Theory Of Evolution Essay Research Paper Theory — страница 3

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geographicalisolation ofpopulations of species. Isolated populations undergo differentmutations, andselection pressures and may evolve along different lines. If theisolation is sufficientto prevent interbreeding with other populations, these differences maybecomeextensive enough to establish a new species. The evolutionary changesbroughtabout by isolation include differences in the reproductive systems ofthe group.When a single group of organisms diversifies over time into severalsubgroups byexpanding into the available niches of a new environment, it is said toundergoAdaptive Radiation . Darwin’s Finches, in the Galapagos Islands, west of Ecuador,illustrateadaptive radiation. They were probably the first land birds to reach theislands, and,in the absence of competition, they

occupied several ecological habitatsanddiverged along several different lines. Such patterns of divergence arereflected inthe biologists’ scheme of classification of organisms, which groupstogether animalsthat have common characteristics. An adaptive radiation followed thefirst conquestof land by vertebrates. Natural selection can also lead populations of differentspecies living insimilar environments or having similar ways of life to evolve similarcharacteristics.This is called convergent evolution and reflects the similar selectivepressure ofsimilar environments. Examples of convergent evolution are the eye incephalodmollusks, such as the octopus, and in vertebrates; wings in insects,extinct flyingreptiles, birds, and bats; and the flipperlike appendages of the seaturtle

(reptile),penguin (bird), and walrus (mammal). MOLECULAR EVOLUTION An outpouring of new evidence supporting evolution has comein the 20thcentury from molecular biology, an unknown field in Darwin’s day. Thefundamental tenet of molecular biology is that genes are coded sequencesof theDNA molecule in the chromosome and that a gene codes for a precisesequence ofamino acids in a protein. Mutations alter DNA chemically, leading tomodified ornew proteins. Over evolutionary time, proteins have had histories thatare astraceable as those of large-scale structures such as bones and teeth. The further inthe past that some ancestral stock diverged into present-day species, the moreevident are the changes in the amino-acid sequences of the proteins ofthecontemporary species. PLANT EVOLUTION

Biologists believe that plants arose from the multicellulargreen algae(phylum Chlorophyta) that invaded the land about 1.2 billion years ago. Evidence isbased on modern green algae having in common with modern plants the samephotosynthetic pigments, cell walls of cellulose, and multicell formshaving a life cycle characterized by Alternation Of Generations. Photosynthesis almostcertainlydeveloped first in bacteria. The green algae may have been preadapted toland. The two major groups of plants are the bryophytes and thetracheophytes;the two groups most likely diverged from one common group of plants. Thebryophytes, which lack complex conducting systems, are small and arefound inmoist areas. The tracheophytes are plants with efficient conductingsystems; theydominate the landscape

today. The seed is the major development intracheophytes,and it is most important for survival on land. Fossil evidence indicates that land plants first appearedduring the SilurianPeriod of the Paleozoic Era (425-400 million years ago) and diversifiedin theDevonian Period. Near the end of the Carboniferous Period, fernlikeplants hadseedlike structures. At the close of the Permian Period, when the landbecame drierand colder, seed plants gained an evolutionary advantage and became thedominantplants. Plant leaves have a wide range of shapes and sizes, and somevariations ofleaves are adaptations to the environment; for example, small, leatheryleaves foundon plants in dry climates are able to conserve water and capture lesslight. Also,early angiosperms adapted to seasonal water

shortages by dropping theirleavesduring periods of drought. EVIDENCE FOR EVOLUTION The Fossil Record has important insights into the history oflife. The orderof fossils, starting at the bottom and rising upward in stratified rock,corresponds totheir age, from oldest to youngest. Deep Cambrian rocks, up to 570 million years old, containthe remains ofvarious marine invertebrate animals, sponges, jellyfish, worms,shellfish, starfish,and crustaceans. These invertebrates were already so well developedthat they musthave become differentiated during the long period preceding theCambrian. Somefossil-bearing rocks lying well below the oldest Cambrian strata containimprints ofjellyfish, tracks of worms, and traces of soft corals and other animalsof uncertainnature. Paleozoic waters were