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  GREGOR JOHANN MENDEL

  Born Johann Mendel in 1822 on a farm in Silesia—then part of the Austrian Empire and now in the Czech Republic—Mendel studied philosophy and physics at the University of Olomouc (1840–43). At this time, he became interested in the work of Johann Karl Nestler, who was researching hereditary traits in plants and animals. In 1847 Mendel entered a monastery, where he was given the name Gregor. He then went on to study science further at Vienna University (1851–53).

  When Mendel returned to his monastery in 1853, the abbot Cyril Napp gave him permission to use the gardens for his research into hybridization. Mendel himself became an abbot in 1868 and no longer had time for his experiments. Although he never received credit for his discoveries during his lifetime, he is widely regarded as the founder of modern genetics.

  Key works

  1866 “Experiments with Plant Hybrids,” Verhandlungen des naturforschenden Vereines in Brünn

  See also: Early theories of evolution • Evolution by natural selection • The role of DNA • The selfish gene

  IN CONTEXT

  KEY FIGURES

  Francis Crick (1916–2004), Rosalind Franklin (1920–58), James Watson (1928–), Maurice Wilkins (1916–2004)

  BEFORE

  1910–29 US biochemist Phoebus Levene describes the chemical components of DNA.

  1944 US researchers Oswald Avery, Colin Macleod, and Maclyn McCarty show that DNA determines inheritance.

  AFTER

  1990 British researchers, led by embryologist Ian Wilmut, successfully clone an adult mammal—a sheep named Dolly.

  2003 Scientists complete the mapping of the entire human genome.

  The discovery of the structure of DNA (deoxyribonucleic acid) in 1953 is one of the most important scientific breakthroughs to date. It offered the key to understanding the very building blocks of life and explained how genetic information is stored and transferred. Englishman Francis Crick and American James Watson famously celebrated their joint discovery in a low-key fashion at their local pub in Cambridge, followed by a letter published in the journal Nature. Their discovery had enormous potential for scientific advances and had an important impact on many fields of research, from medicine to forensic science, taxonomy, and agriculture. The ramifications of their work still reverberate today, as methods of handling genetic material advance and we learn more about how individual genes operate.

  Crick and Watson’s breakthrough was the culmination of decades of research by numerous scientists, including Rosalind Franklin and Maurice Wilkins. While Crick and Watson worked with 3-D models to figure out how the components of DNA fitted together, at King’s College, London, Franklin and Wilkins were developing methods of X-raying DNA to view its structure. Watson had seen examples of Franklin’s work that hinted at DNA’s helical shape shortly before he and Crick announced their breakthrough.

  In 1962 Crick, Watson, and Wilkins were awarded the Nobel Prize for Physiology or Medicine. Franklin, who died in 1958, never received recognition for her part in the discovery during her lifetime, although Crick and Watson openly acknowledged that her work was essential to their success.

  Molecular biologists James Watson (left) and Francis Crick (right), pictured in 1953 with their double helix model of DNA. Watson called DNA “the most interesting molecule in all nature.”

  Double helix structure

  DNA is a molecule featuring two long, thin strands that coil around each other to resemble a twisted ladder, in a shape known as a double helix. Using the ladder analogy, the sides of the ladder are made up of deoxyribose (a sugar) and phosphate, while the rungs of the ladder consist of paired nitrogenous bases, adenine (A), guanine (G), cytosine (C), and thymine (T). A always pairs up with T to form base pair AT, and G always pairs with C to form base pair GC.

  DNA is the blueprint for life. Sequences of bases along the DNA strand constitute the genes that provide the information that determines the complete form and physiology of an organism. A triplet of bases is known as a codon, and each codon specifies the production of one of 20 amino acids; the order in which the amino acids join together in a chain determines the type of protein they go on to make. For example, the combination GGA is the codon for glycine. Sixty-four possible triplets can be made from the four base pairs, and 61 of them code for a particular amino acid. The other three act as signals such as “start” and “stop,” which govern how information is read by the cellular machinery. DNA is also organized into separate chromosomes, of which there are 23 pairs in the human cell.

  A DNA molecule consists of a double helix formed by two strands, made up of sugars and phosphates, linked by paired base nucleotides: adenine and thymine or cytosine and guanine.

  “DNA is like a computer program but far, far more advanced than any software ever created.”

  Bill Gates

  Genetic engineering

  A scientist analyzes a sample of DNA. Genetic manipulation in medicine is standard practice and DNA profiling is a vital forensic tool.

  Understanding the structure of DNA has enabled scientists to change or “engineer” the genetic material in cells. It is possible to cut out a gene from one organism (the donor) and place it into the DNA of another organism. When this practice was first attempted in the 1970s it was both difficult and time-consuming, but technological advances—such as Clustered Regularly Interspaced Short Palindromic Repeats, or CRISPR, which has been particularly useful—have greatly simplified and accelerated the process. In theory, geneticists can now splice any gene with any other. They have attempted some intriguing combinations, such as the insertion of the gene for producing spider silk into goat DNA so that goats produce milk rich in proteins. Other substances that can be produced by modifying genes are hormones and vaccines.

