Essay: Evidence for Evolution
The Evidence for Evolution is the body of observations and experimental results that collectively support the modern theory of biological evolution. As the unifying theory of all modern biology, this evidence is drawn from a wide variety of biological disciplines and the theory itself is continuously updated and refined as new evidence becomes available.
Just like virtually every other extant field-unifying scientific paradigm, many of the more esoteric aspects of the theory of evolution (e.g. specific mechanisms and evolutionary relationships) are still areas of active research. However, the major hypotheses underlying the theory have been consistently well-supported by ever-growing bodies of evidence for almost a century.
In this essay, I have arranged the summarized three lines of evidence, morphological, paleontological, and genetic. Obviously, this essay is far from a comprehensive discussion of the evidence supporting the theory of evolution (such an essay would need to be at least several thousand pages long), however I have done my best to cover as many points as the consideration of brevity would allow.
The most readily visible evidence for the common descent and shared ancestry of modern organisms comes from observing the morphology of complex organisms. For example, major groups (clades) of animals tend to share a common body plan and common anatomical characteristics. The isolated geographic distribution of many of these clades, which implies common ancestry among the member species, was one of the primary observations that led naturalists from the late 18th and early 19th centuries to start postulating a theory of evolution.
Add to this the observation that offspring inherit their physical traits from their parents, along with a mechanism for determining which traits are more likely to be transmitted to future generations (natural selection), and the nature of morphological change within populations over time becomes clear.
Providing a clear time scale for this process, and identifying unambiguous case studies, requires us to delve into the next level of evidence, paleontology.
If two species are descended from a common ancestor, then fossils of a common ancestor, with traits common to both species, might exist.
In fact, tens of thousands of fossils have been found with traits corresponding to predicted common ancestors of modern species. Furthermore, additional thousands of fossils have been found cataloguing the divergence and gradual adaptation of ancestral forms into modern species. Commonly cited examples of both ancestral and "transitional" fossils are found in the evolutionary history of whales.
Whales are estimated to have diverged from the lineage leading to modern ungulates around 55-60 million years ago. Fossils of an animal, called Pakicetus, with unambiguous ancestral whale-like traits are found in strata dated to ~50-53 million years ago. Pakicetus was a land-dwelling carnivore, somewhat similar in size and appearance to a modern wolf (more illustratively, imagine a carnivorous dog-sized goat).
Moving into younger strata, the fossil record provides a detailed stepwise transition from Pakicetus, to Ambulocetus (a family of animals very similar to Pakicetus, but more adapted to a semi-aquatic lifestyle), to Remingtonocetus (clearly well-adapted to a mostly aquatic lifestyle), to Protocetus (the earliest whale fossils found outside of the area around western India), to the ancestral divergence of modern whale families.
Many other evolutionary transitions have been documented in the fossil record, but even more compelling evidence for common ancestry is found in the field of genetics.
Beyond providing a set of "blueprints", an organism's genome provides a sort of history book on the organism's descent and ancestry. The theory of evolution would predict that species with a more recent common ancestor should have more similar genomes than species with more distant common ancestors. Further, common descent would predict that all species should have some highly conserved genetic similarities. Once again, both of these predictions have been found to be correct.
Variation in physical traits (phenotype) is driven by variations in genes (genotype). When a random genetic variation occurs, it is called a "mutation". The vast majority of mutations have absolutely no effect on phenotype whatsoever, those mutations that do affect phenotype provide are the "raw materials" of the evolutionary process.
Mutations that impair an organism's ability to survive and reproduce in its environment are selected against and usually do not persist in a population for more than a few generations. Mutations that improve an organism's ability to survive and reproduce in its environment become more prevalent within a population over generations, as individuals carrying the mutation will tend to have more offspring than individuals without it.
That more closely related species are more genetically similar to one another has been established for a few decades. An entire discipline of biology, comparative genomics, is devoted to studying the differences in genomic structure and function between species (or, sometimes, populations of the same species), in order to better understand the mechanisms of evolution and the mechanisms of gene regulation.
One surprising finding to come out of the earliest studies in comparative genomics was just how genetically similar most species are. This finding led to a major paradigm shift (spawning, in its wake, the field of evo-devo), and, after much research, revealed for the first time the major molecular (that is, genetic) mechanisms of evolution.
Some of the most highly conserved genes (that is, genes with the least variation between species) are the genes encoding for ribosomal RNAs (rRNA). Comparing rRNA sequences allows investigators to infer the evolutionary relationships between distantly related species. For example, rRNA studies have been used to estimate the relationships between the eukaryotic taxa (e.g. plants, animals, and fungi), and even delve into the complex relationship between the three major domains of life (bacteria, archea, eukarya).
Transposable DNA elements (noncoding sequences which are, almost always, nonessential) can also be used to determine the relationship between more closely-related species. In fact, this technique of using transposable elements to determine phylogeny uses the same principles as the DNA fingerprinting techniques used in human paternity tests. That separate species would share some non-coding and nonessential DNA sequences strongly implies shared ancestry.
There is, in fact, a very large body of evidence supporting the theory of evolution. It is because of this large body of evidence that acceptance of the theory of evolution is virtually unanimous among biologists.
- Godfrey (1984). Scientists confront creationism.
- Seriously, just google "ancestral fossil" or "transitional fossil" if you can't wrap your head around this one.
- Gilbert (2006). Developmental biology.