When we hear “evolution”, we often think of a progressive change. In general sense, evolution refers to some sort of development. In biological context, evolution can be construed as the sequence of events depicting the gradual progression of changes in the genetic composition of a biological population over successive generations. Or, it may also pertain to the genetic change itself over time. Evolution may also pertain to the change in the genetic composition of biological populations over successive generations.
Genetic variation. Genes are vital in evolution. Their expression manifests into heritable characteristics. By heritable, it means that the characteristics can be passed on from parents to offspring. These heritable characteristics are part of the phenotypic traits, or simply traits, of an organism. Hence, mutation in these genes will likely affect the manifestation, as well as the passing, of heritable traits from one organism to the next.
For instance, the eye color is a character of varying traits, such as brown, green, blue, hazel, etc. In humans, the color of the eye varies from brown to black, depending on the melanin content in the iris and the cellular density of the stroma. The gene associated with melanin production is the OCA2 gene. All humans were originally brown-eyed.¹ However, humans evolved to have other eye colors.
The blue eye color in humans is not caused by blue pigments. Scientists tracked down the trait and found that a specific mutation within the HERC2 gene was partly responsible. It altered the function of the OCA2 gene that resulted in the production of fewer melanin.¹, ²
Further studies suggested that all humans with blue eyes may have come from a single European ancestor who lived around 6,000 to 10,000 years ago, and who had the mutation responsible for the blue eye color, which eventually became widespread in modern population.²
Apart from mutations, genetic recombinations are another source of genetic variation. In sexual organisms, genetic recombination occurs during meiosis. Pair of chromosomes that have the same genes at the same loci but possibly different alleles are called homologous chromosomes. These homologous chromosomes pair up and exchange genetic material at prophase I of meiosis. Apart from homologous recombination, homologous chromosomes assort independently towards the end of meiosis. These meiotic events that produce new allelic combinations increase genetic variation. Sexual reproduction, thus, escalates evolution.
Natural selection. By natural selection, individuals with advantageous traits are more likely to reproduce with greater success. Thus, these individuals could pass on their traits to the next generation. Evolution that is driven by natural selection is called adaptive evolution.
Genetic drift. Another mechanism that drives evolution is genetic drift. It produces random changes in the frequency of traits in a population. Evolution that arises from genetic drift is called neutral evolution.
Evolution occurs when natural selection and genetic drift act on genetic variation. Other key players are also at work. For instance, gene flow, which refers to the flow of genes between populations as well as between species, can change the course of evolution. When two organisms of different species mate, they could produce an offspring, which could become a new species.
Biologically, evolution is important because it drives biodiversity. Some traits will become predominant while others will become rare over the course of time. Without evolution, life may not be the way as we know it. It will not be as diverse as it is now.
The Earth itself goes through a series of changes. At one point, the Earth was an inhabitable planet. The primitive condition of the Earth was hostile to life. It was presumed that only after about one billion years from the time that the Earth first came into being that life began. RNA-based, self-replicating entities are held as the descendants of all living things. Over a significant period of time, these life forms evolved into single-celled organisms. Multicellular forms came next. They first appeared about 600 million years ago.
Tracking the history of life at various geologic eras, one would find several mass extinctions occurring in between the spurts of life. For instance, during the Permian period of the Paleozoic era, the Earth had a supercontinent called Pangaea surrounded by Panthalassa ocean. This caused the inlands to be very dry and arid. Because of this, reptiles flourished as they were able to thrive even in such habitats. A reptile group Dimetrodon evolved and gave rise to therapsids. The therapsids, in turn, evolved and gave rise to cynodonts, which were the early ancestors of animals. During this period, the early dinosaur ancestors, archosaurs, also appeared. Apparently, a mass extinction called “the Great Dying” occurred and wiped about 90% of life on Earth. The next era (Mesozoic era) is called “the Age of the dinosaurs”. These animals dominated the land, the seas, and the air of the Earth. However, a mass extinction occurred and caused the death of the dinosaurs as well as other large animals. Nevertheless, mammals took the open niche and expanded.
Evolution is crucial for life to persist in the ever-changing Earth. The organisms need to have the capacity to adapt genetically and phenotypically. Engaging in symbiotic relationships with other organisms could also help increase the propensity to survive and thrive. Along with evolution, speciation transpired. Species diverge into two or more descendant species throughout the course of evolution. Unfortunately though, most of the species that lived on Earth already met their demise. 99% of the Earth's species is now extinct. These organisms perished and their species entirely disappeared. Thus, it would seem that extinction of species is inevitable.
Evolutionary History of Life
A diagram, called evolutionary tree, shows the evolutionary relationships of organisms. Grouping is based upon similarities and differences in genetic and physical characteristics. The pattern of branching represents how species or entities evolved from a particular common ancestor. Tracking down the course of evolution of all living things that lived on Earth would altogether lead to the common ancestor, LUCA (last universal common ancestor). LUCA is the hypothetical ancestor of all living things and it is presumed to emerge some 3.5 to 3.8 billion years ago.
There is no consensus as yet as to how life originated on Earth. However, many believed that the RNA-based, self-replicating entities were likely the descendant of all living things. These entities evolved into single-celled organisms containing cytoplasmic structures but lacking the internal compartmentalization. Single-celled organisms lacking membrane-bound organelles are referred to as prokaryotes.
The prokaryotes emerged before the eukaryotes did. They were able to bear up the primitive hostile conditions of the Earth. Later on, single-celled eukaryotes appeared around 1.6 to 2.7 billion years ago. The endosymbiotic theory proposes that larger cells took in smaller cells, such as bacteria and cyanobacteria, for a cooperative association (endosymbiosis). Together, they underwent coevolution. In due course, the smaller prokaryotes evolved into semi-autonomic organelles. The bacteria evolved into mitochondria whereas the cyanobacteria, into chloroplasts. The presence of membrane-bound organelles inside the cell led to the advent of eukaryotes.
