Untitled Essay Research Paper BODYINTRODUCTION TO EVOLUTION — страница 3

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species. Splitting, thepredominant mode of species formation, results from the geographical isolation ofpopulations of species. Isolated populations undergo different mutations, andselection pressures and may evolve along different lines. If the isolation is sufficientto prevent interbreeding with other populations, these differences may becomeextensive enough to establish a new species. The evolutionary changes broughtabout by isolation include differences in the reproductive systems of the group.When a single group of organisms diversifies over time into several subgroups byexpanding into the available niches of a new environment, it is said to undergoAdaptive Radiation . Darwin’s Finches, in the Galapagos Islands, west of Ecuador, illustrateadaptive radiation. They were

probably the first land birds to reach the islands, and,in the absence of competition, they occupied several ecological habitats anddiverged along several different lines. Such patterns of divergence are reflected inthe biologists’ scheme of classification of organisms, which groups together animalsthat have common characteristics. An adaptive radiation followed the first conquestof land by vertebrates. Natural selection can also lead populations of different species living insimilar environments or having similar ways of life to evolve similar characteristics.This is called convergent evolution and reflects the similar selective pressure ofsimilar environments. Examples of convergent evolution are the eye in cephalodmollusks, such as the octopus, and in vertebrates; wings in

insects, extinct flyingreptiles, birds, and bats; and the flipperlike appendages of the sea turtle (reptile),penguin (bird), and walrus (mammal). MOLECULAR EVOLUTION An outpouring of new evidence supporting evolution has come in the 20thcentury from molecular biology, an unknown field in Darwin’s day. Thefundamental tenet of molecular biology is that genes are coded sequences of theDNA molecule in the chromosome and that a gene codes for a precise sequence ofamino acids in a protein. Mutations alter DNA chemically, leading to modified ornew proteins. Over evolutionary time, proteins have had histories that are 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 of thecontemporary species. PLANT EVOLUTION Biologists believe that plants arose from the multicellular green 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 forms having a lifecycle characterized by Alternation Of Generations. Photosynthesis almost certainlydeveloped first in bacteria. The green algae may have been preadapted to land. The two major groups of plants are the bryophytes and the tracheophytes;the two groups most likely diverged from one common group of plants. Thebryophytes, which lack complex conducting systems, are small

and are found inmoist areas. The tracheophytes are plants with efficient conducting systems; theydominate the landscape today. The seed is the major development in tracheophytes,and it is most important for survival on land. Fossil evidence indicates that land plants first appeared during the SilurianPeriod of the Paleozoic Era (425-400 million years ago) and diversified in theDevonian Period. Near the end of the Carboniferous Period, fernlike plants hadseedlike structures. At the close of the Permian Period, when the land became drierand colder, seed plants gained an evolutionary advantage and became the dominantplants. Plant leaves have a wide range of shapes and sizes, and some variations ofleaves are adaptations to the environment; for example, small, leathery leaves foundon

plants in dry climates are able to conserve water and capture less light. Also,early angiosperms adapted to seasonal water shortages by dropping their leavesduring periods of drought. EVIDENCE FOR EVOLUTION The Fossil Record has important insights into the history of life. 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, contain the remains ofvarious marine invertebrate animals, sponges, jellyfish, worms, shellfish, starfish,and crustaceans. These invertebrates were already so well developed that they musthave become differentiated during the long period preceding the Cambrian. Somefossil-bearing rocks lying well below the oldest Cambrian strata