by DK
In England, Nehemiah Grew used microscopy to examine a wide range of plants. He was the first to identify flowers as being the sexual organs of plants. In The Anatomy of Plants (1682), Grew named the stamen as the male organ and the pistil as the female organ. Grew also spotted pollen grains and noted that they were transported by bees.
Since the early days of microscopy, devices have grown in sophistication. The electron microscope, first used in 1931, uses beams of electrons—rather than light—to reveal objects, allowing scientists an even closer look. Electron microscopes provide views of up to one million times actual size—600 times greater than most modern light microscopes.
The compound eye and brain of a bee, drawn by Jan Swammerdam and published in A Treatise on the History of Bees, shows the eye exterior (left) and the eye dissected (right), with the brain cross-sectioned below.
“[Micrographia is] … the most ingenious book that I ever read in my life.”
Samuel Pepys
English diarist
ROBERT HOOKE
Born on the Isle of Wight, England, Hooke showed an early interest in science. A small inheritance allowed him to attend the prestigious Westminster School, where he excelled, earning a place at Oxford University. There he assisted the natural philosophers John Wilkins and Robert Boyle. In 1662 Hooke became the first curator of experiments for the Royal Society of London. In 1665 he became Professor of Physics at Gresham College.
Like many scientists of his day, Hooke had a broad range of interests. His achievements include some early insights into the wave theory of light; the construction of some of the earliest telescopes; and the formulation of Hooke’s Law. Hooke was also a respected architect, an activity that made him a wealthy man.
Key works
1665 Micrographia
1674 An Attempt to Prove the Motion of the Earth
1676 A Description of Helioscopes and Some Other Instruments
See also: Classification of living things • A system for identifying all nature’s organisms • Microbiology • Thermoregulation in insects
IN CONTEXT
KEY FIGURE
Carl Linnaeus (1707–78)
BEFORE
1682 John Ray, an English botanist, proposes that the plant kingdom be divided into trees and two families of herbaceous plants.
1694 French botanist Joseph Pitton de Tournefort publishes Eléments de Botanique. This beautifully illustrated book becomes the botanical classification benchmark for half a century.
AFTER
1957 Sir Julian Huxley is the first to use the term “clade” to describe a common ancestor and all of its descendants.
1969 Robert Whittaker, an American ecologist, argues for a five-kingdom categorization of life: Monera, Protista, Fungi, Plantae, and Animalia.
Before the 18th century, there was no consistent naming system for animals and plants. Botanists and zoologists often did not know if they were discussing the same organism. To overcome the problem, Swedish botanist Carl Linnaeus invented a revolutionary system, which is still in use today. He is known as the “father of taxonomy”—the science of naming and classifying organisms.
Linnaeus divided both the plant and animal kingdoms into classes, orders, genera, and species. Organisms were placed in these levels on the basis of shared characteristics, such as similarity of body parts, size, shape, and methods of getting food. Linnaeus also adopted a precise two-word (binomial) name for each species.
Early insights
By 1730, while still a student, Linnaeus began to have issues with the system for classifying plants developed by Joseph Pitton de Tournefort more than 30 years earlier. For Linnaeus, the characteristics of individual species needed to be analyzed more closely in order to produce a more thorough taxonomic system.
In 1732, Linnaeus joined an expedition to Lapland, where he collected about 100 unidentified species. These formed the basis of his book Flora Lapponica, in which he aired his ideas about plant classifications for the first time.
Three years later, Linnaeus wrote about his idea for a new hierarchical classification of plants in a further book, Systema Naturae, and thereafter in arguably his greatest work, Species Plantarum, published in 1753, which covered 7,300 species. Previously, plants had been known by long impractical names—for example, Plantago foliis ovato-lanceolatis pubescentibus, spica cylindrica, scapo tereti. Linnaeus called this plant Plantago media, which was sufficient to identify it. As well as being concise, the Linnaean system describes relationships between species.
“In natural science, the principles of truth ought to be confirmed by observation.”
Carl Linnaeus
Later developments
Linnaeus constantly expanded Systema Naturae; its 10th edition (1758) became the starting point for modern animal classification. It was he who suggested that humans were members of the primate family. Much later, aided by Charles Darwin’s theory of evolution by natural selection, biologists accepted that a classification should reflect the principle of common descent, which led to the methodology known as cladistics.
Whales were once thought to be fish, and were classified as such in an early edition of Linnaeus’s Systema Naturae. Only later was it understood that they are actually mammals.
