3.15: The Diversity of Life - Biology

3.15: The Diversity of Life - Biology

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Biological diversity is the variety of life on earth. It is increased by new genetic variation and reduced by extinction and habitat degradation.

What Is Biodiversity?

Biodiversity refers to the variety of life and its processes, including the variety of living organisms, the genetic differences among them, and the communities and ecosystems in which they occur. Scientists have identified about 1.9 million species alive today. They are divided into the six kingdoms of life shown in Figure 2. Scientists are still discovering new species. Thus, they do not know for sure how many species really exist today. Most estimates range from 5 to 30 million species.

Figure 2. Click for a larger image. Known life on earth

Cogs and Wheels

To save every cog and wheel is the first precaution of intelligent tinkering.

—Aldo Leopold, Round River: from the Journals of Aldo Leopold, 1953

Leopold—often considered the father of modern ecology—would have likely found the term biodiversity an appropriate description of his “cogs and wheels,” even though idea did not become a vital component of biology until nearly 40 years after his death in 1948.

Literally, the word biodiversity means the many different kinds (diversity) of life (bio-), or the number of species in a particular area.

Biologists, however, are always alert to levels of organization, and have identified three unique measures of life’s variation:

  • The most precise and specific measure of biodiversity is genetic diversity or genetic variation within a species. This measure of diversity looks at differences among individuals within a population, or at difference across different populations of the same species.
  • The level just broader is species diversity, which best fits the literal translation of biodiversity: the number of different species in a particular ecosystem or on Earth. This type of diversity simply looks at an area and reports what can be found there.
  • At the broadest most encompassing level, we have ecosystem diversity. As Leopold clearly understood, the “cogs and wheels” include not only life but also the land, sea, and air that support life. In ecosystem diversity, biologists look at the many types of functional units formed by living communities interacting with their environments.

Although all three levels of diversity are important, the term biodiversity usually refers to species diversity!

Biodiversity provides us with all of our food. It also provides for many medicines and industrial products, and it has great potential for developing new and improved products for the future. Perhaps most importantly, biological diversity provides and maintains a wide array of ecological “services.” These include provision of clean air and water, soil, food and shelter. The quality—and the continuation— of our life and our economy is dependent on these “services.”

Try It

The long isolation of Australia over much of the last 50 million years and its northward movement have led to the evolution of a distinct biota. Significant features of Australia’s biological diversity include:

  • A high percentage of endemic species (that is, they occur nowhere else):
    • over 80% of flowering plants
    • over 80% of land mammals
    • 88% of reptiles
    • 45% of birds
    • 92% of frogs
  • Wildlife groups of great richness. Australia has an exceptional diversity of lizards in the arid zone, many ground orchids, and a total invertebrate fauna estimated at 200,000 species with more than 4,000 different species of ants alone. Marsupials and monotremes collectively account for about 56% of native terrestrial mammals in Australia.
  • Wildlife of major evolutionary importance. For example, Australia has 12 of the 19 known families of primitive flowering plants, two of which occur nowhere else. Some species, such as the Queensland lungfish and peripatus, have remained relatively unchanged for hundreds of millions of years.

A Biologist Explains: What Is Life?

Although biology is the study of life, even biologists don't agree on what 'life' actually is. While scientists have proposed hundreds of ways to define it, none have been widely accepted. And for the general public, a dictionary won't help because definitions will use terms like organisms or animals and plants -- synonyms or examples of life -- which sends you round in circles.

Instead of defining the word, textbooks will describe life with a list of half a dozen features based on what it has or what it does. For what life has, one feature is the cell, a compartment to contain biochemical processes. Cells are often listed because of the influential cell theory developed in 1837-1838, which states that all living things are composed of cells, and the cell is the basic unit of life. From single-celled bacteria to the trillions of cells that make up a human body, it does seem as though all life has compartments.

A list of features will also mention what life does -- processes like growth, reproduction, ability to adapt and metabolism (chemical reactions whose energy drives biological activity). Such views are echoed by experts such as biochemist Daniel Koshland, who listed his seven pillars of life as program, improvization, compartmentalization, energy, regeneration, adaptability and seclusion.

But the list approach is let down by the fact it's easy to find exceptions that don't tick every box on a checklist of features. You wouldn't deny that a mule -- the hybrid offspring of a horse and donkey -- is alive, for example, even though mules are usually sterile, so no tick for reproduction.

