Welcome to Diversity of Organisms: Making Sense of Life!
Hello Biologists! This chapter, "Diversity of Organisms," is all about organization. The living world is incredibly vast—scientists estimate there are millions of species! To study, compare, and protect this incredible variety of life, we need a system. Think of classification as the ultimate filing system or the Dewey Decimal system for all living things on Earth.
We will explore how scientists group organisms, how these groups reflect evolutionary history (phylogeny), and why understanding this diversity is crucial for the future of our planet.
1. Classification: Ordering the Living World
1.1 The Need for Classification (Taxonomy)
Taxonomy is the branch of science concerned with classifying and naming organisms. Without a universal system, scientists in different countries might use different names for the same organism, leading to chaos!
The goals of classification are simple:
- To identify known species accurately.
- To predict characteristics based on group membership.
- To reflect the evolutionary relationships between organisms.
1.2 Binomial Nomenclature
We use a two-part naming system established by Carl Linnaeus in the 18th century. This system gives every species a unique, universally accepted name.
The Rules of the Binomial System:
1. The first name is the Genus (always capitalized).
2. The second name is the species (always lowercase).
3. Both names are written in italics (or underlined if handwritten).
Example: Humans are Homo sapiens. Homo is the genus; sapiens is the species.
Quick Tip: If you don't know the exact species, you can abbreviate the species name (e.g., E. coli for Escherichia coli).
1.3 The Hierarchy of Taxa (The Linnaean System)
Organisms are grouped into a nested hierarchy of taxa (classification levels), ranging from the very broad Domain to the very specific species.
The Seven Main Taxa (KPCOFGS)
1. Kingdom (Broadest)
2. Phylum
3. Class
4. Order
5. Family
6. Genus
7. Species (Most specific)
🔥 Memory Aid (Mnemonic): King Philip Came Over For Good Soup.
Key Takeaway: The lower down the hierarchy two organisms are grouped (e.g., sharing the same Family), the more characteristics they share and the more closely related they are evolutionarily.
1.4 Domains and Kingdoms
All life is first divided into three massive groups called Domains, based primarily on cellular structure.
The Three Domains
1. Archaea: Single-celled, prokaryotic organisms (no nucleus). Often live in extreme environments (extremophiles).
2. Bacteria: Single-celled, prokaryotic organisms. Includes common bacteria and cyanobacteria.
3. Eukarya: Organisms composed of eukaryotic cells (cells with a nucleus and membrane-bound organelles). This domain includes everything from yeast to humans.
The Four Kingdoms of Eukarya
Within the Domain Eukarya, we typically study four kingdoms:
1. Protista: Mostly single-celled eukaryotes (e.g., Amoeba, algae). Often described as the "junk drawer" kingdom.
2. Fungi: Heterotrophs (obtain food from outside sources) with cell walls made of chitin (e.g., mushrooms, yeast).
3. Plantae: Autotrophs (make their own food via photosynthesis) with cell walls made of cellulose (e.g., trees, mosses).
4. Animalia: Multicellular heterotrophs that lack cell walls and typically show locomotion.
2. Classification Methods: Natural vs. Artificial
2.1 Artificial Classification
This method groups organisms based on superficial or arbitrary characteristics, such as color, habitat, or whether they can fly.
Example: Grouping all flying creatures (insects, birds, bats) together. This system is convenient but does not reflect evolutionary relationships.
2.2 Natural Classification
Natural classification groups organisms based on shared ancestry and homologous structures—structures that have a similar origin but may have different functions (e.g., the pentadactyl limb in vertebrates).
Benefit: It allows the prediction of shared characteristics and provides insight into the path of evolution.
Common Mistake Alert: Do not confuse homologous structures with analogous structures (like the wings of a bat and the wings of an insect). Analogous structures have similar functions but evolved independently, showing no close common ancestry.
Key Takeaway: Modern classification is primarily natural because we want our groups to represent shared evolutionary history (phylogeny).
3. Cladistics: Tracking Evolutionary Ancestry (SL/HL)
Cladistics is a method of classifying organisms based on the sequence in which different lines of organisms diverged from a common ancestor.
3.1 Clades and Cladograms
A clade is a group of organisms that includes a single common ancestor and all of its descendants (living and extinct). Clades are central to natural classification.
A cladogram is a tree diagram that visually represents the hypothetical evolutionary history (phylogeny) of a clade.
How to Read a Cladogram:
1. Root: Represents the common ancestor of all species in the diagram.
2. Nodes (Branching Points): Indicate a speciation event or a divergence from a common ancestor.
3. Branches: Represent different lineages or species.
4. Outgroup: A species or group known to have diverged before the lineage of interest. Used for comparison.
The organisms at adjacent branches are the most closely related, having diverged most recently.
