The earth is approximately 4.5 billion years old. What started off as a primordial soup is now a bustling world teeming with millions of species of plant and animal life, dominated by a species of higher primates known as Homo Sapiens [CITE1]. Throughout the young life of the planet, and even younger existence of intelligent life on Earth, there has been much introspection, debate and controversy surrounding the very basic question of how various organisms came to be, survive and adapt in particular manners. Upon much retrospection, one particular human by the name of Charles Darwin, came to the somewhat simplistic solution that all species of life originated from common ancestors and were thereby genetically modified as they reproduced via a various mechanisms colloquially known of as survival of the fittest. Evolution could therefore be described as a process in which an organism slowly changes by degrees to different stages over the course of millions of years.
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In this paper, I will explain Darwin's theory of evolution and how it has led to the diverse population. I will discuss the various mechanisms and processes involved in evolution, how they affect various species and most importantly, how evolution portrays the ecologically landscape of planet earth.
The key to Darwin's theory of evolution is the concept of natural selection. Natural selection is the driving force behind evolution it's the mechanism that ensures the creation of new species and the adaptive changes as well as the perpetuation/support of the existing level of adaption within existing species. In any given population of any species there is a certain degree of variation within the genetic makeup of each member of said population. This genetic variation manifests itself as variation in phenotypes (observable characteristics of an organism) and thus some members of the population will have phenotypes that make them more likely to reproduce (produce offspring) and other members of the population will have phenotypes that make them less likely to reproduce (Gregory, 2009).
A number of factors affect any organism's ability to reproduce, the most fundamental of these is the ability of that organism to survive long enough to reproduce and then survive even longer to reproduce multiple times. This type of selection is summed up with the phrase 'survival of the fittest'; an organism's ability to survive depends on that organism's level of adaptation to the environment. Thus members of the population that have phenotypes that make them more adapted to the environment are more likely to reproduce and other members that have phenotypes that make them less adapted to the environment are less likely to reproduce (Gregory, 2009).
Figure Natural Selection (Gregory, 2009)
Another factor that affects an organism's ability to reproduce is that organism's physical ability to reproduce. Certain phenotypes will make an organism more/less fertile or increase/decrease the fecundity (capacity to reproduce) of the organism (Gregory, 2009). Thus members of the population with phenotypes that increase that organisms ability to reproduce are of course more likely to reproduce and organisms that have phenotypes that decrease that organisms ability to reproduce are less likely to reproduce.
Finally the last major factor that affects species ability to reproduce, in this case species that need a mate to reproduce, is the attractiveness of that species to the opposite sex. Often referred to as 'sexual selection' this mechanism is the process by which one mate chooses its corresponding mate and vice versa. Typically males will compete with each other through fighting or just displaying of 'ornaments' for the attention of females and the 'winner' of said competition will get to mate while the loser will not (Darwin, The Origin of Species, Chapter 4 - Natural Selection, 1859). Members of the population with phenotypes that make them more attractive to the opposite sex are more likely to reproduce and the members with phenotypes that make them less attractive are less likely to reproduce.
Natural selection depends on two concepts in order for it to work and influence the genetic makeup of a species/create new species. One is the concept of heredity; the process by which an organism passes its traits on to its offspring in reproduction. The other is the concept of mutation, the process that creates genetic variation in the first place.
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In Darwin's day not much was known about the process of reproduction and so he used the concept of breeding to explain heredity where a breeder would choose animals with desirable traits and try to breed them with as many other animals as possible in order to produce offspring with those same traits (Darwin, 1859). So for example a farmer with a cow that produces more milk than usual will try to breed that cow with as many times as possible in order to produce more cows with higher milk producing capacity. The offspring of that cow could be said to have inherited the trait of having a higher capacity for milk production from their mother. That said a lot more is known about the process of reproduction now so the concept of heredity can be explained with more accuracy.
We now understand heredity in the context of genetics. Traits are expressed through genes that are located at specific loci in an organisms DNA, the variants of these genes are referred to as alleles (Stanford, 2004). So for example at a particular location or Locus (singular of loci) in a human DNA code there is the gene that expresses eye color, some of the variants of this gene or alleles include green brown and blue. DNA, short for deoxyribonucleic acid, then is a collection of these genes that in summation form the blueprint for an organism's growth and functioning.
In reproduction the DNA of an organism is copied and given to the offspring, who thus inherits the genes and alleles of their parent. In sexual reproduction both parents DNA is copied and given to the offspring. When a conflict of allele occurs, for example the mother has blue eyes and the father has brown eyes, the dominant allele is the one that becomes that offspring's phenotype, in this case brown is dominant, although the offspring still carries the blue eye allele in its genetic code (Stanford, 2004).
