Darwin’s Other Theories and Their Criticism

Darwin is best known for natural selection, but his thinking about evolution went well beyond a single idea. He proposed several additional theories that together formed a broader framework for understanding how species change. Some of these ideas proved remarkably durable, while others collapsed under later scientific scrutiny. Understanding both the strengths and the weaknesses of Darwin’s work gives a fuller picture of how evolutionary theory grew and matured.

Artificial Selection: Shaping Life by Human Choice

Long before nature’s selective pressures became the centre of scientific debate, humans were already reshaping the living world around them. Darwin pointed out that the most common method of producing a new race of individuals is through artificial selection (selection carried out under human control). Farmers and breeders have been doing this for thousands of years, choosing specific animals or plants with desirable traits and allowing only those to reproduce, for example, breeding useful varieties of animals suited to particular purposes.

This process works at two levels:

• Conscious selection — A breeder deliberately picks individuals with a specific trait, such as cows that give more milk or sheep with thicker wool, and breeds them together to strengthen that trait in the next generation
• Unconscious selection — Over time, people naturally tend to keep the best-performing animals and discard the rest, even without a deliberate breeding plan. The cumulative effect across many generations still produces noticeable change in the population

Darwin saw artificial selection as a powerful analogy for what nature does on a much larger scale. If humans can reshape species in just a few hundred years of selective breeding, consider what natural forces could achieve over millions of years.

Sexual Selection: The Competition for Mates

Darwin observed that in many species, males do not simply wait for a chance to reproduce. They compete actively. His theory of sexual selection proposes that there is a constant contest among males for the possession of females.

This competition takes many visible forms. A stag grows elaborate, beautiful horns not primarily for defence against predators but to impress females and intimidate rival males. Male alligators engage in fierce physical combat, wrestling and snapping at each other for access to females. In birds, vivid plumage, complex songs, and dramatic courtship dances all serve the same underlying purpose.

The evolutionary consequence is significant: inferior males, those that lose these contests or fail to attract a mate, are effectively shut out of reproduction. They do not get to contribute their genes to the next generation. Over time, the traits that help males win mates, whether strength, display, or fighting ability, become more common in the population. Sexual selection works alongside natural selection, but its driving force is reproductive access rather than survival against the environment.

Common Descent: One Family Tree for All Life

One of Darwin’s most far-reaching ideas was deceptively simple. His theory of common descent proposes that related groups of organisms share common ancestors. Species that look similar, such as humans and apes, do so not by coincidence but because they descended from the same ancestral population at some point in the distant past.

This idea carries profound implications. It means that all life on Earth is connected through a vast, branching family tree. The more closely two species resemble each other in anatomy, genetics, and behaviour, the more recently they split from their shared ancestor.

Pangenesis: Darwin’s Attempt to Explain Inheritance

Darwin understood that natural selection could only work if favourable traits were somehow passed from parents to offspring. But in his era, the laws of inheritance had not yet been discovered. Gregor Mendel’s groundbreaking work on pea plants, though published in 1866, went largely unnoticed until 1900.

Facing this gap, Darwin proposed the hypothesis of pangenesis. According to this idea, every somatic cell (body cell) in an organism produces tiny particles called pangenes. These miniature particles carry information about the cell that produced them and travel through the body to eventually be collected in the germ cells (reproductive cells). When an organism reproduces, these pangenes are passed to the offspring, theoretically carrying a blueprint of the parent’s entire body.

The idea was creative, but it turned out to be wrong. Francis Galton tested pangenesis in 1875 by performing blood transfusion experiments between rabbits of different colours. His reasoning was straightforward: if pangenes circulated through the blood, then transfused rabbits should produce offspring with mixed traits. They did not. Galton’s experiments provided several strong proofs against the hypothesis.

The final blow came with the rediscovery of Mendelian genetics in 1900. Mendel’s work showed that inheritance operates through discrete units (what we now call genes) located on chromosomes, not through particles shed by every cell in the body. With this discovery, pangenesis was completely rejected.

