Evolutionary Biology Terms Starting With E
Evolutionary Biology Glossary: E
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Endosymbiotic Theory
/ en-doh-sim-bee-OT-ik THEE-uh-ree / · Greek endon meaning within and symbiosis meaning living together
Endosymbiotic theory proposes that mitochondria and chloroplasts originated when free-living bacteria were engulfed by ancestral eukaryotic cells and became permanent internal structures billions of years ago.
Lynn Margulis formally developed and championed endosymbiotic theory in her 1967 paper, arguing that the ancestors of mitochondria were proteobacteria and the ancestors of chloroplasts were cyanobacteria. Several independent lines of evidence support this origin: both organelles carry their own circular DNA, replicate by binary fission rather than by the cell cycle, have double membranes consistent with engulfment, and contain ribosomes that more closely resemble bacterial ribosomes than eukaryotic ones. Mitochondrial DNA in humans spans approximately 16,569 base pairs and encodes 37 genes, a drastically reduced genome compared to free-living bacteria, reflecting the transfer of most ancestral bacterial genes to the host nucleus over roughly 1.5 billion years.
Chloroplasts retain even more genes than mitochondria, consistent with a more recent endosymbiotic origin estimated at around 1 billion years ago.
Secondary endosymbiosis extended the process further: certain algae were themselves engulfed by other eukaryotic cells, passing chloroplasts to entirely new lineages. Kelp, diatoms, and dinoflagellates all acquired photosynthesis this way, and their chloroplasts are surrounded by three or four membranes rather than two, each membrane layer recording a separate engulfment event in evolutionary history.
Mitochondria and chloroplasts are ordinary cell compartments assembled from scratch by eukaryotic cells. Both organelles descended from free-living bacteria, retaining their own DNA, bacterial-type ribosomes, and binary fission as evidence of that independent ancestry.
Green plant cells contain chloroplasts descended from cyanobacteria-like ancestors that were engulfed more than 1 billion years ago. A single chloroplast in a spinach (Spinacia oleracea) leaf cell carries a circular genome of roughly 150,000 base pairs, encoding about 80 proteins, while the remaining several thousand chloroplast proteins are encoded by the plant's nuclear genome and imported after translation.
Differences Between Plant and Animal Cells →Evolutionary Arms Race
/ ev-uh-LOO-shuh-nair-ee ARMZ RAYS / · Latin evolvere meaning unfold and Old French armes meaning weapons
Evolutionary arms race is the ongoing reciprocal process in which two interacting species repeatedly evolve new adaptations in response to each other, with each evolutionary change in one species imposing new selection pressure on the other.
Reciprocal selection between interacting species can drive rapid trait evolution in both lineages simultaneously, a dynamic Leigh Van Valen described in his 1973 Red Queen hypothesis. Predator-prey systems, parasites and hosts, and competing plants and herbivores all show this pattern. Garter snakes (Thamnophis sirtalis) in the Pacific Northwest have evolved resistance to tetrodotoxin, a potent neurotoxin produced by rough-skinned newts (Taricha granulosa), while newt populations in areas with resistant snakes produce tetrodotoxin levels up to ten times higher than newt populations facing non-resistant snakes.
This geographic mosaic of coevolution demonstrates that arms races do not proceed uniformly across a species’ range but instead produce a patchwork of local adaptations.
The coevolution between fig trees and fig wasps represents one of the most specialized arms races documented in nature. Each of the roughly 750 fig species (genus Ficus) depends on a specific wasp species for pollination, and the wasps depend entirely on figs for reproduction. Over approximately 60 million years, both partners have evolved traits that enforce cooperation while each side simultaneously evolves mechanisms to exploit the other.
Arms races always end with one side winning permanently. The balance shifts repeatedly as each population keeps evolving, and neither side reaches a stable endpoint as long as both lineages persist and interact.
Rough-skinned newts (Taricha granulosa) in Benton County, Oregon, carry enough tetrodotoxin in a single individual to kill several adult humans. Local garter snake (Thamnophis sirtalis) populations have evolved sodium channel mutations that confer resistance to this toxin, while newt populations in the same area produce toxin levels far exceeding those found where resistant snakes are absent.
Extinction
/ ek-STINK-shun / · Latin extinguere meaning quench or wipe out
Extinction is the permanent disappearance of every individual of a species, ending that lineage's evolutionary history.
Extinction is a normal outcome of evolution: paleontologists estimate that more than 99 percent of all species that have ever lived are now extinct. Background extinction rates average roughly one to five species per year across geological time, but at least five mass extinction events have raised rates far above this baseline, the most severe being the end-Permian event approximately 252 million years ago, which eliminated an estimated 90 to 96 percent of marine species. Habitat destruction, introduced predators, climate shifts, and disease can each drive extinction independently, and their combined effects accelerate loss dramatically.
Current extinction rates are estimated at 100 to 1,000 times the background rate, leading many researchers to describe the present as a sixth mass extinction event.
The passenger pigeon (Ectopistes migratorius) once formed flocks estimated at more than 3 billion individuals, darkening the sky for hours as they passed overhead. Intensive commercial hunting and forest clearing reduced the entire species to a single captive bird, Martha, who died at the Cincinnati Zoo on September 1, 1914, making the passenger pigeon one of the most dramatic collapses from abundance to extinction in recorded history.
Extinction always happens because a species is weak or poorly adapted. Climate shifts, asteroid impacts, volcanic eruptions, and human habitat destruction have eliminated well-adapted, ecologically dominant species that had persisted for millions of years.
The woolly mammoth (Mammuthus primigenius) survived repeated glacial cycles over roughly 700,000 years before going extinct. A small island population on Wrangel Island in the Arctic Ocean persisted until approximately 4,000 years ago, long after mainland populations disappeared, suggesting that human hunting pressure combined with habitat loss, rather than climate change alone, drove the final extinction.