Lamarckism: MDCAT Biology notes
Lamarckism MDCAT notes: Lamarck's use and disuse, inheritance of acquired characters, the giraffe example, Weismann's rejection, and contrast with Darwin.
Who was Lamarck?
Jean Baptiste de Lamarck (1744–1829) was a French naturalist. He was among the first to propose a full theory saying that species change over time. His theory was published in 1809, fifty years before Darwin's Origin of Species. Its mechanism was later rejected. He is still remembered because his theory of heredity was appealing, even though it was wrong, and because he was an early champion of the idea that evolution happens at all.
The two main ideas
1. Use and disuse of organs
Organs that an organism uses a lot become larger and stronger. Organs that are not used weaken and shrink. If an organ goes unused for several generations, it gradually disappears.
2. Inheritance of acquired characteristics
Changes an organism acquires during its lifetime through use or disuse are passed on to its offspring. According to Lamarck, this is the basis of evolution.
The giraffe example
Lamarck explained the long neck of the giraffe like this:
- Ancestral giraffes had short necks.
- They stretched their necks to reach leaves high on trees.
- The stretching slightly lengthened each giraffe's neck during its life, which is an acquired character.
- The longer neck was passed to the offspring, and the process repeated over many generations.
Lamarck's ideas were also used to explain the loss of tails in humans (through disuse) and human bipedalism.
What is an acquired character?
An acquired character is a trait developed by an individual through its own actions or environment during its lifespan, without any change to its genes (genome). Examples are strong muscles from hard labour, calluses on a manual worker's hands, and a suntan. Because the genome is not modified, acquired characters are non-heritable.
Why Lamarckism was rejected
- August Weismann cut off the tails of mice for many generations (about 20). Every new generation was still born with normal tails. This showed that mutilations, which are acquired changes, are not inherited.
- Weismann's germ plasm theory explains why. Only changes in the germ cells (gametes) pass to offspring. Changes in body (somatic) cells do not.
- Everyday evidence agrees. A mason's children do not inherit the strong muscles their father built by labour.
Lamarck compared with Darwin
| Lamarck | Darwin |
|---|---|
| Use and disuse; inheritance of acquired characters | Natural selection of inherited variations |
| The organism changes itself in response to need | Variation exists first, and the environment selects |
| Giraffes stretched their necks | Giraffes born with longer necks survived and bred more |
| Rejected (Weismann) | Accepted and developed into Neo-Darwinism |
Overproduction, struggle for existence, survival of the fittest and natural selection are Darwin's ideas, not Lamarck's.
Common MDCAT traps
- "Use and disuse" and "inheritance of acquired characters" point to Lamarck. "Survival of the fittest" points to Darwin.
- Weismann, not Darwin or Haeckel, rejected Lamarckism experimentally.
- The mouse tail-cutting experiment rejects Lamarckism.
- Acquired characters are non-heritable because the genome is unchanged.
Quick revision
- Lamarck proposed the inheritance of acquired characteristics.
- Unused organs disappear over generations, according to Lamarck.
- The giraffe's neck grew by stretching, according to Lamarck.
- Weismann cut mouse tails to disprove Lamarck.
- Only changes in germ cells are inherited.