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This text was drafted by award-winning MMW TAs Tara Carter and Beth Peterson, (Anthropology) in the summer of 2008, supported by the MMW-based research funds left in his account on the death of Prof. Donald F. Tuzin, long an instructor and supporter of MMW. The text was substantially modified by Professor David K. Jordan in summer, 2011, and again in 2013 to fit it to the needs of MMW-11. It incorporates some passages written for MMW-1 by Margaret Meredith, a TA in 1998-99 and at that time a graduate student in the history of science.
This text may be freely reproduced for non-commercial educational purposes.
The word “evolution” refers to both a natural process (the process of evolution) and the understanding that we have of that process (the theory or theories of evolution). While evolution is one of the most fundamental of biological processes, understanding how it works involves constant research, model building, and the scientific testing and refinement of the conceptual models.
Not surprisingly, evolution is, for many lay people, one of the most misused and misunderstood concepts in science. Commonly held myths and even fears about evolution typically stem from a lack of understanding about what evolution is and what it is not.
We have all heard statements like, “If humans evolved from chimpanzees then why are there still chimpanzees?” And, “If evolution does occur then why don’t we ever see new species?” And the most widely used slogan, “Evolution is a theory not a fact!”
The idea of evolution is simple enough: “Species of living things change over time, and under the right circumstances this change can produce new species of living organisms from existing ones.”
It is perhaps easy to jump to the conclusion that this means it is possible that humans could have evolved from chimpanzees, but such a view would assume that evolutionary theory only applied to humans and, therefore, the chimpanzees we see today would have existed in the same form millions of years ago. But this could not be further from the truth. Every living species we see today is the product of processes that go back millions of years. If we could travel back in time we would find animals somewhat familiar-looking but decisively different from modern humans and apes. This is because the earliest human ancestors evolved from a species that lived 5 to 8 million years ago, an ancestral species that was the Last Common Ancestor that we share with chimpanzees and other apes. (The Last Common Ancestor, or “LCA,” is the subject of Essay 8.)
At that point in time our family tree split, with one branch leading towards modern humans, and the other towards modern apes. Humans did not evolve from apes anymore than apes evolved from humans, but rather we share a common ancestor. The causes of this split are hotly debated, but the fact that it occurred is well supported. (Caution: Many writers use the term “ape” to refer both to modern apes and to pre-modern ancestral forms, including the common ancestors of humans and apes.)
Evolution takes a lot of time, and because of this we rarely witness the appearance of new animal species. We do, however, see the rapid evolution of micro-organisms such as bacteria and viruses. Evolution is still an active agent shaping our ever-changing planet.
If this is true, then why is there a debate over whether evolution is a fact or just a theory? Calling evolution “just a theory” is intended to imply that it is merely a guess. However the phenomenon, the fact, of evolution is well supported by scientific investigations, passing every rigorous test applied to it. What scientists are still exploring and debating are the details of how exactly the processes of evolution operate, refining the theory on the mechanisms of evolution.
The reluctance of some to see evolution as a fact reflects a thornier issue: is evolution in conflict with religion? If so, must one be discounted because of the other? Most academics would answer that there is no reason or need to view evolution as a rebuttal of religion or in conflict with one’s personal sense of the spiritual. Likewise, acknowledging that humans are susceptible to the same sorts of biological processes as any other species does not make us any less extraordinary. That answer is facile, of course, for it ignores the content of many religious systems, and the way in which believers make use of religious ideas in understanding the world. However those are topics for a different essay.
In this essay, we will explore how the understanding of evolution itself evolved in scientific discourse. In other words, this essay is about the history of science. How the process is now understood to work is treated in the next essay, on genes.
The individual most associated with evolution is Charles Darwin, a nineteenth century English naturalist and theorist. We all have heard of his voyages on the HMS Beagle, and perhaps can even recall his “Eureka Moment” formulating the concept of natural selection after documenting the incredible degree of variation that exists among living finches and pigeons.
While it is entirely correct to credit Darwin with these ideas, his observations were the culmination of thousands of years of investigating and speculating about why there is so much variation within a species. Think of how many different shapes, sizes, and colors dogs come in today and you can appreciate this question. Darwin’s ideas, therefore, were based on the work of many individuals who came before him, standing as Isaac Newton said of himself, “on the shoulders of giants.” As we will see, Darwin’s genius and contribution to the debate lay in his ability to synthesize all of these ideas, using massive amounts of data, creating a logical and cohesive theory that made sense and could be examined by the scientific method.
