Environmental Problems, Their Causes, and Sustainability 1

According to the theory of evolution, all speciesdescended from earlier, ancestral species. In otherwords, life comes from life. This widely accepted scientifictheory explains how life has changed over thepast 3.7 billion years and why life is so diverse today.Religious and other groups offer other explanations,but this is the accepted scientific explanation.Biologists use the term microevolution to describethe small genetic changes that occur in a population.They use the term macroevolution to describe longterm,large-scale evolutionary changes through whichnew species form from ancestral species and otherspecies are lost through extinction. Macroevolutionalso involves changes that occur at higher levels thanspecies, such as the evolution of new genera, families,or classes of species (see Appendix 4).How Does Microevolution Work?Changes in the Gene PoolA population’s gene pool changes over time whenbeneficial changes or mutations in its DNA moleculesare passed on to offspring.The first step in evolution is the development of geneticvariability in a population. Every population of a specieshas a gene pool: its collection of genes or genetic resourcespotentially available to members’ offspring inthe next generation. Microevolution is a change in a population’sgene pool over time.Members of a population generally have the samenumber and kinds of genes. But a particular gene mayhave two or more different molecular forms, calledalleles. Sexual reproduction leads to a random shufflingor recombination of alleles. As a result, each individualin a population (except identical twins) has adifferent combination of alleles.Genetic variability in a population originatesthrough mutations: random changes in the structure ornumber of DNA molecules in a cell that can be inheritedby offspring. Mutations can occur in two ways. One isby exposure of DNA to external agents such as radioactivity,X rays, and natural and human-made chemicals(called mutagens). The other is the result of random mistakesthat sometimes occur in coded genetic instructionswhen DNA molecules are copied each time a cell dividesand whenever an organism reproduces.Mutations can occur in any cells, but only those inreproductive cells are passed on to offspring. Somemutations are harmless but most are lethal. Every so often,a mutation is beneficial. The result is new genetictraits that give the bearer and its offspring betterchances for survival and reproduction under existingenvironmental conditions or when conditions change.Mutations are random and unpredictable, are theonly source of totally new genetic raw material (alleles),and are rare. Life is a genetic shuffle. Once createdby mutation, new alleles can be shuffled togetheror recombined randomly to create new combinations ofgenes in populations of sexually reproducing species.In addition to mutations, another source of evolutionarychange is the trading of genes between bacteria.What Role Does Natural Selection Playin Microevolution? Backing ReproductiveWinnersSome members of a population may have genetictraits that enhance their ability to survive andproduce offspring with these traits.Natural selection occurs when some individuals of apopulation have genetically based traits that increasetheir chances of survival and their ability to produce offspringwith the same traits. Three conditions are necessaryfor evolution of a population by natural selection.There must be genetic variability for a trait in a population.The trait must be heritable, meaning it can bepassed from one generation to another. And the traitmust somehow lead to differential reproduction. Thismeans it must enable individuals with the trait to leavemore offspring than other members of the population.An adaptation, or adaptive trait, is any heritabletrait that enables organisms to better survive and reproduceunder prevailing environmental conditions.Natural selection causes any allele or set of alleles thatresult in an adaptive trait to become more common insucceeding generations and alleles without the trait tobecome less common.When faced with a critical change in environmentalconditions, a population of a species has three possibilities:adapt to the new conditions through naturalselection, migrate (if possible) to an area with more favorableconditions, or become extinct.The process of microevolution can be summarizedsimply: Genes mutate, individuals are selected, and populationsevolve.What Is Coevolution? An Arms Racebetween Interacting SpeciesInteracting species can engage in a back-and-forthgenetic contest in which each gains a temporarygenetic advantage over the other.Some biologists have proposed that interactions betweenspecies can also result in microevolution in each of theirpopulations. According to this hypothesis, when populationsof two different species interact over a longtime, changes in the gene pool of one species can leadto changes in the gene pool of the other. This process iscalled coevolution. In this give-and-take evolutionarygame, each species is in a genetic race to produce thelargest number of surviving offspring.One example is the interactions between bats andmoths. Bats like to eat moths, and they hunt at nightand use echolocation to navigate and locate their prey.90 CHAPTER 5 Evolution and Biodiversity