During evolutionary history, some things have been preserved while others have not. This is due entirely to natural selection. Yes, we are still made up of the same kind of atoms and molecules that existed billions of years ago, but look how much has changed! Evolution and natural selection have provided us with diversity while still maintaining the foundations that have been built upon since the beginning of life. Without this unity and diversity, life would not be so spontaneous and dynamic.

Molecules and Cells

• Mitotic Cell Cycle

The mitotic cell cycle is the process that produces two daughter cells that are genetically identical to the parent cell. Thus, mitosis perpetuates the genetic make-up of the parent and is an example of continuity.

• Meiotic Cell Cycle

The meiotic cell cycle is the process that produces four genetically different daughter cells with half the number of chromosomes as the parent and one fourth the amount of cytoplasm as the parent. Meiosis allows for variation in the characteristics of the offspring. Therefore, meiosis is a process that facilitates change.

• The Cell Theory

The ideas represented in the cell theory establish the basic unity of life. No matter how unique organisms may seem, they are all made up of one or more cells. All life depends upon the production of more cells from pre-existing cells. Each cell exhibits the properties of life.

• DNA

DNA is the hereditary material in all organisms.

DNA replication provides for continuity in all organisms.

Mutations in DNA are responsible for change.

• Plasma Membrane

Cells depend upon the presence of a plasma membrane to regulate the flow of materials into and out of the cell.

Heredity and Evolution

• Sexual Reproduction

Sexual reproduction provides for change in multiple ways.

1. Sexual selection (which female mates with which male)
2. Meiosis (production of gametes)
3. Crossover (during meiosis)
4. Fertilization (which egg fuses with which sperm cell)

The enormous variety of combinations in which organisms, gametes, or alleles can become paired, provides for variation and change.

• Speciation

The process of forming new species is another example of change. Natural selection is the single most important mechanism of change.

Organisms and Populations

• Hardy-Weinberg Principle

Under the Hardy-Weinberg principle, evolution is defined as a change in allele or genotype frequencies. The frequencies will remain the same if the following conditions are met:

1. No natural selection
2. No mutation
3. No migration (immigration or emigration)
4. Large population
5. Random mating

Evolution can be studied or examined mathematically using the Hardy-Weinberg principle. To determine the extent of evolution, one must compare the frequencies of a population at different times. These are the two equations that you need to know.

p + q = 1

p² + 2pq + q² = 1

Variable Equivalents:

p = frequency of the dominant allele

q = frequency of the recessive allele

p² = frequency of homozygous dominant genotype

2pq = frequency of heterozygous genotype

q² = frequency of homozygous recessive genotype

Given any statistics concerning a population, you can apply these equations and determine the various frequencies and then determine whether or not the allele frequencies and genotype frequencies have remained constant or have changed.

• Natural Selection

Populations are dynamic and changing.

Changing environmental conditions select for variations within populations.

Thus, natural selection can change the make-up of a population and determine which organisms will survive.