Chapter 13 Section 3

Chapter 13 Section 3 Examples of Evolution Grade 10 Biology Spring 2011

Objectives • Identify four elements in the process of natural selection • Describe how natural selection has affected the bacteria that cause tuberculosis • Relate natural selection to the beak size of finches • Summarize the process of species formation

Bell Ringer • In small groups brainstorm to identify characteristics of birds that provide information about their diets

Natural Selection at Work • Natural selection is the mechanism that drives evolution • Environment dictates the direction and amount of change • If the environment changes in the future, the set of characteristics that most help an individual reproduce successfully may change

Factors in Natural Selection (4 points) • All populations have genetic variation • In any population there is an array of individuals that differ slightly from each other in genetic makeup

Factors in Natural Selection • The environment presents challenges to successful reproduction • An organisms that does not survive to reproduce or whose offspring die before the offspring can reproduce does not pass its genes on to future generations

Factors in Natural Selection • Individuals tend to produce more offspring than the environment can support • Individuals of a population often compete with one another to survive

Factors in Natural Selection • Individuals that are better able to cope with the challenges presented by their environment tend to leave more offspring than those individuals less suited to the environment do

Example of Natural Selection • Mycobacterium tuberculosis: • In the 1950’s effective antibiotics, isoniazid and rifampin, became available and saved many lives • In the late 1980’s new strains of M. tuberculosis that are largely or completely resistant to the antibiotics appeared • Rates of TB began to skyrocket

Example of Natural Selection • How did M. tuberculosis evolve? • A man was treated with rifampin for an active TB infection • After 10 months the antibiotics cleared up the infection • Two months later, man was readmitted to the hospital with a severe TB infection, and despite rifampin treatment, he dies 10 days later • Strain of M. tuberculosis isolated from his body was completely resistant to rifampin

Example of Natural Selection • How did M. tuberculosis evolve? • Comparison of DNA of rifampin-sensitive M. tuberculosis to rifampin-resistance M. tuberculosis • Only one difference, a single base change from cytosine to thymine in a gene called rpoB

Evolution of Antibiotic Resistance • Rifampin acts by binding to M.tuberculosis RNA polymerase, preventing transcription and so killing the bacterial cell • Mutation in polymerase’s rpoB gene prevents rifampin from binding to the polymerase • Mutation does not destroy polymerase’s ability to transcribe mRNA

Evolution of Antibiotic Resistance • Because its polymerase function was no longer normal, the mutant bacterium could not divide as rapidly as normal bacteria can, but it still could divide • The antibiotic caused the normal bacterial cells to eventually die • The mutant bacteria continued to grow and reproduce in the antibiotic-containing environment

Evolution of Antibiotic Resistance • Because the total number of M. tuberculosis bacteria was reduced drastically by the first antibiotic treatment, the patients infection had seemed to clear • Mutant antibiotic resistant bacteria had survived and continued to grow • The mutant bacteria could produce more effectively in the presence of the antibiotic than the normal bacteria • Mutant bacteria became more common in the bacterial population and eventually became the predominant type

Evolution of Antibiotic Resistance • When the patient became ill again the M. tuberculosis were the rifampin-resistant cells • In this way, natural selection led to the evolution of rifampin resistance in M. tuberculosis

Evolution in Darwin’s Finches • Darwin collected 9 distinct species of Finches, all very similar to one another except for their bills • Large bill- fed on seeds they crush in their mouths • Narrow bill- eat insects • One was a fruit eater • One picks insects out of cactus • Another creeps up on sea birds and drinks their blood

Evolution in Darwin’s Finches

Evolution in Darwin’s Finches • Darwin suggested the Finches all evolved from an original ancestral species • Changes occurred as different populations accumulated adaptations to different food sources

Evolution in Darwin’s Finches • Study over 25 years showed: • During wet years birds with different beak sizes ate the plentiful small, soft seeds • During dry years, plants produce few seeds, large or small • During dry years, few small, soft seeds were available • Difference between survival and starvation is the ability to get the larger, tougher seeds that most birds usually pass by

Evolution in Darwin’s Finches • Study over 25 years showed: • Measured beaks, found that after several dry years, the birds that had longer, more massive beaks had better feeding success and produced more offspring • When wet season returned birds tended to have smaller beaks again • The numbers of birds with different beak shapes are changed by natural selection in response to the available food supply

Formation of a New Species • Divergence: the accumulation of differences between groups • Divergence leads to the formation of a new species • Speciation: process by which new species forms

Forming Subspecies • Separate populations of a single species often live in several different kinds of environments • In each environment, natural selection acts on the population • Over time, populations of the same species that different genetically because of adaptations to different living conditions become subspecies • First step toward speciation • If get so different they cannot interbreed are considered separate species

Maintaining New Species • Once subspecies become different enough, a barrier to reproduction, prevents different groups from breeding with each other • i.e. time of peak mating activity could be different

Maintaining New Species • Types of barriers that may isolate closely related species: • Groups may be geographically isolated • May reproduce at different times • Physical differences may prevent mating • May not be attracted to one another for mating • Hybrid offspring may not be fertile or suited to the environment of either parent

Review • List four examples of natural selection • Describe how speciation takes place • The beaks of finches on the Galapagos Islands enlarged over generations in response to: • Isolation • Pollution • Rain • Limited food supply

Answers • Four examples of natural selection: genetic variation, environmental challenges to reproduction, overproduction of offspring and a struggle for survival, an increase in the number of individuals with characteristics suited to the environment • Speciation takes place as populations of species spread through an environment, they are exposed to varying conditions; over time the separate populations become distinct and split into subspecies, and eventually separate species • The beaks of finches on the Galapagos Islands enlarged over generations in response to: • Isolation • Pollution • Rain • Limited food supply

Chapter 13 Section 3