Making Sense of Evolution

Evolution and Environment

Fossils can give us lot of information about organisms and paleoenvironments, but it is through the coordinated efforts of paleontology, geology, and evolutionary sciences that the most complete picture of Earth’s history can be formed. The basic idea of evolution is that species change on a genetic level, generally over many generations, and those individuals whose changes increase their ability to reproduce will result in the overall population becoming more and more suited to their environment.¹⁹ “Everywhere we look in nature, we see animals that seem beautifully designed to fit their environment, whether that environment be the physical circumstances of life, like temperature and humidity, or the other organisms – competitors, predators, and prey – that every species must deal with.”²⁰

Natural Selection

The main driving force behind evolution is natural selection. Misunderstandings and misconceptions around natural selection are quite common in both the general public and in the classroom. Many people may describe the environment as acting on an organism when perhaps the better description is that it is selecting for traits in an organism (the exceptions perhaps being environmental factors that directly cause genetic mutations, like UV radiation). Abhijeet Bardapurkar makes a very simple analogy to describe the difference between natural selection and what he calls transformative action. Imagine you were to act upon a stone by hitting it with a hammer in the attempt to produce grains of sand. On the other hand, you may also choose to sift through rocks for smaller and smaller samples, as they are available, until you have accumulated rocks that are the size for which you were looking. The first scenario is an example of transformative action – acting upon something to create or make what you desire. The second scenario is what Darwin was describing as natural selection – selecting from what is already available.²¹ Of course, one must be careful in anthropomorphizing the selective pressures organisms face in nature. There is no conscious choosing or selecting of traits; simply environmental conditions that occur in which organisms with well-suited adaptations stand a higher chance of passing on their genetic material. Therefore, having variety in a population is, almost literally, the spice of life. Selection cannot occur when there is nothing from which to select. Transformative action does have its place in the theory of evolution as the random mutations that occur in genes (changing what is already there) are what provide the variety needed for natural selection to take place.²²

We can see numerous examples of natural selection in action today. Coyne illustrates a more modern case of natural selection in the coat color in wild mice. There are mice, “oldfield” mice, that have brown coats. They like to burrow in dark soils. In Florida, there are mice called “beach mice” that are the same species but who have light colored coats that naturally obscure them from predators. Over time, and being better suited to survive and pass on their genetics in the lighter, sandy environment, there are more copies of the “light” forms of pigmentation genes found in the populations of mice living in Florida’s Gulf Coast.²³ If you look at a particular environment and study the organisms that live there successfully, you will find adaptations that allow them do so. When looking into the past, we can use a similar logic to recreate ancient environments and events by looking at the organisms found in the fossil record from that time.

Biogeography

The role that the environment takes in shaping evolution cannot be overstated. The field of biogeography looks at the effects of geography and environment on the types of adaptations and the eventual speciations that can occur. Some biogeographers will argue that earth and life evolve together, in tandem. Observations of adaptations of closely related species can allow us to make inferences on environmental conditions during the time their common ancestors began to differentiate. Essentially, the study of biogeography looks for patterns in distribution and how these patterns developed. These patterns may arise due to geologic features (e.g. mountain formation, introduction or removal of water barriers), but can also be the result of meteorological (e.g. new areas of drought) or oceanographic (e.g. changing ocean currents) features.²⁴ Once separated, species respond to their unique environments and may adapt with traits that eventually prevent them from sharing genetic material with their sister species.

A classic example in the study of evolution and biogeography comes from Darwin’s peer, Alfred Russel Wallace. Wallace realized that differences in all species could not be accounted for simply based on differences in climate, as organisms that lived in the same climates did not exhibit the same adaptations. Much of his work was done in Southeast Asia and Australia. He noticed, through detailed observations, which life forms in these areas differed even though the climate and terrain were very similar. Therefore, Wallace had to conclude that that there was some other mechanism at work to explain the distribution of life forms. In 1915, German geologist, Alfred Wegener, found that two identical species of plant fossils were found on completely different sides the Atlantic Ocean. Since the ocean is much too large for these species to have traveled on their own, Wegener realized that at some point in Earth’s history, these two continents must have been physically joined. Wegener was able to propose a reconstructed ancient environment based on the identical plant fossils he found on the two continents. Although he did not know it at the time, the study of plate tectonics proved that these continents had, in fact, been one in ancient Earth’s history.²⁵ This is one example of how scientists can recreate paleoenvironments using clues from the fossil record.