Darwin was not the first individual to propose a theory for the origins of human life. Anaximander, a Greek philosopher living in the eighth century BC, suggested that human beings had arisen from other forms of life, specifically fish. While Anaximander was incorrect about a fishy beginning for humans (at least as he imagined it), he was perhaps one of the first philosophers to explain the origins of life without a supernatural agent. Most individuals, however, viewed the world in supernatural terms. In the Western world, scholars, like other people, believed that the entire realm of living beings had been created at once, during the Creation of the world. In Europe the Bible served as the cornerstone of cosmology, providing an explanation for all natural phenomena, including the origins of the first two human beings: Adam and Eve. The biblical book of Genesis was one of the few human (as opposed to natural) records of this event. (It was during the course of the eighteenth century that “savants” began to question the use of the biblical account as an authoritative record for understanding the history of the natural world. Indeed, the term “scientist” did not come into being until the nineteenth century. The term “savant” was the word used to refer to persons considered to be experts in any area of knowledge, including that of the natural world.)
This literal interpretation of the Bible was also influenced by the persuasive ideas of another Greek philosopher, Aristotle (384-322 BC). Aristotle believed that all living animals could be ranked in a hierarchical organization, which he called the Great Chain of Being. In a way, the Great Chain of Being is like a classification system that arranges all life forms into a structure that resembles steps on a ladder, or links in a chain. At the bottom of Aristotle’s ladder were the least complex, least perfect creatures, while the most complex and most perfect creatures —human beings naturally— were situated at the top.
The Great Chain of Being was, logically, the predecessor of the efforts to produce a comprehensive system of classification for all living things. Although the precise place of every being in the Great Chain of Being was not necessarily known, each one was thought to be linked to another in a graduated sequence, organized by God from the lowest to the highest form of life: human beings. Humans were, by implication, invited to explore this world, and to discover the underlying order in it.
A second, but equally important influence on Biblical interpretations of the world is the medieval belief in the fixity of species, the idea that all life on earth that currently exists has always existed just as it exists today. It was generally accepted that all life on earth had been created by God, and, therefore, could never change. To suggest that life forms could change over time would have been viewed as a challenge to God’s perfection and could have been considered heresy, a crime potentially punishable by a fiery death.
What results from these literal Biblical interpretations is not just a view of an unchanging world organized through God’s master plan, seasoned with a hint of Aristotle’s Great Chain of Being, but also an idea that the earth itself was young. Using the “begat” sections in the Book of Genesis as a human family tree, Irish archbishop James Ussher (1581-1656) calculated that the earth was created on the morning of October 23, 4004 BC.
There was nothing in Judeo-Christian scriptures (or, for that matter, in Aristotle) that restricted efforts to discover and classify the world of living forms, any more than there was anything to suggest that the forms changed over time. Overlapping in time with Bishop Ussher was English naturalist and minister John Ray (1627-1705). Ray had recognized that plants and animals could be arranged into groups by their ability to mate with one another and produce fertile offspring. He called these species (Latin for “types”). Ray also observed that species often shared physical similarities, so he arranged groups of species into broader categories called genera (singular: genus, Latin for “descent line”).
The system of names is called “binomial nomenclature” and provides terms like “Homo sapiens” for modern humans: Homo is our genus; sapiens is our species. Because the terms are Latin, some editors prefer that they always be in italics. It is a convention of biologists that the name of a species is always capitalized, while the following name of a species is not: Homo sapiens.
A famous Swedish scholar, born two years after Ray’s death, added two more categories to Ray’s system: class and order. He was Carl von Linné (1707-1778), usually known under his Latin name Carolus Linnaeus , since he wrote in Latin. We in fact still use Linnaeus’ four-level system today. Although we have added a few more categories, it is still called Linnaean. It is this system that has served as the basis for taxonomy, the branch of science concerned with description and classification of organisms. As we will see, this exercise is crucial in evolutionary studies.
A “Reference Table of Hominid Classification”shows the categories of the system as used today and as applied to our own line. (Link)
But this gets ahead of our story. It is a testament to their ingenuity and observational powers that Ray and Linnaeus could produce a single logical scheme capable of accomplishing this enormous intellectual task. But they were not evolutionists.
