Alleles are one of a number of different forms of a gene. How and when they occur in an organism’s genes can change. Many factors determine the how and when. The main source for humans is, of course, the various results that occur in sexual reproduction. Genes are “shuffled” during gamete production. The 23 pairs of chromosomes that each new baby gets from its parents move independently during meiosis, and there are millions of possible combinations of genes. During the DNA replication process, mistakes can occur, causing mutations. This can happen naturally or as a result of environmental damage, for example, from radiation or chemicals. Some mutations can cause changes in phenotype, or its physical, behavioral, and biochemical characteristics. Sometimes those mutations will harm an organism’s fitness, and sometimes not.
Mutations are not the only allele change that occurs by chance. Genetic drift does too. When a portion of a larger, established population of an organism divides off, that smaller population may, by chance, pass on a particular allele. For example (and completely imagined), if there is an allele for reproducing sets of twins, and the Pilgrims had that allele more than the overall English population they left behind, there may have been an occurrence of twins being born at Plymouth Rock over the generations that occurred more frequently than in the original English population. The Pilgrims could also be used as an example of allele change from migration. When a population with a set of possible alleles moves, or migrates, into another population with a different set, the genes passed on can change if the two populations mingle. If the Pilgrims were more open to intermarriage, and had mated with the native population in great numbers, the resulting population’s alleles may have changed from migration.
Another allele-changing process is natural selection, which occurs naturally, of course. This process happens because organisms that have allele forms that help it survive better than others in their species are more likely to be selected for reproduction. Those genes can then be passed on. Those without them will slowly disappear. An example is a lizard that once had bright skin coloring that made it easier for predators to see. The brighter, the more likely it is to be eaten, and then not around to breed. The trait would not be around to be passed on.
There are three types of natural selection: directional, stabilizing and disruptive. Directional selection occurs when a particular trait in a species helps them to survive in greater numbers and then reproduce. An example is a finch from the Galapagos Islands that had a bigger beak and could compete better for the insects it ate. The bigger beak made the frequency of this trait occur more on one end of the curve, so the curve moved in a particular direction. Next is the stabilizing selection. This is similar to directional selection in that a particular trait occurs more frequently at one point on the curve. It is different, because it occurs at the center of the curve, so it stabilizes the species’ curve overall. The last, and really the coolest, is disruptive selection. The organisms within a species that have higher fitness occur at two ends of the curve, disrupting its shape. If the curve gets too heavy, in theory, at its two ends, the single curve can split into two, creating two different phenotypes.
The Evolution of the Blue-Ringed Octopus
Like all octopi, the blue-ringed octopus is part of the cephalopod class. Over time, it developed adaptations that other octopi do not have. This includes its lethal toxin, tetrodotoxin, which can kill a human adult in ninety minutes after being bitten. This is rare, but there is no antidote. The toxin in the venom is 10,000 times more powerful than cyanide. Another adaptation – and from what this octopus derives its name – is the blue-ringed characteristic. At rest, the rings are brown or beige. When the blue-ringed octopus is threatened or mad, its skin stretches and releases an electric-blue pigment.
Similar to many other octopi, the blue-ringed octopus adapted chromatophores that allow them to blend into their surroundings. Chromatophores are specialized cells that some organisms, like chameleons, utilize to either hide themselves from predators or sneak up to capture their prey.
Cephalopods have an evolutionary history that spans an impressive 500 million years. They have left behind abundant fossils, mostly shelled nautiloids and ammonoids, that record repeated speciation and extinction events. There are about 17,000 named species of fossil cephalopods, compared to the 800 identified living species. Graph “A” below shows the varied evolutionary history of cephalopods.