9.2: Development of the Theory of Evolution

Organizing the Tree of Life: Linnaean Classification

The beginnings of recorded classification go back to Aristotle in the 4th century BC, with his observations of invertebrates while living on the islands of Greece. His method of classifying organisms began by separating animals from plants, then separating animals into two further groups that we still use loosely today: Anhaima (animals without blood, or invertebrates) and Enhaima (animals with blood, or vertebrates). This system would persist through the middle ages and right up to the time of Linnaeus in the 18th century.

Classification of organisms in the years just prior to Linnaeaus’ work was very disorganized, with resulting names of organisms becoming very long and inconsistent. By introducing the system of binomial nomenclature, which we still use today (Genus followed by species, e.g. Homo sapiens), Carl von Linné simplified and unified taxonomy. His system relies on shared characteristics - what organisms have in common - to create descriptive names and organize them into groups or “bins.”

Modern science owes its understanding of evolutionary processes to the earlier work of luminaries like Linnaeus, who tried to provide order to the living world around him. His approach to taxonomy, the scientific classification of organisms, is a nested hierarchy, with “Kingdoms” as at the highest taxonomic rank and most inclusive category. Kingdoms get divided up into “Phyla,” then “Classes,” “Orders,” “Families,” “Genera,” and finally “Species.”

In 1990, Carl Woese added a higher taxonomic rank, the Domain, because of the discovery of three distinct forms of RNA that separate the Archaea, Eukarya, and Bacteria from one another. Other than this change, the system has remained largely unchanged since Linnaeus created it. Though Linnaeus only used anatomy as the basis of his classification decisions, his scheme has largely been validated through later insights derived from DNA sequencing. In a few cases, small refinements have been provided through genetic insights, but overall, Linnaeus’ system provides clarity for the patterns we see!

Figure \(\PageIndex{1}\): Red Fox classification according to the Linnaean System. Including the later addition of the “Domain”, wolves belong to the Kingdom Animalia and, as the system progresses, end with the genus and species names Vulpes vulpes.

Evolutionary Foundations: Lamarckian Inheritance

Determining a mechanism for why some “modifications” helped create success and others did not would rely on uniting these ideas with the environment in which an organism lived. In late 18th century France, Jean-Baptiste Lamarck tried to explain such observations by attributing parents with the ability to pass on traits or behaviors through use or disuse of an organ, anatomical feature, or behavior picked up during their lifetime. This late 18th century idea would become quite a popular way to explain the length of a giraffe’s neck, for instance, which would become progressively longer during its lifetime as it strained to reach high branches. This long neck in the parent could then be inherited by the giraffe’s offspring. Continued use of this trait would enable further development of the feature.

Figure \(\PageIndex{2}\): Lamarckian inheritance of acquired characteristics says that these giraffe, through their own efforts, lengthened their necks and passed the trait of a slightly longer neck to their offspring. (CC BY-SA 4.0, Sandritaverooka, Wikimedia Commons.)

A modern example of Lamarck's inheritance of acquired traits is a parent that undergoes plastic surgery to change their body. A mother who alters the shape of her nose through surgery will not pass the altered shape down to her children, because that new shape is not coded in her genetics. We know now that Lamarckian inheritance is a flawed understanding of inherited characteristics, but Lamarck correctly argued that life took its form through adaptation to the environment (an idea that was quite controversial at the time!). Though aspects of his ideas would later be proven wrong, the concept of traits being inherited by subsequent generations through natural processes was a major step forward.

A New Paradigm Emerges: Darwin and Wallace

Scientific discovery often occurs over time, but always occurs through the accumulation of data. Thomas Kuhn (1962) outlined how scientific “revolutions” are structured as changes in a paradigm. A paradigm is a pattern of thinking. Old models and theories give way and evolve as new information arises. Sometimes, the entire pattern of thinking shifts in a major way, marking an entirely new way of approaching scientific problems. Charles Darwin and Alfred Russel Wallace both discovered, pretty much at the same time, the key process that would lead to a new paradigm in evolution. This key process was natural selection, the variation in survival due to advantages and disadvantages in certain traits within an organism.

