Darwinism

Disruptive selection splits the population's phenotypic profile into two distinct groups by selecting against the average/intermediate phenotype.

This is natural selection operating through differential reproductive success driven by pollinator preference, leading to a higher frequency of red-flowered offspring.

The selective pressure (tough seeds) favors birds with larger beaks. These individuals survive and reproduce more, increasing the average beak depth in the next generation.

Natural selection is the cause/mechanism, while adaptation (a phenotypic trait that improves fitness in an environment) is the evolutionary effect or outcome.

Stabilizing selection reduces phenotypic variation by selecting against both extremes (very light and very heavy birth weights), maintaining the intermediate phenotype.

Darwin emphasized that natural selection relies on the pre-existing, heritable variations that naturally occur among individuals within any wild population.

Natural selection only acts on traits that influence differential reproductive success. If a trait is completely neutral with respect to fitness, natural selection does not influence it directly.

The inheritance of acquired characters is a postulate of Lamarckism, which Darwinism does not rely on (although Darwin did occasionally speculate on soft inheritance, it is not a core tenet of natural selection).

Because both phenotypic extremes (very long and very short tails) are selected against, stabilizing selection acts to maintain intermediate tail lengths.

Natural selection requires a choice of variants to "select" from. If all individuals are genetically identical, any differential survival is due to chance, and no evolutionary adaptation can occur.

Individuals do not adapt by mutating their own genomes in response to environmental demands. Mutations are random; natural selection simply filters pre-existing genetic variations.

Darwin argued that complex structures evolve via the gradual accumulation of many slight, heritable modifications, with each step improving the organism's fitness compared to the previous state.

The drought alters the environment, making the brown phenotype more advantageous. Brown insects will survive predation better, reproduce more, and increase the frequency of the brown allele.

The rapid development of resistance in bacteria and pests is a documented, direct observation of natural selection occurring in contemporary times under strong selective pressures.

Selective pressure (e.g., predators, climate, disease) is the environmental force that drives natural selection by favoring certain phenotypes over others.

Neo-Darwinism combines Darwinian selection with Mendelian genetics, defining evolution as a change in allele frequencies within a population's gene pool over generations.

Darwin noted that organisms have a massive reproductive potential (overproduction), yet environmental resources remain stable and limited. This discrepancy inevitably leads to competition or a struggle for existence.

Disruptive (or diversifying) selection occurs when environmental conditions favor individuals at both extremes of a phenotypic range over those with intermediate phenotypes.

Natural selection is context-dependent; a trait that is highly beneficial in one environment (e.g., thick fur in the Arctic) may be detrimental in another (e.g., a hot desert).