Mutation is the process by which random changes in the DNA sequence of an organism occur resulting in the duplication, deletion, reorganization, transportation, substitution and/or insertion of genetic traits of that organism. In the context of evolution the mutation occurs when there are errors during the replication of DNA (Synthesis phase) and, in organisms that require a mate to reproduce, during the combination of the two mates DNA (Prophase 1) (O'Neil, 2010). There is also such a thing as horizontal gene transfer in which, through processes like cross pollination, genes are transferred between organisms without producing offspring (Maloy, 2002). These mutations are random and are more often than not detrimental to the organism, but can result in both neutral and beneficial traits. This is the driving force behind the genetic variation from which natural selection makes its choices.
Evolution of Populations Not Individuals
In evolution all the factors that affect natural selection balance out over long periods of time and multiple generations of offspring to produce more adapted populations not individuals (Gregory, 2009). For example let's say that a male organism is born with a phenotype that gives it the instinct to kill all the other male organisms of its species in their sleep, now without competition from other males that individual will probably mate/reproduce more but over multiple generations this trait will harm the population as a whole. Probably in this situation what would happen is that because only one male is matting with the females the genetic variation in the population would decrease resulting most immediately in weak immune systems and the population would probably be wiped by a virus. So although that trait was beneficial to the individual it was detrimental to the population as a whole and so over multiple generations that trait was eliminated by natural selection.
Not every phenotype of organisms can be attributed to natural selection some can be attributed to random sampling of gene variants (allele) and chance, this mechanism is referred to as genetic drift. The so called 'random aspect' of evolution, genetic drift occurs because the alleles that end up in offspring are a random sample of the allele of their parents and chance, despite any genetic advantage/disadvantage an organism might have, is a factor in determining whether an organism reproduces (Moran, 1993). Probability then plays a role in shaping the genetic makeup of populations as well as natural selection. In probability studies there is something called the law of large numbers (more commonly referred to as the law of averages) which states that as the sample size increases the closer the average of the sample will be to the expected value, essentially the larger the sample the closer the outcome to what one would expect the outcome to be. Thus the larger the population of organisms the smaller the influence of genetic drift will have on the genetic makeup (Moran, 1993).
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Figure Genetic Drift (Marginalia, 2009)
For example consider two populations; population A has a total size of 10 organisms and population B has a total size of 1000 organisms. Now imagine in both populations 50% of the population has the allele for blue eyes and the other 50% has the allele for brown eyes. It is far more likely that in population A only the 50% that have the blue eye allele reproduce and pass on that allele (thus eliminating the brown eye allele) than in population B. In population A only 5 organisms need to die before they can reproduce or not pass on that allele during reproduction while in population B 500 organisms would have to fail to pass on the allele.
Creation of New Species (Speciation)
Thus far genetic variation and specialization within existing species has been explained, evolution explains the creation of new species as well. In evolution the process creating new species is referred to as speciation. In nature speciation occurs in at least 3 but possibly 4 fundamental processes which correspond to different extents of geographical isolation (University of California Museum of Paleontology, 2008).
Figure Speciation (BenB4, 2006)
The first process and the most common is allopatric speciation; here an organism's population is split and each group geographically isolated from each other. This can occur for a number of reasons, perhaps a mountain range pops up and splits the habitat populated by the organism in two or perhaps food runs low one year so part of the population emigrate across the desert to find more food. Once isolated over multiple generations each population becomes more adapted to their now separate habitat and undergo a degree of genetic drift until one day the two populations have diverged so much genetically that they would no longer be able to mate and exchange genetic information. Thus over vast periods of time and subject to natural selection/genetic drift and geographic isolation one common ancestor split into separate species (University of California Museum of Paleontology, 2008).
The next two processes are peripatric and parapatric speciation. Peripatric speciation is similar to allopatric speciation in that the two populations are geographically separated. The difference is that in peripatric speciation one population is significantly smaller and periphery to the larger population and thus genetic drift would play a larger role in the divergence of their genetic makeup from the larger population (University of California Museum of Paleontology, 2008). In parapatric speciation the two populations are separate but not completely isolated from each other. At each extreme of the population different ecological niches exist and each separate population becomes more adapted to their particular niche until the two separate populations can no longer exchange genetic information. Essentially in parapatric speciation the distance between the separated populations is so vast that the separate populations adapt and change faster than they can exchange that genetic information threw interbreeding until the two separate populations become separate species and can no longer exchange genetic information at all (University of California Museum of Paleontology, 2008).