Before Darwin and Wallace, Linnaeus’ ideas showed the relatedness of life and Lamarck’s ideas about inheritance supposed that life has changed over time. These ideas represented the paradigm of the time. Darwin’s publication of “The Origin of Species” in 1859 initiated a major shift in the view of how traits are transmitted from parent to offspring. Rather than parents passing traits directly to offspring through use or disuse, Darwin and Wallace would argue that this occurred through natural selection. Their views on this were influenced by two major factors. First, Thomas Malthus’ writings on economic ideas regarding population and resources emphasized that population growth eventually far outstrips available resources in an environment. Second, both Darwin and Wallace had opportunity to conduct extensive fieldwork in locations filled with biodiversity relatively untouched by intensive human development.

Figure \(\PageIndex{3}\): Photo of Alfred Russel Wallace (left) and Charles Darwin (right), co-discoverers of natural selection.

Charles Darwin famously spent five years (1831-1836) as a naturalist aboard The Beagle, a British ship with a mission to circumnavigate the globe. During this time, Darwin was able to spend extended periods of time on land, exploring regions around the South American coast, from Brazil to the Galapagos and beyond. He collected rocks, fossils, animal specimens, planet specimens, and more. Sending them on to England prior to his return, these collections made him famous and would become his life’s work. Ultimately, the immense variation in plant and animal species across South America was puzzling. He found many completely different species living in environments that were very similar. Why was there such variation across distances if the environments were so similar? The classic example of this are his observations of finch variation across the Galapagos Islands (Darwin’s Finches). The best way to characterize this variety was by invoking a common ancestor sometime in the past that, as its descendants moved from island to island, evolved into new species.

Another example from his voyage came from his explorations of the greater rhea and lesser rhea, two flightless birds that local guides often ate as food in the area around Bahia Blanca, Patagonia. These two birds were very similar, but different species. Because one lived north of the Rio Negro (River) and the other south, their ancestors must have pursued separate evolutionary trajectories. Darwin’s insights were not relegated only to living specimens. Darwin also collected many fossils. In subsequent study back home, he made connections between his living specimens and fossil ancestors. Such evidence would become very important for the development of his ideas around natural selection.

Figure \(\PageIndex{4}\): Stops along Charles Darwin’s second voyage of “The Beagle” (1831-1836). These includes locations from Australia to Cape Town, South Africa, to South America and, eventually, back to England.

Alfred Russel Wallace conducted most of his work in two places. Like Darwin, he spent a great deal of time in South America, particularly Brazil. He also conducted field work in what is today Malaysia, Indonesia, and nearby places. Inspired by the field work of Darwin and others, he funded his expeditions, running between 1848 and 1862, through the sales of collected specimens. He methodically planned his field work to explore ideas he had surrounding biogeography, or the geographic distribution of species. If evolution occurs, he reasoned, closely related species should live close to one another. His field work played this out, noting that rivers and other bodies of water did indeed separate related but distinct species. His classic example, called the “Wallace Line,” notes the separation of species of Asian and Australasian descent by the Sunda Straits.

Figure \(\PageIndex{5}\): Map of Indonesia, Australia, and environs depicting “Wallace’s Line”. This line between landmasses separates Asian from Australasian faunas and was described by Wallace. The differences in species across this line provided Wallace with the patterns he used to describe his views on natural selection and evolution.

He noted that while some species likely descended from a common ancestor had evolved very differently on either side, certain species of flora were the same on both sides. Given these straits were about 35 km in width, the mobility of an organism was key. Plant pollen and seeds, blown by the wind or transported by water, allowed growth on both sides of the strait. Large mammals and most birds, by contrast, could not traverse it. This separation, over time, led to new species where once an ancestral form existed. At the time, Wallace would not have known what we now know about the role of plate tectonics in this biogeographical separation. But, he was able to correctly observe here, and elsewhere in his work, that biological change occurs as new species arise from prior species. He recognized natural selection as the mechanism for these observations. Favorable traits, or traits that gave organisms an adaptive advantage, were retained while unfavorable traits would make organisms unfit for their environments.

