Characteristics of Life

Defining What It Means to Be Alive

Biologists use several key characteristics to determine whether something is considered alive. These features distinguish living organisms from non-living matter.

• Organization: Living things are composed of one or more cells, which are the basic units of life.

• Metabolism: All living organisms carry out chemical reactions to obtain and use energy.

• Homeostasis: The ability to maintain a stable internal environment despite changes in external conditions.

• Growth and Development: Living things grow and develop according to specific instructions coded in their DNA.

• Reproduction: Organisms reproduce, passing genetic information to offspring.

• Response to Stimuli: Living things respond to environmental changes.

• Adaptation through Evolution: Populations of organisms change over time in response to environmental pressures.

Example: A plant grows toward sunlight (response to stimulus), uses photosynthesis to make food (metabolism), and produces seeds (reproduction).

Cell Theory

Modern Cell Theory: The Foundation of Biology

Cell theory is one of the two major unifying theories in biology. It describes the properties and significance of cells in all living organisms.

• All living things are composed of one or more cells.

• The cell is the basic unit of life. It is the smallest structure that can carry out all the processes associated with life.

• All cells arise from pre-existing cells. This means that new cells are produced by the division of existing cells.

Example: In multicellular organisms, all cells descend from a single ancestral cell through repeated cell division.

Additional info: The cell theory was developed in the 19th century by scientists such as Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.

Scientific Meaning of Theory

In science, a theory is not a guess or speculation. It is a well-substantiated explanation of some aspect of the natural world, based on a body of evidence and repeatedly tested and confirmed through observation and experimentation.

• Everyday use: "Theory" may mean a guess or hypothesis.

• Scientific use: A theory is a comprehensive explanation supported by evidence.

Evolution

The Theory of Evolution: Explaining Unity and Diversity of Life

Evolution is the process by which populations of organisms change over time. It is the second major unifying theory in biology and explains both the unity and diversity of life.

• Pattern: Characteristics of populations change over time.

• Mechanism: Evolution occurs through processes such as natural selection.

• Common Ancestry: All species are related by descent from a common ancestor.

• Speciation: New species arise from pre-existing species.

Example: The finches on the Galápagos Islands evolved different beak shapes to exploit different food sources, illustrating adaptation and speciation.

Natural Selection: The Driving Force of Evolution

Natural selection is the primary mechanism by which evolution occurs. It explains how certain traits become more common in a population over time.

• Variation: Individuals in a population have different heritable traits.

• Differential Survival and Reproduction: Some traits confer advantages that increase an individual's chances of surviving and reproducing.

• Inheritance: Successful traits are passed on to offspring, leading to changes in allele frequencies in the population.

• Population Change: Over generations, populations evolve as advantageous traits become more common.

Equation:

Additional info: Artificial selection is a form of selection where humans choose which traits are passed on, such as in dog breeding.

Levels of Biological Organization

Evolution acts on populations, not individuals. Understanding the levels of biological organization helps clarify this concept.

• Organism: An individual living thing.

• Population: A group of individuals of the same species living in a specific area.

• Community: All populations of different species in an area.

• Ecosystem: The community plus the non-living environment.

• Biosphere: All ecosystems on Earth.

Summary Table: Unifying Theories of Biology

Explaining Unity and Diversity of Life

The theory of evolution by natural selection explains both the unity (shared features due to common ancestry) and diversity (differences due to adaptation and speciation) of life on Earth.

• Unity: All living things share fundamental characteristics because they descend from a common ancestor.

• Diversity: Populations adapt to different environments, leading to the formation of new species.