CARL LINNAEUS
Born in rural southern Sweden, Linnaeus was educated at the University of Uppsala, where he began teaching botany in 1730. He spent three years in the Netherlands, and, on returning to Sweden, he divided his time between teaching, writing, and plant-collecting expeditions. At Uppsala, 17 of his students embarked on expeditions all over the world. Linnaeus was a friend of Anders Celsius, the inventor of the temperature scale. After his friend’s death, Linnaeus reversed the scale so that freezing point was 32°F (0°C) and boiling point 212°F (100°C). Linnaeus has been described as the “prince of botanists,” and the philosopher Rousseau said of him “I know no greater man on Earth.” Linnaeus is buried in Uppsala Cathedral; his remains constitute the type specimen—the specimen that represents a species—used for Homo sapiens.
Key works
1735 Systema Naturae
1737 Flora Lapponica
1751 Philosophia Botanica
1753 Species Plantarum
See also: Classification of living things • Biological species concept • A modern view of diversity
IN CONTEXT
KEY FIGURE
Ernst Mayr (1904–2005)
BEFORE
1686 Naturalist John Ray defines individual plant and animal species as those that derive from the same seed.
1859 Charles Darwin’s On the Origin of Species introduces the idea that species evolve through natural selection.
AFTER
1976 The Selfish Gene by Richard Dawkins popularizes gene-centered evolution: natural selection at a genetic level.
1995 The Beak of the Finch by Jonathan Weiner follows the work of biologists Peter and Rosemary Grant on the Galapagos Islands.
2007 Massimo Pigliucci and Gerd B. Müller use the term “eco-evo-devo” to suggest how ecology is among the factors affecting evolution.
By the early 20th century, it was accepted that multiple species could evolve from a common ancestor. However, it was not clear how this evolution process actually occurred. In fact, there was some debate about precisely what a “species” was. In 1942, evolutionary biologist Ernst Mayr proposed a new definition of species: groups of interbreeding natural populations that are “reproductively isolated from other such groups.”
What this means is that two populations of the same species living in the same area may at some point become separated by geography, mate choice, feeding strategies, or other means, and then begin to change through natural selection or genetic drift. Over time, as a result of this initial separation, two distinct species evolve, which cannot interbreed. This type of speciation commonly occurs in small populations of creatures on remote islands.
Key differences
The biological species concept
is primarily focused on the breeding potential between organisms. Two organisms may appear identical and live in the same place, but this does not mean that they are the same species. For example, the Western Meadowlark (Sturnella neglecta) and Eastern Meadowlark (Sturnella magna) look similar and have overlapping ranges, but they have evolved to produce different songs. This prevents them from mating with each other, making them two distinct species.
Another scenario is when members of the same species look very different, but because they can mate and reproduce they are still considered to be the same species. The most obvious instance of this is the domestic dog (Canis familiaris), a species in which there are great differences between individuals. However, as is also evident, different breeds are capable of reproduction with each other, and therefore belong to the same species.
Complex permutations
According to the biological species concept, the potential for inter-breeding is key to the definition of a species. Geographical separation alone does not prevent species from reproducing if they are brought together. Evolutionary divergences—such as the different mating songs of the Western and Eastern Meadowlarks—are what prevents interbreeding.
The biological species concept is not applicable to asexual organisms, such as bacteria, or asexual creatures—for example, species of whiptail lizard. Sometimes, too, different animal species are able to mate and produce offspring, as is the case of a female horse (Equus ferus caballus) and a male donkey (Equus africanus asinus), which together can produce a hybrid—the mule. However, mules themselves are generally incapable of reproduction, and therefore the horse and donkey remain different species. Another example is the liger, a zoo-bred hybrid of a female tiger and a male lion.
Such anomalies highlight the complexities of defining a species. The biological species concept remains the most popular, but scientists are now looking at the idea of shared genes, and using DNA sequence analysis. To date, no one has come up with a single definition that covers every known species, and it seems unlikely that anyone ever will. In the absence of better models, Ernst Mayr’s biological species concept provides an extremely useful way of thinking about species and evolution.
“Endless forms most beautiful and most wonderful have been, and are being, evolved.”
Charles Darwin
Alternative species concepts
Male fireflies are an example of a typological species. They emit a pattern of flashes to attract females, who recognize their species’ code and flash back—if they wish to mate.
Although Mayr’s idea about biological speciation is perhaps the most common way to define species and explain how they evolve, it is far from the only one. In fact, there are more than 20 recognized species concepts, ranging across two broad groups: typological and evolutionary concepts. Typological species concepts are based on the idea that a population of individuals of the same type—or sharing the same set of traits—are what makes up a species. The traits can be based on genetics, such as DNA or RNA base sequences, or on phenotypes, such as the size of certain body parts or particular markings, such as the arrangments of spots on insects’ wings. The evolutionary species concept is based on species lineages. A species is defined as the organisms that share a lineage from the time when the species initially split off until extinction, or until an additional splitting off and creation of a new species.