  In gene therapy, a genetically modified vector (often a virus) is used to carry a gene into the DNA of an organism to replace a faulty or unwanted gene.

  Copying the code

  When cells divide, DNA needs to be copied. This is achieved by the splitting of base pairs, which cuts the ladder down the middle to produce two single strands. These act as templates for the production of a second complementary DNA strand on each of them by matching up the appropriate base pairs. The process results in two strands of whole DNA that are exactly the same as the original.

  Since DNA remains in the nucleus of the cell, a related molecule called messenger ribonucleic acid (mRNA) copies segments of DNA coding sequence and carries the information to the regions of the cell where new proteins are made. RNA is chemically related to DNA, but the thymine base (T) is replaced by the base uracil (U), which is less stable but requires less energy to make. Stable living organisms benefit from having DNA genomes, but RNA makes up genomes of some viruses, where stability can be less advantageous.

  DNA is found in all living things on Earth, from amoebae to insects, to trees, tigers, and humans. Of course, the sequence of base pairs varies, and this difference allows geneticists to trace relationships between different species.

  Genetically modified food

  New kinds of rice are being developed through genetic modification. This may improve the nutritional value of the crop or its resistance to disease.

  In agriculture, crops may be engineered to enhance them in some way. A genetically altered crop is known as a genetically modified organism (GMO). Companies that operate in this sector may modify a plant’s DNA so that it produces more of a certain nutrient or a toxin specific to a particular insect pest. The DNA of a plant may also be altered to become resistant to a particular herbicide, so that use of the chemical kills only the weeds and not the crop.

  Some ecologists argue that there is a risk of genetically unmodified plants becoming contaminated by GMOs. They also point out that the long-term effects of eating such foods are as yet not properly understood. Another concern is that in the future large agrochemical companies could control the world’s food supply by patenting the GMOs that they produce, to the detriment of poorer
nations.

  Good and bad errors

  DNA is a highly stable molecule, but sometimes mistakes, known as mutations, occur. These can be in the form of an error, duplication, or omission in the order of the nucleotides A, C, G, and T. Mutation can be spontaneous—the result of errors that occur when the DNA is copied—or may be induced by external influences such as exposure to radiation or cancer-causing chemicals. Some mutations have no effect, but others may change what the gene produces or inhibit the functioning of a gene. This can lead to problems in the organism as a whole. Examples of disorders caused by gene mutations include cystic fibrosis and sickle-cell disease.

  Although many mutations are harmful, occasionally a mutation will confer an advantage on an individual, enabling it to survive in its environment better than others of the same species. This type of mutation may end up being passed on through the process of natural selection. Over many generations, mutation is a mechanism for diversification, survival of the fittest, and ultimately evolution.

  Mutated blood cells occur in sickle-cell disease—a genetic disorder passed on when both parents carry the faulty gene. It can be painful and increases the risk of serious infections.

  The human genome

  On April 14, 2003, scientists completed the lengthy task of mapping (sequencing) the entire human genome. Geneticists worked out the precise position of all the base pairs in a chain of some three billion of the base nucleotides comprising an estimated 30,000 individual genes. This has allowed geneticists to identify new genes and the role they play in organisms.

  Armed with this knowledge, an individual can find out if they have inherited a faulty gene from a parent. Additionally, with access to such data it is possible to screen embryos for known genetic disorders before implantation in the womb. By March 2018, the DNA of around 15,000 organisms had been sequenced. Such information can help show how animals are related in the evolutionary line and how they have diversified.

  While the discovery of the composition and structure of DNA has revolutionized the science of heredity, it is worth noting that the regions of DNA used for coding proteins account for just 2 percent of the entire human genome. The nature of the other 98 percent is not yet fully understood by geneticists, but it is believed that at least some of these regions involve the regulation of the way genes are expressed, or activated. It seems that many more discoveries await future geneticists.

  “With genetic engineering, we will be able … to improve the human race.”

  Stephen Hawking

  DNA barcoding

  The idea of DNA barcoding was first raised in 2003 when a team at the University of Guelph, Canada, suggested that it would be possible to identify species by analyzing a common section of their DNA. Led by Dr. Paul Hebert, researchers chose a region in the gene known as cytochrome c oxidase 1 (“CO1”), made up of 648 base pairs. This region is quick to analyze, but the sequence is still long enough to differentiate between and within animal species. Different gene segments can be used for other forms of life.

  The first part of the barcoding system involves cataloguing samples of known species. The DNA is extracted and organized into a sequence of base pairs, a process known as “sequencing.” The sequence is then stored in a computer database, so that when a DNA sample from an unknown species is sequenced and entered into the database, the computer will match it with existing records. The barcoding technique has proved useful for taxonomy, helping classify animals and plants.

