ScienceDuo

Welcome to the ScienceDuo blog by Chris Wallis and Rhiannon Morris. Screeds on science and sanity from two people who understand neither.

Common misconceptions about Evolution

Before we start tackling some of the misconceptions about evolution it is important to address the question: What is evolution?

In the spirit of biology and our lack of distinct and straightforward definitions for just about anything, there are a number of ways you can define evolution. Strictly, the biological definition of evolution is: A change in allele frequencies within a population over time. Others may define evolution as decent with modification or simply as change that occurs in a population over time. While each of these are correct in some ways, in other ways they can be somewhat misleading. Perhaps the best way to think about evolution is to think of it as a process that must meet at least two conditions:

Firstly, changes must be stably inherited. This means that traits must be passed on from one generation to another; essentially, the change must occur in the organism’s genome.

Secondly, any genetic change must spread through a population. A change in an individual may contribute to the variation within a population, however unless that change is spread through the population, it cannot be considered evolution. After all, all individuals are born with additional mutations and a different combination of alleles from their parents, but being born doesn’t constitute evolutionary change.

To quote an evolutionary biologist: “In the broadest sense, evolution is merely change, and so is all-pervasive; galaxies, languages, and political systems all evolve. Biological evolution … is change in the properties of populations of organisms that transcend the lifetime of a single individual. The ontogeny of an individual is not considered evolution; individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are inheritable via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportion of different alleles within a population (such as those determining blood types) to the successive alterations that led from the earliest protoorganism to snails, bees, giraffes, and dandelions.” – Douglas J. Futuyma in Evolutionary Biology, Sinauer Associates 1986

So now that we have attempted to define evolution and clear up any misunderstandings as to what evolution is let’s get in to some common misconceptions about evolution

Misconception 1: We (Humans) evolved from monkeys.

Had to answer this one after seeing the good old argument from people who don’t accept evolution: If evolution is true, then why are there still monkeys??

We share a common ancestor with monkeys, we did not evolve from them. This is the worst argument against evolution anybody could ever make and shows a complete lack of understanding of any evolutionary biology. Evolution predicts that all life on Earth shares a common ancestor, which of course includes humans and monkeys. At different points in time each new species branched off from the original species and we ended up with the diversity we have today. For humans and Chimpanzees (our most recent common ancestor) it seems we diverged from one another around 7 million years ago. The population we both diverged from was not human or chimpanzee. Although many (actually most) species have become extinct over time, this is not because other species “came from those”.

Misconception 2: Evolution is just a theory.

Although this is strictly true, evolution is indeed a theory, people misunderstand what is meant by theory in this case. In science we deal with levels of certainty based on observed evidence. Generally, the more evidence we have for something, the more certain we are that what we have observed is “true”. A theory in science refers to a comprehensive explanation about an aspect of nature that is supported by overwhelming evidence. Within science, the definition of a theory is different than its colloquial counterpart, which usually means a best guess. Scientific theories explain of a body of facts that are supported by a lot of evidence and have been thoroughly tested. In this way, a theory is superior to any single fact, a theory can never be promoted to a fact.

Evolution is often considered to be both a fact and a theory. Evolution is a process that we have extremely good evidence for, on both small and large scales. Evolution describes both a historical process that has resulted in all the biological forms we see today, and an ongoing process of change that still occurs. In this sense then, evolution is a fact because we have good reason to believe it happened, but in another sense the theory of evolution also explains this process and makes predictions about what we expect to see

Examples of other theories in science:
Gravity- Because we aren’t all floating around, I think we can assume this one is true.
Heliocentric theory- Quite sure that all the planets in our solar system orbit the sun.
Cell theory- We are pretty certain that all living things are made up of cells.

Misconception 3: Humans are no longer evolving.

In some ways yes, in other ways no. Because of advancements in medicine and other technologies we have  removed many of the selective pressures that our ancestors faced For example, because we can treat many diseases that would usually be selected against, we have not evolved natural immunity to  many diseases prevalent in our populations anymore. However modern humans continue to generate genetic diversity through mutation and  now face different evolutionary pressures including: New diseases caused by viruses like HIV or malarial parasites (sickle cell), changes in brain size, changes to where we live and dietary changes (lactose tolerance).

Misconception 4: Evolution only occurs slowly over many millions of years.

