In our opening article to the Human Origins section, we bring you a discussion of what evolution is and how evolution works so that you can understand the principles and mechanisms behind it, to later journey with us as we extrapolate these concepts, along with an estimation of mankind’s proclivities, to where the future of human evolution may be headed.
On This Page:
- 1. Summary
- 2. What is Evolution?
- 3. The Evolution Process
- 3.1 Gene Flow and Genetic Drift
- 3.2 Natural Selection
- 3.2.1 The Species
- 3.2.2 Variation
- 184.108.40.206 Sexual Recombination
- 220.127.116.11 DNA Damage
- 18.104.22.168 DNA Replicating Errors
- 22.214.171.124 DNA Self Editing (MGEs)
- 3.2.3 The Environment
- 3.2.4 Mate Selection
- 4. Genetic Engineering and the Future of Human Evolution
- 5. References
Evolution is the process by which organisms change through successive generations.
Evolution occurs through gene flow, genetic drift, and natural selection. Gene flow is when separated populations reunite and mix their traits. Genetic drift is the random, benign process of certain hereditary traits becoming more common in a species through generations, regardless of the environment. Natural selection, on the other hand, involves a species, new variations appearing in that species, the environment culling out unfit variations, and mate selection which ultimately determines which characteristics get propagated.
There are a finite number of ways variation can be introduced into a species’ gene pool: the natural processes of sexual recombination and DNA editing, externally-caused DNA damage, and internally-caused DNA replicating errors.
Mankind is changing the evolutionary rules with regard to the introduction of variation into the gene pool and the environment’s role in the natural selection process –
It is speculated that such manipulations will have an impact on the resultant Homo sapiens and potential future derivatives.
2. What is Evolution?
In the broadest sense, evolution is merely change, and is therefore universal; galaxies, languages, and political systems all evolve. When focusing on biological evolution, as we do in this article and on this website, evolution is the process by which organisms’ DNA change through successive generations (1)(2).
Note that the above definition does not yet say “to be better suited to the environment”. That’s a specific aspect of evolution- Read on.
3. The Evolution Process
There are three broad mechanisms for how species evolve: gene flow, genetic drift, and natural selection (3).
3.1 Geneflow and Genetic Drift
For the purposes of our discussion, let us quickly address gene flow and genetic drift. Both gene flow and genetic drift homogenize, or promote “sameness”, across a population.
Gene flow requires a situation where two (or more) populations of a species have been separated over long enough periods of time to form distinct traits. If they mix again, the genes will “flow” between populations. While initially providing variation to the two groups as seen separately, gene flow ultimately spreads genes across the two groups homogenizing them as a whole. Gene flow is occurring today in humans as modern transportation closes the relative distance between previously-separated populations (4).
Genetic drift is simply the statistical shift over time of how often traits (genes) that already exist, show up (frequencies). Certain traits either become more or less prevalent in a species as a result of random genetic processes irrespective of the environment.
Say there’s a spot on the underside of your dog’s tongue that doesn’t affect its sense of taste or smell and is no way associated with reproductive ability, but after several generations it’s showing up in all the pups of the bloodline. Then, as the spotted tongue dogs mix with other breeds, it starts showing up there too. If in several thousand years all dogs had the spot, that “drift” will have become “fixed”. At that point, if you’re a dog, you’ve got a spot on the bottom of your tongue.
3.2 Natural Selection
Neither of the former two ‘mechanisms’ or processes really have anything to do with the long-term evolution of a species. Natural selection is the crux of evolution theory.
Now, a key concept often misunderstood, is that evolution actually does not help shape the “best” organism. It does not produce the most “fit” organism as is often heard. Natural selection has no goal to optimize an organism’s potential. What it does is allow the organism best suited to the immediate environment to survive or in some cases even flourish. It in no way intentionally prepares any organism for future demands or changes in the environment.
There are four parts to natural selection in sexually-reproducing organisms: 1) you have a species, 2) you have a variation that “happens” to a member of the species 3) you have the environment the species lives in that is either friendly, neutral, or hostile toward the variation, and 4) you have the mating phenomenon of that species that ultimately determines the heritable characteristics that get passed on to successive generations. If a variation is particularly advantageous in terms of survival and/or mate selection, it will become more and more prevalent throughout the population.
To simplify the thought, let us write it in expression form:
[species] + [variation] - [environment] x [mate selection] = [adapted species]
Now let us examine each of the four Natural Selection factors.
