Natural selection is the process by which individuals with traits better suited to their environment tend to survive and reproduce. This leads to a gradual increase in the frequency of advantageous traits in the population.

Charles Darwin is widely recognized for developing the theory of natural selection after his observations during the voyage of the HMS Beagle. His ideas revolutionized the biological sciences and formed the foundation of modern evolutionary theory.

An adaptation is a heritable trait that increases an organism's fitness in its environment. Such traits are favored by natural selection because they improve the chances of survival and successful reproduction.

Evolution refers to the change in the heritable characteristics of biological populations over successive generations. This process is driven by mechanisms such as natural selection, mutation, gene flow, and genetic drift.

Mutations are random changes in the DNA sequence that create genetic diversity within a population. This new variation is essential for evolution, as it provides the substrate upon which natural selection can act.

Genetic drift is a random process that can lead to significant changes in allele frequencies especially in small populations. It acts independently of the usefulness of traits and can result in evolutionary changes that do not necessarily favor adaptation.

Gene flow is the process by which alleles are transferred from one population to another. This movement of genes can introduce new genetic material to a population and tends to reduce genetic differences between distinct populations.

Allopatric speciation occurs when a population is divided by a physical barrier, leading to genetic isolation and divergent evolution. Over time, the separated groups accumulate different genetic changes, potentially resulting in the formation of new species.

Transitional fossils show intermediary forms between ancestral and modern organisms, providing concrete examples of evolutionary change. They help to bridge the gaps between different major groups, supporting the concept of gradual evolution over time.

Mutations introduce new genetic variations into a population, which are essential for evolutionary change. Without mutations, there would be no new traits for natural selection to favor or disfavor over generations.

Sexual selection is the process where traits that improve an individual's chances of securing a mate are favored. This can lead to the development of characteristics that may not necessarily improve survival but do enhance reproductive success.

Adaptive radiation occurs when a single ancestral species diversifies into several new species that occupy different ecological niches. This process is often triggered by the availability of new resources or environments.

Comparative embryology reveals that many species share similar stages in early development, even if they appear very different as adults. This similarity supports the idea of a common ancestry and shared developmental pathways among diverse organisms.

Homologous structures are similar in form or function across different species because they were inherited from a common ancestor. Their presence supports the evolutionary relationships among species despite differences in their current functions.

Convergent evolution is when unrelated species develop similar traits due to similar environmental pressures, whereas divergent evolution is the process where related species evolve differences, often because they occupy different ecological niches. This distinction highlights the ways in which evolution can produce both similarities and differences among organisms.

Molecular phylogenetics utilizes genetic data, such as DNA and protein sequences, to infer the evolutionary relationships among organisms. This approach allows scientists to construct detailed evolutionary trees that reveal patterns of common ancestry and divergence.

Epigenetic changes modify how genes are expressed without altering the underlying DNA sequence. Some epigenetic modifications can be passed to future generations, influencing traits and potentially contributing to evolutionary change.

Punctuated equilibrium is a theory that proposes species remain in stasis for long periods, with short, rapid bursts of change in between. This model contrasts with the idea of constant, gradual evolution and is supported by certain patterns seen in the fossil record.

Co-evolution refers to the process where two or more species exert selective pressures on each other, leading to reciprocal adaptive changes. Examples include the evolutionary arms race between predators and prey or the mutual adaptations between pollinators and flowering plants.

Although the fossil record offers valuable insights into the history of life, it is inherently incomplete due to factors like fossilization bias and geological processes. These gaps can make it difficult to trace every transitional stage in the evolution of organisms.