Where Darwin’s Theory Falls Short: Key Criticisms

Despite its revolutionary impact, Darwin’s theory had real gaps that later scientists identified and challenged. These criticisms do not erase Darwin’s achievement, but they reveal the limits of what could be understood without modern genetics.

• Survival, not arrival — Darwin explained beautifully how organisms with advantageous traits survive and reproduce. But he could not explain how entirely new traits or new organs appear in the first place. His theory addressed the “survival of the fittest” but not the “arrival of the fittest.” What causes a completely new feature to show up when it has never existed before? Darwin had no answer for this.

• The puzzle of overspecialisation — Natural selection predicts that traits helpful for survival should be favoured. Yet some species developed features so extreme that they actually caused their own extinction. The extinct Irish deer, for instance, evolved antlers so enormous that they became a crippling handicap rather than a benefit. Darwin’s theory could not explain why natural selection would permit the development of such self-destructive overspecialisation.

• Undervaluing mutations — Darwin noticed sudden, dramatic variations in organisms, which he called “sports.” He did not consider them particularly important. Modern genetics, however, has shown that these “sports” are actually mutations (random changes in DNA), and they are the primary means by which entirely new genes arise. By dismissing their significance, Darwin missed a fundamental source of evolutionary novelty.

• Borrowing from Lamarck — Even though Darwin was a prominent critic of Lamarck’s theory, he was not entirely free of its influence. He accepted that the use and disuse of organs could produce certain variations. This idea fits more comfortably in Lamarck’s framework than in Darwin’s own, creating a notable internal inconsistency in his theoretical system.

• Overemphasis on individual differences — Darwin placed great importance on small individual differences as the primary raw material for natural selection. Modern genetics has clarified that these individual differences arise mainly from recombination (the reshuffling of existing genes during sexual reproduction), which plays only a secondary role in producing truly new variation. The primary source of novelty is mutation, not recombination.

• Pangenesis rejected — Darwin’s attempt to explain inheritance through the pangenesis hypothesis was experimentally disproven by Galton in 1875 and completely negated by the rediscovery of Mendelism in 1900. This left a significant hole in Darwin’s framework until modern genetics filled it.

• No explanation for the origin of variation — Darwin described what happens to variation once it exists: natural selection acts on it, favouring some traits and eliminating others. But he could not explain where variation comes from in the first place. The genetic basis of variation, including the roles of mutation, recombination, and chromosomal changes, was unknown in his time.

• Aquatic to terrestrial transition unexplained — Natural selection, as Darwin described it, does not adequately explain major evolutionary transitions such as the evolution of terrestrial animals from aquatic ancestors. Moving from water to land requires a suite of coordinated changes in breathing, locomotion, reproduction, and water balance. Accounting for all of these through gradual selection acting on random variation remains a challenge.

Putting It All Together: How Natural Selection Works

Despite these criticisms, the core logic of Darwin’s natural selection remains a cornerstone of evolutionary biology. The process follows a clear chain of cause and effect that can be traced step by step.

It begins with overproduction: organisms produce far more offspring than the environment can sustain. This creates a struggle for existence, since resources like food, space, and mates are limited. Within any population, individuals show variation, small and large differences in their traits.

Some of these variations are favourable, helping the organism survive and reproduce in its particular environment. Others are unfavourable, putting the organism at a disadvantage. Natural selection acts as the filter. On one path, individuals with unfavourable traits are gradually eliminated from the population. On the other path, individuals with favourable traits survive.

The survivors pass their beneficial traits to their offspring. This is the inheritance of favourable characters. Over many generations, these favourable variations build up in the population through gradual accumulation. Given enough time and enough selective pressure, the accumulated changes become so significant that the population can no longer be considered the same species it once was. A new species has emerged, which is the origin of new species.

This chain captures the elegant simplicity of Darwin’s greatest insight: nature does not plan or design. It simply selects, generation after generation, and the result is the remarkable diversity of life we see all around us.