Environmental Problems, Their Causes, and Sustainability 1

5-4 SPECIATION, EXTINCTION,AND BIODIVERSITYHow Do New Species Evolve? Moving Outand Moving OnA new species arises when members of a populationare isolated from other members so long that changesin their genetic makeup prevent them from producingfertile offspring if they get together again.Under certain circumstances, natural selection canlead to an entirely new species. In this process, calledspeciation, two species arise from one. For sexually reproducingspecies, a new species is formed whensome members of a population can no longer breedwith other members to produce fertile offspring.The most common mechanism of speciation (especiallyamong animals) is called allopatric speciation. Ittakes place in two phases: geographic isolation and reproductiveisolation. Geographic isolation occurswhen different groups of the same population becomephysically isolated from one another for long periods.For example, part of a population may migrate insearch of food and then begin living in another areawith different environmental conditions (Figure 5-7).Populations also may become separated by a physicalbarrier (such as a mountain range, stream, lake, orroad), by a change such as a volcanic eruption orearthquake, or when a few individuals are carried toa new area by wind or water. Other causes are theadvance of glaciers during ice ages, changes in sealevel that can create islands, shifts in ocean currents,a warmer or cooler climate that shifts vegetationnorthward or southward or up or down slopes, and adrier climate that divides a large lake into severalsmall lakes.The second phase of allopatric speciation is reproductiveisolation. It occurs when mutation andnatural selection operate independently in the genepools of two geographically isolated populations. Ifthis divergence process continues long enough, membersof isolated populations of a sexually reproducingspecies may become so different in genetic makeupthat when they get together again, they cannot producelive, fertile offspring. Then one species hasbecome two, and speciation has occurred through divergentevolution.For some rapidly reproducing organisms, thistype of speciation may occur within hundreds of years.For most species it takes from tens of thousands to millionsof years, which makes it difficult to observe anddocument the appearance of a new species.A less common form of speciation is called sympatricspeciation. It is the creation of a new species whengroups in a population living close together are unableto interbreed because of a mutation or subtle behavioralchanges. Some insects are candidates for this typeof speciation perhaps when two populations experiencedifferent types of mutations by feeding on differenttypes of plants.What Is Extinction? Going, Going, GoneA species becomes extinct when its populationscannot adapt to changing environmentalconditions.After speciation, the second process affecting the numberand types of species on the earth is extinction, inwhich an entire species ceases to exist. Fossil recordsindicate that species tend to exist for 1-10 million yearsbefore becoming extinct, although catastrophic eventsaccelerate the extinction process.For most of the earth’s geological history, specieshave faced incredible challenges to their existence.Continents have broken apart and moved over millionsof years (Figure 5-8). The earth’s land area has re-Early foxpopulationSpreadsnorthwardandsouthwardandseparatesNorthernpopulationDifferent environmentalconditions lead to differentselective pressures and evolutioninto two different species.SouthernpopulationGray FoxArctic FoxAdapted to coldthrough heavierfur, short ears,short legs, shortnose. White furmatches snowfor camouflage.Adapted to heatthrough lightweightfur and long ears,legs, and nose, whichgive off more heat.Figure 5-7 How geographic isolation can lead to reproductive isolation, divergence, and speciation.94 CHAPTER 5 Evolution and Biodiversity