Unit 2: Evolution

Learning Goal 2: Describe the effects of environmental factors on artificial and natural selection.

Artificial Selection = Selective Breeding

Nature provides the variation or humans select the variations that they find useful. Breeders often choose a characteristic

that they want passed on and only breed those animals that have that characteristic.

Artificial Selectionaka: Selective Breeding

Darwin’s pigeon experiment

Artificial Selectionaka: Selective Breeding

Used today in agriculture and animal breeding.

Artificial Selectionaka: Selective Breeding

Natural Selection

Survival of the Fittest Struggle for Existence

Members of each species compete regularly to obtain food, living space, and other necessities of life.

Natural Selection

Survival of the fittest. Those species best

adapted to their environment will survive and reproduce.

Species with greater variety have an increased chance of surviving environmental changes.

Natural Selection

Fitness is the ability of an individual to survive and reproduce.

Adaptation is any inherited characteristic that increases an organism’s chance of survival.

Natural Selection We Can SeeThe Case of the Peppered Moth

oSpecies become endangered when the population is decreased to the point that a stress can affect survival.oStresses may be natural or human-made.

Case of the Peppered Moth Peppered moths are a

common moth in England. There are variations in the

coloration of peppered moths from silvery white with gray speckles to dark gray/black.

The most common form before 1850 was a mixture of white and black called “peppered.” About 90% of the moths found in nature were peppered.

Case of the Peppered Moth There are two alleles for the color of

silvery moths: B for black and b for white. The black color is melanin.

In the laboratory, the moths occur in a ratio of 1 black:2 peppered: 1 white.

BB= black, bb= white, and Bb= peppered.

The black is the dominant allele causing speckling when present.

Case of the Peppered Moth

B bB BB

Black

Bb

Pepperedb Bb

Peppered

bb

White

Case of the Peppered Moth Peppered moths lived on

trees covered by silvery lichen.

During the Industrial Revolution, carbon-fuel pollution killed the silvery lichen on urban trees.

Coal soot coated tree trunks causing the trunks to become darker.

Case of the Peppered Moth

In urban industrial areas, silvery-white peppered moths became less common and the dark gray/black moths became more common.

In rural forest regions, the silvery-white peppered moth remained the most common type.

Silvery lichen was still on trees in the rural areas.

Case of the Peppered Moth

The next slide shows a tree with silvery lichen.

Try to find the dark gray peppered moth in the next slide.

Case of the Peppered Moth

Try to find the silvery-white colored peppered moth in the next slide.

Case of the Peppered MothMoth Population

1850 1950 1972

Light Colored

95% 5% 25%

Dark Colored

5% 95% 75%

Today, about 90% of the population is light colored. Why did the population shift back to pre-Industrial Revolution composition?

Natural Selection

Natural selection results in changes in the inherited characteristics of a population. These changes

increase a species’ fitness in its environment.

Natural Selection

Descent with Modification Each living species has

descended, with changes, from other species over time.

Common descent is the principle that all species – living or extinct – were derived from common ancestors.

Summary of Darwin’s Theory

Individual organisms differ, and some of this variation is heritable.

Organisms produce more offspring than can survive, and many that do survive do not reproduce.

Summary of Darwin’s Theory

Because more organisms are produced than can survive, they compete for limited resources.

Each unique organism has different advantages and disadvantages in the struggle for existence.

Summary of Darwin’s Theory

Species alive today are descended with modification from ancestral species that lived in the distant past.