Evolution: The Mechanisms of Biological Change and Biodiversity

Evolution is the process by which the heritable characteristics of biological populations change over successive generations. This fundamental biological phenomenon occurs when evolutionary processes—such as natural selection and genetic drift—act upon genetic variation. Over time, these processes cause certain traits to become more or less common, ultimately giving rise to the vast biodiversity observed across every level of biological organization.

The scientific foundation of evolution was established in the mid-19th century. Two British naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived the theory of evolution by natural selection to explain how organisms adapt to their physical and biological environments. Darwin later detailed these findings in his seminal work, On the Origin of Species.

In 1842, Charles Darwin penned his first sketch of On the Origin of Species.[303]

Key Facts

Natural Selection: Occurs when individuals with favorable traits are more likely to survive and reproduce.
Heritability: For evolution to occur, traits must be capable of being passed from parents to offspring.
Genetic Variation: Differences in morphology, physiology, and behavior among individuals provide the raw material for selection.
Modern Synthesis: The 20th-century integration of Darwinian evolution with Mendelian inheritance and population genetics.
DNA: The molecule that serves as the basis for heredity, passing information across generations.

The Mechanics of Natural Selection

Male moor frogs become blue during the height of mating season. Blue reflectance may be a form of intersexual communication. It is hypothesised that males with brighter blue coloration may signal greater sexual and genetic fitness.[129]

Evolution by natural selection is supported by four observable facts about living organisms. First, populations often produce more offspring than the environment can support, leading to competition. Second, individuals within a population exhibit variation in their traits. Third, these variations result in differential fitness, meaning some traits provide a better chance of survival and reproduction than others. Finally, these advantageous traits are heritable.

Consequently, in each successive generation, individuals with characteristics best suited to their environment are more likely to leave offspring, gradually shifting the characteristics of the entire population.

Mutation followed by natural selection results in a population with darker colouration.

The Modern Evolutionary Theory

The hominoids are descendants of a common ancestor.

In the early 20th century, the scientific community refined these ideas into the "modern synthesis." This framework combined natural selection with population genetics and Mendelian inheritance, identifying DNA (deoxyribonucleic acid) as the primary carrier of genetic information.

DNA structure. Bases are in the centre, surrounded by phosphate–sugar chains in a double helix.

Several key processes drive changes in a population's DNA:

Mutation: Random changes in the DNA sequence that create new genetic variants.
Genetic Drift: Random fluctuations in allele frequencies, which have a more pronounced effect in smaller populations.
Gene Flow: The transfer of genetic material between separate populations.
Natural Selection: The non-random process where beneficial traits increase in frequency.

Simulation of genetic drift of 20 unlinked alleles in populations of 10 (top) and 100 (bottom). Drift to fixation is more rapid in the smaller population.

Types of Genetic Selection

Selection can act on a population in different ways depending on which phenotypes (observable traits) are favored:

Directional Selection: Favors a single extreme phenotype.
Stabilizing Selection: Favors intermediate traits over extreme ones.
Disruptive Selection: Favors both extreme phenotypes over the intermediate.

These charts depict the different types of genetic selection. On each graph, the x-axis variable is the type of phenotypic trait and the y-axis variable is the number of organisms. Group A is the original population and Group B is the population after selection. · Graph 1 shows directional selection, in which a single extreme phenotype is favoured. · Graph 2 depicts stabilising selection, where the intermediate phenotype is favoured over the extreme traits. · Graph 3 shows disruptive selection, in which the extreme phenotypes are favoured over the intermediate.

Outcomes of Evolutionary Processes

Evolutionary tree showing the divergence of modern species from their common ancestor in the centre.[285] The three domains are coloured, with bacteria blue, archaea green and eukaryotes red.

Adaptation and Coevolution

Adaptation occurs when a population becomes better suited to its habitat. For example, the common garter snake evolved resistance to tetrodotoxin, a defensive substance used by its amphibian prey. In some cases, species undergo coevolution, where two or more species reciprocally affect each other's evolution.

The common garter snake has evolved resistance to the defensive substance tetrodotoxin in its amphibian prey.

Speciation and Extinction

Speciation is the process by which new species arise, often through geographic isolation. A classic example is the diversification of finches on the Galápagos Islands. Conversely, extinction occurs when a species ceases to exist, such as the non-avian dinosaurs during the Cretaceous–Paleogene extinction event.

Geographical isolation of finches on the Galápagos Islands produced over a dozen new species.

Tyrannosaurus rex. Non-avian dinosaurs died out in the Cretaceous–Paleogene extinction event at the end of the Cretaceous period.

Common Descent and Homology

The theory of common descent posits that all living organisms share a common ancestor. This is evidenced by homologous structures—organs or skeletal elements that share a basic structure but have been adapted for different functions. For instance, the limb bones of tetrapods or the flipper bones of a baleen whale (adapted from land-dwelling leg bones) demonstrate this shared ancestry.

Homologous bones in the limbs of tetrapods. The bones of these animals have the same basic structure, but have been adapted for specific uses.

A baleen whale skeleton. Letters a and b label flipper bones, which were adapted from front leg bones, while c indicates vestigial leg bones, both suggesting an adaptation from land to sea.[166]

Summary of Evolutionary Concepts

Core Components of Evolutionary Biology
Concept	Definition	Primary Driver
Natural Selection	Survival of the fittest individuals	Environmental Pressure
Genetic Drift	Random change in gene frequencies	Chance/Population Size
Mutation	Permanent change in DNA sequence	Chemical/Physical Errors
Speciation	Formation of new distinct species	Isolation/Divergence

Frequently Asked Questions

Hesperornis regalis, a species of ancient flightless bird with teeth, as drawn by Othniel Marsh, and published in his book, Odontornithes: A Monograph on the Extinct Toothed Birds of North America.

As evolution became widely accepted in the 1870s, caricatures of Charles Darwin with an ape or monkey body symbolised evolution.[347]

What is the difference between natural selection and genetic drift?

Natural selection is a non-random process where traits that improve fitness become more common. Genetic drift is a random process where allele frequencies change by chance, which is particularly impactful in small populations.

How does the "modern synthesis" differ from Darwin's original theory?

While Darwin proposed natural selection, he did not know the mechanism of heredity. The modern synthesis integrated his theories with Mendelian genetics and the discovery of DNA to explain how traits are actually passed and modified.

What are homologous structures?

Homologous structures are physical features shared by different species because they were inherited from a common ancestor, even if those features now serve different purposes (e.g., a whale's flipper and a human's arm).

What causes speciation?

Speciation often occurs through geographic isolation, where populations are separated by physical barriers, preventing gene flow and allowing them to evolve independently until they can no longer interbreed.

What is differential fitness?

Differential fitness refers to the fact that different traits confer different rates of survival and reproductive success, meaning some individuals are more likely to pass their genes to the next generation than others.