5.4 Continued

*5.4 Continued

Fundamental Basis for Darwin’s Theory of Evolution

All organisms are potentially capable of producing large numbers of offspring and far more than the environment can support.

Trees, coral, fish all release thousands of seeds/eggs, yet few of will survive to maturity, and population explosions are rare.

Both plants and animals in a growing population will compete for resources. These may be food, territory, or mates.

Disease and predators may also take their toll.

This competition will bring about the struggle for survival between members of a population.

Organisms that are well adapted to the conditions will out compete others of the population and tend to survive.

Different members of the same species are slightly different and this variation is due to the mechanism of sexual reproduction.

The process of meiosis produces haploid gametes and the genes in the gametes an individual produces may be in different alleles.

When an egg is fertilized, the zygote contains a unique combination of genetic material from both parents.

Sexual reproduction gives enormous source of genetic diversity, which gives rise to a wide variation within the individuals of a species.

As a result of variation, some members of a population may be better suited to their surroundings than others.

They may have better eyesight, or better camouflage.

These individuals will out-compete others; they will survive better, live longer, and pass on their genes to more offspring.

Gradually, as the process is repeated generation after generation, the proportion of these genes in the population as a whole increases.

This is Natural Selection, this allows the fittest to survive to reproduce.

Natural Selection

Species that are living today have evolved to be suited to their environment.

If the environment changes, a population will need to adapt if it is to survive in the new conditions.

2 examples can be seen in a moth population and new strains of bacteria.

Industrial Melanism

The peppered moth is a night flying moth that rests during the day on the bark of trees, mostly on branches that are covered with grey-green lichen.

It is a light speckled grey, and relies on camouflage against the tree branches to protect it from predatory birds.

In Britain in the mid 19th century, a black form of the moth was noticed.

The appearance of this new color coincided with the period of the industrial revolution when many factories were built and added to the growing pollution in the atmosphere.

This pollution killed the lichens that grow on the bark of trees, which became blackened with soot.

The color of the moth is due to a single gene, which can be present in two forms.

The common recessive form gives rise to a light speckled color. The much less common dominant form gives rise to the black, melanic moth.

In the polluted areas, the speckled form was no longer camouflaged on the blackened tree bark, and was easily seen by birds.

The black moths were better suited to the change in environment as they were better camouflaged. Black moths bred and the proportion of black months with the dominant allele grew in population

In 1956, the Clean Air Act became law in Britian and restricted air pollution.

Lichen grew back on trees and their bark became lighter.

As a consequence, the speckled form of the peppered moth has increased in numbers again, and the black form has been less frequent.

Antibiotic Resistance

Antibiotics are drugs that kill or inhibit bacterial growth.

Usually treating a bacterial infection with an antibiotic kills every invading cell.

But, because of variation within a population, there may be a few bacteria cells that can resist the antibiotics.

These will survive and reproduce.

Because bacteria reproduce asexually, all offspring are also resistant, and will also survive in the presence of the antibiotic.

The resistant strain has a huge selective advantage and will quickly out-compete the regular strain.

Treating a disease caused by resistant strains becomes very difficult.

Doctors may have to prescribe stronger doses or try different antibiotics.

The problem with antibiotic resistance is made more complex because bacteria frequently contain additional genetic information in the form of plasmids, which they can transfer with or bacteria.

Potentially dangerous bacteria can become resistant to antibiotics by receiving a plasmid from a harmless species.

Many bacteria are resistant to many types of antibiotics. Forcing pharmaceutical companies to come up with new antibiotics.

MRSA

Methicillin-resistant Staphylococcus aureusMRSA is a “staph” germ that does not get better with the first-line antibiotics that usually cure staph infections.

It’s transmitted from skin to skin contact.

MRSA has arisen because of the result of overuse antibiotics.

Antibiotics used incorrectly or too frequently, help “select” the resistant individuals, which increase in numbers.

Patients failing to take a complete course of medication can also encourage the survival of slightly resistant bacteria that may have been killed if taken properly.