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.
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.
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.
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.
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.