Although evolution often does occur gradually over very long time scales, it can also occur rapidly. We can see evidence for rapid evolution all around us- an obvious one is changed in the reproductive cycle of many species due to changes in the environment and global temperature. Thanks global warming! Also, processes like horizontal gene transfer may confer an immediate and rapid change in a population, such as instant utilization of a new carbon source or resistance to antibiotics in bacterial populations.

Rapid evolution is caused by small population size, short generation time and changes in environmental conditions.

Misconception 5: Evolution is just about mutation and selection.

Charles Darwin’s seminal and most celebrated contribution to evolutionary biology was his theory of natural selection which explains how nature changes populations by matching them to their environment. Natural selection is essentially a combination of differential reproduction and inheritance. More offspring are born that can survive and reproduce and so nature “selects” organism that are best fitted to their environment. This is a brilliantly simple explanation and indeed natural selection is considered to be one the most important forces in evolution, especially in producing adaptive changes. However, despite Darwin’s brilliant idea, natural selection is not the whole story. For example, we now know that most changes at the molecular level (mutations) are neutral or nearly neutral with respect to fitness, this means that most changes spread through a population by random genetic drift. Surprisingly this also applies to slightly deleterious and slightly advantageous mutations, these are maintained in a population by the random noise of drift which overwhelms the power of natural selection to remove or accelerate their fixation. This has important implications for evolution, for example in protein evolution, a new, useful function may only arise if a combination of multiple mutations occurs, some of which may be neutral or slightly deleterious on their own. In other words drift allows the fixation of neutral or slightly deleterious mutations that later on are required for a new and selectable feature to evolve. This process, termed “potentiation” was one the many interesting results of the famous Long Term Evolution Experiment (LTEE) where a population of  E.coli evolved that could utilise citrate as a sole carbon source.

Misconception 6: Evolution is random.

Random mutation is an important non-directional process that produces variation in a population. This is the raw material on which various forces act. These forces are however decidedly not random. Consider selection, this is clearly not a random process. Populations are suited to their niches by adaptation and selection not by chance. Although favourable traits may have occurred by random mutation, selection of said traits is non-random. It is also important to note that his does not mean that evolution has goals, which we tackle below.

Misconception 7: Evolution has goals and always results in progress meaning that organisms get better due to evolution.

Evolution is often depicted as a march of progress that continues to make organisms more complex and more perfect. If this statement were true, surely we would all be free of disease, no? The process of evolutionary change leads to a variety of traits, some of which may be better, or progressive, others are not. These traits are more a product of our history than because of progress. If the tape of life was replayed, then it is likely that a completely different set of organisms would evolve with completely different traits. Also, evolution allows individuals with a large range of traits to survive, however that does not mean these traits are necessarily the best traits. Some populations have a high rate of “bad” genes because there is a high frequency of that gene in the population- this is not exactly progress. Natural selection doesn’t even produce organisms that are perfectly suited to their environment, some are just ok at surviving in that environment

A common misconception is that humans are the ultimate goal of evolution. This is not true, we are like everything else, a happy accident that occurred via evolution over millions of years and we are by no means perfect. The human body has all kinds of things that are poorly “designed”. Here is a really great example of poor design, albeit in a giraffe: https://www.youtube.com/watch?v=cO1a1Ek-HD0

Misconception 8: Individuals evolve.

Because the very definition of evolution is changes in a population, this means that individuals themselves cannot evolve. Individuals can develop over their lifetime, but those developments are not passed on to their offspring.

Misconception 9: Gene trees and species trees don’t always match, so phylogenetics must be inaccurate.

Richard Dawkins once said that you can take any pair of genes from any pair of species and correctly reconstruct its evolutionary tree. This is correct in a sense, but there is a problem, namely that not all phylogenetic trees drawn from genes match the actual species tree. An example of this (much misinterpreted by Creationists) is that up to 30% of Gorilla genes are more closely related to Humans or Chimpanzees than the two are related to each other. In other words, even though the standard phylogenetic tree of primates shows that Humans are most closely related to Chimps, trees drawn from some genes shows that humans are more related to Gorillas. I’ll explain why this is not a problem from evolution at all.

