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In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution     , where similar traits evolve independently in species that do not share a recent common ancestry. The two species came to the same function, flying, but did so separately from each other.

These physical changes occur over enormous spans of time and help explain how evolution occurs. Natural selection acts on individual organisms, which in turn can shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for the genotype of an entire species to evolve. It is over these large time spans that life on earth has changed and continues to change.

Natural selection can take a long time to occur, but artificial selection     can occur much faster. Artificial selection is like natural selection, but humans are the ones selecting which traits are passed on to the next generation. For example, all dog breeds are the same species, but they can look dramatically different and can have different behaviors based on the selective breeding that humans have controlled. Humans have selectively bred many plants to produce more fruit, or withstand the elements, or fight pests. Humans have selectively bred many different animals: sheep, cattle, chickens, dogs, cats, pigeons, etc. Darwin's theory of evolution by natural selection was strongly influenced by his observation of the success of selective breeding.

17.1c evidence of evolution

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species , to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader.

Fossils

Fossils are mineralized structures or imprints indicating ancient life. Fossils form when sediment or volcanic ash accumulates quickly over an organism or its traces (for example, footprints). Over time, that sediment becomes rock, with the mineralized remains left inside. It can take millions of years to rock to form, and therefore scientists need a much larger timescale than people are used to. The geologic timescale is a timeline of Earth's history, see [link] ), and [link] ).

This photo shows the major divisions and subdivisions that make up Earth’s history. The first eon is called the Pre-Archean, or the Hadean Eon, and ranges from Earth’s formation, 4.6 billion years ago to about 3800 million years ago. The next eon is the Archean, which ranges from 3800 to 2500 million years ago. The next eon is the Proterozoic eon, which ranges from 2500 to 570 million years ago. The next eon is called the Phanerozoic, and ranges from 570 million years ago to present day. It is broken up into three eras: the Paleozoic, Mesozoic, and Cenozoic. Within each of the three eras are series of periods. Within the Paleozoic era, the Cambrian period ranges from 570 to 500 mya (millions of years ago), the Ordovician ranges from 500-435 mya, the Silurian ranges from 435-410 mya, the Devonian ranges from 410-360 mya, the Mississipian ranges from 360-330 mya, the Pennsylvanian ranges from 330-290 mya, and the Permian ranges from 290-240 mya. Within the Mesozoic era, the Triassic Period ranges from 240-205 mya, the Jurassic ranges from 205-138 mya, and the Cretaceous ranges from 138-66 mya. Within the Cenozoic era, the Tertiary period ranges from 66-1.6 mya, and the Quaternary period ranges from 1.6 mya to present day.
Major divisions within the geologic time scale (attribution: By United States Geological Survey (http://pubs.usgs.gov/gip/fossils/fig15.gif) [Public domain], via Wikimedia Commons)
This picture shows major events depicted on the geological time scale. 4527 millions of years ago (mya), the Moon formed. Around 4000 mya, the first life formed. Around 3200 mya, we have evidence of photosynthesis.  Around 2300 mya, the atmosphere becomes oxygen-rich. Around 530 mya was the Cambrian Explosion, where marine biodiversity rapidly increased.  Around 380 mya, we have evidence of the first vertebrate land animals. From 230-66 mya, non-avian dinosaurs dominated land environments. Around 2 mya, we have evidence of the first hominins.
Geologic time scale with the addition of various important events on Earth (attribution: By Woudloper Derivative work: Hardwigg (File:Geologic_clock.jpg) [Public domain], via Wikimedia Commons

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Source:  OpenStax, General biology part i - mixed majors. OpenStax CNX. May 16, 2016 Download for free at http://legacy.cnx.org/content/col11749/1.5
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