Biodiversity and Evolution

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Biodiversity is a relatively recent term in science, only coming into common usage in the 1980s. Since then, it has become one of the most important concepts in environmental science.

What is Biodiversity and Why is it Important?

The total number of species present on Earth has increased steadily over time, punctuated by five periods of mass extinction, and periodic minor extinction events (Figure 1).

Figure 1: Pattern of change in number of Families (groups of related genera) over the last 550 million years, with mass extinction events indicated by the 6 concentric circles.1

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    Credit: Margaret Wilson

From a single species which came into existence over 3.6 billion years ago, an estimated 8.7 million species have evolved to populate the planet today.1 Of these, only about 2 million (14% of terrestrial species and 9% of those from marine environments) have actually been described and classified by scientists. This tremendous number and variety of species that exist on the planet is referred to as biological diversity or biodiversity.

Looking Ahead

In the Biodiversity and Ethics section you will learn that biodiversity has ‘intrinsic value’. Here, you see a reason for that value: the indispensable services biodiversity provides for life on Earth.

The term biodiversity can apply to diversity at all levels of biological organization: diversity among individuals within a particular species (genetic diversity), the diversity of species in an ecosystem, and the diversity of ecosystems on Earth. It applies to terrestrial, aquatic (freshwater and marine), and atmospheric environments, including those arising from extensive human modifications such as agricultural landscapes and urban areas.

The Benefits of Biodiversity

The ‘value’ of a species is often determined by the degree to which it benefits humans. These include tangible benefits, like food, building materials, and medicine and indirect benefits, such as aesthetic, spiritual, and cultural enrichment. However, just as valuable is the role biodiversity plays in the structure and function of the ecosystems on which all life depends. It is vital that the fragile interconnections among species of every kind are maintained. Biodiversity makes this possible.

Closer Look

Read more about the nitrogen cycle.

This is true for even the most inconspicuous biological processes. While organisms that can be seen with the naked eye get the most attention, those only visible with a microscope are extremely important to life on Earth. Photosynthesis by microalgae in the oceans and freshwater systems, for example, accounts for between 45 – 80% of the oxygen in the atmosphere. Critical ecosystem processes, such as the nitrogen cycle and decomposition, would not be possible without microscopic bacteria and fungi. With the development of new techniques in molecular genetics, scientists have only recently begun to appreciate the vast genetic diversity of these groups of microscopic organisms and are beginning to discover their importance to securing the proper functioning of ecosystems.

Looking Ahead

Later in this chapter, you will learn in the Biodiversity and Spirituality section that many religions of the world see species interdependence as a sacred quality.

All species contribute to ecosystem structure and function in distinct ways, yet have co-evolved in a relationship of interdependence. Often the loss of a single species can have a dramatic effect on an entire ecosystem. These species are referred to as keystone species because they are critical for maintaining overall ecosystem balance.

Sea otters are an excellent example of a keystone species. Off the coast of central California, when sea otter populations are healthy, lush kelp (macro algae) forests flourish and support a diverse and complex food web. However, when sea otters are removed, populations of sea urchins, their favorite prey, expand to very high densities, consuming the kelp and creating ‘urchin barrens’ (see Figure 2). Loss of lush kelp forests causes populations of species that directly or indirectly rely upon them to enter an imbalance and ultimately to collapse, reducing ecosystem health and stability.

Closer Look

Watch this video from the Stockholm Resilience Centre to lean more about resilience.

Another way biodiversity helps an ecosystem is through resilience. The more an ecosystem is organically diverse, the more resilient it is to disturbances such as drought, flooding, storms, or insect population explosions. Biodiversity increases an ecosystem’s ability to recover from a disturbance by increasing the chances that a threat to one species can be compensated by the endurance of another.

Figure 2: Reduction of diversity and food-web complexity upon removal of sea otters, a keystone predator, on sub-tidal communities. Arrows indicate direction and quantity of nutrient and energy flow; dash-lined arrows indicate minimal energy/nutrient flow. Light grey organisms indicate small/unstable populations.1

Ecosystem Services

Closer Look

Read more about Ecosystem services.

Without biodiversity, the properties and processes of Earth on which human and non-human life depends would not exist. Scientists identify several fundamental benefits of biodiversity to life on Earth. Since the 2005 Millennium Ecosystem Assessment (MEA), these benefits have commonly been referred to as “ecosystem services”. In the MEA document Ecosystems and Human Well-Being: Synthesis, these services are divided into four categories: provisioning services, supporting services, cultural services, and regulating services (Figure 3).

