Introduction
Evolution is the change animals and species undergo over time to adapt to certain environmental changes and eventually produce multiple descendent species. Evolution and natural selection are often taken as complimentary to each other. However, evolution, by definition, is a historical process of change, while natural selection is merely a mechanism that results in this change. Advances in technology and biology has thrown light on several causes and consequences of evolution. Some of the deepest understandings of evolution come from understanding fossils and examining phylogeny and from the study of natural selection. Evidences suggest that when natural selection is strong, the rate of evolution is rapid, more than what was suggested by Darwin. Developments in biotechnology and molecular biology have enabled scientists to recreate evolution under laboratory conditions and provide a more comprehensive understanding of evolution and bio-diversity.
Around 375 million years ago, a large salamander-like creature stepped out of its watery home into the land and resulted in the creation of land vertebrate creatures, birds, insects and everything that we see today. Studies show that life originated on earth nearly 4 billion years ago when the first molecule replicated itself and began the evolutionary process. Fast forwarding several hundreds of millions of years, the last few hundred million years saw several determining factors in evolution such as the rise and fall of the dinosaurs, the Permian extinction, where over 90 percent of all species got wiped out from the surface of the earth. These events are significant milestones in the evolutionary process of life on earth and are corroborated by solid evidences. These help evolutionary biologists understand how life on earth took this particular path.
Evolution is an ongoing process, one that’s working even now, possibly at a much faster rate than in the past due to developments in technology driven by a human-dominated society. Consequently, evolutionary biology is also moving ahead actively studying and investigating how today’s actions affect the process of evolution and what possible path could evolution take in the future. However, evolution is not purely a biological process. This is going to affect humans in many significant ways such as production of advanced crops in agriculture to understanding the human genome. Thus, evolutionary processes has a wide relevance that plays primary roles in the advancement of many areas such as computer programming, medicine, engineering, etc.
Defining Evolution
The Oxford English Dictionary defines evolution in 11 distinct ways. Mathematically, it defines evolution as the successive transformation of a curve by altering the conditions that define it. In chemistry, it is defined as the emission or release of gas and energy in the form of heat, light, etc. The military defines evolution as a particular manoeuver performed tactfully to adapt to different strategies. This wide range of definitions extend even into evolutionary biology with scientists accepting every possible definition of evolution. Jerry Conyne in his book Why Evolution is True defines evolution as “genetic change in population that produce observable change in the traits of the organism over time” (Coyne 2009). Futuyma in his book, Evolution, defines evolution as “changes in properties of groups of organisms over the course of generations” (Futuyma 2005).
Despite all these definitions, the use of the word ‘evolution’ did not appear in Darwin’s On the Origin of Species until the sixth edition. In his earlier editions he referred to the process as descent with modification, which in its own way encompasses everything that evolution stands for: the metamorphosis of plant and animal species through time, including the changes that happen internally to a particular species as well as the creation of new species.
The Concept of Design
One of the key strongly argued, thought about and debated aspect of evolution is design. English philosopher William Paley suggested that the existence of well adapted organisms with specific features and characteristics is a result of some kind of divine intervention, or the hand of God. This was a lucrative proposition for many theologians who supported the concept of involving God’s work in creating design. Darwin, who had formally trained to be a minister, was also tempted to go ahead on this train of thought before eventually dismissing it. The designs plants and animals possess specifically fit their way of life and it was a difficult task not to hold divine powers responsible for this. However, Darwin did look beyond the easier option and this resulted in the creation of what is accepted now – natural selection and evolution.
Darwin’s methods were scientific, citing nature to give credence to his theories. Consequently, what was widely accepted as natural theology for many centuries until 1859 was dismissed in just a few years with the publication of On the Origin of Species. This way of thinking, called Darwinism, gives an extremely simple explanation of how all creatures and organisms adapted and changed into how they appear today. However, this concept is often misunderstood and, in many cases, taken out of proportion and damagingly misinterpreted. In simple terms, life on earth happened after a single molecule replicated itself about 3.5 billion years ago before diversifying and creating many new species in the process. The mechanism through which these organisms evolve over the years is called natural selection.
However, interestingly, the process of evolution does not apply to every organism that exists. For example, the gingko plant and the horseshoe crab have not evolved over millions of years. Thus, the theory of evolution does not imply that all species will constantly evolve over the years. Species like humans and whales have evolved rather rapidly on this time scale compared to many other species.
An important component of the theory of evolution is the concept of gradualism, which states that the change of species from form to another is a gradual process which takes many generations to develop. For instance, the metamorphosis of reptiles into birds was one that took several thousands of millennia to complete. Gradualism doesn’t mean, however, that species will develop and evolve at an even pace. Microbes, for instance, have a short lifespan and consequently have the ability to evolve much faster than, say, humans. When the effects of natural selection are strong, the evolutionary processes speed up. This happens when species occupy new environments and begin to adapt.
