INTRODUCTION TO ENVIRONMENTAL SCIENCES

LECTURE NOTES

 

Ecosystems II: Life, Evolution, and Species Roles

 

 

I. Life and Its Origins

 

All life on earth is either eukaryotic (have a cell nucleus) or prokaryotic (no cell nucleus). All life on earth, except bacteria, now is eukaryotic.

 

There are now five recognized kingdoms of life: monera (prokaryotic bacteria), protista (one-cell eukaryotes like diatoms and amoebas), fungi (multi-celled eukaryotes like mushrooms and molds), plants (photosynthetic green algae and all trees, grasses, flowers, mosses, etc.), and animals (invertebrates and vertebrates).

 

Life began within the first 500 million years of earth history, probably through chemical processes. Initially inorganic compounds combined to form organic ones (they are found all over space) and then these combined to form the first living things capable of utilizing nutrients and energy from their surroundings and of replicating themselves. Mineral surfaces may have acted as catalysts and/or templates for this activity.

 

Where this took place is uncertain. It may have occurred in shallow seas, in the atmosphere, in deep ocean hydrothermal vents, in rocks, or perhaps in all of them. It also is possible that the earth was "seeded" with life from elsewhere in the solar system or galaxy by impacting comets and meteorites. It has taken another 4 billion years of biological evolution to create all of the species diversity we see today from these very simple beginnings.

 

 

II. Evolution

 

Evolution is the way that species respond to changes in their environment. Takes many generations of individuals for this to occur. As a result, new species may form by a process known as speciation and old species may die off by a process known as extinction.

 

A. How Evolution Works

 

Evolution results from changes in the species' gene pool or genetic variability. Affects populations, not individuals; an individual does not evolve, populations do. Different individuals in a population have slightly different sets of genes or alleles (different molecular forms of the same gene).

 

Alleles change through mutations. These are random occurrences due to mistakes during DNA replication or to exposure to mutagens (i.e. radiation). Mutations are random, unpredictable, the only source of new genes, and generally rare.

 

Some mutations produce a harmful adaptation and the individual will not survive. Others produce a beneficial one (at least for that individual in that environment) and will increase its chances for survival and, therefore, the passing of that trait or adaptation to subsequent generations. The gene pool for that species has changed and evolution has occurred.

 

The establishment of a beneficial adaptation that gives some members of a population a better chance of surviving and reproducing is known as natural selection ("survival of the fittest"; does not mean strongest, but best able to produce large numbers of off-spring). Those individuals best suited for a particular set of environmental conditions will have a higher survivorship and reproduction rate.

 

There are three types of natural selection:

 

1)   Directional: Species changes towards an extreme of some trait (becomes larger or smaller, darker or lighter, etc.).

 

2)   Stabilizing: Species changes towards the average; becomes less diverse in some trait (larger and smaller individuals become rarer).

 

3)   Diversifying: Species changes towards both extremes of some trait and the average becomes rarer (large and small are more common)

 

There are limits to adaptations. Traits have to be part of the variety found in the gene pool and they must be inheritable. If a species can't reproduce fast enough it may become extinct before the adaptation is established (fast reproducing species favored). Individual without adaptation have to die off at a faster rate. That is, the adaptation must lead to differential reproduction. If it doesnąt there is no selective advantage for the trait.

 

Species may evolve independently from other species or in tandem with another species. This is called co-evolution. For example, a newt may evolve by becoming more toxic to protect it from predation by snakes. The snakes, in turn, evolve by becoming more resistant to the newt toxin.

 

Evolution may occur as a slow steady process or in bursts of adaptation (punctuated equilibrium) in response to severe environmental stress.

 

 

B. Speciation

 

Speciation is the emergence of new species from a common ancestor. Most commonly it is a two-step process: Geographic isolation causes reproductive isolation. Known as divergent evolution. Usually takes many generations.

 

Geographic isolation: Two populations of the same species isolated from each other by some natural (mountains) or human-caused (highway) barrier. The environments differ on either side of barrier, causing the selected traits to be different. Plate tectonics may isolate entire continents, allowing for large-scale geographic isolation and unique species to occur.

 

Reproductive isolation: Natural selection cause genetic divergence in the two separate populations. At some point the individuals from each population can't (or wont) interbreed (or produce fertile offspring). Two new species now established.

 

 

C. Extinction

 

Adapt or die. Extinction is the ultimate fate of all species. 99.9% of all species in earth history have become extinct.

 

Three main causes: shifting position of the continents (plate tectonics), long-term climate changes, and catastrophic events (impacts or volcanic eruptions).

 

First two generally cause background extinctions (at a low, constant rate). Latter causes mass extinctions (sudden spike in extinction rate; as high as 25%-70% of all living species). Have been at least 6 mass extinction events in the past 500 my.

 

Mass extinctions lead to many empty niches and opportunities for species that survive. These survivors evolve rapidly to fill available niches. Known as an adaptive radiation. It is a reshuffling of the deck. Extinction of dinosaurs allowed for the emergence of mammals (us). Speciation - extinction = biodiversity.

III. Species Roles

 

Each species plays a role in a given ecosystem. This is its ecological niche. May not be fully realized (fundamental niche) due to competition from other species that play a similar or competing role (realized niche)

 

There are two broad categories of species:

 

1) Generalist species: Can live in a variety of conditions or environments (humans, raccoons, coyotes, deer, and rats). They have broad tolerance limits and are less susceptible to extinction when conditions change.

 

2) Specialist species: Can live only in a limited range of conditions or environments (pandas, mountain gorillas, and corals). They have very narrow tolerance limits and do better when conditions are constant.

 

Native species are those that normally live and thrive in a particular environment or habitat. They are found in the location and environment under which they evolved.

 

Non-native, exotic, or alien species have either migrated or been accidentally introduced into a particular environment or habitat. They may be very harmful to native species. Examples include kudzo, zebra mussels, and africanized bees. They are becoming very common due to human mobility, even between continents.

 

Indicator species are good at giving early signs of environmental change and stress. Examples are birds, frogs, and coral.

 

Keystone species are those whose presence is required in order for the environment or habitat to continue. Examples might be birds or insects that pollinate flowers. Lose them and the flowers then die off. Others may be top predators that keep prey species in check.