Figure 3: Ecosystem services-The four types of services that ecosystems provide to support human life and well-being include provisions, supportive, cultural, and regulatory services.1

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    Source: Millenium Ecosystem Assessment 2005

Looking Ahead

Nature has instrumental value for human life. However, when ‘using’ nature we must respect its sustainability. This issue is discussed in the upcoming Biodiversity and Ethics section.

Provisioning services supply goods that directly benefit people, such as food, timber, medicinal plants, and freshwater. Supporting services are processes of nature essential to the functioning of the ecosystem as a whole. These include the formation of soils and the cycling of nutrients. Cultural services are the educational, recreational, aesthetic, and spiritual contributions that nature makes to the richness of human life. Regulating services are the range of functions carried out by ecosystems that modulate climate, decompose waste, filter and water, and pollinate plants.

Humans are inextricably dependent on nature. Maintaining ecosystem health is essential for ensuring a continued supply of ecosystem services.

Questions to Consider

Imagine interconnections that make your everyday life possible—those you see, like your relationships with family and friends, and those you may not see, like the people at the power plant that provides light to your room, or the people who cleaned the street you take to school. Now visit a natural ecosystem in your community, like a park, an empty lot overgrown with plants, a stream or river, or a pond or lake.

  • What are the interconnections in this natural ecosystem that make life possible?
  • Which interconnections do you see?
  • Which do you know are there, but you cannot see them?

How Did Biodiversity Come About?

The rich diversity of life on Earth has arisen from the process of evolution, defined as the successive change in inherited traits and adaptations of biological populations of organisms over time. Largely responsible are two processes, mutation and natural selection, acting on a template of environmental change, some of which is caused by the evolving life itself.

Mutations

Mutations are changes in genes which code for different traits, sometimes resulting in a different physical variation of that trait. Mutations, therefore, are the original source of variation in attributes among individuals and, thus, are the base cause of genetic diversity.

Figure 4: Habitat-specific differences in favorability for different color forms in peppered moths. Note the moth circled in red on the left-hand panel.1

Such changes can be beneficial or harmful, or they can have no influence on survivability, depending on the environment. For example, a mutation in the white-colored peppered moth of England produced a dark-colored form that is easily seen by predators on light-colored lichen covered bark, but well camouflaged on trees with darker bark (Figure 4). During the Industrial Revolution when coal was burned as fuel in London homes and industry, the soot in the air killed the lichens on the bark, so the light-colored peppered moth became vulnerable to predation by birds, while the dark colored mutant had a competitive advantage in the new environment.

Natural Selection

As a consequence of genetic diversity, members of a given species differ from one another in their physical traits (phenotype). It is this variation in phenotype within species that allows for the occurrence of natural selection, a mechanism of evolution postulated by Charles Darwin in his book On the Origin of Species: By Means of Natural Selection (first published in 1859).

Populations are always composed of individuals of one species that differ slightly in physical attributes. Some traits possessed by certain individuals will be better suited to a given set of environmental conditions than others. Individuals with favorable traits, known as adaptations, are more likely to survive, reproduce, and pass their genes on to their offspring. Over many generations the percentage of individuals with adaptive traits increases, thus adapting the whole population to a given habitat. Therefore, natural selection operates on the natural genetic variation within a population, selecting for the traits that are most well adapted to the environmental pressures of that habitat.

Figure 5: Examples of camouflage coloration- a. Brimstone (Live-Leaf) Butterfly, b. Dead-Leaf Butterfly, c. Batfish, d. American Bittern.1

Camouflage or cryptic coloration is an example of an adaptation. Through the process of natural selection, many species have evolved shapes and colorations that provide camouflage, either to be less visible to their predators or to their own prey. Leaf butterflies are able to conceal themselves from bird predators due to their resemblance to a leaf (Figure 5). Those individual butterflies that look less like a leaf are more likely to be eaten by a bird than those more effectively camouflaged. Over time, natural selection narrows the variation among individuals to maintain effective camouflage.

The environmental factors that influence the survival and reproductive success of individuals with different phenotypes are called selection pressures. These can be abiotic, like changes in ambient temperature or soil type, or biotic, like predation or competition among individuals for resources.

Human activities can also result in very strong selection pressures. For example, the use of pesticides to control crop pests can rapidly cause a pest species to develop resistance to the pesticide. Those few individual pests that, by chance, have genes that make them less susceptible to the toxin are the only individuals who survive and, therefore, the only individuals to pass on those genes to their offspring. Thus, through the process of natural selection, the population develops resistance to that pesticide.

