The Contingency of Evolution: What Determines the Outcome?

Earthly organisms must be made from materials that are present on Earth. Natural selection favors the individuals in a population that survive and reproduce the best relative to others present, in the current environment. The italics in the previous sentences highlight the three main contingencies of evolution: 1) Only certain materials were present in the early Earth to serve as raw materials for life. 2) Natural selection is limited by the range of possible biological variation and by the variation present at a particular time, and 3) The environmental circumstances at a given time determine the characteristics favored by natural selection. Changes in the environment lead to changes in the type of characteristics that are favored. The outcome of the evolutionary process is determined by these three contingencies. Thus, it is worth examining them in more detail to determine whether we can reasonably expect the evolutionary outcome that has occurred so far on our Earth to be similar to the outcome of an evolutionary process in another world.

1) The outcome of evolution depends on the materials available at the start of life. The elements that were common on Earth at the time that life was thought to have originated were carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorous (P) and sulfur (S). All biological molecules are combinations of these fundamental elements. Presumably, during the early years of the Earth's history, many different kinds of combinations of these fundamental elements occurred under various physical circumstances. Eventually, the molecules we know as nucleotides were formed, and then they combined into self-replicating RNA-based forms called ribozymes. If we take the ability to self-replicate as the defining feature of life, then these ribozymes were probably the first life on Earth. Further modifications led to the formation of the true nucleic acids (RNA and DNA). DNA is a giant polymer with a sugar-phosphate backbone of nucleotides and nitrogenous "rungs" made of different combinations of C, N, H and O. DNA forms the universal genetic code of information used by all life. RNA then plays a key role in the transfer of information from the DNA to the proteins that are the building blocks of every organism's body.

Once the trial and error process of early biochemistry produced self-replicating RNA, then it appears likely that the evolution of DNA as the genetic code may have occurred quite rapidly. At that point, other types of self-replicating molecules that could possibly have occurred were at a disadvantage, because a self-replicating system that worked was already beginning to dominate the use of resources. So, the first main contingency of life on Earth may be the genetic code itself on which all life is based. Though it is probably not the only way that information could be coded in a molecule, it is the way that became established on the early Earth, and nothing has come along to supplant it.

Another influence of the early materials present on Earth was the fact that water is the universal solvent. Water has very special chemical properties that we largely take for granted, and these properties have dramatically shaped the chemistry of biological life. It is important to recognize, however, that water is not the only possible solvent. Life based on a different set of initial elements, and a different solvent, might be entirely different than the life we know on Earth.

2) Natural selection can only act on biological variation that is present at a given time. Most characteristics of organisms that matter to survival and reproduction can be described in a given population by an average value and a range of genetic variation around the current average. Evolution proceeds when natural selection favors certain variants that survive or reproduce better than others within their environment, and thus contribute more than their share of progeny to the next generation. This process of differential survival and reproduction of individuals with different characteristics leads to a change in the average value of characters so that the population comes to look more like the favored variants (change over time and descent with modification). The key here is that this variation involves slight differences from the current average. Thus, descent with modification means that as evolution proceeds, the average characteristics are modified into new forms. The modifications of the bones in the vertebrate limb for very different functions (figure 3) are an example of this type of evolutionary change. Even a huge evolutionary step like the colonization of land probably proceeded in this way, when a few lobe-finned fishes began to spend time scooting along muddy shores on their weak fins. Later lineages show modifications of this crude beginning of terrestriality, leading to the amphibians (only partially terrestrial), and later to the fully terrestrial reptiles. Change over time and descent with modification can be seen clearly in the evolution of terrestrial vertebrates.

Rarely, a variant form that is radically different appears in a population - one that does not fit within in the usual distribution of variation. Such radically different forms may have been very important in the evolution of major innovations during the history of life. It is probable, however, that the first occurrence of a major new characteristic was only a crude version of its final evolutionary form that permitted organisms to take on a new function and that was then further modified by natural selection acting on small variations.

The key point here is that if a variant is not produced for some reason, that characteristic cannot and will not evolve in real populations, no matter how advantageous it might be. For example, it would be handy for large mammals to construct their skeletons from titanium instead of bone. This would allow a much lighter weight skeleton, and would probably permit the evolution of much larger body sizes, since the size of terrestrial vertebrates is largely limited by the strength of bone. However, titanium is not a material that is used by organisms (with the exception of a few prokaryotes, which appear to use almost every element on Earth, see the next chapter). Thus, a variant within a mammal population that has a titanium skeleton is highly unlikely to ever appear. Instead, the best-working form of a skeleton that can be made with biological materials will be favored by natural selection.

3) The environmental circumstances at a given time determine the form of natural selection and thus the outcome of evolution. Natural selection is the differential survival or reproduction of individuals within a particular population in a particular set of environmental circumstances. No single set of characteristics is universally favored in all organisms, because the characters that lead to high survival in one situation will not necessarily lead to good survival in a different environment. As the evolution of life on Earth has proceeded, there have been changes in the environment due to periodic physical changes on Earth. Radical climate changes have been caused by movement of the continents. Changes in orbital cycles and the earth's tilt have led to ice ages and dramatic changes in sea level. These enormous physical events have led, in some cases, to major extinctions, and they have certainly affected the characters that have been favored in organisms at particular times. Even more importantly, life has changed its own environment. As living things have diversified, they have become inextricably part of one another's environments. Consider the first life on land, colonizing bare rock. Soil was formed as the first bacterial colonists died and decomposed. This allowed plants to take root, whose decomposing bodies further added to the soilbuilding process. The evolution of flowering plants on land has provided not only food but also habitat for many different kinds of terrestrial organisms. The diversity of insects, for example, would not be at current levels, were it not for the diversity of flowering plants that have evolved. In turn, plant diversity has increased due to the effects of the insects themselves on speciation. In life on Earth, many species evolve together ("coevolve"). They acts as part of one another's environment, determining for each other the characteristics that lead to high levels of survival and reproduction.

Contributed by: Dr. Sara Via