Genes, inheritance and selection Study Guide

Evolution is the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the Earth. The idea that species change over time, or evolve, is supported by a wide range of scientific evidence. This evidence comes from several key areas: palaeontology, which is the study of fossils; comparative anatomy, where similarities in body structures of different species are examined; and bioinformatics, which involves comparing the DNA and protein sequences of different organisms. Biogeography, the study of the distribution of species and ecosystems in geographic space and through geological time, also provides insights. Together, these lines of evidence paint a consistent picture of life on Earth changing gradually over millions of years, with new species arising from existing ones through natural selection. For example, the fossil record shows a progression of life forms, from simpler organisms in older rock layers to more complex ones in newer layers, with transitional forms providing links. Similar bone structures in the limbs of mammals, despite their different functions, suggest a common ancestor. Comparing DNA sequences can reveal how closely related different species are. This collective evidence makes the theory of evolution one of the most robust and widely accepted scientific theories.

All variations in living organisms, from slight differences in hair colour to significant differences in disease susceptibility, originate from changes in the genetic material called mutations. A mutation is essentially a change in the DNA sequence. These changes can occur spontaneously during DNA replication or can be induced by environmental factors like radiation or certain chemicals. While mutations are the ultimate source of all new genetic material, their effects on an organism's observable characteristics, known as its phenotype, vary greatly. Most mutations are neutral, meaning they do not lead to any noticeable change in the phenotype because they occur in non-coding regions of DNA or result in a protein that functions identically to the original. However, some mutations do influence the phenotype, causing minor alterations that might be beneficial, harmful, or simply different without a clear advantage or disadvantage. A very small number of mutations have a profound effect, directly determining a specific phenotype. These can range from causing genetic disorders to conferring a clear evolutionary advantage. The wide spectrum of effects highlights the complex relationship between an organism's genetic makeup (genotype) and its observable traits (phenotype).