{"id":2845,"date":"2016-06-13T17:54:29","date_gmt":"2016-06-13T17:54:29","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/biologyxwaymakerxmaster\/?post_type=chapter&#038;p=2845"},"modified":"2024-04-26T00:16:30","modified_gmt":"2024-04-26T00:16:30","slug":"reading-darwin-and-descent-with-modification","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/chapter\/reading-darwin-and-descent-with-modification\/","title":{"raw":"Darwin and Descent with Modification","rendered":"Darwin and Descent with Modification"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Outline the work of Charles Darwin as a naturalist aboard the HMS Beagle<\/li>\r\n \t<li>Summarize the prior work and new evidence Darwin used to develop the idea of \"descent with modification\"<\/li>\r\n<\/ul>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1465\" align=\"alignright\" width=\"400\"]<img class=\"wp-image-1465\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02191627\/Figure_18_01_01.jpg\" alt=\"Illustration shows four different species of finch from the Gal\u00e1pagos Islands. Beak shape ranges from broad and thick to narrow and thin.\" width=\"400\" height=\"361\" \/> Figure 1. Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources.[\/caption]\r\n\r\nIn the mid-nineteenth century, the actual mechanism for evolution was independently conceived of and described by two naturalists: <strong>Charles Darwin<\/strong> and Alfred Russel Wallace. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on\u00a0<strong><em>H.M.S. Beagle<\/em><\/strong>, including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin's journey, like Wallace's later journeys to the Malay Archipelago, included stops at several island chains, the last being the Gal\u00e1pagos Islands west of Ecuador. On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Gal\u00e1pagos Islands comprised several species with a unique beak shape (Figure 1).\r\n\r\nThe species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the mainland of South America. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that the varied beaks of each finch helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey.\r\n\r\nWallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection.\u00a0<strong>Natural selection<\/strong>\u00a0is the process by which organisms that are better adapted to their environment survive and pass those adaptations onto their offspring more than those organisms that are less well adapted.\u00a0This leads to evolutionary change.\r\n\r\nFor example, a population of giant tortoises found in the Galapagos Archipelago was observed by Darwin to have longer necks than those that lived on other islands with dry lowlands. These tortoises were \"selected\" because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn't reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.\r\n\r\nNatural selection, Darwin argued, was an inevitable outcome of <span style=\"text-decoration: underline;\">three principles<\/span> that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace's understanding of this principle came from reading an essay by the economist Thomas Malthus who discussed this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment; it is the only mechanism known for adaptive evolution. It is important to keep in mind that natural selection leads to adaptation to a\u00a0<em>specific<\/em> environment, not all environments or even a different environment.\r\n\r\nPapers by Darwin and Wallace (Figure 2) presenting the idea of natural selection were read together in 1858 before the Linnean Society in London. The following year Darwin's book,\u00a0<em><strong>On the Origin of Species<\/strong>, <\/em>was published.\u00a0His book outlined in considerable detail his arguments for gradual changes and adaptive survival by natural selection.\r\n\r\n[caption id=\"attachment_1466\" align=\"aligncenter\" width=\"800\"]<img class=\"size-full wp-image-1466\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02191726\/Figure_18_01_02ab.jpg\" alt=\"Paintings of Charles Darwin and Alfred Wallace are shown.\" width=\"800\" height=\"541\" \/> Figure 2. Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented together before the Linnean Society in 1858.[\/caption]\r\n\r\nDemonstrations of evolution by natural selection are time consuming and difficult to obtain. One of the best examples has been demonstrated in the very birds that helped to inspire Darwin's theory: the Gal\u00e1pagos finches. Peter and Rosemary Grant and their colleagues have studied Gal\u00e1pagos finch populations every year since 1976 and have provided important demonstrations of natural selection. The Grants found changes from one generation to the next in the distribution of beak shapes with the medium ground finch on the Gal\u00e1pagos island of Daphne Major. The birds have inherited variation in the bill shape with some birds having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Ni\u00f1o, the large hard seeds that large-billed birds ate were reduced in number; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, survival and reproduction were much better in the following years for the small-billed birds. In the years following this El Ni\u00f1o, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the size of bills had evolved to be smaller. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased.\r\n<div class=\"textbox learning-objectives\">\r\n<h3>In Summary: Darwin and Descent with Modification<\/h3>\r\nWhile Charles Darwin is generally called \"<strong>the father of evolution<\/strong>,\" the basic idea for this concept was actually developed by both Darwin and Alfred Russel Wallace. Both scientists based their hypotheses on observations of diversity among natural populations. Darwin\u2019s work in particular focused on animals of the Galapagos islands, especially finches. Over time, the idea that species changed from natural selection pressures through \"descent with modification\" gave rise to the idea of evolution. Data accumulated over time, for example the long study of the Galapagos finches by the Grant research team, has supported this idea and moved it into the realm of a supported theory of biology.\r\n\r\n<\/div>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/66cdf7e1-80cf-43b1-9287-6c03a902d953\r\nhttps:\/\/assess.lumenlearning.com\/practice\/1da761af-a4ff-4056-9910-7ca374f62165\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Outline the work of Charles Darwin as a naturalist aboard the HMS Beagle<\/li>\n<li>Summarize the prior work and new evidence Darwin used to develop the idea of &#8220;descent with modification&#8221;<\/li>\n<\/ul>\n<\/div>\n<div id=\"attachment_1465\" style=\"width: 410px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1465\" class=\"wp-image-1465\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02191627\/Figure_18_01_01.jpg\" alt=\"Illustration shows four different species of finch from the Gal\u00e1pagos Islands. Beak shape ranges from broad and thick to narrow and thin.