9D PP - Speciation

U4 AOS2

Species

A biological species is a group of similar looking organisms in a specific location that can interbreed to create viable and fertile offspring.

 

Species are recognized on the basis of their morphology (size, shape, and appearance) and genomes.

Speciation

Speciation is the process by which new species are formed from existing, or ancestral species.

 

Speciation occurs when genetic difference accumulate between two populations through evolutionary processes such as:

• Mutations

• Natural selection

• Genetic drift

 

This eventually leads to reproductive isolation where the two populations can no longer successfully interbreed to produce viable and fertile offspring.

Isolating Mechanisms

• Pre-reproductive

• Post-reproductive

Examples of Pre-reproductive isolating mechanisms

Geographical: Separation by barriers e.g. body of water

Ecological: inhabit different niches and don’t interact

Temporal: time of day or year when individuals breed differs

Behavioural: type of mating behaviours vary e.g. mating calls

Structural: physical characteristics differ, physically preventing breeding

Examples of Post-reproductive isolating mechanisms

Gamete mortality: sperm unable to penetrate ovum for fertilization

Zygote mortality: fertilization occurs but zygote does not survive

Hybrid sterility: viable offspring produced but is infertile

Types of Speciation

Several models have been proposed to account for new species among sexually reproducing organisms, including:

• Allopatric speciation

• Sympatric speciation

Allopatric Speciation

Allopatric speciation involves the formation of a new species as a result of a geographical barrier/isolation, such as:

• Presence of a mountain range

• Development of a river

• Continental drift

• Rising sea levels, etc.

 

Geographical barriers that isolate populations from each other prevent gene flow.  

Each population experiences different selection pressures resulting in the accumulation of genetic differences. 

The population diverge, forming two separate species that can no longer interbreed.

This is the most common form of speciation.

Formulaic Answer: Allopatric Speciation

• A geographical barrier isolates two populations and prevents gene flow.

• The isolated populations are exposed to different selection pressures, which causes genetic differences (mutations) to accumulate and allele frequencies change.

• Overtime, sufficient genetic differences accumulate so that if the two populations were brought back together, they are no longer able to interbreed to produce viable and fertile offspring; they are two different species.

Sympatric Speciation

Sympatric speciation occurs within populations sharing the same geographical location, where different selection pressures exist, causing individuals with certain phenotypes to diverge and form a new species.

 Note: there is no geographical barrier preventing gene flow

 

This type of speciation is rare.  

Can arise from genetic abnormalities that occur during gamete formation, producing polyploid variants. 

Plants are generally more tolerant to changes in sets of chromosomes than animals. Therefore, sympatric speciation arising from genetic abnormalities is nearly exclusively seen in plants.

VIMSAN

Variation

Isolation

Mutations

Selection Pressures

Allele frequencies change

New species

Galapagos Finches

The Galápagos Islands are made up of 19 islands off the coast of Ecuador.

Each of the 19 islands have their own selection pressures and species.

The islands are separated by the Pacific Ocean, serving as a geographic barrier.

Finch species, known as the Galápagos finches, or Darwin’s finches, inhabit these islands. 

These finches have a variety of beak shapes and sizes, tailored toward the specific food sources on their island.

How Galapagos Finches came about

It has been hypothesised that the formation of the Galápagos finches has been the result of allopatric speciation:

1. An ancestral population of finches from the mainland colonized the islands which are separated by the ocean, preventing gene flow.

2. Each island has different selection pressures and food sources, selecting for different phenotypes (e.g. beak shape), allowing genetic differences to accumulate.

3. Once sufficient mutations accumulated (over millions of years), viable and fertile offspring could no longer be produced through interbreeding; new finch species were formed.

Howea Plants - Reproductive Isolation & Speciation

After inhabiting alkaline soil, physiological differences developed between the palms. Their flowering times changed, creating reproductive isolation and preventing gene flow. Over several generations, genetic differences accumulated until they could no longer interbreed to produce viable, fertile offspring, forming a new species. As Lord Howe Island is relatively small, the palms were unlikely to have been geographically isolated, supporting sympatric speciation.

Lord Howe Island Palms

Lord Howe Island is a small Australian island in the Tasman Sea. Two palm species, Howea forsteriana and Howea belmoreana, provide evidence for sympatric speciation. Researchers hypothesised that differences in soil pH acted as the selection pressure, with H. belmoreana inhabiting neutral and acidic soils while H. forsteriana inhabits alkaline soil. This suggests H. forsteriana diverged from H. belmoreana after the original population colonised alkaline soil.