What is Life? Diversity, Unity, and Evolution in Biology

What is Life?

Introduction

This section introduces the foundational question in biology: What defines life? It explores the diversity of living organisms, the unifying features that all life shares, and the central role of evolution in explaining both unity and diversity.

Diversity of Life

Scope of Biological Diversity

• Species Identified: Approximately 1.8 million species have been formally described by scientists.

• Estimated Total Species: Estimates of total living species range from 10 million to 100 million, with most species still unidentified.

• Extinction: The vast majority of species that have ever lived are now extinct.

• Major Groups:

  • Vertebrates: >50,000 species

  • Fungi: >100,000 species

  • Plants: >290,000 species

  • Insects: ~1,000,000 species

Example: The diversity of beetles is so great that it inspired the quote, "The creator, if he exists, has an inordinate fondness for beetles."

Diversity and Unity

Unity Amidst Diversity

• Despite the immense diversity, all living things share certain fundamental features.

• These shared features point to a common ancestry and the underlying unity of life.

Unifying Features of Life

Core Characteristics of Living Organisms

• Cellular Organization: All living things are composed of one or more cells. The cell is the basic unit of life, with an internal environment distinct from the external environment, regulated by the cell membrane.

• Requirement for Energy: All living things require energy to maintain their organization and carry out life processes. Energy can be harvested from light (photosynthesis) or chemicals (cellular respiration).

• Response to the Environment: Organisms respond to environmental stimuli to obtain and conserve energy and matter. Examples include plant growth toward light (phototropism) and Venus flytrap closing on prey.

• Possession of Genetic Material: All living organisms possess DNA, which stores genetic information. DNA is copied and transmitted to daughter cells, and its imperfect replication introduces genetic variation.

• Growth: Living organisms grow by increasing in size. Unicellular organisms double in size before dividing; multicellular organisms grow mainly by increasing cell number.

• Development: In multicellular organisms, development involves cell specialization and differentiation, triggered by the expression of different genes.

• Reproduction: Living organisms reproduce, either asexually (producing identical offspring) or sexually (producing genetically unique offspring).

• Evolution: Populations of living organisms evolve over generations due to genetic variation and differential reproductive success.

Viruses: At the Boundary of Life

Are Viruses Alive?

• Definition: Viruses are biological agents that infect cells and use them to replicate.

• Diseases: Many diseases (e.g., smallpox, chickenpox, polio, influenza) are caused by viruses.

• Life Criteria: Viruses share some features of life (genetic material, evolution) but lack others (cellular structure, independent metabolism, growth).

Feature	Cellular Life	Viruses
Cellular Structure	Yes	No
Energy Requirement	Yes	Only when infecting
Response to Environment	Yes	Some, not all
Genetic Material	DNA	DNA or RNA
Growth	Yes	No
Reproduction	Yes	Only within host
Evolution	Yes (populations)	Yes, often rapidly

Discussion: The status of viruses as living or non-living is debated because they blur the boundary between life and non-life.

Evolution: The Unifying Theme of Biology

Evolution Explains Unity and Diversity

• Unity: Similarities among organisms are due to shared ancestry (common descent).

• Diversity: Differences arise from evolutionary changes and adaptations to different environments.

• Definition: Evolution is the change in the characteristics of a population over generations.

• Mechanism: Natural selection is the primary mechanism of evolutionary change.

Key Points from Darwin's Theory

• Common Descent: All species arise from a series of ancestral species, explaining life's unity.

• Descent with Modification: Species are modified over time through natural selection, explaining life's diversity.

Natural Selection in Action

• Variation exists within populations (e.g., beetle coloration).

• Environmental pressures (e.g., predation) favor certain traits.

• Individuals with advantageous traits are more likely to survive and reproduce.

• These traits become more common in subsequent generations.

Example: In a population of beetles on dark soil, dark-colored beetles are less likely to be eaten by birds and thus become more prevalent over time.

Summary Table: Evolution by Natural Selection

Step	Description
Variation	Individuals in a population vary in traits
Inheritance	Traits are heritable and passed to offspring
Selection	Some traits confer survival/reproductive advantage
Adaptation	Advantageous traits increase in frequency

Conclusion

Biology is unified by the study of life’s diversity, the shared features of living things, and the central role of evolution. Understanding these concepts provides a foundation for all further study in biology.