Entities on the border between living and non-living also undermine lists. Viruses are the most well-known fringe case. Some scientists claim that a virus isn't alive as it can't reproduce without hijacking the replication machinery of its host cell, yet parasitic bacteria such as Rickettsia are considered alive despite being unable to live independently, so you can argue that all parasites can't live without hosts. Meanwhile Mimivirus -- a giant virus discovered in an amoeba that's large enough to be visible under a microscope -- looks so much like a cell that it was initially mistaken for a bacterium. Humans are also creating fringe cases -- designer organisms like Synthia, which has few features and wouldn't survive outside a lab -- through synthetic biology.

Are entities such as viruses really life-forms, or merely life-like? Using a list definition, that largely depends on the criteria you choose to include, which is mostly arbitrary. An alternative approach is to use the theory considered to be a defining feature of life: Charles Darwin's theory of evolution by natural selection, the process that gives life the ability to adapt to its environment. Adaptability is shared by all life on Earth, which explains why NASA used it as the basis for a definition that might work in helping to identify life on other planets. In the early 1990s, an advisory panel to NASA's astrobiology program, which included biochemist Gerald Joyce, came up with a working definition: Life is a self-sustaining chemical system capable of Darwinian evolution.

The 'capable' in NASA's definition is key because it means astrobiologists don't need to watch and wait for extraterrestrial life to evolve, just study its chemistry. On Earth, the instructions for building and operating an organism is encoded in genes, carried on a molecule like DNA, whose information is copied and inherited from one generation to the next. On another world with liquid water, you would look for genetic material that, like DNA, has a special structure that might support evolution.

Detecting alien life is a harder task than collecting samples, however, as illustrated by the Viking mission. In 1977, NASA put landers on Mars and performed a variety of experiments to try and detect signs of life in the Martian soil. The results were inconclusive: while some tests returned positive results for the products of chemical reactions that might indicate metabolism, others were negative for carbon-based organic molecules. Decades later, astrobiologists are still limited to looking for life indirectly, searching for biosignatures -- objects, substances or patterns that might have been produced by a biological agent.

Given that scientists who look for life are fine with signatures, some say we don't actually need a definition. According to philosopher Carlos Mariscal and biologist W Ford Doolittle, the problem with defining life arises from thinking incorrectly about its nature. Their strategy is to search for entities that resemble parts of life and to think of all life on Earth as an individual. That solution might suit astrobiologists, but it wouldn't satisfy people who want to know whether or not something strange, like a virus, is alive.

A major challenge for both detecting and defining life is that, so far, we've only encountered one example in the Universe: terrestrial life. This is the 'N = 1 problem'. If we can't even agree on the distinction between living and non-living things, how can we expect to recognize weird forms of life?

It's life, but not as we know it

As science hasn't provided conclusive proof of extraterrestrials, we must turn to science fiction, and few series have explored such possibilities better than Star Trek: The Next Generation. The voyages of the starship Enterprise and "its continuing mission to explore strange new worlds and seek out new life and new civilizations" gave us everything from the god-like being Q to a huge Crystalline Entity that converts living matter to energy (a kind of metabolism). Perhaps most interestingly, as researchers get closer to creating an artificial intelligence that's smarter than a person, there's Data -- an android who had to prove human-like sentience but didn't reproduce until he built his own daughter. Would a god who exists beyond time, a spaceship-sized crystal or a robotic AI be considered 'alive'?

Is Data from 'Star Trek: The Next Generation' alive?

'What is life?' is not simply a question for biology, but philosophy. And the answer is complicated by the fact that researchers from different fields have differing opinions on what they believe ought to be included in a definition. Philosopher Edouard Machery discussed the problem and presented it as a Venn diagram with circles for three groups -- evolutionary biologists, astrobiologists and artificial-life researchers -- using hypothetical features upon which they would converge (some biologists think viruses are alive while others believe the cell is essential, so assuming members would agree is controversial). Machery claimed that no criteria could fall within the overlap of all three circles, concluding that "the project of defining life is either impossible or pointless."

But while philosophers can sidestep the problem without consequences, the conclusion that it's futile to define life is both unsatisfying and frustrating for regular folk (and also for those like me, who care about the public understanding of science). Regardless of whether researchers ever reach a consensus on a scientific definition, we still need a folk definition for practical purposes -- a sentence to explain the concept of life that the average person can understand.

Life may be a fuzzy concept, but that doesn't mean its meaning should be vague. As computational biologist Eugene Koonin pointed out, defining life isn't scientific because it's impossible to disprove, as we can always find an entity that meets all criteria but is 'clearly' not alive, or lacks certain features but is 'obviously' a life-form, and so "some kind of intuitive understanding of the living state superseding any definition is involved [. ] we seem to 'know it when we see it'." Koonin focused on whether a definition can provide biological insights (such as identifying novel life-forms) but mentions another area where defining life might be useful: "better teaching of the fundamentals of biology."

So how do we get a definition that teaches biology? This is partly an exercise in semantics. First, a popular definition should avoid technical jargon and use everyday language. Next we need a starting point. Since Aristotle first tried to define life around 350 BC, thinkers have engaged in seemingly endless philosophical discussions, In 2011, biophysicist Edward Trifonov tried to break the deadlock by comparing 123 definitions to find a consensus, grouping words into clusters and counting the ones used most frequently to produce a minimal or concise definition: Life is self-reproduction with variations.

The 'variations' in Trifonov's definition are mutants, the result of mutations (errors in copying) that occur during reproduction, which is what creates the variety in a population that allows 'survival of the fittest' individuals through evolution by natural selection. While Trifonov's consensus and NASA's working definition don't use the same words, they're two sides of the same coin and share a central concept: life is able to adapt to its environment.

Darwinian evolution is the way that life as we know it adapts. But what about things that might use alternative mechanisms of adaptation? As a narrow definition will exclude fringe cases and being broad would let us include a wide range of potential life-forms, our popular definition drops Trifonov's inclusion of 'self-reproduction' (allowing for immortal AIs that don't need to replicate) and also NASA's requirement for a 'chemical system' (allowing for organisms that don't carry genes on a DNA-like molecule). An 'environment' implies a habitat or ecosystem, not simply the surroundings, which rules-out a robot that adjusts its body to traverse a terrain and virtual objects that navigate a digital domain.

Lastly, we need a word for the 'thing' we describe as living. Scientists and philosophers use 'entity' without acknowledging that, just as a dictionary uses 'organism', it's effectively a fancy synonym for 'life' (Can you think of an 'entity' that doesn't imply some sort of life-form?) This slight logical circularity may not be ideal, but I can't think of a better option. An entity is a self-contained thing, which means the word can work whatever the level -- whether that's an individual organism, an AI, or all life on a planet.

Any definition should be necessary and sufficient, but it's important to first identify for whom. Because this article is aimed at a general audience (non-scientists), the goal is a folk definition. So what is life? Here's a suggestion:

Life is an entity with the ability to adapt to its environment.

While I think my 'popular definition' makes intuitive sense, it could still join the hundreds of scientific proposals that have failed to find acceptance. Unlike dictionary definitions, at least it isn't wrong, but only time will tell whether people think it's actually right.

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Biology of Life

Biology of Life: Biochemistry, Physiology and Philosophy provides foundational coverage of the field of biochemistry for a different angle to the traditional biochemistry text by focusing on human biochemistry and incorporating related elements of evolution to help further contextualize this dynamic space. This unique approach includes sections on early human development, what constitutes human life, and what makes it special.

Additional coverage on the differences between the biochemistry of prokaryotes and eukaryotes is also included. The center of life in prokaryotes is considered to be photosynthesis and sugar generation, while the center of life in eukaryotes is sugar use and oxidative phosphorylation. This unique reference will inform specialized biochemistry courses and researchers in their understanding of the role biochemistry has in human life.

Biology of Life: Biochemistry, Physiology and Philosophy provides foundational coverage of the field of biochemistry for a different angle to the traditional biochemistry text by focusing on human biochemistry and incorporating related elements of evolution to help further contextualize this dynamic space. This unique approach includes sections on early human development, what constitutes human life, and what makes it special.

Additional coverage on the differences between the biochemistry of prokaryotes and eukaryotes is also included. The center of life in prokaryotes is considered to be photosynthesis and sugar generation, while the center of life in eukaryotes is sugar use and oxidative phosphorylation. This unique reference will inform specialized biochemistry courses and researchers in their understanding of the role biochemistry has in human life.

Part 1: Defining Terms


Species is a Latin word meaning “kind” or “appearance.” No doubt, we learn to distinguish among different types of plants and animals—between cats and dogs, for instance—by their appearance. Today biologists use many aspects other than an organism’s appearance to characterize species: body functions, biochemistry, behavior, and genetic make-up. As such there are many ways to define what a species is. The most common species concept is the “biological species concept.”

Lab Question


Taxonomy is the identification and classification of species. The taxonomic system developed by Linnaeus in the eighteenth century is still used today. It has two main features. First, it assigned to each species a two-part Latin name. The first word of the name is the genus to which the species belongs. The second part of the name, the specific epithet, refers to one species within the genus. For example, humans are Homo sapiens while the black rat is Rattus rattus and the Norwegian rat is Rattus norvegicus. Notice each species has its own unique name, but the two rat species have a similar genus name. This means that the two rat species are in the same genus and suggests that they are more closely related to each other than either of them are to humans which are in a different genus.

The second component of the taxonomic system developed by Linnaeus was adopting a filing system for grouping species into a hierarchy of increasingly general categories. Taxonomists place related genera in the same family, groups of related families into orders, groups of related orders into classes, classes into phyla (phylum, singular), phyla into kingdoms, and kingdoms into domains.

Today we are going to focus on three of the four kingdoms in the domain Eukarya (organisms with nuclei): kingdom Plantae, kingdom Animalia, and kingdom Fungi.

Course Subjects

Course Description:

Introductory biology course designed for non-science majors who desire

a conceptual approach to biological topics. An introduction to the diversity of life: viruses,prokaryotes, protists, fungi, plants, and animals. Topics will include structures and functions, evolution, environmental and human interactions, and origin of life.

Lecture Topic

Lab number & Lab Topic

Introduction to Biodiversity

Darwin & Natural Selection

Lab 2: Fossils & Evidence of Evolution

Lab 3: Natural Selection Game

Lab 4: Evolutionary History

Lab 6: Bacteria 1: Collecting Samples

Lab 7: Bacteria 2: Bacteria in our Envir

* Bacteria scavenger hunts due

Lecture Exam 1

Ch. 1, 16, 17, 18

Lab 8: Protists I: The Algae

Prokaryotes: Bacteria & Archaea

Lab 9: Protists II: Protozoans & Fungal-Like Protists

Prokaryotes: Bacteria & Archaea

Eukaryotes: Protist Kingdom

Algae &ndash Plant-like Protists

* Protist scavenger hunts due

Protozoans & Fungal &ndashlike Protists

Lab 10: Fungi I: The Fungal Body &

Lab 11: Fungi II: Symbiotic Fungi

Lecture Exam 2

Ch. 19, 20, 22

* Fungi scavenger hunts due

Human population article summary.

Lab Practical 1: Bacteria, Protists, Fungi

Lab 12: Plant Diversity I: Mosses, Ferns, & Horsetails

Lab 13: Plant Diversity II: Gymnosperms

Article summary due

Lab 14: Plant Diversity III: Angiosperms

Plants: Angiosperm Reproduction &

Lab 15: Flowers, Fruits, & Seeds

Plant nutrition & transport

Lecture Exam 3

40, 21, 27, 25, 26

* Plant scavenger hunts due

Lab Practical 2: Plants

Eukaryotes: Animal Kingdom

Lab 17: Introduction to Animal Bodies

Cnidarians, Mollusks, Annelids

Lab 18: Animal Diversity I: Porifera, Cnidaria, & Platyhelminthes

Nematodes, Arthropods, Echinoderms

Lab 19: Animal Diversity II: Nematodes, Annelids, & Mollusks

Lab 20: Animal Diversity III: Arthropods & Echinoderms

Lab 21: Animal Diversity IV:

Animals: Vertebrates &ndash Mammals

* Field Trip Due

Lab 21: Animal Diversity IV:

Animals: Vertebrates &ndash Mammals

Lab 22: Animal Diversity V: Vert. 2

* Animal scavenger hunts due

Lab Practical 3: Animals

Biology: The Unity and Diversity of Life (14th Edition) – eBook

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Watch the video: Diversity of Life (December 2022).