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generations, or populations. The gene is the basicunit in thecell for transmitting hereditary characteristics to offspring. Individuals are unitsupon which natural selection operates, but the trend of evolution can betracedthrough time only for groups of interbreeding individuals, populationscan beanalyzed statistically and their evolution predicted in terms of averagenumbers. The Hardy-Weinberg law, which was discovered independentlyin 1908 bya British mathematician, Godfrey H. Hardy, and a German physician,WilhelmWeinberg, provides a standard for quantitatively measuring the extent ofevolutionary change in a population. The law states that the genefrequencies, orratios of different genes in a population, will remain constant unlessthey arechanged by outside forces, such as selective reproduction and mutation. Thisdiscovery reestablished natural selection as an evolutionary force. Comparing theactual gene frequencies observed in a population with the frequenciespredicted, bythe Hardy-Weinberg law gives a numerical measure of how far thepopulationdeviates from a nonevolving state called the Hardy-Weinberg equilibrium. Given alarge, randomly breeding population, the Hardy-Weinberg equilibrium willholdtrue, because it depends on the laws of probability. Changes areproduced in thegene pool through mutations, gene flow, genetic drift, and naturalselection. Mutation A mutation is an inheritable change in the character of agene. Mutationsmost often occur spontaneously, but they may be induced by some externalstimulus, such as irradiation or certain chemicals. The rate of mutationin humans isextremely low; nevertheless, the number of genes in every sex cell, isso large thatthe probability is high for at least one gene to carry a mutation. Gene Flow New genes can be introduced into a population through newbreedingorganisms or gametes from another population, as in plant pollen. Geneflow canwork against the processes of natural selection. Genetic Drift A change in the gene pool due to chance is called geneticdrift. Thefrequency of loss is greater the smaller the population. Thus, in smallpopulationsthere is a tendency for less variation because mates are more similargenetically. Natural Selection Over a period of time natural selection will result inchanges in thefrequency of alleles in the gene pool, or greater deviation from thenonevolvingstate, represented by the Hardy-Weinberg equilibrium. NEW SPECIES New species may evolve either by the change of one speciesto another orby the splitting of one species into two or more new species. Splitting,thepredominant mode of species formation, results from the 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