Darwinism Modern Synthesis

7.9 Theories Explaining the Mechanism of Evolution

That evolution has occurred is no longer in scientific doubt. The deeper question is how? Multiple thinkers — Lamarck, Darwin, Wallace, de Vries — proposed mechanisms over the 19th and early 20th centuries. The synthesis of these ideas with Mendelian genetics, achieved in the 1930s–40s, produced the Modern Synthetic Theory of Evolution that we use today.

7.10 Lamarckism (1809)

The French naturalist Jean-Baptiste Lamarck proposed — even before Darwin — that evolution of life forms had occurred, but he attributed the driving mechanism to use and disuse of organs and the inheritance of acquired characters.

Lamarck's Giraffe Example

Lamarck explained the long neck of the giraffe as follows: ancestral giraffes, in their attempts to reach leaves on tall trees, repeatedly stretched their necks. The stretched necks (an acquired character) were passed on to offspring. Over generations, giraffes gradually acquired the very long necks we see today.

Nobody believes this conjecture any more. Modern genetics has shown that changes to body cells during an individual's life (somatic changes) are not transmitted to offspring — only changes in the gametes (germline) are inherited. A weightlifter's children are not born muscular; a blacksmith's son does not inherit a strong arm.

7.11 Darwinism — Theory of Natural Selection (1859)

Based on observations made during a five-year voyage on HMS Beagle (1831–1836), Charles Darwin published On the Origin of Species in 1859. He concluded:

• Existing organisms share similarities to varying degrees, both among themselves and with extinct life forms.
• New forms of life have arisen and old ones have gone extinct over the long history of earth.
• Within any population there is built-in heritable variation.
• Those individuals whose characteristics make them better suited to survive in their natural conditions outbreed the others. This is the principle of fitness — measured ultimately by reproductive success.
• The more-fit individuals leave more progeny and are therefore selected by nature — Darwin called this natural selection — the mechanism of evolution.

The naturalist Alfred Russel Wallace, working in the Malay Archipelago, arrived at the same conclusion independently and at the same time. Their joint paper was presented in 1858; Darwin published the full theory in his book in 1859.

Darwin's Logic — A Compact Summary

Darwin was influenced by the work of Thomas Malthus on populations — the recognition that populations could grow exponentially but reproduce in a world of finite resources.

7.12 de Vries — Mutation Theory (1901)

In the first decade of the twentieth century, the Dutch botanist Hugo de Vries, based on his work on Oenothera lamarckiana (evening primrose), proposed that evolution is driven not by small Darwinian variations but by mutations — sudden, large, heritable changes.

de Vries called his single-step large mutation saltation (Latin saltus — a leap). His original interpretation of the Oenothera results was later shown to involve unusual chromosomal behaviour rather than ordinary mutations, but he is correctly credited with placing mutation at the centre of evolutionary thinking.

7.13 The Modern Synthetic Theory of Evolution (1930s–40s)

In the early 20th century the rediscovery of Mendel's work on inheritance (1900) was integrated with Darwin's theory of natural selection and de Vries' mutation theory. The result — assembled by Theodosius Dobzhansky, Ernst Mayr, R. A. Fisher, J. B. S. Haldane and others — is called the Modern Synthetic Theory of Evolution (or neo-Darwinism).

7.14 Types of Natural Selection

Natural selection can act on a continuously varying trait in three different ways:

• Stabilising selection: Intermediate phenotypes are favoured; extremes are removed. Example: Human birth weight — very low and very high birth weights both reduce survival.
• Directional selection: One extreme is favoured over the other; the population mean shifts. Example: Industrial melanism in peppered moths — dark phenotype favoured after pollution.
• Disruptive selection: Both extremes are favoured over intermediates; the population splits into two distinct forms. Example: Beak size in finches during severe drought (large beaks for hard seeds; small beaks for small seeds; medium beaks find nothing).

7.15 Comparing the Main Theories of Evolution

Worked Examples