Evolution

Evolution is the gradual change in the heritable characteristics of biological populations over successive generations. It is the process by which life on Earth has diversified from common ancestors.

Variation refers to the differences that exist between individuals within a species. These differences arise primarily from mutation (changes in DNA) and sexual reproduction (recombination of genes). Variation is crucial because natural selection can only act on existing differences.

Natural selection is the primary mechanism by which evolution occurs. It is based on the following principles: 1. Overproduction: Organisms produce more offspring than can survive. 2. Variation: Individuals within a population show variation in their characteristics. 3. Competition: There is a struggle for existence due to limited resources. 4. Survival of the Fittest: Individuals with characteristics best suited to their environment are more likely to survive and reproduce. 'Fitness' in this context refers to reproductive success. 5. Inheritance: Favourable characteristics are passed on to the next generation, leading to a gradual change in the population over time.

Charles Darwin, along with Alfred Russel Wallace, proposed the theory of evolution by natural selection. Darwin's seminal work, 'On the Origin of Species by Means of Natural Selection,' published in 1859, detailed how species change over time through the accumulation of small, heritable variations that enhance survival and reproduction in a given environment. His theory provided a scientific explanation for the diversity of life and the adaptations of organisms.

Multiple lines of evidence support the theory of evolution: * Palaeontology (Fossils): Fossils are the preserved remains or traces of organisms from the past. They show a progression of life forms over geological time, with simpler organisms appearing in older rock layers and more complex ones in newer layers. Transitional fossils (e.g., Archaeopteryx) show intermediate characteristics between different groups. * Comparative Anatomy: * Homologous Structures: Structures that have a similar basic anatomical plan but may have different functions (e.g., the pentadactyl limb in vertebrates - human arm, bat wing, whale flipper). They indicate common ancestry. * Analogous Structures: Structures that have similar functions but different anatomical origins (e.g., bird wing and insect wing). They arise from convergent evolution, where unrelated species adapt to similar environments. * Vestigial Structures: Reduced or non-functional structures that are remnants of functional structures in ancestral organisms (e.g., human appendix, wisdom teeth, pelvic bone in whales). * Embryology: The study of embryonic development shows striking similarities in the early stages of development among different vertebrate species, suggesting a common ancestor. * Genetics and Biochemistry: All living organisms share a common genetic code (DNA/RNA), and many fundamental biochemical processes (e.g., respiration) are conserved. The degree of similarity in DNA sequences and protein structures (e.g., cytochrome c) between species reflects their evolutionary relatedness. * Geographical Distribution: The distribution of species across the globe can be explained by their evolutionary history and the movement of continents (plate tectonics). Closely related species are often found in geographically close areas.

Speciation is the evolutionary process by which new biological species arise from existing ones. It typically involves: 1. Isolation: A barrier (geographical, ecological, behavioural) prevents gene flow between populations of the same species. 2. Natural Selection: Different selective pressures act on the isolated populations, leading to the accumulation of different adaptations. 3. Reproductive Isolation: Over time, the genetic differences become so significant that individuals from the two populations can no longer interbreed successfully, even if the barrier is removed. They are now considered separate species.