Evolutionary Patterns
As discussed earlier, it is unscientific to equate evolution and natural selection. Natural selection is just the mechanism which brings about evolutionary changes in organisms. There are also other mechanisms through which evolution can take place. However, it is necessary to understand what natural selection means. This is a mechanism through which certain populations that possess a certain trait make offspring that do not possess that trait. For instance, a group of people with blue eyes can start having offspring that do not have blue eyes. This can occur through several mechanisms. Such selections can happen if the variant leads to enhanced characteristics such as longer lifespan, improved attractiveness to the opposite sex, improved fertility, etc. It is difficult to logically explain natural selection. However, if one trait variant is somehow associated with improved reproducibility, then more members of the population will possess the variant in the next generation. This process when continued over several generations, eventually change the construction of that particular population.
However, it is possible that natural selection does not cause any evolutionary change if the differences among individuals are not genetically connected. Natural selection is a biological process where traits are transferred from a parent to an offspring. This is a mandatory requirement for natural selection to result in evolutionary change. In this process, the offspring resemble the parent and the trait variants they possess will increase in frequency the subsequent generations. However, again, offspring do not always possess the same traits as their parents. In some cases, there are phenotypically different individuals. This is not due to changes in their genetic structure, but an adaptation to a possibly new environment during their growth. These kinds of selection where genetic material is not exchanged between parent and offspring, do not result in evolutionary change. Species surviving and reproducing will produce offspring successfully that do not differ genetically from those that fail to prosper. Consequently, the gene pool does not change.
It is important to also understand that natural selection alone is not the only mechanism responsible for evolution. Processes such as mutation, genetic drift, immigration of individuals with different genetic constructions, etc. result in changes to the gene pool or the genetic constitution of a population from one generation to the next. This means that natural selection can result in adaptive evolution, but not all evolutionary changes are adaptive. However, a century and a half of rigorous research has shown conclusively that natural selection is a powerful mechanism that is responsible for evolving species into how we know them today.
For example, wooly mammoths roamed parts of Eurasia and North America and were able to survive and adapt due to its thick and wooly coat. These could be descendants of the mammoths that existed before them, animals with scanty hair like modern day elephants. Changes in genetic structure or mutations in ancestral species could have resulted in these species having their thick coats. As the climates cooled down and the animals migrated north, these species were better suited to adapt and survive and consequently, left offspring that was enriched in genes for hairiness. Thus, subsequent generations were hairier than the previous ones.
A Closer Look into Evolution
There was a time when many scientists associated evolution to gene frequencies. This ideology changed towards the mid part of the 20th century when scientists trained their focus on the genetics of population giving priority to natural selection (Losos 2013). This was enhanced with developments in molecular biology and molecular methods were used in studying evolution. Through these methods, biologists found that populations contained more variations that what was initially assumed. This observation showed that natural selection was not the only driving force that led species towards evolution but also directed focus towards the genetics of populations. There are three primary aspects in understanding evolutionary biology.
Firstly, it is necessary to understand that phenotypic changes occur in the developmental processes that transform a single cell fertilized egg into a fully developed adult individual. Even though controlled genetically, development of an organism is an intricate process that cannot be clearly understood by merely studying the DNA sequence of the organism. It is more important to focus attention to the development of the phenotype, the extent to which these developmental systems enhance or restrain evolutionary change. For this, a deep molecular and embryological understanding is essential.
Secondly, without knowledge of paleontology and history, it is difficult to proceed and understand evolution. The study of fossils gives direct evidence and insights into how life existed in the past. Paleontology has witnessed a surge in the past few decades with many significant discoveries being made. These new discoveries have shed new light on theories of evolution with respect to new data, statistics, species selection, mass extinction, etc. this branch of science is an integral part of evolutionary biology. This has enables scientists to paint what is known as the tree of life, the pattern of descent of different species and the relationships between different species.
The final aspect to be noted is that life is hierarchically organized. Genes are located within individuals, individuals exist within a population, populations exist within a species, and species within clades. Although population genetics deals with changes within a population, evolutionary changes happen at all the different levels. Natural selection cannot be answered or understood completely by just studying a population alone. For instance, over 2000 species of rodents exist while there are just three species of monotremes, a particular type of mammals which include the platypus. It is important to analyze the entire species to be able to answer a few questions. Another example is the presence of useless noncoding of DNA in many species. One reason could be that just a few genes are capable of mutating and multiplying their own copies efficiently. This DNA might increase in frequency if the particular gene is beneficial to the individual. Thus, similar to the process by which selection of heritable traits among individuals lead to evolutionary changes in a population, selection among the other levels can also lead to this change provided the traits are transmitted to the offspring and these traits are beneficial in some way or another.
