Chapter 14 Principles of Evolution Evolution is a change in the genetic makeup of a population over time as a result of natural selection The main ideas of evolution were not widely accepted until after Charles Darwin published On the Origin of Species in 1859 Pre Darwinian thought held that all organisms were created simultaneously by God and that each distinct life form was permanently fixed and did not change over time So how did the concept of Evolution come about Exploration of new lands revealed a staggering diversity of life Early explorers often took naturalists along to catalogue the plant and animal life they found The number of species was far greater than expected The vast numbers of species observed allowed naturalists to see patterns that had not emerged before Each area had its own distinctive set of species Some species closely resembled one another yet differed in some characteristics These patterns seemed inconsistent with the idea that species were fixed and unchanging Fossils revealed that life has changed over time Fossils the preserved remains or traces of organisms that died long ago Discovered in many forms Petrified remains of bones wood shells or their impressions left in mud Most are found in sedimentary rock William Smith a British surveyor recognized that certain fossils were always found in the same layers of rock Many rocks occur in layers with newer layers positioned over older layers Also the organization of fossils and rock layers was consistent with fossils of a given type always in the same layers Most fossils found in the oldest deepest layers were very different from modern organisms The resemblance to modern organisms gradually increased in progressively younger more shallow rocks Many of the fossilized species were extinct The fossil evidence led to a revolutionary conclusion Different types of organisms had lived at different times in the past This countered the view that species were created at one time and did not change afterwards Fossils of Extinct Organisms Youngest rocks Oldest rocks a Trilobite b Seed ferns c Allosaurus Fig 14 4 Some scientists devised non evolutionary explanations for fossils To account for the existence of extinct species while preserving the notion of a single creation by God Georges Cuvier 1769 1832 proposed the theory of catastrophism High numbers of species were created originally A series of catastrophes produced rock layers and destroyed many species preserving some as fossils Modern day species are the survivors of these catastrophes Geological evidence led to several conclusions Earth is far older than the 6 000 years proposed by theologians There was enough time for evolution to occur Modern geologists estimate that the Earth is about 4 5 billion years old Charles Darwin and Alfred Russel Wallace proposed a mechanism of evolution By the mid 1880s a growing number of biologists had concluded that present day species had evolved from earlier ones But how 1858 Charles Darwin and Alfred Russel Wallace independently provided evidence of the occurrence of evolution and proposed the mechanism for how it occurs Darwin and Wallace both had traveled extensively and studied plants and animals in detail Both had observed that some species differ in only a few traits Both knew that fossils showed a trend of increasing complexity Both were aware that the Earth is extremely old Darwin s Finches Residents of the Gal pagos Islands Fig 14 5 Darwin and Wallace independently proposed that organisms evolved by natural selection Both presented papers to a biological journal in London in 1858 Darwin published On the Origin of Species by Means of Natural Selection in 1859 Proposed that individuals evolved through a process of descent with modification Individuals in each generation differ slightly from the members of the preceding generation Over long time periods small genetic differences accumulate to produce major transformations Darwin and Wallace s theory was based on 4 principles Principle 1 Individual members of a population are different from one another We now know that variations arise purely by chance resulting from random mutations in DNA The differences are obvious in many physical characteristics and extend to the genetic level Variation in a Population of Snails Fig 14 6 Principle 2 At least some of the differences between members of a population are characteristics that may be passed from parent to offspring Inheritable However the mechanism of inheritance was not understood at this point in time Mendel 1890 s Principal 3 In each generation some individuals in a population survive and reproduce successfully Others do not Darwin observed that many more individuals are born than survive to reproduce Some individuals have more offspring than others Principle 4 Individuals with advantageous traits survive longest and produce the most offspring a process known as natural selection In the competition to survive and reproduce winners are determined not by chance but by the traits they possess Natural selection modifies populations over time Natural selection acts on individuals within a population however it is the population that changes over time Over generations the population changes as the percentage of individuals inheriting favorable traits increases An individual cannot evolve but a population can How Do We Know That Evolution Has Occurred An overwhelming body of evidence in multiple areas of science supports the theory of evolution 1 Fossils provide evidence of evolutionary change over time 2 Comparative anatomy gives evidence of descent with modification 3 Embryological similarity suggests common ancestry 4 Modern genetic analyses reveal similarities in genes among diverse organisms 1 Fossils provide evidence of evolutionary change over time Several series of fossils have been found that exhibit the evolution of body structures over time One series reveals that modern whales evolved from land dwelling ancestors The Evolution of the Whale 0 o g a s r a e y f o s n o i l l i M 40 45 Modern whales Basilosaurus Dorudon Rhodocetus Ambulocetus 50 Pakicetus Fig 14 7 2 Comparative anatomy gives evidence of descent with modification Comparing the bodies of organisms of different species has revealed differences that could result only from evolutionary change during descent from a common ancestor Homologous structures are structures that have the same evolutionary origin despite their current appearance or function
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