From the tiniest microbe to the largest whale, all species on earth are constantly evolving, albeit some more quickly than others. Just a single mutation can start a ripple effect, establishing the transformation of one species into another. But why and how do some organisms evolve faster than others?
The slowest evolving animal alive is the elusive elephant shark. This fish inhabits the waters around southern Australia and New Zealand at depths of up to 200 meters. Using its snout, it hunts for shellfish living in the sand. The elephant shark is one of few animals in a group called the Chimaeras, whose skeletons are made from cartilage rather than bone. Chimaeras are closely related to sharks, rays and skates but the elephant shark split off from these other fish some 420 million years ago and has changed little since then.
Surpassing all norms, the elephant shark managed to escape evolutionary pressures through great periods of change. For example, through analysing its genome, a team of scientists were recently able to confirm that this species is missing the gene family responsible for the development of bone. It appears that this gene eventually occurred through a duplication event, which then gave rise to the evolution of all bony vertebrates.
Evolution is a complex maze, so there is no one reason why one particular species shouldn’t evolve while another stays the same. It is likely that the elephant shark has been in fairly stable environmental conditions over the past 420 million years, or else it might have needed to adapt to changes that could have included climate, new predator-prey structures or food scarcity.
Several situations create the rapid evolution of species. When a group of animals become stranded in a small area such as an island, a cave or an isolated mountain, a remarkable event occurs: the larger bodied animals tend to shrink and the smaller bodied animals tend to enlarge. This situation has been called ‘The Island Rule’. When stranded, the larger animals have an evolutionary incentive to eat less and reach sexual maturity earlier in order to reproduce more frequently. Both of these pressures rely on an animal becoming smaller. Although now extinct, there are many examples of dwarf elephants colonising Mediterranean islands throughout the ages. Island dwarfism in elephants has been known to take place in less than 100,000 generations, a relatively short evolutionary time for these large mammals.
Meanwhile, smaller animals are free from many of the predatory pressures they might have previously succumbed to on the mainland. For this reason they are naturally selected for size since they no longer need to hide from predators, and being larger means being able to fight the competition for food. A famous example of island gigantism is the dodo, a gigantic flightless pigeon which was prevalent on the island of Mauritius until around 1680.
The guppy, a small freshwater fish, currently holds the gold medal among vertebrates for its extremely short evolutionary timescale. In an experiment to measure how fast it could evolve, a team of scientists split a group of guppies into two and placed them into distinct parts of a river. One half inhabited the top of a waterfall where all predators were excluded whilst the other half inhabited the lower area of the stream which was abundant with predators.
The guppies at the top experienced little predatory pressure and within just ten years had evolved to become much larger and produce fewer offspring. In a similar case to the dodos, the guppies were selected to become much larger since this helped them to compete for the limited food supply.
Despite ten years being exceptionally fast for a vertebrate to evolve, microbes far exceed this timespan with an ability to do so over a matter of days. Bacteria have a number of characteristics that help them to evolve this fast, including being able to reproduce quickly through short generation times. Under optimal conditions, some bacteria can multiply their number by a million within twenty generations.
A typical bacterial strain such as Staphylococcus aureus, which is the non-antibiotic-resistant version of MRSA has a generation time of about half an hour. This means that within just ten hours, one microbe could reproduce to create an army of a million.
The second reason for their fast-paced evolution is their large population sizes and close living quarters. In these large communities, there is a much greater chance of mutations arising as well as a greater ability to spread them quickly. Bacteria, like all other animals, are able to pass genes down vertically from parent to offspring. Yet they also possess another interesting trait which helps them to transfer mutations exceptionally fast; the ability to transfer genes horizontally.
Horizontal gene transfer is the process of passing DNA between each other through injection. Bacteria are able to put their DNA into small self-replicating circles called plasmids and so pass it to others, even of different species. Once received, bacteria can incorporate the new DNA into their own genome.
This is particularly handy for the bacteria, and particularly dangerous for humans, since these microbes do not even have to reproduce in order to pass their beneficial genes on; they can simply transfer some of their DNA to another species that lacks it. In the case of antibiotic resistance, it therefore takes just one microbe to have the resistance gene and it can spread like wildfire, especially when the population encounters the antibiotic and so the rest of the population die out, leaving space and nutrients for the resistant population to expand into.
Studying evolutionary speeds can help scientists in a number of ways. Investigating the traits of both extremely old species and of constantly evolving species tells us about our genes and which ones give us certain characteristics, which can help us develop new treatments for diseases. Additionally, we can exploit microbes that evolve quickly to create potential new medicines by using them as little protein factories.
Bacteria will probably be here for as long as life survives on Earth and the elephant shark may well remain unchanged for another 420 million years, but our big question is; how will we evolve during that time?
IMAGE: Ted Murphy