The Linnaean scheme for classifying the living world lacked (and still lacks) the dimension of time. That is, it cannot take into account any changes in any particular kind of plant or animal which occur over the course of history. Linnaeus' classification scheme, continuously updated, has remained in scientific use (as have parts of many other very old theories and schemes) because it works, and its static dimension can be modified.
Linnaean classification was intended to represent a living world assumed to be changeless. The notion that any type of being could be lost (extinct) was deemed unlikely, for a lost species would represent a broken link or blank in the Great Chain of Being. To be sure there seemed to be some gaps in the chain. But savants could easily explain the gaps as yet undiscovered species. They believed that generations of concerted effort would eventually yield a discernible graduated and hierarchical relation among all living things.
Gaps in the record were not the only problem. There were also odd animals and plants represented only by fossils and not by living examples. By the late 1700s, the discovery of an increasing number of huge elephantine bones found in shallow deposits in the earth's surface (when plowing the soil, mining, digging foundations, or building roads) and the discovery of shells embedded in the earth's strata brought the static, “creationist” scheme of the natural world under strain. Although these bones and shells resembled living species, such as elephants and ammonites, they were far larger than any known living creatures.
Some savants argued that these were simply pathological forms, the odd mistakes of nature. Others argued that the animals to which these bones and shells belonged still dwelled in the interiors of yet unexplored continents, such as North and South America, Australia, and Africa, or in the depths of the seas. Or perhaps in Tibet. But further exploration failed to find them in most cases. Some people, conceding that the evidence didn’t “add up,” suggested that perhaps the fossils were simply “planted” by God in order to test our faith.
Robert Hooke (1635-1703) rightly recognized that fossils of plants and animals represented the remains of creatures that had become extinct rather than just simply “odd mistakes” of nature. This may be a simple observation to us, but in his day, Hooke’s observations suggested that the living things, as a totality, do in fact change over time rather than remaining static. Hooke even went so far to suggest that the extinction of plants and animals might be related to changes in the environment, such as the flood described in the Biblical story of Noah, where God deliberately wipes out all animal life and “reseeds” the earth from animals Noah has preserved in his ark.
Between 1796 and 1812, the influential French anatomist, Georges Cuvier (1769-1832), published a series of papers in which he argued even more persuasively for Hooke’s insight. He built a case for the idea that these bones and shells could only be relics of a former world. In other words, the animals to which they belonged had lived in a former period of the earth's history and were truly extinct. He argued, further, that the history of the world had been punctuated by a number of different catastrophic events which had annihilated several species of animals at various different times, and not merely during Noah’s flood. This seemed logically to require some hypothetical re-creation after each catastrophe, or else a whole series of Noahs, one per catastrophe.
In the study of the natural world, the direct observation of nature, as opposed to reliance on revered texts, is an idea that we take for granted today. But it was not until the 1800s that people referred to as “scientists” (rather than “savants”) began to use the fossil record to understand scientifically the history of the earth, and the modern discipline of geology came into being.
Thus, “science” as we understand it today, science as the study of the natural world using evidence from nature, is a rather recent as well as modern idea. During the 1700s, savants relied upon both textual and natural records as authoritative accounts of the natural world.
It is fair to say that, in Europe anyway, there simply was no notion of evolutionary change before the late 1800s. Before Darwin, scientific understanding of the natural world did not seek to account for the creation of new species during historical (well … pre-historical) time.
The existence of fossils inspired a number of scholars to reason that if plants and animals could change over time, than so too, could the earth itself.
Nicholas Steno (1638-1686) provided proof that the earth had undergone, and was continuing to undergo, change. Steno recognized that below its surface the earth could be sorted into strata (“layers”) and that these strata represented different periods. Imagine a layer cake. Each layer of the cake looks entirely different, since one layer is chocolate, another is strawberries, and still another is whipped cream. The layer on the bottom is the layer first put down in the making of the cake, while whatever is on top is the last layer added. Just like our layer cake, the landscape of the earth is made-up of layers or strata and the deepest layers must be older, it was reasoned, than layers above it. What Steno (and Charles Lyell, whom we will meet in a moment) discovered by examining these layers was that the earth’s history was complex and changed significantly over time. For example, it is not uncommon to find fossils of sea-life at the top of mountains. (There are more species of sharks represented by fossils in the mountains above Ensenada on the Baja California peninsula than there are in the ocean today.)
