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Natural selection is a mechanism of evolution. Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success. This process causes species to change and diverge over time.
Natural selection is one of the ways to account for the millions of species that have lived on Earth.
In Darwin and Wallace's time, most believed that organisms were too complex to have natural origins and must have been designed by a transcendent God. Natural selection, however, states that even the most complex organisms occur by totally natural processes.
Prof Adrian Lister, a researcher at the Museum says, 'It's not that biologists don't understand that organisms are complex and functional, and it does seem almost miraculous that they exist. We realise that, but we think we've found another way of explaining it.'
In natural selection, genetic mutations that are beneficial to an individual's survival are passed on through reproduction. This results in a new generation of organisms that are more likely to survive to reproduce.
For example, evolving long necks has enabled giraffes to feed on leaves that others can't reach, giving them a competitive advantage. Thanks to a better food source, those with longer necks were able to survive to reproduce and so pass on the characteristic to the succeeding generation. Those with shorter necks and access to less food would be less likely to survive to pass on their genes.
Adrian explains, 'If you took 1,000 giraffes and measured their necks, they're all going to be slightly different from one another. Those differences are at least in part determined by their genes.
'The ones with longer necks may leave proportionally more offspring, because they have fed better and have maybe been better in competing for mates because they are stronger. Then, if you were to measure the necks of the next generation, they're also going to vary, but the average will have shifted slightly towards the longer ones. The process carries on generation after generation.'
An adaptation is a physical or behavioural characteristic that helps an organism to survive in its environment.
But not all characteristics of an animal are adaptations.
Adaptations for one purpose can be co-opted for another. For instance, feathers were an adaptation for thermoregulation - their use for flight only came later. This means that feathers are an exaptation for flight, rather than an adaptation.
Adaptations can also become outdated, such as the tough exterior of the calabash fruit (Crescentia cujete). This gourd is generally thought to have evolved to avoid being eaten by Gomphotheres, a family of elephant-like animals. But these animals went extinct around 10,000 years ago, so the fruit's adaptation no longer has a survival benefit.
Selection for adaptation is not the only cause of evolution. Species change can also be caused by neutral mutations that have no detriment or benefit to an individual, genetic drift or gene flow.
In terms of evolution, an animal that is 'fit' is one that is adapted to its environment. This concept is at the core of natural selection, although the term 'survival of the fittest' has often been misunderstood and may be best avoided.
There is also a degree of randomness to evolution, so the best-adapted animal won't always be the one to survive.
Adrian explains, 'If you're going to get hit by a rock or something, it's just bad luck. But on average and over time, the ones that survive are the ones that are fittest - the ones that have the best adaptations.'
Darwin collected many animal specimens during the voyage of HMS Beagle (1831-1836). Among his best-known are the finches, of which he collected around 14 species from the Galápagos Islands. The birds sit within the same taxonomic family and have a diverse array of beak sizes and shapes. These correspond to both their differing primary food sources and divergence due to isolation on different islands.
The green warbler-finch (Certhidea olivacea), for example, has a sharp, slender beak which is perfect for feeding on small insects. In comparison, the large ground finch (Geospiza magnirostris) has a short, stocky beak to crack seeds and nuts.
Darwin's finches are often thought of as inspiring a 'eureka moment', but it was actually mockingbirds that impacted Darwin's thoughts on evolution.
Darwin had collected mockingbirds in South America before travelling to the Galápagos. On the first island, San Cristóbal (then known as Chatham Island), he saw a bird he recognised as a mockingbird. But on nearby Floreana Island he saw that the mockingbirds were considerably different.
Darwin realised that differences between species of mockingbird on the islands were greater than between those he'd seen across the continent. He began contemplating while aboard HMS Beagle, but it took several years before he came up with his theory of evolution by natural selection.
The finches - once they had been identified as different species by the British ornithologist John Gould - became one useful example among the many other animals he saw.
The finches are of scientific interest today. The study of Daphne Major, a volcanic island in the Galápagos archipelago, began in 1972 and found that natural selection has resulted in changes in the beak shape and size of two species of finch: the medium ground finch (Geospiza fortis) and common cactus finch (Geospiza scandens). Both species' beaks have been seen to shrink over time, but followed different patterns.
Darwin thought that natural selection progressed slowly and only occurred over a long period of time. This may often be true, but it has been shown that in some cases a new species can evolve within a lifetime.
For 31 years, scientists studied the survival of a male finch that emigrated from Santa Cruz Island as well as six generations of its descendants on Daphne Major. From the second generation onwards, the birds behaved as a separate species to the others on the island.
Lamarckism is a theory named after French naturalist Jean-Baptiste Lamarck (1744-1829). It proposes that animals acquire characteristics based on use or disuse during their lives, rather than through hard-coded genetic changes.
In Lamarckian theory, giraffes stretch their necks to make them longer. These animal's offspring would inherit longer necks as a result of their parents' efforts.
Adrian says, 'If you tried to stretch your neck for 10 minutes each morning, then you would probably end up with your neck being a few millimetres longer for a few years. But your children would not inherit it. That's where this theory fails.'
For millennia, the world was viewed as static. The ideas that mountains could rise, and climate and organisms could change didn't exist. Earth was thought to exist in an optimal form.
But natural selection relies on the fact that the world is constantly changing. Evolution occurs automatically for survival and for millions of years it has been playing catch-up with our dynamic world.
'Organisms are either adapted enough to survive and reproduce, or they are sub-optimal and the population shrinks. It may even shrink to zero, and that means extinction,' states Adrian.
Scientists have been able to predict natural selection over short terms. But it is almost impossible to accurately determine its effects in the future due to unpredictable fluctuations of the environment.
Natural selection implies that if organisms are surviving, they are adapted. But as the environment changes, we may find that what was once an adaptation may no longer be useful.
Although it is possible for evolution to occur quickly, the more rapidly the planet changes, the harder it is for evolution to keep pace and the more serious the risk of a massive rise in extinctions becomes.