Evolution, Extinction, and the Fossil Record

Using the fossil record to determine evolutionary steps, extinctions, and ancient environments is not without its challenges. It would seem that one should be able to directly infer relationships by reading the strata as one would a book, with the ancestors and descendants in unique strata and with ancestors in lower (older layers). However, one may find species that appear to be more closely related to ancient ancestors in younger layers along with their apparent descendants. Unfortunately, evolution does not always transpire in a nice step-wise manner, especially when you come across organisms that are so well-suited to their environments that they are not supplanted by another organism for an extended period of time. Additionally, geologic processes do not always occur uniformly, especially when major events, like storms, disrupt the formation of sedimentary rocks. Rock records may also experience shifts in facie through regular geologic processes, moving the evidence far away from its origin. This can throw off time estimates and make connecting ancestors, descendants, and environments more difficult. For most fossils, the best bet is to look at the features the fossils show us and determine which may have been acquired through evolution. Then, fashion a cladogram (often not thought of as true phylogenetic trees) that hypothesizes about the relationships between organisms.²⁶ These facts aside, do not discount the value and necessity of using fossils to determine evolutionary relationships and environmental conditions of the past. Despite the challenges, it would not be possible to learn about ancient Earth and its life forms without the information that can be gleaned from the fossil and rock records.

Perhaps as important as illustrating changes in evolution, the fossil record also provides information on extinctions that ultimately opened niches for new species to fill. There are two types of extinction – final extinction in which no genetic material is passed on and the type that occurs as a result of the generation of a new species. In the first, this is akin to the end of a branch in the Tree of Life while the second is part of a longer branch that includes the ancestors connecting living species.²⁷ Written into the rocks are records of numerous extinctions (mass and otherwise) as well as evidence for the evolution of life forms. In Earth’s history there have been five major mass extinctions –446 Ma at the end-Ordovician, 371 Ma at the Frasnian-Famennian (FFB), 251 Ma at the Permian-Triassic (PTB), 200 Ma at the Triassic-Jurassic (TJB), and 65 Ma at the Cretaceous-Paleogene boundaries (KPB). Although we have evidence in the fossil record for these mass extinction events, the jury is still out as to the exact causes for each. It is believed that abiotic events, such as massive climate change, were the main catalysts for the catastrophic loss of life. The fossil record shows a clear indication of the magnitude of loss of marine and terrestrial fauna during these extinction events. It was long held that the flora found during that time weathered the extinction events relatively intact; however, palaeoecological studies of three (PTB, TJB, and KPB) of the big five mass extinction events have indicated that even plant communities collapsed in conjunction with the highest level of faunal extinction at those times.²⁸

 Mass extinctions are of great interest due to the extreme loss of life forms all at once. As significant as mass extinctions seem, and they are for numerous reasons, extinctions on Earth are more far more common. In fact, it is estimated that approximately 99.9% of life forms that have ever existed on Earth are now extinct.²⁹ Paleontologists call the slower, more ongoing disappearance of life, background extinction. The mechanisms behind background extinctions are not well understood as they are phenomena of the past that are not directly observable. It is believed that most background extinctions occur when an organism’s environment changes in such a way that individual survival trumps reproductive fitness, essentially causing a decline in populations over time. In essence, extreme changes in the environment, abiotic or biotic, occur faster than the reaction of the organism. Background extinctions account for the majority of all extinctions which have occurred on Earth. As communities adapt to their environment, some will continue along the path of evolution, while others will continue their population declines.³⁰