Darwinism

Malthus's work on human populations made Darwin realize that in nature, overproduction of offspring coupled with limited food and space leads to a struggle for survival.

Evolutionary fitness is defined by reproductive success; an organism must not only survive but also successfully pass its genetic material to fertile offspring.

Under Darwinism, variation exists prior to environmental change. Penicillin acts as a selective agent, removing susceptible cells and allowing pre-existing resistant cells to populate.

The differing beak morphologies are adaptations developed over generations as ancestral finches colonized islands with distinct food resources (seeds, insects, nectar).

Natural selection can only change the genetic makeup of a population over generations if the phenotypic variations being selected have an underlying heritable genetic basis.

Natural selection is a non-random, directional process that adapts populations to their environments, whereas genetic drift is random and non-adaptive.

Stabilizing selection favors intermediate variants by acting against extreme phenotypes, reducing variation and maintaining the status quo of a phenotypic trait.

Lyell's principle of slow, ongoing geological processes made Darwin realize that the Earth is ancient, providing the vast timescales required for gradual biological evolution via natural selection.

When humans actively select which individual animals or plants reproduce based on desired traits, the process is known as artificial selection (selective breeding).

"Descent with modification" is Darwin's definition of evolution, indicating that species are related by common ancestry but have changed over time in response to natural selection.

This is the classic case of industrial melanism (peppered moth). The change in tree bark color causes directional selection, shifting the population curve toward the dark phenotype.

While Darwin observed heritable variation, he lacked knowledge of Mendelian genetics, chromosomes, and DNA, and could not explain how traits were passed down or how new genetic variations arose.