Microevolution & Macroevolution

Evolution involving small-scale changes, i.e., within the species level, occurring over a short period of time that results in the formation of new taxa.
produces minor changes to the gene poolGene Pool:
The complete set of unique alleles in a species or population.
of a population. These changes are caused by mutation and genetic recombination, which generates variationVariation:
The differences among individuals in a population.
. These genetic variations result in different genotypesGenotype:
The genetic makeup of an organism or group of organisms with reference to a single trait, set of traits, or an entire complex of traits. (adj. genotypic)
within the population that appear as different phenotypesPhenotype:
The observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences; what an organism “looks like.” (adj. phenotypic)
among the individuals. Genetic drift acts upon the genotype, whereas natural selection is only able to act upon the phenotype.

Over time, natural selection and genetic drift result in macroevolution. MacroevolutionMacroevolution:
Evolution happening on a large scale, i.e., at or above the level of species, over geologic time resulting in the formation of new taxa.
results in large changes to a gene pool often resulting in the evolution of a new species. Macroevolution can occur very gradually over a long period of time, called gradualismGradualism:
The process of gradual evolutionary change over time.
, or very quickly after a long period of stability, called punctuated equilibriumPunctuated Equilibrium:
The evolutionary process involving long periods without change (stasis) punctuated by short periods of rapid speciation.

During the Early Paleozoic Era, certain bivalve species developed long siphons and a muscular foot that gave them the ability to bury themselves within the sediment, providing them with a strong advantage against predators on the surface of the seafloor. The minor genetic mutation that resulted in longer siphons is an example of microevolution. Over thousands of generations, and many additional favorable mutations, burrowing bivalves diversified through the process of macroevolution. You can find more information about the early evolution of bivalves here.

Short and long siphons of living bivalves are illustrated by the six images above. Short siphons (top row) are found in the Heart Cockle (Fragum unedo), Galeommatid Clam (Scintillona cryptozoica; the excurrent siphon is between the orange-tipped tentacles at left), and Coquina Clam (Donax veruinus). Long siphons (bottom row) are found in the Razor Clam (Solen vaginoides), Pipi Clam (Donax deltoides), and Dipper Clam (Cuspidaria latesulcata; the long siphons are encased in the shelly “rostrum”). All of these species were photographed alive at Moreton Bay, Queensland, Australia.