  See also: Early theories of evolution • Evolution by natural selection • The rules of heredity • The selfish gene • A system for identifying all nature’s organisms • Biological species concept

  IN CONTEXT

  KEY FIGURE

  Richard Dawkins (1941–)

  BEFORE

  1963 British biologist William Donald Hamilton writes about the “selfish interests” of the gene in The Evolution of Altruistic Behavior.

  1966 American biologist George C. Williams proposes in his book Adaptation and Natural Selection that altruism is a result of selection taking place at the level of the gene.

  AFTER

  1982 Richard Dawkins argues in The Extended Phenotype that the study of an organism should include analysis of how its genes affect the surrounding environment.

  2002 Stephen Jay Gould critiques Dawkins’ theory in The Structure of Evolutionary Theory, which revisits and refines the ideas of classical Darwinism.

  The concept of the “selfish gene” was popularized by British evolutionary biologist Richard Dawkins in his 1976 book of that name. It states that evolution is fundamentally based upon the survival of different forms of a particular gene at the expense of others. The forms that survive are those that are responsible for the bodily types and behaviors (phenotypic traits) that successfully promote their own propagation. Supporters of the theory argue that because heritable information is passed through the generations by the genetic material of DNA, both natural selection and evolution are best considered from the perspective of genes.

  Dawkins was strongly influenced by the work of William Donald Hamilton on the nature of altruism and closely examined the biology of selfishness and altruism in The Selfish Gene. He argued that organisms were simply vehicles that supported their genes, or “replicators.” Genes that help an organism survive and reproduce tend also to improve those genes’ own chances of being replicated.

  Successful genes often provide a benefit to the host organism. For example, a gene that protects an animal or plant against disease thereby helps that particular gene to spread. However, the interests of the replicator and the vehicle may sometimes seem to be in conflict. Genes drive the male black widow spider to mate despite the risk of being eaten by her. However, the male’s sacrifice nourishes the female and improves the prospect of his genes being passed on.

  Selfishness and altruism

  Gene selfishness usually gives rise to selfishness in the behavior of an individual organism, but there are circumstances in which the gene can achieve its own selfish goals by fostering apparent altruism in the organism. One example is kin selection, the evolutionary strategy that favors the reproductive success of an individual organism’s relatives, even at the cost of the individual’s own reproduction or survival.

  An extreme example of genetically based altruism is eusociality. Honey bees are a eusocial species. They live in colonies which include breeding and non-breeding individuals. By helping the colony survive, the many thousands of non-breeding worker bees ensure the reproduction of the genes they have in common with the sole breeding individual, the queen.

  Critics of Dawkins’ theory argue that since individual genes do not control behavior, they cannot be said to be acting selfishly. Dawkins has maintained that he never meant to suggest that genes had their own conscious will. He later wrote that “the immortal gene” might have been a better title for both his concept and the book.

  A male black widow spider gingerly approaches a huge female to mate. This genetically driven act will reproduce his genes but will lead to his death.

  “The theory of evolution is about as much open to doubt as the theory that the Earth goes around the Sun.”

  Richard Dawkins

  RICHARD DAWKINS

  Richard Dawkins was born in Kenya to British parents. After the family returned to the UK, he developed a strong interest in the natural world and studied zoology at Oxford University. While there, he was tutored by Nobel Prize-winner Niko Tinbergen, who was a pioneer of animal behavior studies. After a brief period at the University of California at Berkeley, Dawkins returned to Oxford to lecture in zoology.

  Richard Dawkins is best known for his book The Selfish Gene, in which he argues that the gene is the principle unit of selection in evolution. His theory later triggered a series of fierce debates with Stephen Jay Gould and other evolutionary biologists. Dawkins is also known as a strong advocate of atheism and feminism.

  Key works

  1976 The Selfish Gene


  1982 The Extended Phenotype

  1986 The Blind Watchmaker

  2006 The God Delusion

  2009 The Greatest Show

  on Earth: The Evidence for Evolution

  See also: Evolution by natural selection • The rules of heredity • The role of DNA • Mutualisms

  INTRODUCTION

  In the 5th century BCE, the Greek historian Herodotus described watching crocodiles open their jaws for plovers to pick food from their teeth. He may have been the first to write about an ecological process—in this case a mutualistic relationship between reptiles and birds. Aristotle and Theophrastus observed many more interactions between animals and their environment in the 4th century BCE.

  Over the next two millennia, countless other observations of the natural world were made, but a deep understanding of how organisms interacted with each other and the world around them was hampered by the inability to observe very small things, those that were active at night, or those living underwater. Additionally, few people with an interest in nature experienced much beyond their own local area. As technology improved and people began to travel the world, scientists such as Robert Hooke, Antonie van Leeuwenhoek, Carl Linnaeus, Alexander von Humboldt, Alfred Russel Wallace, Charles Darwin, and Johannes Warming became increasingly aware of ecological processes and laid the foundations of the science of ecology, even if they didn’t use that word.