Examples of Evolution

Grade 10 BiologySpring 2011

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

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

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

All populations have genetic variationIn any population there is an array of

individuals that differ slightly from each other in genetic makeup

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

Individuals tend to produce more offspring than the environment can support Individuals of a population often compete with

one another to survive

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

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

How did M. tuberculosis evolve?A man was treated with rifampin for an active

TB infectionAfter 10 months the antibiotics cleared up the

infectionTwo 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

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

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

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

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

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

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 eaterOne picks insects out of cactus Another creeps up on sea birds and drinks

their blood

Darwin suggested the Finches all evolved from an original ancestral species

Changes occurred as different populations accumulated adaptations to different food sources

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 smallDuring dry years, few small, soft seeds were

availableDifference between survival and starvation is

the ability to get the larger, tougher seeds that most birds usually pass by

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

Divergence: the accumulation of differences between groups Divergence leads to the formation of a new

speciesSpeciation: process by which new species

forms

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

Once subspecies become different enough, a barrier to reproduction, prevents different groups from breeding with each otheri.e. time of peak mating activity could be

different

Types of barriers that may isolate closely related species:Groups may be geographically isolatedMay reproduce at different timesPhysical differences may prevent matingMay not be attracted to one another for mating Hybrid offspring may not be fertile or suited to

the environment of either parent

1. List four examples of natural selection 2. Describe how speciation takes place3. The beaks of finches on the Galapagos

Islands enlarged over generations in response to:

a) Isolation b) Pollutionc) Rain d) Limited food supply

1. 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

2. 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

3. The beaks of finches on the Galapagos Islands enlarged over generations in response to:

a) Isolation b) Pollutionc) Rain d) Limited food supply