From Inheritance to Natural Selection

The Revolution in How We See Animal Populations

A journey from Lamarck's inheritance of acquired characteristics to Darwin's theory of natural selection

A World of Change

Imagine looking at a giraffe's long neck and asking, "How did it get that way?" For centuries, the answer seemed simple to many: through effort and practice. This intuitive idea was at the heart of early evolutionary thought.

But in the span of just a few decades, a scientific revolution—spearheaded by the competing and contrasting ideas of Jean-Baptiste Lamarck and Charles Darwin—utterly transformed our understanding of animal life. This shift wasn't just about new facts; it was a fundamental change in perspective, moving from a story of individual striving to one of population-wide struggle and survival.

This is the story of how we learned to read the history of life not in individual creatures, but in the shifting fortunes of entire populations.

The Two Visions: Lamarck vs. Darwin

Before Darwin, the prevailing view of life was largely static. Species were created as they were and remained unchanged.

Lamarck's Theory

Jean-Baptiste Lamarck (1744-1829) was one of the first to propose a coherent theory of evolution, or "transmutation" as it was then called.

Inheritance of Acquired Characteristics:

Lamarck proposed two key principles:

  1. Use and Disuse: An organism can change its body parts during its lifetime by using them more or less.
  2. Inheritance of Acquired Characteristics: These individual, lifetime changes are then passed on to the organism's offspring.

In this view, change is driven by need and individual effort, and inheritance is direct.

Darwin's Theory

Charles Darwin (1809-1882), along with Alfred Russel Wallace, proposed a radically different mechanism after his voyage on the HMS Beagle.

Natural Selection:

Darwin's theory rests on a few simple, observable facts:

  1. Variation: Individuals in a population show random variations.
  2. Overproduction: Populations produce more offspring than can survive.
  3. Struggle for Existence: Competition for limited resources.
  4. Differential Survival: Advantageous variations improve survival and reproduction.
  5. Heredity: Advantageous variations are heritable.

In this view, change is driven by environmental pressure and differential survival across a whole population.

Evolution of Evolutionary Thought

1744-1829

Jean-Baptiste Lamarck

Proposed the theory of inheritance of acquired characteristics, suggesting organisms change through use and disuse of organs.

1831-1836

Darwin's Voyage on HMS Beagle

Darwin's observations during this journey, especially in the Galápagos Islands, laid the foundation for his theory of natural selection.

1858

Joint Presentation with Wallace

Darwin and Alfred Russel Wallace jointly presented their papers on natural selection to the Linnean Society of London.

1859

On the Origin of Species

Darwin published his groundbreaking work detailing the theory of evolution by natural selection.

An In-depth Look: Darwin's Moths

While Darwin used many observations to build his case, one of the most powerful and visually compelling validations of natural selection occurred after his death, in the industrial landscapes of 19th-century England.

The Experiment: Nature's Test on Peppered Moths

The peppered moth (Biston betularia) exists in two main forms: a light, speckled variety (typica) and a dark, almost black variety (carbonaria).

Prior to the Industrial Revolution, the light moths were overwhelmingly common, as they were perfectly camouflaged against lichen-covered tree trunks, hiding them from bird predators. The dark moths were rare, as they were easily seen and eaten.

The widespread pollution of the Industrial Revolution acted as a massive, unplanned experiment.

Light peppered moth on light background

Light moth (typica) camouflaged on lichen

Dark peppered moth on dark background

Dark moth (carbonaria) on soot-covered bark

Methodology: A Step-by-Step Observation

Environmental Change

Soot from coal fires killed the light-colored lichens and darkened the tree trunks.

Hypothesis

The change in environment would shift the selective pressure on the moths.

Field Experiments

Mark-release-recapture experiments to track survival rates of different morphs.

Results and Analysis: A Dramatic Shift

The results were stark and clear. In polluted areas, the dark moth population skyrocketed, while in clean areas, the light moths remained dominant. Kettlewell's experiments showed that birds were the selective agent, preying on the more conspicuous moths in each environment.

Moth Population Shift in Industrial Area (Manchester, UK)
Kettlewell's Recapture Experiment Results (1955)
Comparison: Polluted vs. Unpolluted Forests

The Scientist's Toolkit

Tools for Tracking Animal Populations

Research Tool / Concept Function & Explanation
Field Observation & Note-taking The foundational tool. Darwin's detailed notes on the variation in finch beaks across the Galápagos Islands were crucial for his later theories.
Population Census & Sampling Estimating the size and structure of a population (e.g., age, sex ratio) without having to count every single individual.
Mark-Recapture Study A key method for estimating population size and tracking survival. Animals are captured, marked, released, and then later a second sample is captured.
Selective Agent Identification Determining the specific environmental factor causing differential survival (e.g., bird predators for the moths, drought for Darwin's finches).
Statistical Analysis Moving from anecdotal observation to robust science. Used to determine if observed changes in population traits are statistically significant.
Fossil Record & Biostratigraphy Provides the long-term, historical evidence for population changes and speciation events over geological time.

The Legacy of a New Perspective

The journey from Lamarck to Darwin was a paradigm shift of monumental importance.

Lamarck saw a world where the individual will of an organism could shape its descendants. Darwin revealed a world where blind, impersonal forces of nature—variation, selection, and time—sculpt entire populations.

The story of the peppered moths is more than just a neat example; it is a microcosm of this grander process. It showed that evolution wasn't a slow, unobservable force from the deep past, but a dynamic and ongoing process.

By shifting the focus from the life story of a single giraffe to the statistical fate of a million moths, Darwin gave us the key to understanding the breathtaking diversity and intricate adaptation of all life on Earth. It is a perspective that, to this day, forms the bedrock of modern biology.

Genetics

Modern genetics has provided the mechanism for inheritance that Darwin hypothesized.

Ecology

Evolutionary theory forms the foundation of modern ecological understanding.

Medicine

Understanding evolution is crucial for combating antibiotic resistance and disease.