The Modern Synthesis is the theory used to explain and predict all of the traits, variation and changes in living things. It combines the older theories of evolution and Mendelian genetics with more recent discoveries in molecular genetics and cellular biology. Pheno- and genotypical changes can be explained and observed on the molecular, cellular, individual and populace scale through the modern synthesis.

Most multicellular plants and animals reproduce sexually; each parent contributing half of an offspringOs total genetic material. An organismOs sex cells are produced by meiosis. In meiosis, a cell containing a full set of genetic information replicates its chromosomes and divides itself twice. The resulting four cells each contain one half of the parent cellOs genes.

Alleles are represented in the genes of an organismOs chromosomes. Which chromosomes and therefore alleles end up in any given sex cell is completely random. Mendel observed this in his Law of Independent Assortment. In genetically complex species two parent organisms can produce millions of genetic combinations in their offspring through this meiosis alone. However, there are two other known processes which affect genetic change; recombination and mutation.

In meiosis, chromosome pairs are connected together to form units known as tetrads. Sometimes sections of genetic material from the chromosomes switch places with one another. This process of genetic recombination adds even greater variability in the production of sex cells.

Although the processes of meiosis and recombination redistribute existing genetic material, only through mutation can new genetic material develop. Mutation is a mistake in the replication of the DNA which makes up an organismOs chromosomes. Many mutations have no phenotypical manifestations and the majority that do are detrimental to an individualOs survival. Some mutations however, are helpful in the survival of an organism, thus the mutation is passed down to subsequent generations.

The genetic make-up of an organism is irrelevant to the species as a whole if that organism does not survive to reproduce. Basically, the process of natural selection is organisms that can survive surviving and producing offspring with their OfavorableO genes. Over time, great changes can occur in a population through natural selection augmented by the processes of genetic drift and migration.

Genetic drift consists of any number of random happenings that affect the genetic variability of a population. In short, an event that kills all or most of the individuals with certain traits, forever alter the populationOs genetic make-up. Genetic drift has very little effect in large populations. The line between genetic drift and natural selection is often unclear. Who, for example, can say whether a group of genetically similar individuals who are in a forest fire were randomly killed or were selected against for not getting out of the burning forest.

Migration is the flow of genes from one population to another. A physical change of location or a change in mate selection patterns can lead to migration between populations. Isolation of groups from one another, also through physical separation or mating patterns, can lead to changes in populations as well.

Variation in and among species is the result of both cellular and environmental factors. Radical changes come from small mistakes that occur in the replication of DNA and are continued by the simple fact that things that can survive will and those that cannot, wonOt. The Modern Synthesis attempts to explain these factors that account for the vast diversity of life on earth