\" width=\"400\" height=\"361\" \/><\/p>\n<p id=\"caption-attachment-1465\" class=\"wp-caption-text\">Figure 1. Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources.<\/p>\n<\/div>\n<p>In the mid-nineteenth century, the actual mechanism for evolution was independently conceived of and described by two naturalists: <strong>Charles Darwin<\/strong> and Alfred Russel Wallace. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on\u00a0<strong><em>H.M.S. Beagle<\/em><\/strong>, including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin&#8217;s journey, like Wallace&#8217;s later journeys to the Malay Archipelago, included stops at several island chains, the last being the Gal\u00e1pagos Islands west of Ecuador. On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Gal\u00e1pagos Islands comprised several species with a unique beak shape (Figure 1).<\/p>\n<p>The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the mainland of South America. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that the varied beaks of each finch helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey.<\/p>\n<p>Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection.\u00a0<strong>Natural selection<\/strong>\u00a0is the process by which organisms that are better adapted to their environment survive and pass those adaptations onto their offspring more than those organisms that are less well adapted.\u00a0This leads to evolutionary change.<\/p>\n<p>For example, a population of giant tortoises found in the Galapagos Archipelago was observed by Darwin to have longer necks than those that lived on other islands with dry lowlands. These tortoises were &#8220;selected&#8221; because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn&#8217;t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.<\/p>\n<p>Natural selection, Darwin argued, was an inevitable outcome of <span style=\"text-decoration: underline;\">three principles<\/span> that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace&#8217;s understanding of this principle came from reading an essay by the economist Thomas Malthus who discussed this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment; it is the only mechanism known for adaptive evolution. It is important to keep in mind that natural selection leads to adaptation to a\u00a0<em>specific<\/em> environment, not all environments or even a different environment.<\/p>\n<p>Papers by Darwin and Wallace (Figure 2) presenting the idea of natural selection were read together in 1858 before the Linnean Society in London. The following year Darwin&#8217;s book,\u00a0<em><strong>On the Origin of Species<\/strong>, <\/em>was published.\u00a0His book outlined in considerable detail his arguments for gradual changes and adaptive survival by natural selection.<\/p>\n<div id=\"attachment_1466\" style=\"width: 810px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1466\" class=\"size-full wp-image-1466\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02191726\/Figure_18_01_02ab.jpg\" alt=\"Paintings of Charles Darwin and Alfred Wallace are shown.\" width=\"800\" height=\"541\" \/><\/p>\n<p id=\"caption-attachment-1466\" class=\"wp-caption-text\">Figure 2. Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented together before the Linnean Society in 1858.<\/p>\n<\/div>\n<p>Demonstrations of evolution by natural selection are time consuming and difficult to obtain. One of the best examples has been demonstrated in the very birds that helped to inspire Darwin&#8217;s theory: the Gal\u00e1pagos finches. Peter and Rosemary Grant and their colleagues have studied Gal\u00e1pagos finch populations every year since 1976 and have provided important demonstrations of natural selection. The Grants found changes from one generation to the next in the distribution of beak shapes with the medium ground finch on the Gal\u00e1pagos island of Daphne Major. The birds have inherited variation in the bill shape with some birds having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Ni\u00f1o, the large hard seeds that large-billed birds ate were reduced in number; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, survival and reproduction were much better in the following years for the small-billed birds. In the years following this El Ni\u00f1o, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the size of bills had evolved to be smaller. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased.<\/p>\n<div class=\"textbox learning-objectives\">\n<h3>In Summary: Darwin and Descent with Modification<\/h3>\n<p>While Charles Darwin is generally called &#8220;<strong>the father of evolution<\/strong>,&#8221; the basic idea for this concept was actually developed by both Darwin and Alfred Russel Wallace. Both scientists based their hypotheses on observations of diversity among natural populations. Darwin\u2019s work in particular focused on animals of the Galapagos islands, especially finches. Over time, the idea that species changed from natural selection pressures through &#8220;descent with modification&#8221; gave rise to the idea of evolution. Data accumulated over time, for example the long study of the Galapagos finches by the Grant research team, has supported this idea and moved it into the realm of a supported theory of biology.<\/p>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_66cdf7e1-80cf-43b1-9287-6c03a902d953\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/66cdf7e1-80cf-43b1-9287-6c03a902d953?iframe_resize_id=assessment_practice_id_66cdf7e1-80cf-43b1-9287-6c03a902d953\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe><br \/>\n\t<iframe id=\"assessment_practice_1da761af-a4ff-4056-9910-7ca374f62165\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/1da761af-a4ff-4056-9910-7ca374f62165?iframe_resize_id=assessment_practice_id_1da761af-a4ff-4056-9910-7ca374f62165\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-2845\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":3,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\"}]","CANDELA_OUTCOMES_GUID":"f3976b20-7072-414c-a16f-9e7a98ed3bc0, de04e842-e7c7-4318-920f-02cad40ea7ee, 2dc93e8f-6685-4b44-8bef-8775591cd6e8","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-2845","chapter","type-chapter","status-publish","hentry"],"part":337,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters\/2845","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":16,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters\/2845\/revisions"}],"predecessor-version":[{"id":6716,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters\/2845\/revisions\/6716"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/parts\/337"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters\/2845\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/media?parent=2845"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapter-type?post=2845"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/contributor?post=2845"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/license?post=2845"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}