BIO 105 Lecture Review: Scientific Method, Evolution, and Key Concepts (Fill-in-the-Blank)
What is science?
A process that logically addresses questions about cause-effect relationships in nature.
Science considers past and current events.
Science can make logical predictions.
The conclusions drawn must be testable and falsifiable.
What science is not
Examples of notions science rejects or does not rely on:
Vitalism: life processes driven by an unmeasurable force.
The idea that species are designed/created by an intelligent, supernatural creator.
Science is not capable of using supernatural explanations.
Why scientists avoid supernatural explanations
Reasons include:
Supernatural explanations are not testable.
They prevent explanation in terms of other testable concepts.
Even if something is genuinely supernatural, it would be an isolated anomaly with little explanatory power.
Science aims to explain the universe using all available data to increase understanding.
Falsifiability and the nature of theory (Popper)
A theory that cannot be refuted by any conceivable event is non-scientific.
Irrefutability is not a virtue; it is a vice in scientific theories.
Quote attribution: Karl R. Popper.
Controlled experiments and variables
A variable is something that can vary.
Examples to consider whether they are variables:
Body length in a fish — variable.
Sex in honey bees — typically a categorical/qualitative variable.
The mass of an electron — constant, not a variable in an experiment.
Scientific control
A scientific control is designed to minimize effects of variables other than the single independent variable being tested.
Purpose: isolate the effect of the independent variable.
Why use a control?
Provides a baseline or starting point for comparison.
Serves as a check for experimental failures.
Positive control should yield a positive result; negative control should yield a negative result.
If expected control results are not observed, something is wrong and reagents/equipment should be checked.
Controls increase confidence in results.
What is a controlled experiment?
A scientific investigation in which both the control group and the experimental group(s) are kept under similar conditions (matched variables) except for the independent variable under study.
Purpose: to identify or determine the effect of the independent variable.
Case study: the Widowbird experiment
Organism: Long-tailed widowbird (small grassland bird of southern Africa).
Males can be polygynous and have long tails; hypothesis: longer tails attract more females to nests.
Experimental setup:
Control group: normal tail length.
Two experimental groups: one with shortened tails, one with lengthened tails.
Birds in similar environments and studied at the same time of year to minimize environmental variation.
Outcome measured: number of female nests in each male’s territory.
Result: males with longer tails had territories with about 2 nests per male; control group averaged about 1 nest per male.
Numerical summary:
Control: ≈ 1 nest/male
Experimental group 1: ≈ 2 nests/male
Experimental group 2: ≈ 2 nests/male
Variables in the Widowbird study (three categories)
Independent variable: Tail length (normal, short, long).
Matched variables (environmental controls): time of year, habitat, study conditions, etc.
Dependent variable: Number of nests per territory.
Features of good experimental design
An independent variable that is carefully manipulated (e.g., tail length).
Other variables that could affect the outcome are matched across all test subjects.
Question: Which variables were matched in the widowbird experiment? (Answer: environmental conditions such as time of year, habitat, and general conditions were matched.)
A practical exercise: DDT in food and rat litter size
Question: How should two rat groups be matched if one group has DDT in the diet and the other does not?
Options included age, proportion of males and females, body size, or all of the above.
Correct: All of the above should be matched.
Independent variable (in this experiment): Dietary DDT.
Dependent variable: Litter size.
Note: The slide also asks which is the independent variable and which is the dependent variable; in this case, the independent variable is Dietary DDT and the dependent variable is Litter size.
Looking at data and correlation vs causation
Data interpretation relies on statistics and graphs to reveal patterns not obvious at first glance.
Correlation does not imply causation.
Misleading example: rising ice-cream sales correlate with increased sunstroke, but this is not a causal relation.
Likewise, coffee consumption correlating with longer life does not necessarily imply causation.
Studying scientific knowledge: past events and fossils
Scientific studies cannot always use controlled experiments to reproduce past events (e.g., dinosaurs giving rise to birds).
Instead, science tests predictions derived from hypotheses about the past.
Example predictions:
Some dinosaur fossils should have bird-like features.
Some early bird fossils should have reptilian features.
Fossil evidence and the dinosaur-bird connection
Fossil evidence can test predictions about the past.
A visual example: fossils showing feathers in certain dinosaurs suggest a link to birds.
A key point: the link between dinosaurs and birds is supported by multiple lines of evidence, including feathers, egg-laying, bone structures, and protein sequences.
Question (conceptual): Which fact is most important in demonstrating the link between dinosaurs and birds? Answer: There are multiple lines of evidence; no single fact is sufficient on its own.
Correlation, evidence, and theory updates
Remember: Correlation does not always imply causation, but accumulating more evidence increases certainty.
Even well-established ideas can be revised in light of new evidence.
The nature of scientific knowledge and theory
Can scientific knowledge be modified and updated?
