Investigating Life: The Scientific Method

Scientific Investigation

Scientific investigation is the process by which scientists explore the natural world, gather data, and develop explanations based on evidence. This process is fundamental to biology and all scientific disciplines.

• Observation: The act of noticing and describing events or processes in a careful, orderly way. Observations can lead to new technological improvements and discoveries.

• Data: Quantified information collected during observations or experiments. Data can be qualitative (descriptive) or quantitative (numerical).

Main Types of Scientific Approaches

• Descriptive Science (Discovery Science):

  • Focuses on observation-based discovery.

  • Involves collecting and analyzing data without manipulating variables.

  • Example: Cataloging species in a rainforest.

• The Scientific Method (Experimental/Hypothesis-Driven Science):

  • Involves proposing and testing hypotheses through experiments.

  • Two main experiment types:

    • Controlled Experiments: One variable is manipulated while all others are held constant.

    • Comparative Experiments: Predicts that groups will be different and compares data from different sample groups.

Key Concepts in Scientific Investigation

• Hypothesis: A tentative, testable answer to a scientific question. Must be falsifiable (able to be proven false).

• Null Hypothesis (H0): States that there is no effect or no difference. Used as a default position that researchers try to disprove, reject, or nullify.

• Statistical Significance: Indicates whether observed differences are likely due to chance. Commonly, a p-value of 0.05 or lower is considered statistically significant.

Theories, Laws, and Principles

• Theory: A well-substantiated explanation of some aspect of the natural world, based on a body of evidence and repeatedly confirmed through observation and experimentation (e.g., Cell Theory, Theory of Evolution).

• Law/Principle: A statement that describes or predicts a range of natural phenomena, often expressed mathematically, and based on repeated experimental observations (e.g., Laws of Thermodynamics, Bernoulli's Principle).

Comparison Table: Theory vs. Law/Principle

Logic in Scientific Investigation

• Inductive Logic: Uses specific observations to develop general principles or hypotheses.

• Deductive Logic: Uses general principles to predict specific results that must be true if the hypothesis is correct.

Example: Inductive logic might lead to the hypothesis that all swans are white after observing many white swans. Deductive logic would predict that the next swan observed will also be white if the hypothesis is true.

Model Systems in Biology

Model systems use one type of organism to understand others, based on the shared ancestry, genetic code, and molecular building blocks of life.

• Example: Using fruit flies (Drosophila melanogaster) to study genetics applicable to other animals.

Experiment Types

• Controlled Experiments:

  • Use samples and groups that are as similar as possible.

  • Manipulate one or more factors (independent variables) while keeping others constant.

  • Compare experimental groups to unmanipulated control groups.

  • Variables:

    • Independent (Predictor) Variable: The factor being manipulated ("X axis").

    • Dependent (Response) Variable: The response that is measured ("Y axis").

• Comparative Experiments:

  • Predict that groups will be different.

  • Gather and compare data from different sample groups.

Statistical Testing in Science

Statistical tests are used to determine if observed differences are meaningful or due to random variation.

• Statistical Test: Calculates probabilities in observed vs. experimental variation. A commonly accepted standard is a p-value of 0.05 or lower.

• Important Considerations:

  • Null hypothesis: no difference exists

  • Data: collection and sample size

  • Quantification

  • Reproducibility

  • Power (the probability of detecting an effect if there is one)

Life and Its Organization

Origin and Diversity of Life

Life on Earth began approximately 4.6 to 4.5 billion years ago, but it took over 600 million years for life to evolve. All living organisms today are descended from a single common ancestor, as evidenced by similarities in gene sequences, genetic code, and amino acids.

• If life had multiple origins, we would not expect to see such striking similarities among all living things.

Characteristics of Life

• It is organized

• It is diverse

• It changes (evolves)

• There are interactions among living things

• It is complex

Levels of Organization of Organisms

Biological classification organizes life into hierarchical categories, reflecting evolutionary relationships.

• Domain (3): Archaea (Prokaryotic), Bacteria (Prokaryotic), and Eukarya (Eukaryotic: Protists, Fungi, Plants, Animals)

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Binomial Nomenclature

Binomial Nomenclature is the system of naming species using two terms: the first indicates the genus, and the second indicates the species. For example, Homo sapiens refers to humans.

• Members of the same species can mate, reproduce, and produce viable offspring with each other.

Summary Table: Levels of Biological Classification

Key Terms and Concepts

• Hypothesis: An "if" statement proposing a possible explanation.

• Prediction: A "then" statement describing what will happen if the hypothesis is correct.

Example of Hypothesis and Prediction:

• Hypothesis: If fertilizer is added to plants, then they will grow taller.

• Prediction: Then, plants given fertilizer will be taller than those not given fertilizer after four weeks.

Equation Example (Statistical Test):

To test for statistical significance, a common test is the t-test:

Where and are sample means, and are sample variances, and and are sample sizes.