Evolution What you need to know

Evolution is the unifying force in modern biology, but it remains a source of misunderstanding and controversy. Start finding out why it is so important with our beginner's guide

There are all sorts of ways to reconstruct the history of life on Earth. Pinning down when specific events occurred is often tricky, though. For this, biologists depend mainly on dating the rocks in which fossils are found, and by looking at the "molecular clocks" in the DNA of living organisms.

There are problems with each of these methods. The fossil record is like a movie with most of the frames cut out. Because it is so incomplete, it can be difficult to establish exactly when particular evolutionary changes 

happened.               

Modern genetics allows scientists to measure how different species are from each other at a molecular level, and thus to estimate how much time has passed since a single lineage split into different species. Confounding factors rack up for species that are very distantly related, making the earlier dates more uncertain.

These difficulties mean that the dates in the timeline should be taken as approximate. As a general rule, they become more uncertain the further back along the geological timescale we look. Dates that are very uncertain are marked with a question mark.

In 1859 Charles Darwin published his theory of natural selection amid an explosion of controversy. Like the work of Copernicus in the 16^th 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 code

Darwin 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.