B.7F-Evolutionary Mechanisms_B.10D on level – 03

The sudden appearance of red varieties in a patch of white flowers might be explained by the immigration of alleles for flower color from another population. Perhaps there exists a population of the same wildflower, but with the red and white varieties present. By chance, wind or insects might transfer pollen to the patch of white flowers, resulting in the red variety emerging in future generations. Gene flow is the evolutionary mechanism that moves genes from one population to another. The flow of genes can go in either direction. Alleles can be removed from a population by emigration or added to a population by immigration. Thus, gene flow can reduce the differences between populations.

What if a white variety of the flower did not exist, yet suddenly a white variety appears in a patch of the red variety? A change in the DNA of the allele, or gene, for flower color may have occurred. Mutation is an evolutionary mechanism that is a permanent, heritable change in the DNA structure. Mutation is considered the raw material for evolutionary change because it introduces new alleles into the gene pool. This changes the allele frequencies of a population and results in an increase in the population’s genetic variation. A mutation does not arise due to an organism’s needs – it is simply a random change of the DNA molecular structure.

Sexual reproduction generates new combinations of alleles within an organism’s offspring that are not found in the parents. This genetic recombination is enhanced by several processes – the crossing over of alleles during meiosis, the independent assortment of chromosomes in the formation of gametes, and the random union of gametes from the two parents. This evolutionary mechanism increases genetic variation of a sexually reproducing population.

Although natural selection is the main mechanism for evolution, the processes of genetic drift, gene flow, mutation, and genetic recombination also affect evolutionary change. Of these evolutionary mechanisms, only natural selection adapts a population to its environment. The other mechanisms occur by random chance and may impact a population in a positive, negative, or neutral ways. All the evolutionary mechanisms impact a population by altering the allele frequencies in the population’s gene pool.