Related Concept Videos

Formation of Species01:31

Formation of Species

Speciation describes the formation of one or more new species from one or sometimes multiple original species. The resulting species are discrete from the parent species, and barriers to reproduction will typically exist. There are two primary mechanisms, speciation with and without geographic isolation—allopatric and sympatric speciation, respectively.
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What is a Species?01:17

What is a Species?

Overview
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Keystone Species01:39

Keystone Species

Measures of species biodiversity, such as richness (i.e., the number of species present) and evenness (i.e., their relative abundance), describe an ecological community’s structure. Many factors affect community structure, including abiotic factors (e.g., sunlight and nutrients), disturbances (e.g., fire or flood), species interactions (e.g., predation or competition), and chance events (e.g., foreign species invasion). Certain species—such as keystone species—also play a...
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Frequency-dependent Selection01:21

Frequency-dependent Selection

When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to as frequency-dependent selection. Frequency-dependent selection may occur between species or within a single species. This type of selection can either be positive—with more common phenotypes having higher fitness—or negative, with rarer phenotypes conferring increased fitness.
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What is Natural Selection?01:32

What is Natural Selection?

Natural selection is an evolutionary process in which individuals with survival-promoting traits reproduce at higher rates. These favorable traits become more common within a population or species. Naturally selected traits initially arise via random genetic mutations. In order for selection to occur, there must be variation within a population, the trait controlling the variation must be heritable, and there must be an evolutionary advantage for variation in the trait.
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Energy-releasing Steps of Glycolysis01:28

Energy-releasing Steps of Glycolysis

Glycolysis is divided into two phases based on whether energy is utilized or released. While the first phase consumes ATP, the second phase produces energy in the form of ATP and NADH. The energy is released over a sequence of reactions that turns G3P into pyruvate. The energy-releasing phase—steps 6-10 of glycolysis—occurs twice, once for each of the two 3-carbon sugars produced during steps 1-5 of the first phase.
The first energy-releasing step—the 6th step of glycolysis...
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