Genetics is the branch of biology that deals with the inheritance of traits. This includes the mechanisms of hereditary transmission and the variation of inherited traits among similar or related organisms.
Genes are the units that carry heredity information for every living organism. Genes are located on DNA - thousands of genes are located on each DNA strand. Chromosomes are pairs of tightly wound DNA strands. Different species have different numbers of chromosomes. Genes, DNA and chromosomes are located in the nucleus of every cell of every living organism.
Gene expression is a gene decoding process leads to the synthesis of proteins-in turn, proteins determine the phenotypic traits of an organism. This is how an organism's genetic makeup (its genotype) determines the physical appearance (phenotype) of an organism.
Biological evolution is change in the genetic makeup of a species or population of organisms -it takes place over successive generations. During evolution, the frequency of specific alleles in a population's gene pool changes. Evolution leads to cumulative genetic change overtime.
The theory of evolution by natural selection is a unifying scientific theory on the biological relationships and common ancestry among various species of organisms, and how populations of organisms have evolved over time. This theory also provides scientists with an opportunity to further redefine the field of biology to include the role of the natural environment in shaping the proliferation, extinction, and geographic distribution of populations of organisms on Earth over time.
Past events which have affected the evolution of life on Earth include climate changes, massive volcanic explosions and asteroid impacts - all which can lead to the destruction of vast areas of habitats and have lead to the extinction of a variety of species of organisms. A species may become extinct if its habitat is altered or destroyed, or because the environment has changed in such a way that it is unfavorable to the species.
Evolution by natural selection can be summarized as follows: