Species

Millions of different species exist on the earth. these millions of species, which have evolved over billions of years, are the result of two distinct but simultaneously occurring processes: the processes of speciation and extinction. One of the processes that affects the number of species on earth is speciation, which results when one species diverges into two distinct species as a result of disparate natural selection in <range type="comment" id="565571023_9">separate </range id="565571023_9"><range type="comment" id="565571023_10">environments</range id="565571023_10">. Geographic <range type="comment" id="565571023_11">isolation </range id="565571023_11">i<range type="comment" id="565571023_12">s one common</range id="565571023_12"> mechanism that <range type="comment" id="565571023_13">fosters </range id="565571023_13">speciation; speciation as a result of geographic isolation occurs when two populations of a species become separated for long periods of time into areas with different <range type="comment" id="565571023_14">environmental </range id="565571023_14">condition. After the two <range type="comment" id="565571023_15">populations </range id="565571023_15">are <range type="comment" id="565571023_16">separated</range id="565571023_16">, they evolve <range type="comment" id="565571023_17">independently</range id="565571023_17">; if this <range type="comment" id="565571023_18">divergence </range id="565571023_18"><range type="comment" id="565571023_19">continues </range id="565571023_19">long enough, members f the two <range type="comment" id="565571023_20">distinct </range id="565571023_20">populations <range type="comment" id="565571023_21">eventually </range id="565571023_21">become so different <range type="comment" id="565571023_22">genetically </range id="565571023_22">that they are two distinct species rather than one. The process of speciation may <range type="comment" id="565571023_23">occur </range id="565571023_23"><range type="comment" id="565571023_24">within </range id="565571023_24">hundreds of years for organisms that <range type="comment" id="565571023_25">reproduce </range id="565571023_25"><range type="comment" id="565571023_26">rapidly</range id="565571023_26">, but for most species the process of speciation can take thousands to millions of years. One example of speciation is the early fox, which over time evolved into two distinct species, the <range type="comment" id="565571023_27">gray fox</range id="565571023_27"> and the <range type="comment" id="565571023_28">arctic fox</range id="565571023_28">. The <range type="comment" id="565571023_29">early </range id="565571023_29">fox <range type="comment" id="565571023_30">separated </range id="565571023_30">into populations which <range type="comment" id="565571023_31">evolved </range id="565571023_31"><range type="comment" id="565571023_32">differently in response to</range id="565571023_32"> very different <range type="comment" id="565571023_33">environments </range id="565571023_33"><range type="comment" id="565571023_34">as </range id="565571023_34">the populations moved in different <range type="comment" id="565571023_35">directions</range id="565571023_35">, one to colder northern climates and the other to warmer southern climates. The northern population adapted to cold weather by developing <range type="comment" id="565571023_36">heavier fur,</range id="565571023_36"> shorter ears, noses, and legs, and white fur to camouflage itself in the snow. The southern population adapted to warmer weather by developing <range type="comment" id="565571023_37">lighter fur</range id="565571023_37"> and longer ears, noses, and legs and keeping its darker fur for better camouflage <range type="comment" id="565571023_38">protection</range id="565571023_38">. Another of the processes that affects the number of species on earth is <range type="comment" id="565571023_39">extinction</range id="565571023_39">, which <range type="comment" id="565571023_40">refers </range id="565571023_40">to the situation in which a species <range type="comment" id="565571023_41">ceases </range id="565571023_41">to exist. When environmental conditions change, a species needs to adapt to the new environmental conditions, or it may become extinct. <range type="comment" id="565571023_42">Extinction </range id="565571023_42">of a species is not a<range type="comment" id="565571023_43"> rare occurrence </range id="565571023_43">but is instead a rather <range type="comment" id="565791271_1">commonplace </range id="565791271_1">one: it has, in fact, been <range type="comment" id="565791271_2">estimated </range id="565791271_2">that more than 99 percent of the species that have ever existed have become <range type="comment" id="565791271_3">extinct</range id="565791271_3">. Extinction may <range type="comment" id="565791271_4">occur </range id="565791271_4">when a <range type="comment" id="565791271_5">species </range id="565791271_5"><range type="comment" id="565791271_6">fails </range id="565791271_6">to <range type="comment" id="565791271_7">adapt </range id="565791271_7">to <range type="comment" id="565791271_8">evolving </range id="565791271_8">environmental conditions in a limited area, a process known as background extinction. In contrast, a broader and more abrupt extinction, known as <range type="comment" id="565791271_9">mass </range id="565791271_9">extinction, may <range type="comment" id="565791271_10">come about </range id="565791271_10">as a result of a catastrophic <range type="comment" id="565791271_11">event </range id="565791271_11">or <range type="comment" id="565791271_12">global climatic change</range id="565791271_12">. When such a catastrophic event or global climatic change <range type="comment" id="565791271_13">occurs</range id="565791271_13">, some species<range type="comment" id="565791271_14"> are able to adapt to the new environment,</range id="565791271_14"> <range type="comment" id="565791271_15">while those that are unable to adapt</range id="565791271_15"> <range type="comment" id="565791271_16">become extinct. </range id="565791271_16">From geological and <range type="comment" id="565791271_17">fossil </range id="565791271_17"><range type="comment" id="565791271_18">evidence</range id="565791271_18">, it <range type="comment" id="565791271_19">appears </range id="565791271_19">that <range type="comment" id="565791271_20">at least five great mass extinctions have occurred;</range id="565791271_20"> the <range type="comment" id="565791271_21">last </range id="565791271_21">mass extinction occurred <range type="comment" id="565791271_22">approximately </range id="565791271_22">65 million years ago, when the dinosaurs became extinct after 140 million years of <range type="comment" id="565791271_23">existence </range id="565791271_23">on earth, <range type="comment" id="565791271_24">marking </range id="565791271_24">the end of the Mesozoic Era and the beginning of the Cenozoic Era. The fact that millions of species are in <range type="comment" id="565791271_25">existence </range id="565791271_25">today is evidence that speciation<range type="comment" id="565791271_26"> has clearly kept well ahead of extinction</range id="565791271_26">. <range type="comment" id="565791271_27">In spite of </range id="565791271_27">the fact that <range type="comment" id="565791271_28">there have been numerous</range id="565791271_28"> periods of mass extinction, <range type="comment" id="565791271_29">there is clear evidence</range id="565791271_29"> that periods of mass extinction have been <range type="comment" id="565791271_30">followed </range id="565791271_30">by periods of dramatic <range type="comment" id="565791271_31">increases </range id="565791271_31">in new species to fill the <range type="comment" id="565791271_32">void </range id="565791271_32">created by the mass extinctions, <range type="comment" id="565791271_33">though </range id="565791271_33">it may take 10 million years or more following a mass extinction for biological <range type="comment" id="565791271_34">diversity </range id="565791271_34">to be <range type="comment" id="565791271_35">rebuilt </range id="565791271_35">through speciation. When the dinosaurs <range type="comment" id="565791271_36">disappeared </range id="565791271_36">65 million years ago, for example, the evolution and speciation of <range type="comment" id="565791271_37">mammals </range id="565791271_37">increased spectacularly over the millions of years that <range type="comment" id="565791271_38">ensued</range id="565791271_38">.





1. processes affecting the development of millions of species: 1,3,6 2. speciation: 2,5 extinction: 1, 4, 7