Evolution – How Do Complex Organs Evolve?

Previously, we looked at some of the fallacies and misconceptions surrounding the theory of evolution, and addressed each one. Finally, we briefly described the common creationist argument against evolution; namely, how do complex organs evolve? This is the question we are going to address today.

To start, let’s examine why the evolution of complex organs could be a potential problem for the theory of evolution. Primarily, evolution occurs over many, many small increments. Single mutations in genes at each generation eventually lead to large differences between organisms over millions of years. Importantly however, is that for the mutation to be passed on to the next generation and therefore be part of the evolutionary process, it must give the organism in question some evolutionary advantage. For example, if we analyse the evolution of the giraffe’s neck, every mutation that led to a longer neck would have been beneficial to the survival of the giraffe. Therefore, the giraffe’s neck gets longer and longer (within reason).

The problem that complex organs evoke however, is that they may not be beneficial at every stage of development. For example, let’s suppose that humans hadn’t yet evolved a spleen. Instead, our blood remained largely unfiltered, and many people would die each year from infection. While a spleen as we know it would be helpful, how would one evolve? The spleen is coded for in our DNA by thousands of genes, with very specific relationships between them. For this reason, it is extremely unlikely (if not functionally impossible) to wait for an individual to be born with all of these mutations arisen in perfect harmony. Instead then, our best bet is to evolve a spleen gradually. And this is exactly where our problem lies. Small genetic mutations that would contribute toward a fully functioning spleen but which, alone, aren’t sufficient to construct an organ that has any benefit to the organism, aren’t beneficial in themselves and so won’t be selected. For example, even if an individual was born with half of the mutations required, the spleen would still not be functional, meaning that the mutations would have no benefit and the individual would be no more likely to pass their DNA (and demi-spleen) on to their offspring. As a result, the spleen evolving gradually in this way seems like a non-starter.

When Darwin was formulating his theory, he considered this problem extensively, and proposed a few solutions to the potential problem. His first, but probably least convincing, was the proposal of incipient organs. He proposed that some organisms would have organs in the process of evolving into complex and functional organs. They may currently be non-functional, or be functional at a lower capacity, but nonetheless they would essentially be a complex organ in the process of evolution. Referring back to our spleen thought experiment, Darwin proposed that there would be organisms with the equivalent of a demi-spleen.

In some cases, this argument seems plausible. For example, the difference in complexity of the photosensitive cell of a eukaryote to the human eye, can be mapped to thousands of small incremental changes, suggesting that the evolution of a complex organ could theoretically take place in this ‘incipient’ fashion. As we’ve previously highlighted however, this is unlikely to happen, and Darwin himself acknowledged this. The resistance of the scientific community to accept that the difference in vision complexity between the species of the world could be explained in this ordinal fashion, alongside the lack of evidence for even finding an incipient organ in any species that has ever been observed, meant that this idea was readily abandoned, with a host of more plausible proposals taking its place.

In truth, there are numerous of genetic and biological mechanisms through which complex organs evolve, which all work in tandem to eventuate in complex organs, but for the sake of keeping things short (kind of), we’ll only go through a few.

 

  1. Exaptation

Whilst the specific nomenclature surrounding the concept of exaptation changes, from ‘co-option’, to ‘recruitment’, to most widely accepted title of ‘exaptation’, the concept remains constant. Essentially, exaptation is the process by which an organ evolves for one function, and then later acquires a secondary function. An apt analogy in a paper I read on the topic, gave the example of the coin. A coin’s primary adaptive function is as currency, however, in the modern world, the coin has also gained the function of a lottery ticket scraper. Whilst a coin would have always have this capability, the function only became apparent when the environment required this function (i.e. a change in the environment made that capability a benefit to have). In this analogy then, the coin had an original function, which remains to this day, but also, at some point in time, developed a secondary function. Taking this analogy further, whilst coins are beneficial in scraping lottery tickets, are they perfectly suited for the job? Not really. In reality, they are probably slightly too small and too hard to hold to be the ‘perfect’ ticket scraper. So, in an ideal world, we would change the coin to be more suited to its secondary function whilst also keeping the coin capable of its original function. This would then be a case of ‘secondary adaption’ – when an organ (or object, in this case analogy) evolved to become better suited to another function.

Moving from the inanimate world back to the biological, the wings of birds and insects provide a good example of exaptation and secondary adaption in evolutionary history. Originally, it is likely that wings were not used for flight. Instead, they would have been useful for catching prey, or in thermoregulation, but would not have been capable of sustaining flight. However, as the wing – which originally would have very little in common with what a biologist would now call a wing – became better adapted to its function of catching prey or regulating body temperature, the ability for the wing to allow for some rudimentary form of gliding would have arisen as a by-product – an exaptation.* As time progressed, the wing would have become better suited to gliding (its secondary adaption), and once fully evolved to support gliding, may have allowed for powered flight – a further exaptation.

