Source: BIOL 110 Learning Guide, Exploring Life, Evolution, and Earth's Changing Environment Difficulty: Introductory | Prerequisites: None. This is foundational material for the course.
This tutorial covers the most fundamental questions in biology: what counts as alive, how life changes over time, and how scientists map those changes. You will need these concepts for every later unit in the course, from genetics to ecology. The material draws heavily on Darwin's framework and updates it with modern phylogenetics, so understanding it now saves you from playing catch-up when the course moves into molecular biology and human evolution. If you have never taken a biology course, this is where you start.
Living things share a set of properties (organisation, metabolism, homeostasis, growth, reproduction, response to stimuli, evolution) that viruses only partially meet. All life traces back to a Last Universal Common Ancestor roughly 3.7 billion years ago, and scientists use phylogenetic trees to map those relationships. Natural selection acts on heritable variation within populations, favouring traits that improve survival and reproduction in a given environment.
Properties of life
The characteristics shared by all living organisms: cellular organisation, metabolism, homeostasis, growth and development, reproduction, response to stimuli, and the capacity to evolve. Think of these as the checklist biologists use to decide whether something qualifies as "alive."
Biological evolution
A change in the heritable characteristics (genetic makeup) of a population over successive generations. In simple terms, this means the gene pool of a population shifts over time, sometimes in ways you can measure (like a change in average beak size).
Last Universal Common Ancestor (LUCA)
The hypothetical single-celled organism from which all current life on Earth descends, estimated to have lived approximately 3.7 billion years ago. Think of it as the root of the entire tree of life.
Phylogenetic tree
A branching diagram that represents the evolutionary relationships among species or groups of organisms, based on similarities and differences in their DNA, anatomy, or other characteristics. In simple terms, it is a family tree for species.
Node (on a phylogenetic tree)
A point on a phylogenetic tree where one lineage splits into two or more descendant lineages. Each node represents a common ancestor shared by everything on the branches above it.
Adaptation
A heritable trait that increases an organism's fitness (its ability to survive and reproduce) in a particular environment. Think of it as a feature that has been "kept" by natural selection because it works well in that setting.
Natural selection
The process by which organisms with heritable traits better suited to their environment tend to survive and reproduce at higher rates than organisms without those traits, leading to a gradual change in the population's characteristics over generations. In simple terms, nature "picks" who thrives based on how well their traits match the current conditions.
Fitness (biological)
An organism's ability to survive and reproduce in a given environment. Higher fitness means more offspring that themselves survive to reproduce. This is not about strength or speed in the everyday sense.
Genetic variation
Differences in DNA sequences among individuals in a population. This variation is the raw material that natural selection acts on.
Heritability
The degree to which a trait is passed from parents to offspring through genetic information. If a trait is heritable, offspring tend to resemble their parents for that trait.
Biologists define life by a cluster of properties rather than a single criterion. An entity must show cellular organisation, carry out metabolism, maintain homeostasis, grow and develop, reproduce, respond to stimuli, and have the capacity to evolve.
Viruses are not classified as living organisms under this framework. They lack cells, they cannot metabolise on their own, and they cannot reproduce without hijacking a host cell's machinery. They do contain genetic material (DNA or RNA) and they do evolve, which is why the question keeps coming up in introductory courses. The short answer for an exam: viruses are not alive because they fail several criteria on the checklist, most critically the absence of cellular structure and independent reproduction.
Biological evolution is a change in the heritable characteristics of a population over generations. The key word is "population," not "individual." Individual organisms do not evolve during their lifetimes; populations do, across generations.
All life on Earth can be traced back to a single common ancestor, LUCA, that lived roughly 3.7 billion years ago. The evidence for this includes shared biochemistry (all life uses DNA, RNA, and proteins), shared genetic code, and homologous structures across widely divergent species.
LUCA matters because it means every living thing on the planet is related. Bacteria, oak trees, mushrooms, and humans all share a lineage. This interconnectedness is the foundation of comparative biology: if two organisms share a recent common ancestor, they are likely to share many genes and traits.
Phylogenetic trees are diagrams that map the evolutionary relationships among organisms. Darwin's original sketch in his notebook represented the history of life and the divergence of species over time (answer B in the tutorial's multiple-choice question).
Key reading rules:
Tips (the ends of branches) represent living or recently extinct species.
Nodes (branching points) represent common ancestors where one lineage split into two or more.
Orientation does not matter. A tree drawn sideways, upside down, or rotated carries the same information, because the branching pattern and the order of common ancestors stay the same.