Some process of evolution are difficult to explain. For instance, there is a lot of focus on changes that are not genetically based. For instance, a process known as phenotypic plasticity, the ability of a single genotype to produce different phenotypes while put under different adaptive conditions, maybe an adaptive process in itself (DeWitt 1998). If individuals of a population are exposed to different environmental conditions as they develop, then it is more likely that its genotype will evolve to produce appropriate phenotypes that are advantageous to the individual. However, this process does not result in any evolutionary change, but the extent of plasticity can evolve provided the differences in extent of plasticity lead to changes in the number of surviving offspring. These processes, however, are extremely difficult to demonstrate.
Evolutionary Residue
The body structure, internal organs, and the physiology of different species hold fundamental clues to understanding evolution. Vestigial organs present in many species gives clear insights into their traits that were useful in their ancestors. DNA sequences can now be read directly and many species have their evolutionary history inscribed in their DNA. Interestingly, a lot of information can be inferred from studying the embryos of different species. Many embryos, during development, develop organs that radically disappear before birth. The form of these embryos are also interesting to analyze. They are contoured, in absurd ways and are highly imperfect in design. This proves, to a large degree, that design or engineering of these species is done by evolution and not by divine intervention.
The wings of an ostrich is an example of a vestigial organ or a vestigial trait, a feature it inherited from its ancestors but does not possess the usefulness it once had. Like all flightless birds, the ostrich descended from birds that could fly. This is a known fact and there are evidences for this statement in the DNA of all flightless birds. The wings, even though useless for flying, have evolved to perform new functions such as provide balance, mating, and threaten their enemies.
Evolutionary theory doesn’t suggest that vestigial characteristics do not have any functions. Just like the wings of an ostrich, a trait can be vestigial as well as have some function. The reason why these traits are vestigial is not because they do not have a function, but because they do not perform the function it was designed to do. An interesting point of study in evolution is understanding these vestigial traits and learning how long back where these traits non-vestigial and what were its functions back then.
Consider the wings of flightless birds for example. What caused these birds to lose their flight? Most birds that turned flightless did so on islands. For example, the extinct dodo of Mauritius, the kiwi in New Zealand, etc. An important feature clearly observable on remote islands is the lack of mammals and reptiles that feed on birds. Birds like the ostrich evolved in the Southern Hemisphere, where the number of carnivorous mammals are much fewer compared to the north.
Flight is an expensive affair, one that consumes significant amounts of energy that could otherwise be harnessed for other useful purposes such as reproduction. If flying is a form of defense mechanism against predators or if flying is a way to reach food which is not easily available on the ground, then chances of evolving into a flightless form is unlikely. However, since such difficulties on islands are minimal, these birds slowly began losing the functionality of their wings. Also, by reducing wing size, these birds also reduce their chances injury. Thus, natural selection would result in producing more birds that are flightless with reduces wingspans.
However, Darwin was not entirely right in his theories. Even, though natural selection does play an important role in transferring traits from parent to offspring, one thing he did not consider was the process of heredity. Mendel’s work was unkown to him during his time and that DNA was the storehouse of all genetic information would elude him for another century. Darwin also erred in calculating the rate at evolution would take place. He concluded that natural selection was a weak process that would take several thousands or millions of years for any visible evolutionary change to occur. However, he inspired many evolutionary biologists to continue working on this field and show that natural selection was not a weak process but a predominating strong force that in some cases gives rise to rapid evolutionary changes.
Conclusions
Although selection plays a major role in giving form to human evolution, natural selection still falls short of explaining all aspects of human evolution. Many traits that humans possess such as the enlarged brain, altruistic behavior, enhanced senses, etc. may have developed as a result of adaptation, but other characteristics maybe a consequence of phenotypic plasticity or some other form of non-adaptive mechanism. Studying evolutionary psychology that focusses on change in human behavior is an even more controversial area of research.
However, most research in evolutionary biology is motivated by the mysteries surrounding human evolution. The questions surrounding human evolution are more philosophical in nature than biological. Interestingly, it is noteworthy that plants and animals, the group under which humans fall under, constitute a very minute part of the evolutionary tree. Microbes and microbial species constitute a major part of this tree of life. Consequently, humans occupy just one in millions of branches on this tree and microbes are as well adapted to their environments as we are to ours. Philosophers argue that had the dinosaurs not gotten extinct, humans and mammals would have found a way to overcome these giant lizards. However, evidences suggest that these reptiles were thriving just before their end and showed no signs of being pushed away. This makes it indeed though provoking to contemplate what life on earth would be like today had that fateful asteroid not crashed into the earth 65.3 million years ago wiping out the dinosaurs and making way for evolutionary diversification of life.
References
Coyne, J. A. (2009). Why evolution is true. Penguin.
Futuyma, D. (2009). Evolution (2nd ed.). Sunderland, Mass.: Sinauer Associates.
Losos, J. (2013). What Is Evolution?. Princeton Guide to Evolution, 3-9.
DeWitt, T. J., Sih, A., & Wilson, D. S. (1998). Costs and limits of phenotypic plasticity. Trends in ecology & evolution, 13(2), 77-81.