Looking Ahead

Species extinction is an issue that affects the common good. This issue is discussed in the upcoming Biodiversity and Ethics section.

Often times, environmental changes caused by humans can be too rapid to allow species to adapt to the changing environment, and may lead to extinction. For example, accidental introduction of the brown tree snake by human visitors into the pacific island of Guam in the early 1950’s resulted in extinction of over 70% of the bird species native to the island as a direct result of predation. Many species have gone extinct or are threatened with extinction because pressures from human activities are altering habitat more rapidly than species can adapt.

Important Evolutionary Events in Planetary History

The evolution of life on Earth has been characterized by ongoing increases in the number of species punctuated by recurrent and mass extinction events that greatly reduce the species number (Figure 1). These patterns are influenced by biological changes in the environment, some of which are caused by activities of life itself. This includes human activities, the most devastating being the present-time Anthropocene Mass Extinction, the largest extinction event in history (this is discussed further below). Over the past 500 years, human activities have destroyed 25% of all mammal species, with one third of all remaining vertebrate species now threatened. Other pre-historic extinctions have been caused by the movement of land masses via continental drift and tremendous meteor impacts to the Earth.

Perhaps the most influential biological innovation to arise since life has existed was the evolution of photosynthesis about 3.4 billion years ago. By utilizing the abundant supply of carbon dioxide (CO2) gas in their surrounding environment, species that could photosynthesize were able to harness the abundant energy of the sun. They used this energy to build sugar, which stored the energy in chemical bonds to use later in building structural molecules such as carbohydrates, lipids, and proteins that contribute to their growth and reproduction. Interestingly, the process of photosynthesis produces oxygen (O2) as a waste product.

Over two billion years, photosynthetic activity by plants and algae allowed O2 concentrations to increase in the atmosphere and oceans. This led to extremely important evolutionary changes.

First, it allowed formation of the ozone layer (O3) in the upper atmosphere, which blocks harmful ultraviolet (UV) radiation from reaching the Earth’s surface. This UV protection allowed photosynthetic species to live in shallower waters or closer to the ocean surface where they had better access to light, supplying more food to non-photosynthetic species.

Second, it allowed evolution of the biochemical process of cellular respiration. Through this process, which requires O2, an aerobic organism can extract 18 times more chemical energy from a molecule of sugar than the earlier, ancestral anaerobic life forms. This increased energy efficiency and made possible the evolution of more complex forms of life, which grew from the first single-celled organisms to complex organisms with cells that differentiated to form tissues of various functions, such as muscle, bone, nerve, skin, etc.

Changes in the orientation of land masses by continental drift have strongly affected patterns of change in biodiversity. For example, the mass extinction at the end of the Permian Era, 250 million years ago, occurred at a time when land masses on Earth were joined into a single ‘super continent’ called Pangaea (Figure 6).

Figure 6: Distribution of land masses at different points in Earth’s history.1

Closer Look

The surface of the Earth is constantly changing via continental drift. Watch how the distribution of land masses have changed over the last 260 million years.

During this time the connected land masses extended to both the North and South poles, allowing formation of ice-caps. At the same time extensive volcanic activity in what is now Siberia covered vast areas of the landscape with lava flows. Evidence also suggests the Earth suffered a large meteor impact at this time.

This combination of catastrophic events caused the extinction of over 90% of marine species and an estimated 70% of terrestrial species, the largest loss of species, on a percentage basis, in the history of life on Earth up until that point. However, back then, relatively few species had yet evolved.

Think about that: all life today has evolved from those species that survived each mass extinction event. If a different set of species survived any of these evolutionary events, life on Earth would look much different than it does today. If any of these events drove the ancestor from which mammals ultimately descended to extinction, humans would not exist.

Questions to Consider

Imagine adaptations you would want in your body and in your behavior if you lived outdoors all year in an arctic environment with an average temperature of -40 to 0˚C (-40 to +32˚F).

  • Would you prefer to be tall and thin, or short and stocky? Why?
  • Would you prefer a large nose or a small nose? Why?
  • Would you prefer spending hours of restful reading or lively hiking? Why?
  • Would you prefer to sleep alone or in a group? Why?

Read this fascinating discussion of physical and cultural adaptations by the Inuit People of the Arctic.

 

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    Mora et al. 2011