# Isolation & Speciation ## Genetic Isolation - When two populations of the same species become **reproductively isolated** from each other, they can eventually become **genetically isolated** - If the two populations are no longer reproducing with each other, then they do not interchange genes with each other in the production of offspring - Changes that occur in the allele frequencies of each group are **not shared**, so they evolve independently of each other - This can lead to the formation of two populations that are **no longer successfully able to interbreed**

When genetic differences lead to an inability of population members to interbreed and produce fertile offspring, **speciation** has occurred

- There are different mechanisms of speciation: - **Allopatric speciation** - **Sympatric speciation** ## Allopatric Speciation - **Allopatric speciation** occurs as a result of geographical isolation - It is the most common type of speciation ### The process of allopatric speciation - Populations of a species become **separated from each other by geographical barriers** - The barrier could be natural like a river, ocean, or desert - It can also be man-made, like a major highway or urban development - This creates two populations of the same species who are **reproductively separated** from each other - As a result, **no genetic exchange** can occur between them

If sufficient selection pressures act to change the gene pools within both populations, they will eventually diverge and form separate species

- The changes in the alleles/genes of each population will affect the phenotypes present in both populations - Over time, the two populations may begin to differ: - **Physiologically** - how their bodies function - **Behaviourally** - how they act - **Morphologically** - their physical structure ### Example: Allopatric speciation in plant populations - Imagine there is a population of flowering plants that are all one species growing in a valley - A river changes course and now runs through the middle of the valley, dividing the population into two groups - The natural barrier prevents the two groups from interbreeding, so there is **no gene flow** between them - The two populations experience different selection pressures: - One side has rockier, drier soil - The other side has moister, nutrient-rich soil - Over thousands of years, the divided populations form two distinct species that can no longer interbreed - Different root structures develop - Flowering times shift - Pollinator preferences change ## Sympatric Speciation - **Sympatric speciation** takes place with **no geographical barrier** - A group of the same species could be living in the same place but for speciation to take place there must exist **two populations within that group** and **no gene flow** occurs between them

Something must split or separate the two populations even though they occupy the same geographical area

### Mechanisms of sympatric speciation This can occur via: 1. **Ecological separation** 2. **Behavioural separation** #### Ecological separation - Populations are separated because they **live in different environments within the same area** - For example: - Soil chemistry can differ greatly in different areas of the same field - Soil pH has a major effect on plant growth and flowering - A population growing in soil with a different pH may flower at a different time from another population - This leads to reproductive separation (and eventually genetic isolation) of the two populations #### Behavioural separation - Populations are separated because they have **different behaviours** - Examples include differences in: - Feeding behaviours - Communication methods - Social behaviours, such as courting displays to attract a mate - Mating times (temporal isolation) ### Example: Sympatric speciation in aquatic organisms - A species of fish lives in a large lake - Some individuals within the population feed near the surface on insects - Others feed on the lake bottom, consuming molluscs and crustaceans - The different feeding behaviours separate the population into different environments - Here, **behavioural separation has led to ecological separation** | Population | Feeding Zone | Selection Pressure | Adaptive Traits | |------------|-------------|-------------------|-----------------| | Surface feeders | Upper water | Catching fast insects | Upward-facing mouths, rapid burst swimming | | Bottom feeders | Lake floor | Crushing hard shells | Downward-facing mouths, powerful jaws | - The separated groups experience **different selection pressures** - Small, upward-facing mouths are advantageous for surface-feeding - Large, powerful jaws are advantageous for bottom-feeding - Over time natural selection causes the populations to **diverge and evolve different courtship displays** - Surface feeders develop elaborate jumping displays - Bottom feeders develop substrate-drumming displays - They can **no longer interbreed** - they are now considered separate species ## The Role of Mutations in Speciation - Both allopatric and sympatric speciation are **reliant on mutations occurring** within individuals in populations - Without mutations, there are **no new alleles** of genes for selection to act on

Mutations are the ultimate source of genetic variation - the raw material for evolution

- The changes in genetic material caused by mutations are important as these changes produce the differences in: - Physiology - Behaviour - Morphology - These differences accumulate between populations over many generations, eventually leading to speciation ### How mutations contribute to speciation 1. Random mutations occur in individuals within a population 2. Most mutations are neutral or harmful, but some provide advantages 3. If populations are reproductively isolated, different mutations accumulate in each population 4. Natural selection acts on these mutations differently in each population 5. Over many generations, the genetic differences become so great that: - Individuals from the two populations can no longer produce fertile offspring - Speciation has occurred ## Comparing Allopatric and Sympatric Speciation | Feature | Allopatric Speciation | Sympatric Speciation | |---------|----------------------|---------------------| | **Geographic barrier** | Required (river, mountain, ocean, etc.) | Not required | | **Initial cause of isolation** | Physical separation | Ecological or behavioural differences | | **Gene flow** | Prevented by distance | Prevented by reproductive barriers | | **Frequency** | More common | Less common | | **Time scale** | Often many thousands of years | Can occur more rapidly | | **Example** | Finches on different islands | Fish in same lake with different feeding habits | ## Key Principles of Speciation - Speciation requires **reproductive isolation** - populations must stop exchanging genes - **Genetic drift** and **natural selection** cause the isolated populations to diverge genetically - Over time, populations accumulate enough differences that they become **reproductively incompatible** - The formation of new species is **gradual** and occurs over many generations

A **species** is a group of organisms that can interbreed to produce fertile offspring

- Once populations can no longer produce fertile offspring together, they are considered separate species - This is known as the **biological species concept**