Steno argued that this layering proved the earth had changed, but he had no explanation for how those changes occurred, for how a landscape with mountains could once have been under of the sea.
The popular explanation for this radical change was a process known as catastrophism. One well-known advocate of catastrophism was Georges Cuvier (1769-1832). Cuvier, like other catastrophists, initially believed in a biblical chronology, that is, a “young earth,” created only a few millennia ago (perhaps even in 4004) BC. And yet the existence of fossils and radical changes in the earth’s landscape over time was at this point hard to deny. To bring about so much change in such a short span of time required global catastrophic events (hence the name “catastrophism”), such as floods and earthquakes —the kinds of events described in the Bible. These global catastrophes explained the presence of fossils and a cause for why some creatures had become extinct, and it explained why some fossils were in places where they didn’t seem to belong. While Cuiver was willing to admit that changes in the earth, and the earth’s life forms had occurred, he had no concept of development. Instead, he saw a series of abrupt starts and finishes, rather than an evolving world.
Catastrophism, because of its easy compatibility with Biblical accounts, initially enjoyed wide popularity, but this fame was not long-lasting. The theory simply could not stand up to further investigations. Some landscape changes were the result of catastrophic events, to be sure, but other changes seemed to happen more gradually.
Georges-Louis Leclerc (1707-1788) was a French scholar who eventually affected the title Comte de Buffon. Historians usually call him by his title, or sometimes shortened to simply Buffon. Buffon suggested that catastrophic events were rather rare, and instead changes in the earth’s landscape could mainly be explained by “uniform” processes: processes like erosion or blowing sand that we can see at work today and that have, in that sense, been “uniform” throughout the earth’s history. This idea is appropriately enough called uniformitarianism. The insight that natural processes have always been the same remains a foundational starting point for scientific inquiry.
But if this was true, argued Buffon, then the earth had to be a lot older than 6,000 years. It was James Hutton (1726-1797) who connected Buffon’s idea of uniformitarianism with the evidence seen in the earth’s strata. Buffon had envisioned most of the earth’s changes occurring at the hands, or rather the waves, of the sea, but Hutton suggested the sea was just one of several general processes, adding to the list the processes of uplift that create mountains, and processes of erosion that break them down again. Hutton envisioned the earth as a factory, with a constant cycle of building up the landscape followed by breaking it down again. All of this took a lot of time.
Charles Lyell (1797-1875) took Steno’s insights into strata and Hutton and Buffon’s ideas about uniformitarianism to the next level, unabashedly declaring all the earth’s features to be the result of long-term processes that continue to operate in the present day as they did in the past. Lyell published his findings in his three-volume Principles of Geology. His work was met with both enthusiasm and controversy. Among those weighing in on the debate was Darwin himself, who took the first volume of Lyell’s book with him as he embarked on the HMS Beagle in 1831.
In the course of his work, Linnaeus had discovered, as Hooke had before him, that some fossils represented species that were now extinct, suggesting that new species could arise as well. But also like Hooke before him Linnaeus had no explanation for how or why changes within a species could occur or how species could arise. Linnaeus had always held to the belief in the fixity of species. By the time of his death in 1778, however, faced with mounting evidence, Linnaeus had begun to question his position. Linnaeus and others recognized that species did in fact change, but exactly how they changed was a mystery.
Perhaps the most influential scholar to attempt an answer for how a species could change was French naturalist Jean-Baptiste Lamarck (1744-1829). Similar to Hooke, Lamarck suggested a dynamic relationship between the environment and plants and animals. If the environment changed —if the global temperature got warmer, say— then plants and animals would adjust to this increase in temperature biologically. Simply put, the body can change to adapt to new environmental circumstances. “If a particular part of the body felt a certain need to change, ‘fluids and vital forces’ would be directed to that point, and the structure would be modified.” Since this new trait would make that plant or animal better suited to its environment, that trait would get passed on to its offspring. Likewise, those parts of the body not being used would disappear over time.