3.2 Molecular Evidence for Cladistics (HL Focus)
While morphology (physical structure) was traditionally used, modern cladistics relies heavily on molecular evidence, especially DNA and protein sequences.
- If two species have very similar base sequences in their DNA or amino acid sequences in a protein, it is assumed they separated from a common ancestor relatively recently.
- Molecular changes (mutations) happen at a predictable rate. This forms the basis of the molecular clock.
Molecular Clock: If the rate of mutation is constant, the number of differences in the DNA or protein sequence between two species can be used to estimate how long ago they diverged.
Example: Humans and chimpanzees share nearly 99% of their DNA, suggesting a recent divergence.
3.3 Reclassification Based on Cladistics
Sometimes, traditional classification systems conflict with the evidence provided by cladograms. In these cases, reclassification occurs to ensure that classification reflects phylogeny.
Example: Historically, plants like orchids were classified separately, but DNA evidence showed they were much more closely related to other plants than previously thought, leading to adjustments in their family groupings.
Key Takeaway: Cladistics uses molecular evidence (DNA/proteins) to build evolutionary trees (cladograms). A valid clade must contain the ancestor and ALL its descendants.
4. Evolution and Speciation
The diversity we classify today is the result of evolution—the cumulative change in the heritable characteristics of a population over successive generations.
4.1 Reviewing Natural Selection
Evolution occurs primarily through the process of Natural Selection (or "survival of the fittest").
1. There is variation within a species (due to mutation, meiosis, and sexual reproduction).
2. Overproduction of offspring leads to competition for resources (the struggle for existence).
3. Individuals with advantageous traits (adaptations) are more likely to survive and reproduce.
4. These advantageous traits are passed on to the next generation, leading to a change in the species over time.
4.2 Speciation
Speciation is the process by which one species splits into two or more distinct species.
A species is defined as a group of organisms that can interbreed to produce fertile offspring.
How Speciation Happens (Divergence)
Speciation often occurs when populations become reproductively isolated:
- Two populations of the same species become geographically separated (e.g., by a new mountain range or river). This is called allopatric speciation.
- The environments they live in are now different, leading to different selective pressures.
- Natural selection acts independently on the two populations, favoring different traits.
- Over long periods, they diverge genetically until they can no longer successfully interbreed, even if they meet again. They are now separate species.
Did you know? Divergent evolution is the process by which two related species become more different over time, often due to adaptation to different environments. This is why we see homologous structures.
Key Takeaway: Speciation requires reproductive isolation, which prevents gene flow between populations, allowing them to evolve independently into new species.
5. Conservation of Biodiversity
Understanding diversity is essential for protecting it. Biodiversity is the variety of life in the world or in a particular habitat or ecosystem.
5.1 Defining Biodiversity
Biodiversity is measured at three main levels:
1. Genetic Diversity: Variation in genes within a single species or population. High genetic diversity increases a population’s resilience to environmental change.
2. Species Diversity: The number of different species present in an area (richness) and the relative abundance of each species (evenness).
3. Ecosystem Diversity: The variety of habitats, communities, and ecological processes within the biosphere.
5.2 The Importance of Conservation
Protecting biodiversity is vital for several reasons:
- Ecological Functions: Every species plays a role. Loss of one species can destabilize entire food webs (e.g., loss of a keystone species). Intact ecosystems provide essential services like clean water, air purification, and nutrient cycling.
- Ethical and Aesthetic Value: Many argue that all species have an intrinsic right to exist, regardless of their usefulness to humans.
- Economic and Medicinal Value: Many current and future medicines, industrial materials, and food crops originate from wild species. Protecting genetic diversity ensures future agricultural resilience.
5.3 Threats to Biodiversity
Human activities are the primary driver of biodiversity loss. Major threats include:
- Habitat Loss and Fragmentation: Clearing land for agriculture, development, or resource extraction breaks large habitats into smaller, isolated patches, severely limiting species movement and population size.
- Invasive Species: Introduction of non-native species can outcompete, prey upon, or introduce disease to native species.
- Pollution: Chemical runoff, plastic waste, and emissions damage habitats and poison organisms.
- Overexploitation: Unsustainable hunting, fishing, or harvesting of resources.
- Climate Change: Shifting temperatures and weather patterns force species out of their historic ranges faster than they can adapt or migrate.
Don't worry if this seems challenging: Conservation is a complex global issue, but recognizing the causes of biodiversity loss is the first step toward finding effective biological and political solutions.
Key Takeaway: Biodiversity is measured at the genetic, species, and ecosystem levels. Its conservation is critical because healthy, diverse ecosystems provide indispensable services and stability to the planet.