Gaia hypothesis Hypothesis that the Earth is alive and can be considered a system that operates and changes by feedback of information between its living and nonliving components.

game species Type of wild animal that people hunt or fish for, for sport and recreation and sometimes for food. 

gangue Waste or undesired material in an ore. See ore. 

gap analysis Scientific method used to determine how adequately native plant and animal species and the existing network of conservation lands protects natural communities. Species and communities not adequately represented in existing conservation lands constitute conservation gaps. The idea is to identify these gaps and then eliminate them by establishing new reserves or changing land management practices. 

gene flow Movement of genes between populations, which can lead to changes in the genetic composition of local populations. 

gene pool Sum total of all genes found in the individuals of the population of a particular species. 

generalist species Species with a broad ecological niche. They can live in many different places, eat a variety of foods, and tolerate a wide range of environmental conditions. Examples are flies, cockroaches, mice, rats, and human beings. Compare specialist species. 

Genes Coded units of information about specific traits that are passed on from parents to offspring during reproduction. They consist of segments of DNA molecules found in chromosomes. 

genetic adaptation Changes in the genetic makeup of organisms of a species that allow the species to reproduce and gain a competitive advantage under changed environmental conditions. See differential reproduction, evolution, mutation, natural selection. 

genetic diversity Variability in the genetic makeup among individuals within a single species. See biodiversity. Compare ecological diversity, functional diversity, species diversity. 

genetic engineering Insertion of an alien gene into an organism to give it a new and usually beneficial genetic trait. Compare artificial selection, natural selection. 

genetically modified organism (GMO) Organism whose genetic makeup has been modified by genetic engineering. 

genome Complete set of genetic information for an organism. 

geographic isolation Separation of populations of a species for long times into different areas.

geology Study of the earth's dynamic history. Geologists study and analyze rocks and the features and processes of the earth's interior and surface. 

geothermal energy Heat transferred from the earth's underground concentrations of (1) dry steam (steam with no water droplets), (2) wet steam (a mixture of steam and water droplets), or (3) hot water trapped in fractured or porous rock. 

global warming Warming of the earth's atmosphere because of increases in the concentrations of one or more greenhouse gases primarily as a result of human activities. See greenhouse effect, greenhouse gases. 

Globalization Broad process of global social, economic, and environmental change that leads to an increasingly similar and integrated world. See information and globalization revolution.

Grassland Biome found in regions where moderate annual average precipitation (25-76 centimeters, or 10-30 inches) is enough to support the growth of grass and small plants but not enough to support large stands of trees. Compare desert, forest. 

green revolution Popular term for introduction of scientifically bred or selected varieties of grain (rice, wheat, maize) that, with high enough inputs of fertilizer and water, can greatly increase crop yields. 

greenhouse effect Natural effect that releases heat in the atmosphere (troposphere) near the earth's surface. Water vapor, carbon dioxide, ozone, and several other gases in the lower atmosphere (troposphere) absorb some of the infrared radiation (heat) radiated by the earth's surface. This causes their molecules to vibrate and transform the absorbed energy into longer-wavelength infrared radiation (heat) in the troposphere. If the atmospheric concentrations of these greenhouse gases rise and they are not removed by other natural processes, the average temperature of the lower atmosphere will increase gradually. Compare global warming.

greenhouse gases Gases in the earth's lower atmosphere (troposphere) that cause the greenhouse effect. Examples are carbon dioxide, chlorofluorocarbons, ozone, methane, water vapor, and nitrous oxide. 

gross domestic product (GDP) Total market value in current dollars of all goods and services produced within a country , usually during a year. Compare gross national product, gross world product. 

gross national income (GNI) Total market value in current dollars of all goods and services produced within and outside a country during a year plus net income earned abroad by a country's citizens. Formerly called gross national product. 

gross primary productivity (GPP) The rate at which an ecosystem's producers capture and store a given amount of chemical energy as biomass in a given length of time. Compare net primary productivity. 

gross world product (GWP) Market value in current dollars of all goods and services produced in the world each year. Compare gross domestic product, gross national income. 

ground fire Fire that burns decayed leaves or peat deep below the ground surface. Compare crown fire, surface fire.

Groundwater Water that sinks into the soil and is stored in slowly flowing and slowly renewed underground reservoirs called aquifers; underground water in the zone of saturation, below the water table. Compare runoff, surface water.