Keshu

1. What is a Species?

Defining a "species" sounds simple, but it is one of the most debated concepts in biology. The most widely accepted definition in modern biology is the Biological Species Concept.

The Biological Species Concept

• Viable: The offspring survive to adulthood.

• Fertile: The offspring can successfully reproduce themselves (e.g., horses and donkeys can mate to produce a mule, but because mules are sterile, horses and donkeys are considered separate species).

Limitations of this Definition

While useful, this definition doesn't work for everything:

• Asexual Organisms: Bacteria and many plants don't mate; they clone themselves.

• Fossils: We cannot observe the mating habits of extinct dinosaurs.

• Ring Species: Neighboring populations that can interbreed, but the "ends" of the geographic loop cannot.

2. Variation Within a Species

No two individuals within a species (except identical twins) are exactly alike. This diversity is called intraspecific variation.

Variation can be categorized by how it looks (types of data) and what causes it.

A. Types of Variation (Data)

1. Continuous Variation: Characters change gradually across a spectrum. There are no distinct categories; features are measurable.

   • Examples: Human height, birth weight, leaf length.

2. Discontinuous Variation: Characters fall into distinct, sharp categories with no intermediates.

   • Examples: Blood types (A, B, AB, O), eye color, ability to roll your tongue. 3. Causes of Variation

     Variation is driven by two main factors, and often a combination of both:

     A. Genetic Factors (Heritable)

     Genetic variation is crucial because it is the raw material for natural selection and evolution. It arises from:

     • Mutation: Random changes in the DNA sequence. This is the ultimate source of new alleles.

     • Meiosis: The process of making sperm and egg cells shuffles genes through:

       • Crossing over (swapping DNA segments).

       • Independent assortment (randomly separating chromosomes).

     • Random Fertilization: Which specific sperm meets which specific egg is entirely up to chance, creating unique genetic combinations.