Examples of ideas and whether they are still accepted:
Matter cannot be created nor destroyed. (A foundational idea in physics with extensive support.)
Continents are fixed in place. (Contradicted by plate tectonics.)
Ulcers are caused by stress, not bacteria. (Updated by microbiology evidence.)
A core strength of science is its ability to update knowledge as new data emerge.
The meaning of theory in science
Theory in science is not the everyday sense of "a guess"; it is a well-supported, major explanatory framework.
Everyday use: a theory may be a guess.
In science: a theory is a major unifying idea with far-reaching implications that has been extensively tested and for which attempts to disprove it have failed.
Examples: theory of gravity, theory of relativity, cell theory, theory of evolution by natural selection.
Theories are built on facts and hypotheses; facts are observable and verifiable.
Facts, hypotheses, and theories
Facts: objective, verifiable observations.
Hypotheses: proposed explanations for observed phenomena; testable predictions.
Theories: well-substantiated explanations that organize a broad range of facts and hypotheses.
Evolution as a concept: fact and theory
Evolution is a fact: observable change in populations over time; can be observed directly in short timeframes (e.g., bacteria) and across species.
Evolution is also a theory: a unifying explanation of the mechanisms driving those changes (e.g., natural selection, mutation, drift, migration, non-random mating).
The E. coli long-term evolution experiment (LTEE)
Setup: 12 identical E. coli isolates started in 1988, subcultured daily due to short generation times.
Current status: over 50,000 generations observed.
Measurements: growth rate, cell size, growth density, genetic markers.
Key results:
Growth rate increased across populations.
Evolved larger cell volumes.
Four populations developed defects in DNA repair mechanisms.
One strain evolved the ability to utilize citrate as an energy source (Cit+), a trait normally absent in E. coli but present in Salmonella; used as a distinguishing lab feature.
Pod Mrcaru lizards: rapid evolution in the wild
Location: Adriatic Sea, Pod Mrcaru island (two small islets).
History: Five pairs of Podcaris sicula introduced in 1971 from Pod Kopiste.
Study in 2008:
Lizard populations on Pod Mrcaru evolved larger heads compared to ancestors.
Diet shift: more vegetation and fewer insects, making plant tissue tougher to chew.
Larger heads imply larger bite force; evolution occurred in about 18–19 generations (roughly <40 years).
Diet data and interpretation for the lizards
Summer diet on the two islands showed shifts in the proportion of plant matter vs arthropods vs rest; data indicate a change in diet composition consistent with selection pressures for biting power.
Graph (illustrative): summer diet composition differences between Pod Kopište and Pod Mrčaru.
Darwin, evolution, and human observation
Darwin suggested that humans would not live long enough to observe evolution in our lifetimes. Discussion prompts: agree or disagree.
Natural selection: the mechanism and its consequences
Definition: the process that results in the survival and reproductive success of individuals best adapted to their environment; leads to perpetuation of advantageous traits.
Natural selection is a central idea in biology and helps explain evolution.
Examples of natural selection in action
Deer mice camouflage and predation:
Typical environment: dark soil; mice are camouflaged.
Mutation causes some mice to have lighter fur; in a dark environment, white mice are more visible to predators.
Result: white mutants have reduced fitness in the dark environment, and frequency of white fur remains low.
If environment changes to lighter ground, selection shifts toward lighter fur, increasing the frequency of lighter mice over generations.
The Nebraska dune example (about 8,000 years): sand dunes formed ~10,000 years ago; lighter-colored mice adapted to dunes through natural selection.
Summary: individuals better adapted to their environment have higher reproductive success (fitness); selection pressure includes predators, disease, resource availability, and other factors.
Other mechanisms influencing evolution
In addition to natural selection, evolution can be driven by:
Mutations (new genetic variation).
Non-random mating (assortative mating).
Genetic drift (random changes in allele frequencies, especially in small populations).
Migration (gene flow between populations).
Important takeaway: natural selection is important but not the only mechanism driving evolution.
Population genetics and drift (illustrative squirrel example)
Genetic drift: changes in gene frequencies due to random sampling events; more pronounced in small populations.
Example narrative: a garden with 10 squirrels (5 black and 5 grey) is trapped; over years, random sampling and relocation can shift coat-color frequencies.
Data example (from a classroom exercise):
Year 1: grey 12, black 0 (total 12)
Year 2: grey 10, black 4 (total 14)
Year 3: grey 8, black 5 (total 13)
Interpretation: allele/frequency changes can occur due to chance alone, illustrating drift; not all changes imply adaptation.
Migration and its role in evolution
Migration (gene flow) involves movement of individuals between populations, introducing new genes and potentially altering the genetic makeup of the recipient population.
Migration is another mechanism contributing to evolution.
Mechanisms causing evolution: recap
The five mechanisms discussed: genetic mutations, natural selection, non-random mating, genetic drift, migrations.
Key reminder: natural selection is important but not the sole driver of evolutionary change.