3.2.1 The Species
There can be several scientific nuances to defining what a species is, including similarity of DNA, their physical construct (morphology), or ecological niche. We’ll use the one most serviceable in the broad discussion of the evolutionary process: “A species is a group of organisms capable of interbreeding and producing fertile offspring” (5).
Naturally, on a site named The Future of Human Evolution, the species we’re most concerned with is Homo sapiens. Feel free at this point to note the idea of subspecies, because one of the concepts we’ll be discussing throughout the site is that of our near-term ability to create adaptive traits specific to whatever biosphere environment in which we may find ourselves. A subspecies is simply a geographically separated instance of a species that develops unique traits (6).
How does one thing become another (the whole premise of evolution)? Without the ability to change, organisms would never have survived on earth through the various severe climate changes that have occurred over the past two billion years (7).
Variations are inheritable differences among the individuals of a single species. Eye color and height are examples of observable variation, whereas resistance to a disease may not show on the exterior but is very important from an evolutionary perspective. There are many such examples of variation (8). In order for variations to be acted upon by natural selection and therefore become a species-wide trait, the variations must be heritable (repeated for emphasis).
For one thing to become something else, this “variation” must be introduced into the genetic code (DNA) that gets transferred to later generations. There are a finite number of ways variation can happen. In fact, to put a point on it, variations in nature are the result of either, a) sex-related recombination, b) externally-caused DNA damage, c) internally-caused DNA replicating errors, or d) DNA’s own self-editing capability (9). Many scientific journals refer to change from any source as “mutation”.
126.96.36.199 Genetic Variation from Recombination
Recombination occurs in sexually reproducing organisms, such as the human. The parent has two sets of chromosomes in each cell, one from its father, the other from its mother. In the body’s process of creating the sperm and the egg at the cellular level, the usual paired chromosomes are split such that the sperm and the egg carry only one set of each to the offspring. At conception, when the sperm and the egg combine, each set of chromosomes are dissected and scrambled, then reformed with entirely new combinations of traits (a.k.a. alleles). This process adds variability to the offspring and allows the testing of new allele combinations (new traits) against the environment (10).
188.8.131.52 Genetic Variation from External Damage
While cells have a remarkable ability to repair DNA damaged by the environment, occasionally a change in an existing cell’s molecular structure caused by the environment survives. This is not usually a hereditary issue unless the cell affected is part of a developing organism and through replication propagates the altered DNA thoroughly enough to affect the reproductive cells and be transferred into the population via normal reproductive means. The term “Mosaicism” refers to an organism comprised of both damaged (altered) and undamaged (unaltered) cells (11).
184.108.40.206 Genetic Variation from Replicating Errors
Every time a human cell divides its DNA replicates. This happens in humans as in most organisms during the maturation process and continuing constantly thereafter as cells are replaced. Cells have to copy and transmit the exact same sequence of 3 billion nucleotides (structural DNA molecules) to its daughter cells. While most DNA replicates with fairly high fidelity, mistakes do happen, with polymerase enzymes sometimes inserting the wrong nucleotide or too many or too few nucleotides into a sequence. Fortunately, most of these mistakes are fixed through various DNA repair processes. Repair enzymes recognize structural imperfections between improperly paired nucleotides, cutting out the wrong ones and putting the right ones in their place. But some replication errors make it past these mechanisms, thus becoming permanent mutations (12).
220.127.116.11 Genetic Variation via Internal Natural Editing
Mobile Genetic Elements (MGEs) are a type of genetic material that does not directly include instructions for organism-building, but does move DNA around within the genome. Scientists do not agree on where this material comes from. Because it is found in all forms of life, some say it was introduced in the form of a virus-like entity in the “last universal common ancestor.” In fact, when first discovered in the 1980’s it was often referred to as “junk DNA” because it appeared to have no function. Since then scientists have discovered that this movement of DNA can in some cases have an indirect affect an organism’s form and function, making evolutionary leaps possible. This leads other scientists to believe that MGEs developed as a natural part of genomic evolution (13) (14).
3.2.2 The Environment
For most of history, the environment simply “was”. It neither catered to nor conspired against any species. Species either got along in the environment, or they died out. As the environment changed, the ice ages for instance, either species were already suited to adapt, or a variation introduced into their lineage helped them cope.
