Question
Explain Mendel’s monohybrid cross with pea plants. How does natural selection lead to speciation? Give an example.
(CBSE Class 10 — Heredity and Evolution)
Speciation Process
flowchart TD
A["Original Population"] --> B["Geographical Isolation"]
B --> C["Population A"]
B --> D["Population B"]
C --> E["Different mutations accumulate"]
D --> F["Different natural selection pressures"]
E --> G["Genetic drift over generations"]
F --> G
G --> H{"Can they still interbreed?"}
H -->|Yes| I["Still same species (subspecies)"]
H -->|No| J["New species formed = Speciation"]Solution — Step by Step
Mendel crossed tall pea plants (TT) with dwarf pea plants (tt).
F1 generation: All plants were tall (Tt). The tall trait is dominant — it masks the dwarf trait. The dwarf trait is recessive.
F2 generation: When F1 plants (Tt) were self-pollinated:
| T | t | |
|---|---|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (dwarf) |
Phenotypic ratio: 3 Tall : 1 Dwarf
Genotypic ratio: 1 TT : 2 Tt : 1 tt
The dwarf trait reappeared in F2 — it was hidden, not destroyed, in F1.
Charles Darwin proposed that organisms produce more offspring than can survive. Those with favourable variations (better suited to the environment) survive and reproduce more — passing these traits to the next generation. Over time, the population shifts toward the better-adapted form.
Example: In a population of beetles, green beetles on green bushes are camouflaged from predators. Red beetles are eaten more often. Over generations, the green colour becomes dominant — not because beetles “chose” to be green, but because green ones survived to reproduce.
Speciation occurs when a single species splits into two or more new species. The process:
- A population gets geographically separated (mountain, river, or any barrier)
- Each subpopulation faces different environmental conditions
- Different mutations and natural selection pressures cause them to change differently
- Over many generations, the genetic differences become so large that they can no longer interbreed
- Two new species have formed
Example: Darwin’s finches on the Galapagos Islands. A single ancestor species spread to different islands. Each island had different food sources, so natural selection favoured different beak shapes — large beaks for cracking seeds, thin beaks for catching insects. Over time, they became distinct species.
Why This Works
Mendel showed that traits are inherited as discrete units (we now call them genes), not blended. This solved a major problem for Darwin — blending inheritance would dilute any new variation within a few generations. With particulate (Mendelian) inheritance, variations are preserved and can accumulate over time through natural selection.
Speciation requires both genetic variation (provided by mutation) and selection pressure (provided by the environment). Geographic isolation gives separated populations enough time to diverge genetically.
Alternative Method — Analogies
We can also study evolution through:
- Homologous organs — Same structure, different function (human arm, whale flipper, bat wing) — suggest common ancestor
- Analogous organs — Different structure, same function (bird wing, insect wing) — suggest convergent evolution
- Fossils — Preserved remains show progression of life forms over time
For CBSE Class 10, the Punnett square for monohybrid cross is a guaranteed question (3-5 marks). Always draw the cross neatly, label the parents, gametes, F1, and F2 generations, and state both phenotypic and genotypic ratios. Full marks require all these elements.
Common Mistake
Students write “acquired characters are inherited” — this is Lamarck’s (rejected) theory, not Darwin’s. A blacksmith’s strong arms do not make his children stronger. Only genetic changes (mutations in DNA) are inherited. Natural selection acts on genetic variations, not on changes caused by an organism’s lifestyle.