See also: Evolution by natural selection • The role of DNA • The selfish gene • Competitive exclusion principle
IN CONTEXT
KEY FIGURE
Carl Woese (1928–2012)
BEFORE
1758 Systema Naturae (10th edition) by Carl Linnaeus classifies known life into two kingdoms: animals and plants.
1937 French biologist Edouard Chatton divides life into prokaryotes (bacteria) and eukaryotes (organisms with complex cells).
1966 German biologist Willi Hennig establishes a system of classification based on clades—groups of organisms based on common ancestry.
1969 American ecologist Robert Whittaker divides life into five kingdoms: bacteria, protists, fungi, plants, and animals.
AFTER
2017 A consensus among biologists accepts a seven-kingdom classification of life.
Before biologists had the equipment and techniques needed to scrutinize the microscopic structure of living things, biological diversity was split simply into animal-like and plant-like organisms. Then, in the 20th century, better microscopes began to reveal deeper differences that could not be seen with the naked eye. By the 1960s, picking up on an idea first proposed by Edouard Chatton in the 1930s, the need for a new division of living things emerged, placed between prokaryotes (such as bacteria, with simple nucleus-free cells), and eukaryotes (such as animals and plants with larger, more complex cells).
In the 1970s, the American biologist Carl Woese claimed that even this system failed to account for the diversity among microbes—the smallest living things. He focused on ribosomes—minuscule grains that all cells need in order to make protein—and devised what he called the “three-domain system.” This gave him a new perspective on the branches of Charles Darwin’s evolutionary “tree of life.” Woese found big differences in the chemical makeup of ribosomes among tiny microbes, with one group as far from other prokaryotes as bacteria are from humans.
Sulfur-dependent archaea organisms thrive in the hot geothermal pools of Yellowstone National Park, Wyoming, in conditions that would kill most other organisms.
Revising the tree of life
Woese’s third domain of organisms, known as archaea, is superficially similar to bacteria, but has some strange properties. Many thrive in extreme habitats. Some—uniquely among living things—generate methane in oxygen-deprived places, such as deep marine sediments, or inside warm digestive cavities, such as those of belching, flatulent plant-eating mammals. Other archaea inhabit lakes that are ten times saltier than seawater, or hot acidic pools fed by geothermal heat that would kill anything else.
A decade before Woese proposed his theory, Robert H. Whittaker had recognized animals, plants, and fungi as separate eukaryotic kingdoms, with all other eukaryotes placed in the protist kingdom, and bacteria constituting a fifth kingdom. Whittaker’s protist kingdom covered eukaryotic organisms such as amoebas that did not fit the other categories. Some protists were closer to animals, some closer to plants, and others not close to either. They did not match the tree of life model, in which clades—groups of organisms with a common ancestry—spring as branches from the previous fork.
Woese sought a classification system that reflected the intricacies of evolution—with main branches on the tree of life splitting into smaller ones, and even tinier twigs that end in the leaves of individual species. In the future, the complex tree of life may reveal even more evolutionary categories.
According to Carl Woese, all organisms can be separated into three main categories or “domains.” These divisions are based on similarities in the ribosome structure found in the cells of the groups of organisms within each domain.
Kingdom of their own
Fungi, such as this bright yellow jelly fungus growing on a fallen tree, are no longer classified as plants. Fungi are genetically closer to animals.
For most of the history of biology, fungi were considered to be plants. Even the great classifier of organisms Carl Linnaeus included them in his kingdom Plantae. It was only with the invention of more powerful microscopes that the differences in fungi began to be better understood. It is now known that chitin, a complex carbohydrate and component of fungus cell walls, is not found in plants. Also, fungi make their food by digesting rotted material, whereas plants make food by absorbing light energy in photosynthesis.
DNA analysis shows that fungi are far removed from plants in the evolutionary tree of life: they are, in fact, genetically closer to the branch that gives rise to animals. These same studies show that certain aquatic molds—traditionally classified as fungi—are not related to fungi, while some disea
se-causing microbes are fungi that have evolved to become microscopic parasites.
See also: Early theories of evolution • Evolution by natural selection • The role of DNA • A system for identifying all nature’s organisms
IN CONTEXT
KEY FIGURE
Edward O. Wilson (1929–)
BEFORE
1993 The UN proclaims December 29 as the International Day for Biological Diversity.
1996 The Song of the Dodo by American science writer David Quammen explores the nature of evolution and extinction as habitats become more and more fragmented.
AFTER
2014 The Sixth Extinction by environmental journalist Elizabeth Kolbert shows how humans are causing a sixth mass extinction of species.
2016 In Half-Earth, Edward Wilson proposes that Earth can be saved by dedicating half of it to nature.
Biodiversity is the variety of life on Earth—in all forms and at every level, from genes to microbes to humans and all other species, including those yet to be discovered. Humans rely on biodiversity for food and fuel, shelter, medicine, beauty, and pleasure. For other species, it also provides nutrients, seed dispersal, pollination, and reproductive success. No living thing could survive without biodiversity.