Firstly it’s important to note that species trees represent a statistical average of all the genes in the population of a species and that because populations contain many alleles of a given gene and that they segregate independently of one another, some genes don’t segregate evenly when a speciation event occurs. Imagine a gene with three alleles A, B and C, gene A is the ancestor of B which formed by a single mutation, then some time later some B alleles mutated to form C. Now we have three versions of a gene in a single species, but that gene itself has its own tree that shows A is more closely related to B than it is to C. Now imagine that this population splits into two populations, just by chance allele A and C may segregate into one population and B into the other, now lets suppose that the new population containing A and C also splits into two populations, one receives the A allele and the other C. Now we have three populations each with a different allele but the relatedness of the population doesn’t necessarily reflect that of some of the alleles in them. This phenomena is called incomplete lineage sorting (ILS) and far from being a problem for evolution, it is actually a prediction of evolution, as it shows the stochasticity of genetic segregation. Also ILS can be used to predict useful parameters of ancient populations, for example ILS has been used to predict the effective population size and effects of bottlenecks on early human populations, these studies have shown that the smallest effective human population size was around 1200 individuals around the time we migrated out of Africa.

Misconception 10: All traits are adaptive.

Although it may seem this way at times, when you really think about it, this is obviously not true. The best example is the colour of our blood. This is not an adaptive trait; it is simply a product of the chemistry of our blood. Also, neutral and nearly neutral evolution teaches us that  most changes have no effect on an organisms fitness and these changes can become fixed by genetic drift, this also means that certain traits can arise via non-adaptive means.

Misconception 11: Epigenetics shows that Lamarck was correct and Darwin was wrong.

Darwin was wrong about many things, this is only to be expected given that he didn’t have access to many ideas only available through more modern discoveries. He nothing about genetics or DNA, never mind epigenetics and horizontal gene transfer. Epigenetics has been touted to overthrow our understanding of evolutionary biology, displacing outdated Darwinian mechanisms like natural selection and drift acting on genetic variation in a spectacular Kuhnian style paradigm shift. This is nonsense.

I won’t go into detail on the mechanism of epigenetics but essentially it involves switching off genes via attaching a methyl group (CH3) to a gene or regulatory region, or changes in histones that can increase or decrease expression of nearby genes. This process is extremely important during development where different cells differentiate and become committed to certain cell types which form different organs and tissues. Epigenetic regulation is also important in health and disease. Epigenetics is not an important mechanism of evolution.

The reason people seem to get so excited by epigenetics is that these changes are not immediately encoded in the DNA sequence (hence the “epi” prefix which means above) and these changes can in some instances be transmitted to offspring. Thus epigenetics is often linked to and conflated with inheritance of acquired characteristics, essentially Lamarckian inheritance.

Lamarckian inheritance was a competing evolutionary model that posits that the experiences of an organism can be transmitted to the germline and thereby passed on to its offspring. Think of a giraffe stretching its neck to reach higher leaves which caused its offspring to have slightly longer necks which they then stretched and so on. This idea was largely abandoned when no plausible mechanism could account for changes being transmitted across the Weismann barrier which segregates germ cells from somatic cells.

The problem with an epigenetic view of evolution is that most epigenetic tags are removed in the germ line by reprogramming which preps germline cells for developmental differentiation. Thus epigenetic changes are rarely passed on to offspring and ones that are have been shown to be unstable and reset after a generation or two. This instability means that meaningful adaptive changes will not spread through a population and effect the structure of that population as proper evolutionary change requires. Epigenetics also has no mechanism for cumulative changes that could result in something like a new biochemical pathway. This is because epigenetics only acts as a switch between two states, a gene is turned on or off and this may be inherited, but new structures cannot be formed in this way.

So although non-genetic changes can be passed from parent to offspring, they rarely are and are removed quickly. Contrast this with the Darwinian view of heritable changes where thousands of studies on speciation and adaptation have shown that changes in the genetic sequence underlies long term population level change.

Heard, E., & Martienssen, R. A. (2014). Transgenerational epigenetic inheritance: myths and mechanisms. Cell, 157(1).

 

And remember kids:

laddervstree

Image here

 

Hope you enjoyed and congrats if you made it to the end.

Rhiannon Morris and Chris Wallis

 

 

 

 

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2 comments on “Common misconceptions about Evolution

  1. This will spur me on to read Matt Ridley’s new book on evolution which I’m stalling on at the moment. I’m glad to have discovered your blog. I never was much good at essay writing myself.

    Like

    • ScienceDuo
      March 29, 2016

      I feel like I should give that one a read at some stage too. Glad you enjoyed this – Rhee

      Like

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This entry was posted on March 29, 2016 by in Uncategorized.
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