*It’s most likely that the wing was originally adapted for thermoregulation, and therefore the bigger the wing, the faster and more easily the organism would have been able to regulate their temperature. As the wing evolved to be bigger and bigger, once it reached a sufficient size, it would have been able to sustain a rudimentary form of gliding. From here, the wing would have evolved to better suit the gliding ability and so on.

As we can see then, certain organs or appendages can evolve in an incipient fashion by virtue of having more than one function. However, this doesn’t really explain the majority of complex organs – for example, exaptation would struggle to explain how the spleen evolved, and even if it did, the question would arise as to how the spleen evolved to serve its primary function in this first place. So, there must be other contributing factors.

  1. Duplication

Evolution and natural selection is presided on the assumption of random mutations in an organism’s genome – small changes at a genetic level that result in changes in the phenotype. One such genetic change that is conducive to evolution is duplication. Duplication occurs when a strand of DNA (of any given length) is duplicated within the genome. For example, the number of copies of the salivary amylase gene, which is involved in breaking down starch, varies from person to person (and interestingly seems to vary depended on starch consumption), demonstrating how a gene can be duplicated.

Duplication at a genetic level can also have a large effect on the structure of the organism. Repeated structures are fairly common within an organism (e.g. teeth, arms, legs, kidneys, etc.), and whilst a duplication could lead to another repeat in a structure without changing its original function – for example, wisdom teeth are a duplication but retain the function of teeth nonetheless – the repeated structure could be available to begin adapting to a new function. Reverting back to the wing example, true flies have a repeated set of wings on their hindquarters. While originally a duplication of the wings the flies use for flight, these hind wings now function as a stabilizer for the fly, rather than as an instrument for powered flight. In this case, the structure that arose as a result of the duplication was surplus to requirement, but was able to adapt to a different function, similar to exaptation but different nonetheless.

Through this method, complex organs can arise from a duplicated structure that has the ability to evolve for a new function. For example, this isn’t true, but the spleen could have evolved from a duplication of the liver. A genetic duplication would have led to a repeat of the liver, with one liver retaining its original function, and the other evolving to serve a different function, ultimately becoming what we know as the spleen.

 

  1. Collage

To fully understand the process of evolution, one must understand that organs and organisms do not exist in isolation. The biological world is a complex web of relationships, where the tiniest change in one aspect will inevitably have a ripple effect, spreading exponentially far. A microcosmic example, is within our own bodies. Our bodies are the culmination of various organs, appendages, cells, neurons, tissues, all functioning together to create a working organism. But, these relationships are not set in stone, and are capable of change, and it is through this ability to change that complex organs can evolve.

Let’s imagine a really simple organism that is currently without a complex organ. Instead, the organism comprises many small components. These components currently each have their own function that they serve well. Here are the components…

Components 1

Whilst they serve their respective functions well, by random mutation, these components come together to form a single structure, either performing each of the three functions more efficiently than before, or by serving the original three functions as well as a further function as a consequence of the amalgamation…

 Components 2

However, this new structure is not perfectly adapted for its new function, and so begins to be modified by natural selection…

 Components 3 

At this stage, another component, previously associated, becomes associated with our organ, allowing the organ to perform yet another function…

Components 4

This organ, once again, changes as a result of evolutionary advantage and natural selection to become more suited to its new function…
Components 5

This same process continues until we observe a fully functioning complex organ. This process of collaging demonstrates how complex organs can results from the amalgamation of many smaller, separate components.

 

  1. Scaffolding

The final process we are going to discuss today is ‘scaffolding’. Scaffolding describes the fact that a complex organ is unlikely to have always existed as some lower form of what it currently is. For example, when a building is constructed, scaffolding is placed around the building to aid with its construction. Once the building is complete however, the scaffolding is removed, leaving a building that appears to have always been in some form of its finished state. The same principle can be applied to the evolution of complex organs. For example, let’s imagine that component X performs a task in an organism. At some point, by the process of collage that we described before, component A is added to X to improve its ability to perform its task. At a later date, component B is also added, so that the observed organ is essentially XAB. As a result of natural selection, the organ becomes sufficiently modified to no longer require X, so X is selected against and eventually removed from the system. To an outside observer, the organ may appear to have gone from A and, separately, B. But, in truth, A and B were only able to be joined by the original presence of Z.

 

The previous descriptions of the 4 methods through which complex organs may have evolved highlight the interdependence of evolutionary pressures. No single process works in isolation, in much the same way that no part of the body, no organism, and no species is an island. In truth, the development of organs is as complex as the organs themselves, and each and every collection of tissue inside of our bodies is the result of all of the above processes, alongside millions of years of evolutionary pressure, millions of mutations and many phases we are yet to discover. If that doesn’t convince you that you’re special, I’m not sure what will.