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More than 99 percent of all species that ever lived (over five billion) [1] are estimated to be extinct. [2] [3] Estimates on the number of Earth's current species range from 10 million to 14 million, [4] with about 1.2 million or 14% documented, the rest not yet described. [5]
Evolution is the change in the heritable characteristics of biological populations over successive generations. [1] [2] It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. [3]
The annual life cycle has evolved in over 120 plant families across the entire angiosperm phylogeny. [213] Notably, the prevalence of annual species increases under hot-dry summer conditions in different families including Asteraceae, Brassicaceae, Fabaceae, and Poaceae. [212] Leaves have evolved multiple times - see Evolutionary history of plants.
The most striking feature of evolution of the pelvis in primates is the widening and the shortening of the blade called the ilium. Because of the stresses involved in bipedal locomotion, the muscles of the thigh move the thigh forward and backward, providing the power for bi-pedal and quadrupedal locomotion.
Figure 1:In mammals, the quadrate and articular bones are small and part of the middle ear; the lower jaw consists only of dentary bone.. While living mammal species can be identified by the presence of milk-producing mammary glands in the females, other features are required when classifying fossils, because mammary glands and other soft-tissue features are not visible in fossils.
This study resolved insect phylogeny of all extant insect orders, and provides "a robust phylogenetic backbone tree and reliable time estimates of insect evolution." [62] Finding strong support for the closest living relatives of the hexapods had proven challenging due to convergent adaptations in a number of arthropod groups for living on land ...
The idea that animal and plant species were not constant, but changed over time, was suggested as far back as the 18th century. [42] Darwin's On the Origin of Species, published in 1859, gave it a firm scientific basis. A weakness of Darwin's work, however, was the lack of palaeontological evidence, as pointed out by Darwin himself.
Trilobites saw great diversification over time. [22] For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished, and surviving groups diversified to fill ecological niches with comparable or unique adaptations.