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Evolution | Definition of Evolution by Merriam-Webster
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If even one part is missing, the entire system will fail to function. Every individual part is integral. The common mousetrap is an everyday non-biological example of irreducible complexity. It is composed of five basic parts: a catch to hold the bait , a powerful spring, a thin rod called "the hammer," a holding bar to secure the hammer in place, and a platform to mount the trap. If any one of these parts is missing, the mechanism will not work. Each individual part is integral. The mousetrap is irreducibly complex. We now know that there are in fact tens of thousands of irreducibly complex systems on the cellular level.
Specified complexity pervades the microscopic biological world. Molecular biologist Michael Denton wrote, "Although the tiniest bacterial cells are incredibly small, weighing less than 10 grams, each is in effect a veritable micro-miniaturized factory containing thousands of exquisitely designed pieces of intricate molecular machinery, made up altogether of one hundred thousand million atoms, far more complicated than any machinery built by man and absolutely without parallel in the non-living world. The eye, the ear and the heart are all examples of irreducible complexity, though they were not recognized as such in Darwin's day.
Nevertheless, Darwin confessed, "To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When individuals compete for limited resources in their environment they are subject to ecological selection. These traits are subject to sexual selection.
These are sometimes correlated to the health of an individual, and are therefore an honest badge of fitness. Sexual selection can even act at a molecular level. Birds are particularly known for showy ornaments that attract mates, but also increase the chances of being spotted by predators. Over eons, and many generations, the process of slow evolutionary change , called anagenesis, can cause one species to evolve into another.
Allopatric speciation happens when a geographical change — a river changing course for example or a new mountain range — splits a species in two. Once separated, as happened to antelope squirrels on either side of the Grand Canyon in the US, the populations evolve independently, eventually becoming distinct and reproductively isolated. These species adapt to different opportunities in the environment, and then cease to interbreed — perhaps due to some isolating mechanism.
Rarely new species can also form through hybridisation, such as sunflowers. Darwinian evolution is a slow, gradual process. But much of the fossil record hints at puzzling long periods of stasis , with scarcely any change. In , evolutionary biologists Niles Eldredge and Stephen J Gould argued instead that species, perhaps even communities , form suddenly in fits and starts of change. They called the theory punctuated equilibrium. Like individuals in a population, species also struggle amongst themselves to survive, and most become extinct over time.
Species can also die out in mass extinctions , such as the one that caused the demise of the dinosaurs. Today we may be in the throes of another mass extinction , caused by human overexploitation of habitats. During his voyage on the HMS Beagle and throughout his life , Darwin gathered evidence that contributed to his theory of natural selection. In Origin of Species he presented support from the fields of embryology , geography , palaeontology and comparative anatomy see interactive graphic. Darwin also found evidence for his theory in examples of convergent evolution, co-evolution and adaptive radiation.