11.2 Adaptations of Organisms and the Theory of Evolution

Classifying Organisms by Evolutionary Relationships

Organisms can be grouped by whether their cells contain a membrane-bound nucleus. Prokaryotes (bacteria and archaea) lack a nucleus and membrane-bound organelles; eukaryotes (protists, fungi, plants, animals) have both. This distinction is foundational because it separates life into the two most fundamental domains. Detailed taxonomy, the three-domain system, binomial nomenclature, and dichotomous keys are covered in Section 9.1; here the focus is on evolutionary relationships — how shared derived traits reveal common ancestry.

The major eukaryotic groups reflect evolutionary innovations: protists are mostly unicellular eukaryotes; fungi are heterotrophs that absorb nutrients; plants are autotrophs that photosynthesize; animals are multicellular heterotrophs that ingest food. Within animals, the presence of a notochord, dorsal nerve cord, and pharyngeal slits at some life stage distinguishes chordates; within chordates, vertebrates evolved jaws, limbs, amniotic eggs, and endothermy in sequence. These nested groups are evidence of descent from common ancestors.

Adaptations That Enhance Survival and Reproductive Success

An adaptation is a trait that increases an organism's fitness — its survival and reproductive success — in a particular environment. Adaptations fall into three categories:

• Structural — physical features such as a cactus's spines (modified leaves that reduce water loss and deter herbivores), a hawk's talons, the beak shape of a Galápagos finch, or the streamlined body of a dolphin.
• Behavioral — inherited or learned actions such as migration, hibernation, courtship displays, pack hunting, or burrowing.
• Physiological — internal processes such as antifreeze proteins in arctic fish blood, venom production in snakes, or the camel's ability to concentrate urine and conserve water.

Classic examples include camouflage (a peppered moth resting on soot-darkened bark is eaten less often) and mimicry (a harmless king snake resembles a venomous coral snake, gaining protection without producing toxin). Both illustrate that the benefit is contextual — camouflage on dark bark becomes a liability on clean bark, which is exactly what the industrial-pollution data from England showed.

How Populations Change Through Time

Microevolution is a change in the frequency of alleles (gene variants) in a population over generations. The unit of evolution is the population, not the individual — an individual's genotype is fixed at birth, but allele frequencies in a population shift as some individuals reproduce more than others. Four mechanisms drive this change:

1. Mutation — the original source of new alleles; a random change in DNA that may be neutral, harmful, or beneficial.
2. Gene flow — movement of alleles between populations through migration or gamete dispersal, which can introduce new variation or homogenize differences.
3. Genetic drift — random change in allele frequencies due to chance, strongest in small populations; a founder effect or bottleneck can lose alleles regardless of their fitness value.
4. Natural selection — differential survival and reproduction of individuals with heritable traits better suited to the current environment; the only mechanism that consistently produces adaptation.

The Four Conditions for Natural Selection

Darwin's mechanism requires:

1. Variation — individuals in a population differ in traits.
2. Heritability — those differences are passed from parents to offspring.
3. Overproduction — more offspring are produced than the environment can support.
4. Differential reproductive success — individuals with traits better suited to the environment leave more surviving offspring.

Sexual selection is a special case in which the selective pressure is mating success rather than survival; it explains traits like a peacock's tail that reduce survival but increase reproductive success. Fitness in this framework means reproductive contribution to the next generation, not strength or health.

Evidence for Evolution

A Common Misconception to Counter in the Classroom

Many students — and many adults — picture evolution as a ladder leading upward, with humans at the top. This is wrong. Evolution has no goal and no direction other than the one set by the current environment. A trait that is adaptive today can become a liability if the environment changes; a 'simple' organism like a bacterium is as fully evolved as a human, because it has been evolving for the same length of time and is superbly suited to its environment. The tree of life is a branching bush, not a ladder.