Planet Earth/7e. Darwin and the Struggle for Existence
Natural Selection[edit | edit source]
A little tombstone sits in the graveyard in the town of Malvern near Worcestershire England. It reads “Anne Elizabeth Darwin, Born March 2 1841 Died April 28 1851. A dear and good child” The death of Charles Darwin’s eldest daughter at the age of 10 came at a time in his life when he was struggling both with his mental and physical health and as a scientist. Since returning from his grand voyage around world in 1836, where he served as a scientific investigator on board the research vessel H.M.S. Beagle, Darwin had been feverously working on scientific reports describing everything that he saw and collected, including the rocks, fossils and animals that he collected onboard the research vessel. He had sent many of his collections off to the experts of the time, Richard Owen (who coined the word dinosaur) wrote about the fossils he collected, George Waterhouse wrote about the mammals, John Gould and George Robert Gray wrote about the birds, Leonard Jenyns the fish, and Thomas Bell the reptiles, each naming new species and varieties discovered during the expedition. These reports were assembled into a large edited volume describing, with wonderment and rich and detailed illustrations, the wide variety of life on Planet Earth. When it was completed around 1844, Darwin was married with a growing family, including little Annie, a joyful little girl. And he was beginning to turn to new projects, including a detailed study of barnacles, and other shells he had collected during his trip.
It was during this time that Darwin read a short essay by the economist Thomas Robert Malthus entitled “Essay on the Principle of Population,” which had been written in 1798, but recently reprinted in 1826. In summary, Malthus wrote “the power of population is indefinitely greater than the power in the earth to produce subsistence for man,” in other words, population sizes will continue to grow exponentially, until the population runs out of resources, and then it will crash. What pertained to bacterial populations, also pertained to human populations. A growing population was limited by the availability of food.
For Darwin, this idea that population growth was limited by available natural resources was something he wondered about in trying to understand how so many varieties of plants and animals came to be on the Earth. His second key insight came from his life in the country and working with livestock, and learning of the ways in breeding cattle, sheep, horses, dogs, goats, chickens and pigeons, which each had been selected for by the various traits that they possessed by famers of the day, who would artificially select which pairs to mate.
In fact, when he bought Annie a pet canary, a little yellow bird with a beautiful song, he was introduced into the world of bird-fanciers and breeders, who selected for the best traits, such as singing, or weird feathers, or large body sizes or unique colors, and only allowing those individuals to breed together. He also bought his children ponies; small horses bred to be tiny for his children to learn to ride on. It was from these homelier observations that he started thinking about the origin of new forms of life in the natural world. In particular, how did a new species come to be? What was the cause of all this variety of life on Earth? Most of his ideas were kept in a notebook, and still not well defined, composed of simple and loose diagrams and observations from talking with animal breeders in the rural country and visiting local county fairs. He also enjoyed exploring the countryside making observations in his notebook about the animals and plants that he witnessed on these trips into nature, but also sharing conversations over a fence on local breeds of dairy cows.
In 1848, Darwin become gravely ill, spending nights vomiting, and struggling to eat. He grew weary and sick. He entrusted his unfinished notebook to his wife, Emma, and told her that if he were to die, she should publish the work it contained. Worried about his condition, the family traveled to Malvern Wells looking for a cure for his sudden illness, where they meet a doctor. His name was Dr. Cully. He had developed a series of homeopathic cures featuring the emersion of one’s body into cold waters and bathing, as well as hot saunas, but also was known to consult ghosts through seances and other unorthodox treatments that were in vogue during the middle 1800s. Darwin was very skeptical of the treatments, but after a few months, the routine at Malvern Wells improved his health and he was able to return home. Much of his health was likely restored from the long walks that were part of the treatment, which he continued to practice into old age. In the Autumn of 1850, Annie became ill on holiday with the family with a fever and the family took her home to nurse her to health. They consulted a local traditional doctor, who prescribed some medicine. Still ill later that winter, she took to spending the day with her father helping to organize his collection of barnacle shells in his study. In early December, Annie developed a horrible sounding cough, and the Darwin’s again visited the doctor in London, who gave them the grim news of her serious illness. This was in the days before antibiotics, and typhoid was a major cause of death at the time. Typhoid is caused by a bacterial infection of Salmonella typhi found in unclean water that leads to a high fever, diarrhea, and vomiting. It can be fatal, especially in children. The worried parents took Annie to Dr. Cully as a last resort, as he had helped with Charles Darwin’s ailment before and maybe the treatment would help his sick daughter. During the early Spring, while staying at Malvern there was some hope of her improvement, but on April 28th, she died. Her death devastated Charles Darwin, and he entered into a dark period of his life.