The two men did write letters to each other. While Darwin came to the idea before Wallace, it was in conversation with Wallace that natural selection, and the new paradigm that would follow, emerged. In 1858, they jointly read papers at a meeting of the Linnaean Society, which would mark the first public description of evolution through natural selection. This was followed one year later by Darwin’s publication, On the Origin of Species. Darwin and Wallace described natural selection as the mechanism that makes survival to the next generation possible, though it does not mean that these offspring are necessarily the “fittest” to survive. Organisms just have to have enough favorable characteristics to be able to survive long enough to pass on their genes, in whatever condition, on to the next generation. Sometimes, genetic mutations passed on are benign. Other genetic changes passed on provide a favorable adaptation for offspring. At still other times, mutations passed on are detrimental to the survival of offspring or even the ability of a parent to reproduce at all.

Figure \(\PageIndex{6}\): A brief timeline of evolutionary thought, from James Hutton to Charles Darwin and Alfred Russell Wallace.

Meanwhile, Back in Europe: Mendelian Genetics

Even with this new understanding of natural selection, it would take an understanding of how genes work to explain how they are inherited by offspring. The foundations for understanding inheritance would come from the 1860s work of an isolated monk in a Czech monastery. While Darwin and Wallace toiled around the globe and in the heat of rainforests, an important part of evolutionary theory was taking shape through humble pea plants.

Gregor Mendel, the son of a farmer, was very interested in plants. Harnessing his curiosity and focusing on pea plants, he crossbred a wide array of varieties and recorded how traits were passed down in the next generation. Applying his mathematics training, Mendel was able to use statistics, applied to his pea plant populations, to predict which traits – smooth skin, wrinkled skin, and so on - would be inherited. As is often the case with new scientific insights, this work was not recognized for its importance right away. But, owing to the work, interests, and curiosity of this isolated monk, natural selection was bolstered with genetics. Natural selection would not only be attributable to environmental factors, but could now also be described by changes in how genes are inherited from one generation to the next.

Figure \(\PageIndex{7}\): Characteristics of pea plants Mendel used in his inheritance experiments. These include seed form, cotyledon color, flower color, pod form and color, flower position on stem, and stem size.

There are three principles in Mendelian genetics. (It is worthwhile to note that, though the term “laws” here are accepted within the scientific community, they may be more accurately referred to as “principles.”)

1. Law of Segregation: Genes come in two forms called alleles.
2. Law of Dominance: Alleles can be dominant (expressed in an organism’s appearance) and recessive (not expressed). You may remember creating Punnett Squares in a previous biology class – as these are a tool that can be used to analyze the expression of one allele over another from one generation to the next.
3. Law of Independent Assortment: The selection of one allele of a gene over another, however, occurs independently of selection occurring among other genes.

These laws, however, do not describe how one allele is transmitted to the next generation, only that alleles exist, are treated independently of other genes during selection, and ultimately determine what variants of such genes are physically expressed. For instance, there are cases where one allele is not completely dominant over another, a situation called “incomplete dominance”. Likewise, there are situations where the phenotype for both alleles is expressed. This is called “codominance”. It is also known that some genes exist in nature with more than two alleles, or “multiple alleles”. Finally, some traits are “polygenic”, such as the color of fruit fly eyes, where several genes contribute to a physically expressed variation.

Figure \(\PageIndex{8}\): Variations in fruit fly eye color (from the top, clockwise): brown, cinnabar, sepia, vermilion, white, wild.

Key Terms

• binomial nomenclature - a system of naming plants and animals by assigning a two-part, Latin-based name to each, consisting of the genus and the species
• invertebrate - an animal that does not have a backbone or spinal column
• Lamarck's inheritance of acquired traits - the theory that an organism's offspring inherit characteristics that the parent acquired during its lifetime through the use or disuse of organs
• natural selection - the method by which organisms with traits better suited to their environment are more likely to survive and reproduce, passing those advantageous traits to their offspring
• vertebrate - an animal with a backbone, or spinal column, and an internal skeleton