals occupying different niches (Figure from: Evolution - The Evidence at http://bioserve.latrobe.edu.au/vcebiol/cat3/u4aos2p4.html#Divergent ) (By the way, this is an excellent site, you should go there and take a look at it.)
Parallel Evolution also involves divergence from a common ancestor, but the descendants evolve similar morphologic characteristics as time goes on (most likely in response to similar environmental parameters, i.e., occupying a similar niche). So, parallel evolution is the development of similar characteristics in closely related organisms.
Parallel Evolution in the elephant and wooly mammoth (From: Evolution - The Evidence at http://bioserve.latrobe.edu.au/vcebiol/cat3/u4aos2p4.html#Divergent)
Convergent Evolution is the development of similar morphology in distantly related groups. As with parallel evolution, this is in response to adapting to similar habitats and niches. An excellent example of this is the modern dolphin and the ichthyosaurs of the Mesozoic Era (see Figure 5.22, p. 118 in Wicander and Monroe).
Some examples of convergent evolution from three very distantly related organisms (From: Evolution - The Evidence at http://bioserve.latrobe.edu.au/vcebiol/cat3/u4aos2p4.html#Divergent).

Comparative Embryology

The Biogenic Law (or Haeckel's Law) - "Ontogeny Recapitulates Phylogeny"
Embryology: similarities in the development of embryos in the early stages for closely related organisms (for example: gill slits and tails in humans, that our ancestors possessed), with later divergence in the development.

Vestigial and Atavistic Structures

Vestigial structures or organs: structures or organs that are nonfunctional or only partially functionally (like the human appendix and wisdom teeth, whale and snake pelvises, horse toes and dewclaws in dogs). These structures or organs may be fully functional in other organsims or were in the ancestors of organisms that have them.

Distributional Evidence (Biogeography and Paleobiogeography)