Darwinian evolution occurs when natural selection changes the heritable characteristics of a population over generations. Lamarckism incorrectly proposes that organisms inherit characteristics acquired by their parents during their parents’ lifetimes.
This distinction is part of A4.1 Evolution and speciation for both SL and HL and links directly to natural selection.
Darwinian evolution begins with pre-existing heritable variation among individuals. Mutation produces new alleles, while sexual reproduction produces new combinations of alleles. These variations arise before a selection pressure acts and are not produced because organisms need them.
Individuals with traits that increase survival or reproductive success in a particular environment tend to produce more offspring. Their alleles are therefore passed on more frequently, so the population’s heritable characteristics change over generations. The individual does not evolve; the population does.
| Darwinian evolution | Lamarckism |
|---|---|
| Variation exists before selection occurs. | The environment supposedly causes an individual to develop a needed characteristic. |
| Natural selection favours individuals with advantageous heritable traits. | Use strengthens a structure, while disuse supposedly weakens it. |
| Alleles associated with higher reproductive success become more common. | Characteristics acquired during life are supposedly inherited. |
| Evolution occurs in populations over generations. | Individuals supposedly change during their lifetime and pass on that change. |
For example, ancestral giraffes varied genetically in neck length. Longer-necked individuals may have obtained more food and produced more offspring, increasing the frequency of alleles associated with longer necks. Lamarck’s explanation would instead claim that giraffes stretched their necks and transmitted the resulting increase in length to offspring.
A common misconception is that organisms evolve because they “need to adapt.” Selection pressures do not create purposeful mutations; they determine which existing heritable variants leave more offspring.
Exam technique: For a command term such as distinguish, state both sides explicitly. Refer to pre-existing heritable variation, differential reproductive success and population-level change, then contrast these with inheritance of acquired characteristics.
