Environmental Problems, Their Causes, and Sustainability 1

Reproductive individuals in populations of specieshave an inherent evolutionary drive to ensure that asmany members of the next generation as possible willcarry their genes. This increases the chance that theirpopulation will undergo evolution through naturalselection.Two types of reproduction can pass genes on tooffspring. One is asexual reproduction, in which alloffspring are exact genetic copies (clones) of a singleparent. This is common in species such as bacteria thathave only one cell. Each cell can divide to produce twoidentical cells that are genetic clones, or replicas of theoriginal.The second type is sexual reproduction, in whichorganisms produce offspring by combining sex cells orgametes (such as sperm and ovum) from both parents.This produces offspring with combinations of genetictraits from each parent.Sexual reproduction has three disadvantages.First, males do not give birth. This means that femaleshave to produce twice as many offspring to maintainthe same number of young in the next generation as anasexually reproducing organism.Second, there is an increased chance of genetic errorsand defects during the splitting and recombinationof chromosomes. Third, courtship and mating ritualsconsume time and energy, can transmit disease, andcan inflict injury on males of some species as they competefor sexual partners.So if sexual reproduction has some serious disadvantages,why do 97% of the earth’s species use it? Accordingto biologists, this happens because of two importantadvantages of sexual reproduction. One is thatit provides a greater genetic diversity in offspring. Apopulation with many different genetic possibilitieshas a greater chance of reproducing when environmentalconditions change than does a brood of geneticallyidentical clones. In addition, males of somespecies can gather food for the female and the youngand protect and help train the young.Species with a capacity for a high rate of populationincrease (r)are called r-selected species (Figure 9-9and Figure 9-10, left). Such species reproduce early andput most of their energy into reproduction. Examplesare algae, bacteria, rodents, annual plants (such as dandelions),and most insects.These species have many, usually small offspringand give them little or no parental care or protection.They overcome the massive loss of offspring by producingso many that a few will survive to reproducemany more offspring to begin the cycle again.Such species tend to be opportunists. They reproduceand disperse rapidly when conditions are favorableor when a disturbance opens up a new habitat orniche for invasion, as in the early stages of ecologicalsuccession.Environmental changes caused by disturbancescan allow opportunist species to gain a foothold. However,once established, their populations may crashbecause of unfavorable changes in environmental conditionsor invasion by more competitive species. Thishelps explain why most r-selected or opportunist speciesgo through irregular and unstable boom-and-bustcycles in their population size.At the other extreme are competitor or K-selectedspecies (Figure 9-9 and Figure 9-10, right). These speciestend to reproduce late in life and have a smallnumber of offspring with fairly long life spans.Typically the offspring of such species developinside their mothers (where they are safe), are bornfairly large, mature slowly, and are cared for and protectedby one or both parents until they reach reproductiveage. This reproductive pattern results in afew big and strong individuals that can compete forresources and reproduce a few young to begin the cycleagain.Carrying capacityKWhat Types of Reproductive PatternsDo Species Have? Opportunists andCompetitorsSome species have a large number of small offspringand give them little parental care while other specieshave a few larger offspring and take care of them untilthey can reproduce.In 1967, Robert H. MacArthur and Edward O. Wilsonsuggested that species could be classified into twofundamental reproductive patterns, r-selected and K-selected species. This classification depends on their positionon the sigmoid (S-shaped) population growthcurve (Figure 9-9) and the characteristics of their reproductivepatterns (Figure 9-10, p. 170).Number of individualsr species;experiencer selectionTimeK species;experienceK selectionFigure 9-9 Positions of r-selected and K-selected species onthe sigmoid (S-shaped) population growth curve.http://biology.brookscole.com/miller14169