Mechanism of Evolution

Why Lamarck's Theory Was Rejected

Lamarck's use-and-disuse theory has an appealing simplicity, but it does not hold up: traits acquired during an individual's lifetime — such as a neck stretched by reaching for leaves — are not encoded in that individual's reproductive cells, so they cannot be passed on to its offspring. Modern genetics rejects this "inheritance of acquired characteristics." Darwin's theory of natural selection does not need individuals to acquire new traits during their own lifetime; it only needs heritable variation that already exists within a population, some of which happens to be advantageous.

From Malthus to Natural Selection

Darwin's own thinking was strongly shaped by Thomas Malthus's essay on populations, which pointed out that populations have the potential to grow exponentially, while the resources that support them — food, space — are always limited. Combining this insight with his own field observations, Darwin built his case for natural selection on a short chain of simple, verifiable facts:

• Natural populations have the capacity to increase in numbers exponentially if left unchecked.
• In practice, population sizes in nature tend to stay roughly stable over long periods.
• Individuals within a population show heritable variation in their traits.
• Because resources are limited, individuals must compete for them, and not all can survive and reproduce equally.

Putting these together, Darwin reasoned that individuals whose heritable variation let them use the available resources more efficiently would, on average, leave more surviving offspring than others. Generation after generation, this shifts the characteristics of the population as a whole — and, given enough time, produces forms different enough to count as new species. This is the essence of evolution by natural selection.

Mendel, de Vries, and the Question of Where Variation Comes From

Darwin's theory explained how favourable variations spread through a population, but it said little about where new variations come from in the first place. Gregor Mendel's work on inheritance, though contemporary with Darwin, showed that heritable traits are carried by discrete "factors" (what we now call genes) passed from parent to offspring — but Mendel's work went largely unnoticed for decades. …