Filled with depression and sadness, Charles Darwin’s productivity decreased during the years after Annie’s death. He published his work on barnacles which was dragged out over a few years, but melancholy kept him from returning to his earlier ideas. He kept thinking about the fragility of life, and how with Annie’s death, it was not just the death of her, as painful as that was, but all her offspring. She would bear him no grandsons and no granddaughters— it was like a great chain of being had been cut off. Life was a struggle- a struggle for survival.
Five years later, with a push from his scientific mentor and hero, Charles Lyell, Darwin started to formula his ideas into an actual book. Like the process of breeders selecting mating pairs by the traits they possess, nature can select for advantageous traits in that not all individuals are able to successfully reproduce and pass on those traits. Only those individuals that leave offspring for the next generation leave behind those traits. With enough time, new species can arise when populations become isolated and exposed to different selective forces, as over time the population will adapt and change to suit the environment. Those individuals in a population that survive are the ones that reproduce and pass on their traits to the next generation. This book became known as “The Origin of Species, by Means of Natural Selection.”
Just when Charles Darwin was about to publish his book, however a letter arrived from a man named Alfred Russel Wallace. Wallace was working on the other side of the world, in Singapore in the 1850s and had discovered the possible evolution by natural selection that Darwin had been working on. Wallace was collecting natural history specimens that he hoped to sell on his return to England and was busy collecting on the Island of Ternate in Indonesia at the time. Here in 1858, he came upon an idea for evolution in which species can become organized by progressive steps checked and balanced by the necessary conditions of life, which leads to an extraordinary modification of form between each generation.
The idea was nearly the same as that of Charles Darwin’s. Wallace had written to him, with the hopes that he would pass it on to Charles Lyell for publication for the Linnaean Society meetings. When Darwin showed the letter to Lyell, the two decided to write back, and mention that Darwin had come to the same conclusion, although Darwin explained his explanation was part of large book he was writing on the topic. Later that year, in 1858 both Darwin and Wallace’s short essay was read to the Linnaean Society meetings. Sadly, Charles Darwin was unable to read it himself, as his young infant son died of fever a few days before, and Charles Lyell presented in his place. While the essay did not garnish much interest, Darwin’s book, which came out the following year quickly becoming a best seller, and altered culture even into the modern age— Charles Darwin is likely the best-known name in the field of biology, as Einstein is in physics. But he is often mistakenly characterized in the modern realm of science.
Darwin knew nothing of genetics, chromosomes, heredity, DNA, or RNA. Instead his writings resulted in our modern understanding of how life adapts by its continued process of being selected by nature. Death removes all of life in the end, but it is only those individuals that survive to at least the stage in life to have offspring and reproduce that succeed in the long run with a new generation after their parent’s death. Each generation growing stronger by the natural selection of traits that make each generation slightly better adapted to the world it confronts. This adaptive process of trial and error is powerful, resulting in the amazing diversity of life on our planet. Evidence for Darwin’s theory of natural selection was powerfully vindicated by fossilized remains of transitional animals preserved in an orderly fashion in the stratigraphy of rocks on the Earth’s surface. Each generation of fossilized lifeforms working to perfect the act of surviving long enough, long enough for a brief moment on Earth, enough to leave its descendants and a new generation.
The Species Problem[edit | edit source]
Darwin left behind more questions than he answered. Darwin viewed that organisms were naturally selected by the harsh environments that they found themselves in, and the best fit individuals of that harsh environment would leave behind the next generation, with each generation overtime becoming better adapted to the environment. If the harsh environment was too harsh, and zero individuals survived, then the lineage or species would become extinct. This trial and error method of change was similar to the artificial selection that animal and plant breeders will employ in cultivating the traits that they personally find advantageous. It is one of the reasons why there are so many different breeds of dogs, and such variety, each lineage was selected for those particular traits that make each breed unique. Darwin viewed differences in shape (morphology) and characteristics of each variety as a unique species of lifeform, which changed as the environment changed, but his ideas did not truly explain what a species is and how they originate. This led biologists to try to define what is a species? And secondarily how do they appear in nature?
