Species and Speciation: PECTAA Class 11 Biology, Punjab Board

Two birds can look almost identical and still never breed with each other. Two dogs can look nothing alike and still belong to one species. Looks alone do not settle what a species is. Section 1.8 of the book gives the test it uses and explains how new species form.

Chapter 1 of the PECTAA Biology 11 book is Biodiversity and Classification. After biodiversity and its assessment (section 1.7), section 1.8 covers species and speciation. This note is for FSc Part 1 (1st year, Class 11) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.

What to know

Species. A species is generally defined as a group of individuals that can interbreed and produce fertile offspring under natural conditions. Members share common characteristics and genetic makeup that set them apart from other species.

The problem with appearance. Identifying species from physical traits can mislead because of cryptic species: organisms that look similar but are genetically distinct.

Mayr's emphasis. The German-American biologist Ernst Mayr made reproductive isolation the key criterion. On this view, species are groups of interbreeding natural populations, and members of different species do not usually mate or produce viable, fertile offspring.

Linking back to classification. Species is the base rank of the taxonomic hierarchy met earlier in the chapter, so a clear species test matters for every level above it. It also links to section 1.7: species diversity is counted species by species, so cryptic species can make an area look less diverse than it really is. Reproductive isolation is the common thread in all four modes below; the modes differ only in how the populations come to stop sharing genes.

Speciation. Speciation is the evolutionary process by which new species arise from a common ancestor. Genetic changes build up until populations become reproductively isolated. The book describes four mechanisms.

1. Allopatric speciation. A population is split geographically into isolated groups. Each group meets a different environment and evolves independently. In time the differences become large enough to stop interbreeding, even if the barrier disappears. Example: Darwin's finches on the Galápagos Islands, whose populations adapted to different environments.

2. Peripatric speciation. A small population becomes isolated at the edge of a larger one. The small group changes rapidly and diverges from the original population. Example: island species that evolve from a small founding population.

3. Parapatric speciation. Populations live next to each other but in different environments along a gradient. Gene flow between them is limited, and each adapts to its own conditions, which can lead to reproductive isolation. Example: the grass Anthoxanthum odoratum, which shows different adaptations to soil conditions across a gradient.

4. Sympatric speciation. New species arise within the same geographical area, with no physical barrier. It can happen through polyploidy (an organism having multiple sets of chromosomes) or niche differentiation (subpopulations using different resources). Some plants become polyploid and are reproductively isolated at once, forming a new species immediately.

Key points

• The book's species test is interbreeding to give fertile offspring in nature.
• Cryptic species show why appearance alone is not enough.
• All four modes end in reproductive isolation.
• Only sympatric speciation happens without any geographical separation.

Worked examples

Example 1 (identify the mode). A river changes course and splits a population of beetles into two. After many generations, the two groups cannot interbreed.

1. A geographical barrier separated the groups completely.
2. This is allopatric speciation.

Example 2 (identify the mode). A plant in a field doubles its chromosome sets and can no longer breed with neighbouring plants of its parent type.

1. No barrier: same field.
2. Polyploidy gives immediate isolation, so this is sympatric speciation.

Example 3 (identify the mode). A few lizards drift on debris to a nearby island and found a new population that soon differs from the mainland form.

1. A small founding group at the edge of the main range.
2. This is peripatric speciation.

Common mistakes in the exam

1. Defining species by appearance alone. Use interbreeding and fertile offspring.
2. Leaving out "fertile". Some crosses give offspring that cannot reproduce.
3. Mixing peripatric and parapatric. Peripatric is a small isolated edge group; parapatric is adjacent populations along a gradient.
4. Saying sympatric speciation needs a barrier. It occurs in the same area.
5. Forgetting the example that the question asks for.

Exam technique

• Define the term, describe the mechanism, then give the book's example.
• Use the comparison table above for "differentiate" questions.
• Underline scientific names or write them in italics.
• When asked to "explain" a mode, say what separates the populations, why gene flow stops, and what the end result is.
• Check your board's notice for the sections in your paper.

Practice questions

1. Define speciation. Answer: The process by which new species arise from a common ancestor through genetic changes that lead to reproductive isolation.
2. Who emphasised reproductive isolation as the key criterion for species? Answer: Ernst Mayr.
3. Name the mode of speciation shown by Darwin's finches. Answer: Allopatric speciation.
4. What is polyploidy? Answer: Having multiple sets of chromosomes.
5. Give the book's example of parapatric speciation. Answer: The grass Anthoxanthum odoratum across varying soil conditions.

Ready to practise this? Continue in 1st Year Biology.