Biology | Evolution and speciation

Welcome to Evolution and Speciation!

Hello future biologists! This chapter is truly the heart of the "Unity and diversity" section, as it explains why life is so incredibly diverse, yet fundamentally connected. Evolution is the unifying theory of all biology, explaining everything from why we look different from our grandparents to how a single-celled ancestor gave rise to whales and redwood trees.

Don't worry if some of the concepts seem abstract at first. We will break down the process of evolution—Natural Selection—into simple, manageable steps. By the end, you'll understand how simple mechanisms acting over vast periods of time created the complexity of life!

Section 1: The Core Concept of Evolution

What is Evolution?

In simple terms, evolution is the cumulative change in the heritable characteristics (traits) of a population over successive generations.

A more precise definition often used in genetics is: A change in the allele frequency within a population's gene pool over time.

• Evolution acts on populations, not individuals. A single organism cannot evolve during its lifetime.
• It requires time—often millions of years—though we can observe microevolutionary changes (like antibiotic resistance) much faster.

The Father of Evolution: Charles Darwin

The modern theory of evolution is credited primarily to Charles Darwin (and Alfred Russel Wallace), who proposed the mechanism of Natural Selection.

Did you know? Darwin’s famous journey aboard the HMS Beagle, particularly his observations of finches and tortoises on the Galápagos Islands, provided crucial evidence that species were not fixed, but changed based on local environmental pressures.

Section 2: The Mechanism of Natural Selection

Natural Selection is the engine of evolution. It is a logical process based on four observable pillars. Think of natural selection as a filtering process, where the environment is the filter, and only the best-adapted organisms pass through to reproduce.

Step-by-Step: The Four Pillars of Natural Selection

1. Variation Exists Within a Population (The Essential Ingredient)

For selection to occur, individuals in a species must show differences (variation) in their characteristics. If everyone was identical, no trait could be selected for or against.

Sources of Variation:

• Mutation: Random changes in the DNA sequence. This is the ultimate source of all new alleles.
• Meiosis: Recombination of genes through crossing over and independent assortment during gamete formation.
• Sexual Reproduction: The combination of genes from two parents (fertilization) results in unique offspring combinations.

2. Overproduction and Competition (The Struggle for Survival)

Species tend to produce more offspring than the environment can possibly support. This leads to competition (a "struggle for existence") for limited resources like food, shelter, and mates.

Analogy: Imagine a tiny pond that can only sustain 100 fish. If 1,000 eggs hatch, 900 fish must die or fail to reproduce. They are competing fiercely.

3. Differential Survival and Selection (The Filtering)

Due to the struggle for existence, some individuals are better equipped to survive and reproduce than others. These advantageous traits are determined by their genes.

Key Concept: Survival of the Better Adapted.

• Individuals with traits that make them slightly better at finding food, avoiding predators, or resisting disease are more likely to survive and reach reproductive age.
• These individuals are considered to have higher fitness. Fitness is defined biologically as the ability to survive and produce viable, fertile offspring.

4. Inheritance and Change (The Outcome)

Since the advantageous traits are heritable (passed genetically to the next generation), the offspring of the fitter individuals will also likely possess those traits.

• Over many generations, the frequency of the advantageous allele increases in the population's gene pool.
• The population is now, on average, better adapted to the environment. This cumulative change over time is evolution.

Section 3: Evidence Supporting Evolution

Evolution isn't just a theory; it is supported by vast amounts of evidence across multiple fields of biology.

1. The Fossil Record

Fossils provide direct evidence of ancestral species and show transitional forms—organisms that link modern groups to their presumed ancestors.

• The fossil record shows that life on Earth is not static but has changed over time.
• Fossils appear in a sequential order that matches their complexity (e.g., prokaryotes before eukaryotes; fish before amphibians).
• Example: The evolutionary sequence of the horse (from small, many-toed ancestors to the large, single-toed modern horse) is well-documented by fossils.

2. Selective Breeding (Artificial Selection)

Humans selecting desirable traits in domestic species demonstrates that selection causes evolution.

• Artificial selection is selection driven by human intervention, rather than environmental pressure.
• Examples: All modern dog breeds (Chihuahuas to Great Danes) originated from the wild wolf through selective breeding for specific traits (e.g., temperament, size, coat color). Similarly, all variations of cabbage, broccoli, and cauliflower derived from a single wild mustard plant.

3. Homologous Structures

These are structures found in different species that have the same basic structure (origin) but have evolved to perform different functions.

• Homologous structures imply a common ancestry.
• The classic example is the pentadactyl limb (the five-digit limb) found in mammals, birds, amphibians, and reptiles. Although a human hand, a bat wing, and a whale flipper perform different jobs, the arrangement of bones is structurally similar, showing they inherited the structure from a common ancestor.
• Convergence vs. Homology: Be careful! Structures with similar function but different origins (e.g., a bat wing and an insect wing) are called analogous structures. These show convergent evolution, where unrelated species adapt to similar environments, but they do not provide evidence of recent common ancestry.

4. Adaptive Radiation

This occurs when a common ancestor gives rise to multiple new species, each adapted to occupy a different ecological niche.

• This often follows a dispersal event to a new area with diverse resources.
• Example: Darwin's finches. A single ancestral finch species colonized the Galapagos Islands and diversified rapidly. Different islands had different food sources, leading to the evolution of varied beak shapes (e.g., thick, crushing beaks for nuts; thin, pointed beaks for insects).

Section 4: Speciation—The Formation of New Species

Evolutionary changes within a population over time (microevolution) eventually lead to the formation of entirely new species (macroevolution). This process is called speciation.

What is a Species?

The most commonly used definition (the Biological Species Concept) states that a species is a group of organisms that can interbreed and produce fertile offspring.

Common Mistake Alert: If two organisms can mate, but their offspring are sterile (like a mule, the sterile hybrid of a horse and a donkey), they are considered different species.

The Key to Speciation: Reproductive Isolation

Speciation requires reproductive isolation, which means that gene flow between two populations stops.

Isolation mechanisms can be categorized as:

• Pre-zygotic Isolation: Mechanisms that prevent mating or fertilization (e.g., different mating seasons, behavioral differences, incompatible genitalia).
• Post-zygotic Isolation: Mechanisms that occur after fertilization (e.g., the resulting hybrid embryo doesn't survive, or the hybrid offspring is infertile).

Modes of Speciation

1. Allopatric Speciation (Geographic Isolation)

This is the most common form of speciation. It involves physical separation.

Step-by-Step Allopatric Speciation:

1. A single large population exists.
2. A geographic barrier (like a river changing course, a mountain rising, or a colonization event) splits the population into two isolated groups.
3. No gene flow occurs between the two groups.
4. Each isolated group experiences different selection pressures in their unique environment, and random mutations accumulate (genetic drift).
5. Over time, the populations diverge so much that even if the geographic barrier is removed, they can no longer interbreed (reproductive isolation is achieved). They are now two separate species.

Example: Separate populations of squirrels on opposite sides of the Grand Canyon.

2. Sympatric Speciation (No Geographic Isolation)

Speciation occurs without a physical barrier while the populations remain in the same geographic area.

• This is much rarer in animals but common in plants, often due to polyploidy.
• Polyploidy is the state of having more than two sets of chromosomes. If an error occurs during meiosis, an organism may become polyploid. This individual often cannot mate successfully with the parent species, immediately creating a new, reproductively isolated species.