The Biological Species Concept[edit | edit source]
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Much of our current understanding of what a species is comes from the work of a famous ornithologist by the name of Ernst Mayr. Mayr grew up in Germany, with a complete fascination of birds, which he fanatically catalogued and described from his numerous bird-watching trips into the mountains of southern Germany. He could easily identify most birds on the basis of color, shape, and song, with particular skill. He would use the nomenclature of paired genus and species scientific names developed by Carl Linnaeus in the 1600s, as well as the local German common names for birds. After his education, he worked in Berlin at the museum continuing to catalogue the occurrence of various birds in the country. In 1927, he was introduced to Walter Rothschild, an eccentric wealthy English aristocrat.
Rothschild was the oldest son of the famous Rothschild banking family in London. Walter Rothschild struggled throughout his life with a stuttering speech, and found it difficult to make connections with fellow humans, both in communicating verbally as well as being socially awkward and self-conscious when he spoke. It was animals that he felt most comfortable being around, and from an early age dreamed of building a private zoo, and taking care of an assortment of animals. As he grew older, he had the finances to make it possible and founded a zoo and natural history collection in London. This private collection housed many living species of animals collected from around the world, including many cassowary birds (large flightless birds from New Guinea). Rothschild had described many of these birds as new species, with slight observable differences in the shape of their crests and skulls. Fascinated with these birds from New Guinea, Rothschild was eager to continue to collect bird specimens from the islands for his collections in England. Ernst Mayr seemed the best choice to send on an expedition to New Guinea to find more bird specimens. For Mayr this was a chance in a lifetime, and he was eager to spend a year or more collecting birds in the tropical islands of the South Pacific and Indonesia and getting paid all the while. The adventure lead to the collection of one of the largest museum bird collections ever made of the region, and quickly Mayr discovered the importance of geography in the occurrence of species. Using the colors and morphology (shape) of individual specimens to name species, Mayr also found that he could use other differences to distinguish individual birds, such as differences in mating calls or behaviors he observed. These traits were localized to particular geographic regions, likely representing interbreeding populations. These traits may not be demonstrated by differences in observable morphology or color on an individual specimen back at the museum. He wondered how such behaviors would affect the localized breeding populations and influence mating or breeding between pairs in the wild.
On returning from the trip, the collections were added to Rothschild’s museum and zoo, but a mysterious scandalous event occurred a few years later which would allow Mayr to study the collections again. Rothschild was being blackmailed, and to make the payment he sold the valuable collection of museum bird specimens to the American Museum in New York, where Mayr was working after his return from New Guinea. For Mayr, it meant that his collection of birds he had personally collected in the field was within his possession again. He could study all of these birds in more detail, and furthermore he knew exactly where each had been collected. It was during this period of cataloguing specimens that Mayr begin to develop a new concept for what defines a species. Rather than distinguishing species based on similarity of appearance, Mayr began to distinguished species based on their ability to interbreed in nature. He developed the modern idea of a species— the biological species concept.
The biological species concept defines a species as individual members of a population that actually or potentially interbreed in nature with viable offspring. Species are defined this way by their ability to share genetic information within a population over time, rather than their physical appearance. The biological species concept differs from the earlier morphological species concept, which simply groups individual organisms by their shared appearance. The biological species concept groups individuals based on their ability to interbreed with each other instead. This more reproductive definition is somewhat difficult to distinguish in practice, because not all individuals will interbreed within a population. It is much easier to compare differences and similarities within individuals, but recent developments in bioinformatics (the study of genetic similarities and differences of organisms) has allowed biological species to be defined based on shared generic information, rather than their appearance.
Species are the fundamental unit of evolution since they pass on information through heredity of specific traits. These traits allow species to recognize each other in the wild, and will restrict interbreeding between species. The concept of what truly defines a species is debated among biologists, but often takes into factors of viability of offspring, potential ability to interbreed in the wild and genetic similarities between individuals, rather than appearance. Although the general appearance of animals and plants can be more directly used by biologists to determine a species in practice in the field, often discoveries of new species rely on genetic information as well.
