Paleontology

Fossil Record of Evolution

Evolution

The word "evolution" is used by paleontologists and biologists to describe three related but distinct ideas:

1. The observation that life has a history. In other words, the deduction made from our studies of the fossil record that the world has been populated by a vast sequence of organisms that are no longer living.

Most of the evidence in support of this observation comes from the fossil record.

2. The inference that the vast sequence of extinct fossils and living organisms is interrelated, i.e. that there is a connectedness to the living world such that all organisms are united by common descent.

Evidence in support of this inference comes from comparisons of both living and fossil organisms and from the temporal sequence of species found in the fossil record.

3. The theory that purports to explain how new species are produced from previously existing species (speciation). Currently, the most widely accepted "theory of evolution" is a hybrid of modern genetics combined with Charles Darwin’s original "theory of descent with modification through the agency of natural selection" called the modern synthesis.

The details of how evolution works to produce new species is still being actively researched and debated, both by biologists and paleontologists. Because of the nature of the fossil record - in particular its relative lack of resolution at short time scales (ecological time) - most attempts to study generation by generation changes in species are carried out by biologists. Studies of microevolution attempt to catch the speciation process in action and dissect its workings on the level of genetic change.

Paleontologists tend to focus their efforts on studying the long term patterns of evolutionary change found in the fossil record - called macroevolution. Of particular interest to paleontologists are questions such as:

• within a single lineage (line of descendent species) or single species, what is the pattern of evolutionary change over hundreds of thousands or millions of years?
• what is the tempo (rate) and mode (style) of evolution over long periods of time? Are new species evolved at steady rate or are there sporadic bursts of evolution? Do some types of organisms appear to evolve faster or slower than others?
• what might be the cause of the patterns seen in speciation in the fossil record?

The New Synthesis model for speciation:

In general, speciation is thought to occur when the following happens:

a. A population of organisms becomes divided so that gene flow between the populations is restricted or stopped.

b. Natural selection works through differential survival and reproduction to change the genetic makeup of each subpopulation in different ways depending on the nature of the selection pressures in each subpopulation’s habitat. This process leads to adaptation.

c. Genetic hanges accumulate in each subpopulation. When the two subpopulations have reached a level of difference that prohibits them from successfully interbreeding they are said to be reproductively isolated. Technically they are now two different species because even if allowed to freely coexist the members of each subpopulation cannot or will not successfully interbreed.

If the subpopulations are recombined before they achieve reproductive isolation they will interbreed and their accumulated genetic difference will be shared and eventually lost.

There are three general models for how a species population becomes divided so that genetic change can accumulate.

allopatric speciation: Allo means different, other - the main species population becomes subdivided by a physical barrier - river, lake, mountain range, ocean current, etc. so that gene flow between the subpopulations is restricted or cut off.

sympatric speciation: Different members of a species population stop interbreeding within the same region. This could occur for a number of reasons, including accidental behavioral differences, mate preferences, feeding differences, etc.

peripatric speciation: Small subpopulations of the main species population continuously form at the periphery of the main population’s habitat. These small subpopulations occupy marginal habitat patches that are isolated from and somewhat different from the main habitat. Because of the small size of these peripheral populations, genetic differences can accumulate rapidly, provided the subpopulations do not go extinct, as most probably do.

In general, biologists study evolution in detail. By breeding successive generations of organisms in the laboratory, or by studying successive generations of natural populations, biologists try to observe how genetic change is channeled and accumulated over time.

Evolution at this level of detail is often called microevolution.

Unfortunately, biologists can only follow detailed species changes for a limited number of generations. In nature, it may take hundreds or even thousands of generations for genetic change sufficient to lead to speciation to accumulate.

Paleontologists have the opposite problem that biologists do. They cannot follow the genetic changes in populations over successive generations. The nature of the fossil record will not allow this, principally for one reason:

Time averaging - it is virtually impossible to discretely fossilize successive generations. Instead, fossil deposits either contain the remains of many generations jumbled together, or they are isolated snapshots of particular generations or a few generations. In other words, the time resolution of the rock record does not allow continuous observations to be made on a scale of years, decades, or even centuries.

Furthermore, paleontologists can never know when the biological species criteria of reproductive isolation has been achieved. Remember, paleontologists deal with morphospecies based on differences in morphology judged to be comparable to differences seen in living species that are reproductively isolated.

Ideally, microevolution and macroevolution should meet somewhere in the middle, and the observations of both should be consistent. In reality, biologists and paleontologists have traditionally had much trouble coming to agreement on the mechanisms of evolution.

This is partly because there are some real inconsistencies between what is observed on the microevolutionary vs macroevolutionary scales, and partly because of miscommunication between the two disciplines, particularly over the time scale at which each type of study operates.

Paleontological studies of species change through time

Gradualism

Although speciation is dependent on genetic change, it is not necessary that most of the genetic change seen between species be tied to the speciation event.

Traditionally it has been assumed that species continually evolve in response to changes in their environment. As the conditions of the habitat or interspecific relations change through time, a species population must change accordingly or risk becoming maladapted and being driven to extinction. The great evolutionary biologist Lee VanValen referred to this as ‘the Red Queen Hypothesis’ after the Red Queen in Through the Looking Glass who had to run as fast as she could just to stay in place.

The implication of this idea is that over the life of a species it should be apparent that the average morphology of the individual members of the species changes slowly through time as the species population evolves to become and remain adapted.

This model of macroevolution was implicitly stated by Charles Darwin himself - he believed that evolutionary change occurred slowly and gradually throughout the life of a species - and is now referred to as gradualism.