Lamarck’s theory is known as the inheritance of acquired characteristics. It proposes that physical changes that occur during the lifetime of an individual, whatever they may be, can be passed on to the next generation. A popular example illustrating Lamarck’s theory is the proposal that giraffes have long necks because at some point their ancestors had to stretch more and more to reach higher leaves as the leaves on lower branches gave out. Since these long-necked giraffes could eat leaves from both the low and high branches of trees, a real advantage, the trait was passed on to their future offspring.
Lamarck’s theory made one incredible breakthrough and one major error: The breakthrough was seeing the dynamic relationship between an organism and its environment. But the critical error was thinking that evolutionary change could occur during an individual’s lifetime.
This mistake is easily seen by taking Lamarck’s model to a logical extension: if a man shaves his body to become a faster swimmer, will his children be born hairless? Today we know this is impossible because we have an understanding of genetics, but there was no concept of genetics in Lamarck’s day. Gregor Mendel (1822-1884) would provide the foundation for the field of genetics, but he died in relative obscurity. Darwin never learned of his work for example. The world would have to wait until the early 1900s when Mendel’s work was rediscovered.
Another element missing from Lamarck’s model is competition. English economist Thomas Malthus (1766-1834) pointed out that the population size of cities and villages increases exponentially, but the amount of food available remains more or less constant. Therefore, the tendency for populations to grow is always checked by the limited availability of food, and this results in a competition for resources, which kills off the less “fit” animals (villagers or giraffes). Malthus was trying to explain economic variables that kept population sizes at a fairly steady rate, but as we will see, his idea has multiple applications.
Charles Darwin (1809-1882) was born into a world that acknowledged that biological change, that is evolution, seemed possible. But people were still searching for a satisfying answer for the mechanism of that change.
Some scholars held to the ideas of Cuvier, that natural catastrophes followed by new creations were the mechanism of change. Many other scholars, however, agreed the notion of catastrophes did not hold up to Hutton and Lyell’s ideas on uniformitarianism.
It was Darwin, however, who would synthesize all of these data and ideas into one argument, an argument that has withstood over a century of scientific scrutiny.
Darwin, perhaps more than anyone else who came before him, appreciated the overwhelming degree of variation that exists within living species. Earlier scholars often viewed this variation as “mistakes” or deviations from the ideal form. Lamarck suggested variation existed as organisms changed in response to environmental alterations, but if this were entirely true then we would expect at some point every member of a species to look more or less the same because they were all responding to the same stimuli. Darwin saw that this was not true.
In England, Darwin had made careful observations of domestic pigeons that “pigeon fanciers” had bred for certain features. Darwin’s discovery was that despite this selective breeding, these pigeon populations still showed a wide range of physical variability with every generation. Even those less desirable traits that breeders tried not to select for still showed up in new generations, forcing breeders to continue the process by which pairs of individuals were selectively mated. Lamarck’s prediction provided no way to explain these facts.
In puzzling over these observations, Darwin began to take clues from a great many previous theories, and in the end Darwin had come up with a new proposal, one made up of elements from a wide range of scholarship.
1. He took his first lead from Thomas Malthus. We recall that Malthus observed that limited resources created competition within human populations. Darwin reasoned the same must be true for all living beings. Variation within a species would make the difference in the fitness, or relative adapted-ness of an individual organism, measured in reproductive success, that is, in its success in leaving descendants in the population.
2. Darwin agreed with Lamarck’s observation that the relationship between an organism and its environment is crucial, but unlike Lamarck, Darwin did not think variation arose when it was needed, but that variation already existed. An organism with better-adapted pre-existing traits would tend to be more reproductively successful, passing on those traits to its offspring.
3. Over time, however, environments can change, so traits that were once adaptive could become neutral or even maladaptive.
It made the best evolutionary sense, then, to have a varied population, since if all individuals were identical, then one change in the environment could potentially send the species into extinction. But how was variation created or sustained?
Darwin did not understand where this variation came from, since, like Lamarck, he lived before genetics were understood, but his observations showed that nature, like a plant or animal breeder, selects or favors better-adapted individuals through more successful reproduction. Darwin called this process natural selection, the mechanism of evolution. (He called it “natural” in contrast to artificial selection done by plant and animal breeders.)
