Biology: The Study of Life

What Does It Mean to Say That Something is Alive?

Biology is the scientific study of life, and all living organisms share five fundamental characteristics that distinguish them from non-living matter.

• Cells: All organisms are composed of membrane-bound cells, which are the basic units of life.

• Replication: All organisms are capable of reproduction, ensuring the continuation of their species.

• Information: Organisms process hereditary information encoded in genes and respond to environmental signals.

• Energy: All organisms acquire and use energy to sustain life processes.

• Evolution: Populations of organisms are continually evolving, adapting to their environments over time.

Theories in Biology

A theory in science is an explanation for a broad class of phenomena, supported by extensive evidence. Three major theories form the framework for modern biological science:

• Cell Theory: All organisms are made of cells, and all cells come from preexisting cells.

• Theory of Evolution by Natural Selection: Explains how organisms are related and how they change over time.

• Chromosome Theory of Inheritance: Describes how hereditary information is transmitted from one generation to the next.

Life is Cellular and Replicates through Cell Division

Discovery of Cells

The cell theory originated from observations made by early microscopists:

• Robert Hooke (1665) observed cork tissue and coined the term "cells."

• Anton van Leeuwenhoek (1670s) observed single-celled organisms, which he called "animalcules."

• By the 1800s, biologists recognized that all organisms consist of cells.

Cell Theory and Spontaneous Generation

Cell theory challenged the idea of spontaneous generation, which posited that life could arise from non-living matter. Louis Pasteur's experiments demonstrated that cells arise only from preexisting cells, not spontaneously.

Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

Hereditary information is encoded in genes located on chromosomes. DNA is the hereditary material, and genes are segments of DNA that code for cell products.

Structure of DNA

DNA is a double helix composed of four building blocks: adenine (A), thymine (T), cytosine (C), and guanine (G). The sequence of these bases encodes genetic information.

The Central Dogma

The central dogma describes the flow of genetic information in cells: DNA codes for RNA, which codes for proteins. Proteins determine an organism's traits.

Genetic Variation and Evolution

DNA is copied accurately during cell division, but mutations can occur. These changes may alter proteins and lead to heritable variations, which are the basis for the diversity of life.

Energy and Nutritional Needs

Organisms require energy for cellular processes and need molecules as building blocks for DNA, RNA, proteins, and other cellular components. The way organisms acquire energy is central to their diversification.

Life Evolves

Evolution and Natural Selection

Evolution is the change in characteristics of a population over time. Darwin and Wallace proposed that species are related by common ancestry and can be modified from generation to generation (descent with modification).

Natural Selection

Natural selection explains how evolution occurs. Two conditions must be met:

• Individuals vary in heritable characteristics.

• Certain traits increase reproductive success in a particular environment.

Natural selection acts on individuals, but evolutionary change occurs in populations. Speciation occurs when populations diverge to form new species.

Fitness and Adaptation

Fitness is the ability of an individual to produce surviving offspring. Adaptation is a trait that increases fitness in a particular environment.

Example: Finches on the Galápagos Islands with small, pointed beaks had higher fitness when small, soft seeds were abundant. This adaptation increased their survival and reproduction.

The Tree of Life Depicts Evolutionary History

Phylogeny and Genetic Variation

The tree of life is a family tree of organisms, describing genealogical relationships among species. Biologists analyze genetic variation by comparing DNA and RNA sequences; fewer sequence differences indicate closer relationships.

Green algae is more closely related to land plants than brown algae.

Phylogenetic Tree of Life

A phylogenetic tree shows relationships between species. Branches sharing a recent common ancestor represent closely related species. The tree of life is estimated from genetic data.

Major Groups of Organisms

The tree of life indicates three major groups:

• Eukaryotes: Organisms with a nucleus (Eukarya).

• Bacteria: Prokaryotes lacking a nucleus.

• Archaea: Prokaryotes lacking a nucleus, distinct from bacteria.

Taxonomy and Classification

Linnaeus’ System and Nomenclature

Taxonomy is the effort to name and classify organisms. Each organism is given a unique two-part scientific name (genus and species), which is always italicized. Genus names are capitalized, species names are not (e.g., Homo sapiens).

Doing Biology: The Scientific Method

Hypothesis Testing

Science involves formulating hypotheses and testing them through experiments. A hypothesis is a testable statement explaining an observation. Experiments allow researchers to test the effect of a single factor.

Experimental Design

Good experimental design includes:

• Control groups to check for other factors.

• Constant experimental conditions.

• Repeating tests and using large sample sizes.

Example: Giraffe Neck Length

Two hypotheses were tested:

• Food competition hypothesis: Long necks evolved to reach food.

• Sexual competition hypothesis: Longer-necked males win more fights and father more offspring.

Data supported the sexual competition hypothesis.

Example: Ant Navigation

Wittlinger and colleagues tested whether desert ants use a "pedometer" to navigate. Manipulating leg length affected the ants' ability to return to the nest, supporting the hypothesis that ants use stride length and step number to calculate distance.

Summary Table: Fundamental Characteristics of Life