Instant Expert: Evolution | New Scientist [From www.newscientist.com]
In 1859 Charles Darwin's published his theory of natural selection amid an explosion of controversy. Like the work of Copernicus in the 16th century revealing the movement of the Earth, Darwin's idea shook the foundations of the establishment and profoundly altered humanity's view of its place in the universe.
Today evolution is the
unifying force in modern biology; it ties together fields as disparate as
genetics,
microbiology and
palaeontology. It is an elegant and convincing explanation for the
staggering diversity of Earth's five million or more living
species.
Evolution has several facets. The first is the theory that all living species are the modified descendents of earlier species, and that we all share a common ancestor in the distant past. All species are therefore related via a vast tree of life. The second is that this evolution is driven by a process of natural selection or the - "survival of the fittest".
Darwin argued that all individuals struggle to survive on limited resourses, but some have small,
heritable differences that give them a greater chance of surviving or reproducing, than individuals lacking these
beneficial traits. Such individuals have a higher
evolutionary fitness, and the useful traits they possess become more common in the population because more of their offspring survive.
Eventually these advantageous traits become the norm. Conversely, harmful traits are quickly eradicated as individuals that possess them are less likely to reproduce. Natural selection therefore works to create a population that is highly suited to its environment, and can adapt to changes.
Sex wars
When individuals compete for limited resources in their environment they are subject to
ecological selection. However, useful traits are not only those that give a survival advantage, but also those that increase a
plant or animal's chance of
reproducing. These traits are subject to
sexual selection.
Sexually selected traits can make a male organism more attractive to females, the peacock's tail for example. These are sometimes correlated to the health of an individual, and are therefore an honest badge of fitness. Another type of sexually selected trait gives males a physical advantage in out-competing other males for mates, the stag's antlers for example. Sexual selection can even act at a molecular level.
Birds are particularly known for showy
ornaments that attract mates, but also increase the chances of being spotted by predators. Other sexually selected traits include:
lion's manes,
great tit's or
budgie's plumage, grouse
mating rituals, insect
love tokens, the height of
human males and
human hair,
intelligence and
facial features.
Species spawning
Over eons, and many generations, the process of
slow evolutionary change, called anagenesis, can cause one species to evolve into another. But most new species form in a
speciation event, when one species splits into two; a process Darwin called the "
mystery of mysteries".
Allopatric speciation happens when a geographical change - a river changing course for example or a new mountain range - splits a species in two. Once separated, as happened to antelope squirrels on either side of the Grand Canyon in the US, the populations evolve independently, eventually becoming distinct and reproductively isolated.
Sympatric speciation occurs when new species emerge
without separation, such as the 13 species of
Galapagos finch or Africa's
cichlid fish. These species adapt to different opportunities in the environment, and then
cease to interbreed - perhaps due to some
isolating mechanism. Rarely new species can also form through hybridisation, such as
sunflowers.
Darwinian evolution is a slow, gradual process. But much of the fossil record hints at puzzling long periods of stasis, with scarcely any change. In 1972, evolutionary biologists Niles Eldredge and Stephen J Gould argued instead that species, perhaps even communities, form suddenly in fits and starts of change. They called the theory punctuated equilibrium.
Like individuals in a population, species also
struggle amongst themselves to survive, and most
become extinct over time. Species can also
die out in
mass extinctions, such as the one that caused the demise of the
dinosaurs. Today we may be in the throes of another
mass extinction, caused by human overexploitation of habitats.
Evolutionary scenarios
During his voyage on the
HMS Beagle and
throughout his life, Darwin gathered evidence that contributed to his theory of natural selection. In
Origin of Species he presented support from the fields of
embryology,
geography,
palaeontology and
comparative anatomy (see interactive graphic). Darwin also found evidence for his theory in examples of convergent evolution, co-evolution and adaptive radiation.
Convergent evolution, is when the same adaptations have evolved independently in different lineages of species under similar selection pressures. Today we see convergent evolution in species as diverse as: shark and camels, shrimps and grasshoppers, flamingos and spoonbills, marsupial and placental mammals and bioluminescent sea creatures. We also see it in the ears and teeth of mammals.
Co-evolution is when the evolutionary history of two species or groups of species is intimately intertwined. Examples include: the co-evolution of
flowering plants and pollinators such as
bees,
lizards and
moths;
pocket gophers and their lice; humans and
intestinal microbes; and the war our
immune systems wage with the
pathogens that attack us.
Adaptive radiation is the rapid speciation of one ancestral species to fill many empty ecological niches. Adaptive radiations are most common when animals and plants arrive at previously uninhabited islands. Examples of adaptive radiation can be found in: the Galapagos finches, Australia's marsupials, Hawaii's honeycreepers and fruit flies, Madagascar's carnivores and other mammals, New Zealand's birds and the prehistoric flying pterosaurs.
Secret codeDarwin was able to establish natural selection, without any understanding of the genetic mechanisms of inheritance, or the source of novel variation in a population. His own theory on the transmission of traits, called pangenesis, was completely wrong.
It was not until
Gregor Mendel and the start of the 20
th century that the genetic mechanism of inheritance began to be revealed. We now know that most traits, such as skin colour, eye colour and
blood group are determined by our
DNA and genes. During the 20
th century, evolutionary biologists such as
Ernst Mayr, J.B.S. Haldane,
Julian Huxley, and
Theodosius Dobzhansky combined Darwinian evolution with our emerging knowledge of genetics to produce the "modern synthesis" that we call evolutionary biology today.
Most genes come in a
variety of forms, one inherited from each parent. The varieties are known as
alleles, and encode slightly different traits. The incidence of different traits, or alleles, in a population is driven by natural selection and
genetic drift, which can randomly
reduce genetic variation. Today, evolution is defined as the change in the frequency of alleles in populations over time.
New traits are introduced into populations by gene flow from other populations or by mutation. Mutation is a change in the structure of a gene and can be caused by errors in copying DNA, carcinogenic chemicals, viruses, UV-light and radiation. Most mutations are neutral, having no effect on gene function; others are harmful, such as the ones that cause inherited diseases like cystic fibrosis. Rarely mutations can lead to beneficial new traits, such as increased resistance to malaria.
Today evolutionary biologists are largely
divided into two camps. The pro-selectionists such as
Richard Dawkins,
Stephen Pinker,
Edward O Wilson,
Matt Ridley,
Mark Ridley and
Jared Diamond believe in the primacy of natural selection as the principle guiding evolution. Others such as Niles Eldredge, Stephen J. Gould,
Brian Goodwin,
Stuart Kauffman and
Steven Rose argue that we are still missing something big, and that natural selection does not explain the full complexity of evolution.
John Pickrell, 11 July 2005