For natural selection to operate, three conditions were necessary, and when all of them were present they were sufficient to produce the effect. Darwin argued the three necessary and (jointly) sufficient conditions were:
It is important to stress two crucial details about Darwinian evolution: First, evolution is not goal-oriented —it is their fit with an environment, regardless of their origin, that determines whether traits are advantageous or not. Second, evolution is a process operating at the population level, not the individual level. Natural selection operates on individuals, but evolution has its effect on a population; it is the population, not the individual, that can be said to evolve. When we speak of the evolution of a trait, whether purple tail feathers or the opposable thumb, we are speaking of its characteristics in a gradually changing population, not in an individual. Nearly all of the popular misunderstanding of evolution derives from the failure to appreciate these two features of the model.
There has been tremendous speculation over how much Darwin knew of the work of Alfred Russell Wallace (1823-1913). Wallace also came up with a theory of natural selection very similar to Darwin’s and at about the same time. In many ways, it was the presence of Wallace that gave Darwin the courage to publish his work in his Origin of Species. Remember that Darwin’s views were in contrast to popularly held beliefs such as the fixity of species, so it easy to see why he was somewhat reluctant to publish his findings.
Did Darwin steal Wallace’s ideas? Did Wallace steal Darwin’s ideas? The answer to both questions is probably no. The reason both came to the same conclusion is one of timing. Both Darwin and Wallace took notice of the major thinkers in this field: Malthus, Lyell, Hutton, Buffon, Ray, Linnaeus, Lamarck, Hooke, and Steno to name just a select few.
Natural selection would not usually be a rapid process. To make the theory of natural selection work, one must demonstrate that the earth is quite old. Steno, Hutton, and Lyell provided his evidence, especially with the theory of uniformitarianism.
Likewise, understanding natural selection is dependent on the ability to classify animals and plants into groups based on their ability to successfully reproduce. John Ray and Carolus Linnaeus provided this with the system of genus, species, and class and order.
One would need to show that organisms could change over time because of changing environmental stimuli, a fact demonstrated by Buffon, Cuvier, and Lamarck.
To make natural selection work there must be a reason why some individuals were reproductively successful and others were not. Malthus and his theory on competition over resources provided this last necessary clue.
Both Darwin and Wallace went on voyages that took them half world away and in sight of a variety of animals rarely seen by westerners. Through the course of their cataloging —finches for Darwin, and a variety of Malaysian and Indonesian plants and animals for Wallace— both men came face to face with the extraordinary degree of natural variation that existed within a species, and realized how much variation could exist within a species inhabiting nearby islands.
Using all of these data and all of their interpretations of these data based on their own observations on variation, both Darwin and Wallace came to formulate an argument for natural selection.
| Summary Table | ||
|---|---|---|
| Robert Hooke (1635-1703) | 1660s-90s | Saw fossils as evidence of environmental change. |
| Nicholas Steno (1638-1686) | 1669 | Developed the method of stratigraphy. |
| John Ray (1627-1705) | Late 1600’s | Arranged plants and animals by species, defined by their ability to mate with one another and produce fertile offspring. (Without knowing it Ray had provided the crucial mechanism of natural selection: reproduction.) |
| Georges-Louis Leclerc, Comte de Buffon (1707-1788) | 1749 | Concept of adaptation and a dynamic relationship between organisms and their environment; laid the foundation for Uniformitarianism (“Old Earth”). |
| Carolus Linnaeus (1707-1778) | 1758 | Developed a classification system for organisms using scientific names; recognized extinction and the possibility for new species. |
| Thomas Malthus (1766-1834) | 1789 | Recognized the relationship between population size and available resources; adds in the element of competition. |
| Georges Cuvier (1769-1832) | 1795- early 1800s | Explained extinction through his theory of “castastrophism” and ideal prototypes linked to climatic alterations. |
| James Hutton (1726-1797) Charles Lyell (1797-1875) | 1795 1830-33 | Endorsed uniformitarianism; controlled scientific investigation. |
| Jean-Baptiste Lamarck (1744-1829) | 1809 | Explained the extinction of rise of new species through his theory of the “Inheritance of Acquired Characteristics,” which explained changes in organisms as a means of adaptation to the environment. |
| Charles Darwin (1809-1882) | 1859 | Discovered natural selection. Wrote Origin of Species. |
| Alfred Russel Wallace (1823-1913) | 1859 | Discovered natural selection |
| Gregor Mendel (1822-1884) | 1860s | Identified regularities of inheritance through genes. |
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