Man has become a different animal, so to speak. We have altered our environment to an amazing degree. In fact, many of the threats inherent in our environment have been nullified. As a result, a new temporary “normal” has been defined where people with genetic debilitations that would previously have made them unfit for the “natural” environment can now lead productive lives and perhaps even transmit what would have been considered genetic disorders to future generations. Some futurists today are concerned about the future of human evolution and point to this situation as a losing scenario in a downward spiral of dysgenics. Humans have removed the culling effects of the environment. Believers in the benefits of germ-line engineering see the same situation and recognize it for what it is: a temporary condition in which humans have learned to alter one factor in the natural selection equation (the environment) without having mastered control over purposeful variation: altering the dysfunctional gene(s) for healthier alternatives. This mastery to correct and improve upon dysfunctional DNA will come in short time. No need to fear dysgenics.
One final word on the environment: we do not mean to imply that man has mastered the global environment, merely many survival-related elements within it. In fact the health of the biosphere remains a top concern of scientists and futurists alike. We are on a race to see if man’s instigation of global warming and the loss of biodiversity will optionally a) upset the balance of earth’s environment to the point that it is hostile to the human organism, b) allow us long enough to learn to mitigate our own destructive forces, c) allow us long enough to colonize extra-solar planets, many of which we are finding are terrestrial in nature, or d) both (b) and (c), allow us long enough to learn to live with the planet while colonizing others in the future.
3.2.4 Mate Selection
Critical to the topic of natural selection in sexually-reproducing organisms is the mate selection phenomenon. From evolution’s perspective mate selection is just as, if not more important than, the environment in deciding what traits are passed on to future generations, sometimes to the detriment of survival capability. The Robin has been used as an example of this. Females at some point took a liking to a hint of red in the male. Multiplied over thousands of generations, the male is now bright red and the easy target of predators (15).
The point is that nature may inadvertently create the most wondrous variation, perhaps immunity to cancer for example. But if the person first gifted with the variation can’t find a mate with which to procreate, the species gene pool will never receive the benefit of nature’s gift.
4. Genetic Engineering
By now, you understand how the whole process of natural evolution works, right down to the molecular level. Genetic engineering is simply the manual editing of the genes, much in the same way that natural editing occurs. By manipulating DNA in a zygote (the single cell resulting from a fertilized egg that grows into a human) we can introduce deliberate, targeted, heritable variation that can then be tested by the environment. Just like nature’s evolution, the future of human evolution is subject to the natural selection equation: (species) + (variation) – (environment) x (mate selection) = (adjusted species). The key differences are randomness and time. We don’t have to wait eons or leave it up to chance to accidentally hit upon desirable or necessary traits. In a free society we can enjoy the opportunity to choose a happier, healthier genetic future for ourselves and our children.
See our genetic engineering section for more.
(1) Northwestern University Online Definitions, http://groups.molbiosci.northwestern.edu/holmgren/Glossary/Definitions/Def-B/biological_evolution.html
(2) Merriam-Webster Dictionary, http://www.merriam-webster.com/dictionary/evolution
(3) Berkeley University, Evolution 101, http://evolution.berkeley.edu/evosite/evo101/IIIMechanisms.shtml
(4) Extensive gene flow in human populations as revealed by protein and microsatellite DNA markers.
Santos EJ, Epplen JT, Epplen C. http://www.ncbi.nlm.nih.gov/pubmed/9156328
(5) Merriam-Webster Dictionary, http://www.merriam-webster.com/dictionary/species
(6) Merriam-Webster Dictionary, http://www.merriam-webster.com/dictionary/subspecies
(7) A journey through climate history. http://www.abc.net.au/innovation/environment/cc_timeline.html
(8) Google search on “examples of genetic variation”. https://www.google.com/search?q=examples+of+genetic+variation
(9) Modern Genetic Analysis.Griffiths AJF, Gelbart WM, Miller JH, et al., New York: W. H. Freeman; 1999. http://www.ncbi.nlm.nih.gov/books/NBK21393/
(11) Chial, H. (2008) Somatic mosaicism and chromosomal disorders. Nature Education
(12) DNA Replication Errors and Causes of Mutation, http://www.nature.com/scitable/topicpage/dna-replication-and-causes-of-mutation-409
(13) A Discussion of Mobile Genetic Elements, http://www.icr.org/article/evolutions-best-argument-has-become/
(14) Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 9.3, Mobile DNA. Available from: http://www.ncbi.nlm.nih.gov/books/NBK21495/
(15) Onelife Website, The Evolution Process, http://www.onelife.com/evolve/evolution.html#EP
Additional resources referenced in the writing of this article:
- What is a gene mutation and how do mutations occur? http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/genemutation
- Mobile Genetic Elements: The Agents of Open Source Evolution. http://people.ibest.uidaho.edu/~bree/courses/30_Frost_2008.pdf