In Neoproterozoic era, particularly in the Ediacaran period (around 600 million years ago), the first multicellular form emerged. How multicellularity came about is still a matter of debate until now. The most popular theory in this regard is that of Haeckel's. According to his Gastraea Theory, multicellularity occurred when cells of the same species group together in a blastula-like colony, and gradually, certain cells in the colony underwent cell differentiation. Also in this period, sponge-like organisms evolved based on the recovered fossils of Ediacaran biota. They were presumed to be the first animals.
The next era, the Paleozoic, is comprised of the geologic periods from Cambrian to Permian, each highlighted by major evolutionary events.
In Cambrian period (around 541 million years ago), a sudden burst of life occurred. This geologic event was called the Cambrian explosion. Diverse plants and animals came into being. Plants and fungi spread to the land. Soon, animals such as arthropods ventured ashore, probably to mate and lay eggs.
In Ordovician period (485 to 440 million years ago), the invertebrates were the dominant animals. Primitive fish continue to evolve that in the next geologic period, Silurian, a mass evolution of fish occurred. Also in Silurian (440 to 415 million years ago), arachnids and arthropods started to colonize the land, not just ventured it. Internal gas exchange systems, waterproof external layers, skeletal systems (endo- or exoskeletons), and a form of reproduction not involving water came about and helped life on land plausible.
Devonian period (415 to 360 million years ago) is called the Age of the Fish. The fish became the dominant marine vertebrate species. On land, plants evolved and primitive plants, trees, and shrub-like forests served as new habitats. With the evolution of land plants, the animals, too, evolved and diversified. Amphibians were the first tetrapods to appear. They emerged around 364 million years ago.
In Carboniferous period (360 to 300 million years ago), a major evolutionary event occurred. Tetrapods that lay amniotic eggs emerged. The laying of amniotic eggs in drier environment allowed the tetrapod amniotes to move father away from the waterside, and thereby dominate farther inlands. Because of this, these early amniotes diversified greatly towards the end of this period.
In Permian period (300 to 250 million years ago), reptiles and synapsids flourished. Soon, a major evolutionary event occurred, which led to the emergence of beast-faced therapsids. These therapsids later gave rise to the cynodonts (the early ancestors of mammals). The first archosaurs (early ancestors of dinosaurs) also appeared in the Permian period.
Following the Paleozoic era is the Mesozoic era (252 to 66 million years ago), which is referred to as "the Age of the Dinosaurs". The dinosaurs roamed and dominated the Earth. However, a mass extinction event occurred. They perished together with the other large animals (>25 kg in weight) by the end of this era.
In the next era, Cenozoic (66 million years ago to the present day) era is called the "New Life". Mammals expanded and diversified. The great apes evolved and led to the evolution of hominids, which was the evolutionary line that led to the Homo species. The only extant species of the genus Homo is the Homo sapiens (anatomically modern humans).
Evolutionary biology is a subfield in biology that focuses primarily in evolution. An expert in evolutionary biology is called an evolutionary biologist. Some of the major topics are natural selection, genetic drift, biodiversity, and speciation.
Charles Darwin (12 February 1809 – 19 April 1882) is considered by many as the "father" of evolutionary biology for his scientific contributions, such as his theory of evolution through natural selection. In 1858, his theory was published in joint publication with Alfred Russel Wallace (8 January 1823 – 7 November 1913), a naturalist who independently formulated a similar theory. This prompted Darwin to publish On the Origin of Species in 1859 where he explicated his ideas in details. In brief, the theory posited that natural selection ('survival of the fittest') caused the branching pattern of evolution. Darwin believed in the common ancestors from where species descended from.
Charles Darwin may be the most iconic person in this field but he was not alone in shaping evolutionary thought. Carl Linnaeus (23 May 1707 – 10 January 1778), for example, helped through his biological classification of organisms in 1735. He classified organisms into hierarchical groups based on physical similarities and differences. Jean-Baptiste Lamarck (1 August 1744 – 18 December 1829) proposed "transmutation theory" where he hypothesized the transmutation of species. Accordingly, the organisms began as simple forms of life through spontaneous generation and then became more and more complex over time. Further, he argued that the organs of a plant or an animal could change based on use or disuse, and this change is heritable. Lamarck's theory is regarded as the first fully formed theory of evolution. But unlike Darwin who believed in common ancestors, Lamarck did not.
Current research in evolutionary biology incorporates the methods of mathematical and theoretical biology and of molecular genetics. The modern synthesis makes use of a mathematical framework that reconciles Darwin's theory of evolution and Gregor Mendel's laws of heredity to come up with a unified theory of evolution.
- Latin evolutio (“an unrolling, unfolding”), ex- (“from, out of”) + volere (“to roll”)
- concerted evolution
- convergent evolution
- cultural evolution
- divergent evolution
- Douglas spontaneous evolution
- quantum evolution
- saltatory evolution
- Evolutionary ecology
- Evolutionary biology
- Evolutionary tree
- last universal common ancestor
- Bryner, J. (31 January 2008). “Genetic mutation makes those brown eyes blue”. MSNBC. Retrieved from http://www.msnbc.msn.com/id/22934464/.
- “All Blue-Eyed People Have This One Thing In Common”. IFLScience. Retrieved from http://www.iflscience.com/plants-and-animals/all-blue-eyed-people-have-common-ancestor-0.
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