One interesting point about gradualism is that it suggests that speciation can occur over time as an entire species population changes to a degree that it can no longer be considered the same species it was before.

Phyletic evolution / pseudoextinction / cladogenesis

This type of change is called phyletic evolution (anagenesis) and it results in the production of chronospecies, each arbitrarily seperated by the pseudoextinction of ancestor species as they change into descendent species.

Gradual evolution can result in the splitting of a lineage to form a new species (cladogenesis) as indeed it must if it is to explain the diversity of life on earth. The important point here is that the speciation event itself does not account for the majority of evolutionary change - this occurs during the normal life of the species.

Unfortunately, it has always been recognized that the fossil record does not seem to agree with the gradual view of evolution. In particular, paleontologists from Darwin onward have noted that, rather than smooth transitions from one species to the next, the fossil record shows jumps - old species are suddenly replaced by new species with little sign of intermediates.

Also, paleontologists from Darwin onward have explained away the seemingly discontinuous nature of the fossil record as an artifact of its incompleteness. In other words, the lineages of fossils seen only appear to be discontinuous because the intermediate forms were not preserved. Continuous change is being made to look discontinuous by gaps in the fossil record.

George Gaylord Simpson discussed and explained away the apparent discontinuities between species seen in the fossil record in his book Tempo and Mode in Evolution (1944).

Data on the evolution of a series of species of the ammonite Zugokosmoceras shows an apparent morphological jump that coincides in the stratigraphy with an apparent disconformity. If the missing time represented by the disconformity is restored, the jump disappears.

Simpson also discusses how the seemingly discontinuous record of horse evolution originally described from old world fossils can be explained by the fact that horses evolved in the new world, gradually and continuously, and that occasionally species would migrate to the old world and appear suddenly in the fossil record.

What Simpson is arguing with these examples is more than just that the fossil record shows continuous, gradual evolution. Simpson’s main underlying point is that macroevolution (as seen by paleontologists) is no different from microevolution (seen by biologists) operating over a very long span of time.

Punctuated Equilibrium

In the 1960’s a number of very detailed studies of evolutionary change in the fossil record failed to demonstrate the expected pattern of gradual change from one species to another. Also, statistical re-examinations of some of the most famous examples of gradual evolution suggested that they were not as clearly gradual as had always been assumed.

Two paleontologists who had been studying lineages of fossil invertebrates - Niles Eldredge (Devonian trilobites) and S.J. Gould (Pleistocene land snails) became convinced that the apparent discontinuities in the fossil record were real.

In 1972 Eldredge and Gould proposed a different model of evolution that they argued better explained what was seen in the fossil record (a similar model had already been recently proposed by Soviet paleontologists - but was not available in English translation). They called this model punctuated equilibrium. E and G based their model on the following observations and arguments:

1. The majority of fossil species lineages studied do not show any significant directional change. Instead change, when it is evident, is nothing more than random fluctuation around a mean. This stasis in species form is not an artifact to be explained away, rather it is a significant pattern (stasis is data).

2. Species appear in the fossil record fully differentiated from other species. This observation, combined with the observation of stasis, suggests that most evolutionary change is associated with the speciation event itself and is rapid relative to the life of the species. In other words, the evolution of species can be seen as long periods of equilibrium punctuated by short bursts of change.

3. This pattern of change is what one would expect if species were forming as small, peripheral subpopulations - as in the peripatric speciation model of microevolution. Such a localized, small population would have little chance of being recorded in the fossil record. Only when the newly evolved species invaded the range of the parent species and grew to large numbers would it appear in the fossil record.

An important point to make here is that the actual scale of time over which species evolve in this model is short only in relation to the life of the species, which on average is about 5 million years. Thus, if the actual speciation event take only 1% of the life of the species, this still leaves 50,000 years for the evolutionary change to take place. For an organism that reproduces annually that means 50,000 generations!

When it was first proposed, punctuated equilibrium was received with skepticism by many biologists (if not with downright hostility - one famous letter to a journal addressing this issue referred to it as ‘evolution by jerks’.) because they failed to appreciate that ‘rapid’ has an entirely different meaning to a paleontologist than it does to a biologist.

Many biologists thought that E and G were rejecting Darwinian evolution in favor of some undescribed process that made leaps from one morphology to a completely new morphology. In reality, given a small population and 50,000 generations it is not difficult at all to imagine evolution by natural selection (microevolution) producing the species level changes seen in the fossil record. What is punctuated is the change seen through the lens of geologic time.

Contrasting the two models, we see the the major difference between them is in where the evolutionary change is concentrated. However, it should be noted that as a model, punctuated equilibrium raises a totally new question:

What is it that is causing the stasis? Why do species persist for so long without any significant directional change? A satisfactory answer to this question has not yet been found.

The controversy to date

Since the proposal of the punctuated equilibrium model, a number of detailed evolutionary studies have been done on species lineages in the fossil record to look for patterns of gradualism or punctuation. Although many would probably argue with me, I think that the weight of evidence and opinion is clearly leaning toward the punctuational model for evolutionary change in the fossil record.

Surpisingly, the best examples of gradual change come from planktonic foraminifera that inhabit, large, relatively monotonous expanses of open water, while the best examples of punctuated change come from very heterogeneous benthic habitats that are subject to continuous environmental change. This is backwards of what might have been expected and in need of a good explanation.

Of course, the importance and commonness of each type of evolutionary pattern is still controversial. Part of the problem is that the data collected to detect patterns in evolutionary lineages are never perfectly suited to the task and often can be interpreted both ways, depending on your preconceptions.

Are the trends gradual (as drawn), or are they punctuational (as suggested by some of the large jumps in rib number)?