The last and controversial process is sympatric speciation; here there is no geographic separation and two populations are said to diverge because of reproductive isolation (University of California Museum of Paleontology, 2008). For example at one extreme of a species population organisms could have larger mouths and eat larger prey while at the other extreme the organisms have could have smaller mouths and eat smaller prey. Perhaps it is somehow advantages in this species to mate with a partner who has a similar diet to yours. The population then begins to polarize around two extremes each population now adapting to a different diet until eventually the two populations can no longer exchange genetic information. Again in sympatric speciation the rate of adaptation and change would have to be faster than the exchanging of genetic information through interbreeding. That said there is no conclusive evidence of sympatric speciation ever occurring and many believe that examples of sympatric speciation are in fact only small scale or 'micro' instances of allopatric speciation (University of California Museum of Paleontology, 2008).
A Brief History of Life as We Know It (or the Evolutionary Timeline)
Any good theory of life as it is should explain the history of life and how it got to this point, from what I can gather the history of life as explained by evolution goes something like this:
4600 MA: Formation of earth.
4100 MA: Earth's crust cooled, atmosphere formed. RNA based organic compounds compete for life in this primordial soup.
3900MA-2500MA: Cells resembling prokaryotes appear. Prokaryotes are cells that lack a nucleus as opposed to eukaryotes which do. First Organisms are therefore "chemautotrophs"; that use CO2 and oxidize inorganic (non-carbon based) materials to get energy.
3500MA; Last Universal Ancestor; this is the most recent organism from which All of life on earth descends from. The Cenancestor had a genetic code based on DNA (the double strand of 4 basic nucleotides), the code was expressed via use of proteins and 20 basic amino acids, glucose, cells now have cellular membranes, and rather importantly, cells multiply via cell division. The split begins - between common bacteria that are still existent today in many forms and Archaea. Archaeans were single celled organisms that were highly resistant and enduring in extreme environments and can still be found in sulphuric volcanic hot springs.
2500 Ma - 542 Ma; The Protozeroic eon; From branch of Archaeans, the Eukaryotes split yet again, as these were now multi celled organisms. It is from the group Eucaryota that the animals, plants and humans that we know are descended. We now have multicellular life.
The Phanerozoic eon; "The Period of well displayed life" begins with the Paleozoic eras which we know for the splendid imagery of a fish slowly gaining land legs. The first footprints on the land of Earth has been found in 530 Ma. (This is the first part of the popular picture of fish to man that we have all come to know, love and abuse). This eon was characterised by mass extinctions, the first major one occuring at the end of this era. The The Permian-Triassic Elimination event led to the extinction of 90-95% of the marine species at the time. This "flood-style elimiation" led to immense diversification and the next 30M years were dedicated to repopulating the planet with lots of fancy plants and animals. In true form to survival, the Mesozoic era which was from 251.4Ma to 68Ma began with a Mesozoic Marine Revolution that led to the power struggle to lean in favour of the terrestrial life. Houston, we now have dinosaurs, let the kids in. Our beloved Jurassic period also occurs here and as we all know, this era ends with a bang, with the Cretaious-Tertiary extinction event ending half of all animal species on earth, including all dinosaur life (except their bird descendants).
Fortunately for us, this meant the beginning of the Cenozoic era which began in 65.5 Ma and is what we are currently stumbling through. Along with certain trees dominating the mountainside, and a time of exciting diversification for ants, mammals became the dominating group on the planet. However at this point we're talking about true primates. Not yet Og.
During this era we see the first bats (52Ma), the first giant whales (40Ma), the first deer (25Ma) and the first Girraffe (20Ma). I suppose, being self interested humans, what is most interesting to us, is that it is during this period that the evolution of various mammals and human ancestors runs rampant. We begin with various groups of low primates, move onto the first Hominin in 6.5Ma, the diversification of the Australopithecenes in 6Ma along with their extinction in 2Ma, the rise and fall of the Neanderthals. At around 2Ma, the first members of the Homo genus appear on earth. Anatomically modern humans only appeared around 200ka in Africa and then around 50,000 years ago, they replaced our only other real rival, the Neanderthal in Europe and other hominins in Asia. This can be traced all the way down to my sitting right here, typing with a potentially longer fingers than my ancestors.
So here we are 4.5 billion years later and in that time the forces of evolution has resulted in adaptation of species to new environments, co-operation between various forms of life, speciation leading to more diversity and extinction which is the culling of species. The theory of evolution is one of the most, if not the most, tested and verified theory in all of human history. Darwin's original idea has lasted for over 200 years when you consider how much even our understanding of physics, the basic laws of this universe, has changed in that same amount of time you must admit the durability of the theory is quite impressive. Huza to 200 years of evolutionary thinking and huza to 200 more!