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The Limitations of Fossils
The Limitations of Fossils
When people think of evolution, often the first thing that comes to mind are fossils - perhaps even dinosaur fossils. While fossils are an important piece of evidence that supports the theory of evolution by natural selection, they are also severely limited and far from the only line of evidence that has informed the theory.
Fossils can show morphological (referring to physical structures) similarities and differences amongst groups, but are unable to display behaviors, genetic similarities, and more. Also, fossils form extremely rarely.
In fact, despite their utility, fossils are actually fraught with biases as a result of how they form. For fossils to form, the remains (or evidence of) an organism must avoid decay and be preserved in a medium. This must lie undisturbed for long enough with sufficient pressure to become mineralised and fossilize.
Soft bodies are much less likely to fossilize compared to hard body structures. As a result, organisms with skeletons are much more likely to fossilize than a jellyfish, for example. Body size also plays a role, of course. Large fossils are easier to find than smaller ones.
Some environments are not very conducive to fossils forming, such as rainforests. Rainforests are teeming with huge arrays of biodiversity, but their warmth and humidity inspires rapid decomposition, making fossilization highly unlikely.
An organism's commonality is also a huge factor. If it is rare, there are fewer individuals that will die and possibly be fossilized, meaning that small populations or populations that are endemic to small areas of the world are unlikely to form fossils as well.
The final bias in the fossil record highlighted here is temporal. Geological processes and fossilization are very slow, but can cause fossils to be buried too deep and lost forever, making old fossils rarer than younger ones.
As a result of the limitations of the fossil record, we will explore some of the other compelling lines of evidence that support the theory of evolution by natural selection.
Biogeography
Biogeography
Biogeography, as its name implies, is the study of living things and where they live. This has evolutionary implications because the closer two species are geographically, the more likely they are to have a recent common ancestor and have traits in common.
We even see this will fossilized organisms, often finding fossils of long-extinct animals in areas where we currently have similar ones such as the giant sloth fossils Darwin found where we currently have modern sloths.
However, some fossils just didn't seem to fit. Occasionally, a group of fossils would be found across continents as if placed there by a higher power. This first seemed to hurt the theory of evolution - how could we find similar fossils across continents when those organisms could not swim nor fly?
Over time, continents have moved apart (a phenomenon called continental drift) and once were a large, combined land mass.
When mapped across our understanding of Pangaea at the time of these organisms, we can see that these organisms actually did share a geographic range.
Homologous Structures
Homologous Structures
Throughout many different taxa, or groups of organisms, we see similar structures with similar evolutionary origins.
These homologous structures are the result of common ancestry, essentially they were inherited from the same common ancestor.
In this image, you can see the homologous bone structures found in tetradpods across diverse organisms. The structures that are common between them are color-coded (i.e. humerus is blue, phalanges are green). These similar structures support the theory of evolution by showing evidence of common ancestry across different groups. The structures are ultimately used for diverse needs, but have a common origin.
Vestigial Structures
Vestigial Structures
Many organisms have obsolete structures that no longer serve a particular function. The classic example is typically the human tailbone. We have a tailbone, but obviously we are unable to move or utilize it in anyway.
So why bother developing these structures? Well, there simply is not enough of a selection pressure to necessitate an evolutionary change. Obviously having a tailbone is a small waste of energy, and it certainly hurts to injure or break it, but that happens so rarely and (nowadays) does not generally result in death. As a result, an individual who breaks their tailbone can still pass their genes on to the next generation and make more tailbones.
In short, there is not enough of a benefit for individuals that do not have these structures for it to become the norm to lose them. Similar things can be said of other human structures such as wisdom teeth or the appendix. Although, there has been recent support for a possible use for the appendix in humans.
Universal Genetic Code
Universal Genetic Code
One aspect of our DNA that is often overlooked is its universality. All DNA, regardless of the organism from which it came, will be transcribed and translated in the exact same manner.
An mRNA strand inserted into a eukaryote will be translated to form the exact same polypeptide as when inserted into an E. coli bacterium.
This is a result of the common ancestry we all share. Our last universal common ancestor (or LUCA, as it is often called) had this genetic system and passed it on to all offspring, eventually (with billions of years of time) resulting in you.
Observing Evolution in Real Time
Observing Evolution in Real Time
Evolution is typically considered to occur at a slow, if not glacial pace. That is because generations take time and many organisms require time to reach sexual maturity.
However, the quicker an organism's generation time, the quicker evolution may occur. Similarly, the more often mutations may occur, the more often traits may appear that natural selection can act upon. These two factors (along with conjugation) make bacteria VERY quick evolvers.
In fact, bacteria evolve on such a rapid timeline that you can actually observe it within a single human body over the course of a few weeks. A significant health concern globally is the evolution if antibiotic resistance in bacteria. That is, bacteria are evolving resistances to our antibiotics that used to kill them much more effectively.
This makes certain diseases much deadlier than they were more recently. In fact, we are noticing that antibiotic-resistant strains of bacteria are popping up independently all over the world do to the misuse of antibiotics. We will explore in class the way by which this evolves in a population of bacteria and how to minimize the risk, but it is important to realize that evolution is not always so slow, and some things are much, much better at it than we are!
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