How Species Originate[edit | edit source]
Ernst Mayr’s work in cataloguing the large variety of birds from New Guinea highlighted something also observed by Charles Darwin, that geographic isolation was an important factor in the origination of new species and varieties of life. During Darwin’s expedition around the world he collected many new bird specimens as well, most notably finches from the Galapagos Islands. The collection of birds was studied by John Gould who realized that they were all closely related to each other (appearing to share traits with the species Geospiza magnirostris, the Galapagos Ground Finch), but differed depending on which island the bird was collected from. Each bird appeared to be adapted to the particular environment of the island it lived on. Some birds had larger beaks to break tough seeds, while others had narrow beaks to feed on small seeds, depending on the local plants that grew on each island. Ernst Mayr discovered that birds were also localized, but this regional environment influence was the source of new species. For example, if a group of individual birds of a particular species arrived at a new island, and survived for many generations in isolation they would be subjected to different natural selection processes, and could come to resemble a different species over time. If they later returned to the original geographic range, they may not be recognized as the same species, and would not interbreed with the general parent population. This process of complete geographic isolation is called peripatric speciation, which was first proposed by Ernst Mayr in 1954, which states that species originate from isolated portions of the general population, which change due to differences in the local environment. This is often cited, for example when a species originates on geographic isolated islands, but can also be applied to regions along the periphery of a species geographic range if the individual population become cut off from the main parent population center. This idea has led to the discovery that most species originate along the periphery of a species geographic range, or when the geographic range of a species becomes fragmented.
Together this style of speciation is collectively called allopatric speciation, which is used whenever biological populations of the same species become geographically isolated from each other to an extent that it prevents or interferes with gene flow. The term parapatric speciation is used when an isolate population is restricted from the parent population, but may not be completely isolated geographically. There is some debate if new species can originate within the population without geographic isolation or restriction. The origin of species from within the parent population is called sympatric speciation. Ernst Mayr viewed that sympatric speciation was rare or impossible, and indeed over the years examples of sympatric speciation have proven elusive, indicating that geographic barriers or isolation is the main driver in the origin of new species on Earth.
Down the long museum halls of the American Museum, where Ernst Mayr worked on his bird collection, an equally large collection of fossil trilobites is housed. Niles Eldridge happily studied these ancient fossils that exhibit a wide diversity of varieties and forms preserved in Paleozoic shales from around the world. Long extinct, these small fossils are highly sought after by collectors who have named and classified many species and forms. Eldridge noticed that the observed appearance of new varieties and species in the fossil record exhibited a pattern which appeared to be influenced by the mechanism of parapatric and peripatric speciation, where new species or forms appeared to change rapidly in the periphery of a species geographic range, within a small isolated population, then expand outward, colonizing larger regions. Since the fossil record is limited geographically, these periphery small populations were not preserved in the fossil record as frequently, but do appear when they expand their range and population size. The temporal pattern of evolution observed in the fossil record is one of rapid change (or colonization) followed by status. In 1972, Nile Eldridge authored with fellow paleontologist Stephen Jay Gould the idea of punctuated equilibrium. Punctuated equilibrium states that species appear suddenly in the fossil record, and exist over time in stable populations that show little morphological change. The pattern is a result of the observation that most species originate from small isolated or restricted populations separated from the parent population These regions are rarely sampled in the fossil record, although examples have been demonstrated. Simply stated, this idea of evolution views that the major driving force for the great variety of life on Planet Earth is a result of peripheral changes occurring along the edges or among isolated small populations, which if successful will sweep out adding new varieties of life, as accumulations of layer upon layer of growing biodiversity.
Geographic reproductive isolation is important for the origin of species within sexually reproducing organisms, but asexually reproducing organisms, like bacteria can also speciate into new forms through isolation as well, but relies on chance genetic mutations or sharing of genetic information between individual cells. This process is much slower, since genetic information is slower to change in frequency over time in these populations, through the lack of intermixing of mating pairs.
Darwin’s Dilemma[edit | edit source]
Charles Darwin struggled to define how heredity of traits worked between mated pairs and their offspring. He recognized, as most of us do, that traits are passed from the parents to the offspring, but the mechanism behind this heredity was a mystery to him. Darwin coined the long disused term pangenesis, for this out of date idea that traits were blended together from both parents, but this presented the problem. The problem is that overtime the diversity or frequency of novel traits would decrease. For example, if you can imagine a population of a single species of a reproducing population of individuals, each individual in this population exhibiting a variety of color, say red ones, blue ones, and yellow ones, and each time one pair would mate and produce offspring, the resulting colored offspring would be a mix or blend of the two parent colors, for example a yellow and red pair, would produce an orange offspring. With each generation the population would began to look a drab gray, as more and more colors would be mixed with each generation. Like mixing or blending all the colors of paint together will also result in a drab gray or brown color over time. How does a population retain its genetic diversity over time? This idea of Darwin’s blended inheritance of traits was clearly incorrect, heredity was not blended or mixed equally from each parent, but there must be some other mechanism behind the heredity of traits. The solution to Darwin’s dilemma would come from a contemporary of Darwin’s, but one a world away in the highly religious Augustinian Order of the St. Thomas Monastery in Brno in the Czech Republic.
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