Branch length may or may not represent time or genetic change, depending on the tree. Check the axis or caption.
The primary purpose of phylogenetic trees in modern science is to hypothesise and interpret relationships between organisms. They are not about geographic distribution, size classification, or physical appearance.
"Organisms are always getting better through evolution." Evolution does not move toward perfection or complexity. Natural selection favours traits that improve survival and reproduction in the current environment. If the environment changes, previously advantageous traits may become neutral or harmful. "Better" is always relative to conditions.
"Organisms evolve in response to a change in an environment." Individuals do not choose to evolve or develop new traits because conditions changed. Variation already exists in the population before the environmental change. The change simply shifts which existing variants survive and reproduce at higher rates.
"Evolution only occurs slowly and gradually." While many evolutionary changes unfold over long timescales, some can happen rapidly. The Grants' research on Galápagos finches showed measurable beak-size shifts in a single generation following drought. Bacterial populations can evolve antibiotic resistance within days.
"Mutations arise because organisms need them." Mutations are random with respect to what would be useful. Applying an antibiotic does not cause bacteria to develop resistance mutations. Rather, resistance mutations that already exist in the population are selected for when the antibiotic kills non-resistant individuals.
⚠️ Know the difference between natural selection and evolution. Natural selection is the mechanism (differential survival and reproduction based on traits). Evolution is the outcome (a measurable change in a population's traits over time). A question describing birds with certain beaks surviving a drought is describing natural selection. A question reporting that the proportion of short-tailed vs. long-tailed birds changed over ten years is describing evolution.
⚠️ Be able to list the properties of life and explain why viruses fail the test. This comes up in multiple-choice and short-answer formats.
⚠️ Nodes on a phylogenetic tree represent common ancestors, not living species. Tips represent living or recently extinct species. Students frequently mix these up.
⚠️ Orientation of a phylogenetic tree is irrelevant to its meaning. A rotated tree still shows the same branching relationships.
⚠️ The LUCA concept (all life shares a common ancestor ~3.7 billion years ago) is testable as a true/false question. The answer is true.
True or False: Viruses are considered living organisms because they contain DNA or RNA. (False – they lack cells and cannot reproduce independently.)
Fill in the blank: A change in the heritable characteristics of a population over generations is called __________. (biological evolution)
True or False: The orientation of a phylogenetic tree changes the evolutionary relationships it depicts. (False – the branching pattern stays the same regardless of orientation.)
Fill in the blank: The points on a phylogenetic tree where lineages split are called __________. (nodes)
True or False: Individual organisms evolve during their lifetimes. (False – populations evolve over generations; individuals do not.)
Q: What are the properties of life, and why are viruses not classified as living organisms?
A: The properties of life include cellular organisation, metabolism, homeostasis, growth and development, reproduction, response to stimuli, and the capacity to evolve. Viruses are not classified as living because they lack cells, cannot carry out metabolism independently, and require a host cell to reproduce. Although they contain genetic material and can evolve, they fail too many criteria to qualify.
Q: What does Darwin's original sketch of the evolutionary tree represent?
A: It represents the history of life and the divergence of species over time (option B). It shows how species branch from common ancestors rather than mapping geography or physical traits.
Q: What is the significance of nodes in a phylogenetic tree?
A: Nodes indicate points where one lineage divided into two or more species (option D). Each node represents a common ancestor shared by all species on the branches that descend from it.
Q: How does the concept of LUCA shape our understanding of the interconnectedness of all life?
A: LUCA establishes that all living organisms, from bacteria to humans, share a single common ancestor from roughly 3.7 billion years ago. This means every living species on Earth is related, which explains shared biochemistry (DNA, RNA, proteins, the genetic code) and allows scientists to use comparative methods across species.
Q: A researcher finds that 30% of a bird population had short tails at the start of a ten-year study and 45% had short tails at the end. Is this an example of natural selection or evolution?
A: This describes evolution, because it documents a measurable change in the population's trait distribution over time. It does not describe the mechanism (which specific selective pressure caused the shift), only the outcome.
This material connects directly to genetics and molecular biology later in BIOL 110. Understanding that evolution acts on heritable variation requires knowing how DNA replication, mutation, and gene expression work, which are covered in upcoming tutorials.
Phylogenetic trees reappear when the course covers biodiversity and classification. The ability to read a tree is a skill you will use repeatedly.
The lactose tolerance case study (covered in the companion study notes on natural selection) ties this material to human evolution and gene expression, topics the course returns to several times.
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