Evolution by Natural Selection (Grade 12 NSC Matric Life Sciences): Revision Notes
Formation of a New Species (Speciation)
What is speciation?
Speciation refers to the process by which new species are formed from existing ones. This fascinating evolutionary process occurs when enough genetic variations accumulate within a population over time, eventually leading to the development of organisms that are distinct enough to be considered a separate species.
For speciation to happen, variations must occur within individuals through several mechanisms including sexual reproduction, genetic mutations, crossing over during meiosis, and random chromosome arrangements. When these variations become established and maintained within a population, they increase the likelihood that the species will change over time.
The accumulation of genetic variations is a gradual process that typically occurs over thousands to millions of years. Each small change contributes to the overall genetic diversity that eventually leads to the formation of new species.
Understanding the biological species concept
Before diving into how new species form, it's important to understand what scientists mean when they talk about a "species."
The Biological Species Concept
The biological species concept defines a species as a group of organisms that share similar characteristics and can interbreed with one another to produce fertile offspring.
This definition is crucial because it helps us understand when speciation has truly occurred - when two groups of organisms can no longer successfully breed together to produce viable, fertile offspring, they have become separate species.
Geographic speciation
Geographic speciation is one of the most common ways new species form in nature. This process occurs when part of a population becomes physically separated from the main population due to geographical barriers such as continental drift, oceans, rivers, mountains, or other natural disturbances like volcanoes or earthquakes.
The diagram above shows how a population of lizards can become separated by a seasonal river, creating the initial conditions for geographic speciation to begin.
The stages of geographic speciation
Geographic speciation typically occurs in several distinct stages that can take thousands of years to complete:
Stage 1: Initial separation (Early isolation)
During this first stage, individuals from the original species become separated from the parent population due to a geographic barrier. Initially, the separated groups are still essentially the same species, but they can no longer freely exchange genes with each other.
Stage 2: Independent evolution (Complete isolation)
Over time, the geographic barrier becomes more permanent and complete. For example, a seasonal river might widen and deepen, flowing year-round and creating complete geographic isolation between the two populations.
Key Changes During Stage 2
At this point, gene flow between the two populations completely stops. Each population is now exposed to different environmental conditions and selective pressures. Natural selection begins working independently on each population, causing them to change both genetically (genotypically) and in their physical appearance (phenotypically).
This diagram illustrates how the separated populations adapt to their different environments, with one group becoming lighter-coloured for desert conditions and the other becoming darker for forest environments.
Stage 3: Speciation complete (Reproductive isolation)
After extended periods of separation and independent evolution, the two populations have changed so much that even if they were brought back together, they would no longer be able to interbreed successfully. The genetic differences that have accumulated make them reproductively incompatible.
This final diagram shows how the two new species may come into contact again but are now genetically unable to produce fertile offspring together, confirming that speciation has occurred.
Generic account of speciation
The process of geographic speciation demonstrates how environmental pressure and genetic isolation work together to create new species. Understanding this process helps us appreciate the incredible diversity of life on Earth.
Key Steps in Geographic Speciation
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Population separation: A single population becomes divided by a geographical barrier, splitting it into two separate groups.
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Gene flow cessation: The two populations can no longer mate with each other, so genetic material stops flowing between them.
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Different selective pressures: Each separated population faces different environmental conditions and challenges.
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Independent natural selection: Natural selection operates separately on both populations at the same time, but in different ways due to their different environments.
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Genetic and physical divergence: Over time, the two populations become increasingly different from the original population in both their genetic makeup and physical characteristics.
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Reproductive isolation established: Eventually, even if the populations were reunited, they would not be able to interbreed and produce fertile offspring, confirming they have become distinct species.
Understanding cladograms and speciation events
Scientists use special diagrams called cladograms (also known as phylogenetic trees) to show how species are related to each other and when speciation events occurred during evolutionary history.
These diagrams help us visualise evolutionary relationships between species. When reading a cladogram, each branching point represents a speciation event where one species split into two. Some species shown on these diagrams may still exist today (extant species), while others may have become extinct.
Reading Cladograms
The length of lines in these diagrams can represent time - longer lines suggest that a species has remained relatively unchanged for longer periods, while shorter lines before branching points indicate more rapid evolutionary changes.
South African example
South Africa provides an excellent real-world example of speciation processes in action.
South African Speciation Example:
Bontebok Antelope Populations Bontebok antelope populations exist in different locations such as Bontebok National Park and Table Mountain National Park. These populations are separated by hundreds of kilometres, and scientists believe that this geographical separation may lead to speciation over time.
Common misconceptions and exam tips
Understanding speciation requires avoiding several common misunderstandings that students often have about this process.
Critical Misconceptions to Avoid
Common misconception: Students often think speciation happens quickly or within a single generation.
Reality: Speciation typically takes thousands to millions of years to complete.
Essential Exam Tips
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When describing speciation, always mention that gene flow stops between populations and that natural selection works independently on each separated group.
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Be able to explain the difference between genotypic changes (genetic/DNA level) and phenotypic changes (observable physical characteristics).
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Remember that for speciation to be complete, reproductive isolation must occur - the populations must be unable to interbreed and produce fertile offspring.
Remember!
Key Points to Remember
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Speciation is the formation of new species from existing ones through accumulated genetic variations over time
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Geographic barriers like rivers, mountains, or oceans can separate populations and prevent gene flow between them
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Independent natural selection acts on separated populations in different environments, causing them to evolve in different directions
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Reproductive isolation is the final test of speciation - when populations can no longer interbreed to produce fertile offspring, they have become separate species
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Cladograms help scientists visualise evolutionary relationships and show where and when speciation events occurred in evolutionary history