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| Tyrannosaurus rex.
Non-avian dinosaurs died out in the Cretaceous–Paleogene
extinction event at the end of the Cretaceous
period. |
Evolution
Evolution is a scientific theory used by biologists. It
explains how living things change over a long time, and
how they have come to be the way they are.
Earth is very old. By studying the layers of rock, we
can find out about its past. That kind of research is
called historical geology.
We know that living things have changed over time,
because we can see their remains in the rocks. These
remains are called 'fossils'. So, we know that the
animals and plants of today are different from those of
long ago. And the further we go back, the more different
the fossils are. How has this come about? Evolution has
taken place. That evolution has taken place is a fact,
because it is overwhelmingly supported by many lines of
evidence. At the same time, evolutionary questions are
still being actively researched by biologists.
Comparison of DNA sequences allows organisms to be
grouped by how similar their sequences are. In 2010 an
analysis compared sequences to phylogenetic trees, and
supported the idea of common descent. There is now
"strong quantitative support, by a formal test", for the
unity of life.
The theory of evolution is the basis of modern biology.
"Nothing in biology makes sense except in the light of
evolution". |
|
Evidence for evolution
The evidence for evolution is given in a number of
books. Some of this evidence is discussed here.
Fossils show that change has
occurred
The realization that some rocks contain fossils was a
landmark in natural history. There are three parts to
this story: |
- Realizing that things in rocks
which looked organic actually were the altered
remains of living things. This was settled in the
16th and 17th centuries by Conrad Gessner, Nicolaus
Steno, Robert Hooke and others.
- Realizing that many fossils
represented species which do not exist today. It was
Georges Cuvier, the comparative anatomist, who
proved that extinction occurred, and that different
strata contained different fossils.
- Realizing that early fossils
were simpler organisms than later fossils. Also, the
later the rocks, the more like the present day are
the fossils.
|
"The most convincing evidence for the occurrence of
evolution is the discovery of extinct organisms in older
geological strata... The older the strata are...the more
different the fossil will be from living
representatives... that is to be expected if the fauna
and flora of the earlier strata had gradually evolved
into their descendants."
– Ernst Mayr |
Evolution of horses
The evolution of the horse family (Equidae) is a good
example of the way that evolution works. The oldest
fossil of a horse is about 52 million years old. It was
a small animal with five toes on the front feet and four
on the hind feet. At that time, there were more forests
in the world than today. This horse lived in woodland,
eating leaves, nuts and fruit with its simple teeth. It
was only about as big as a fox.
About 30 million years ago the world started to become
cooler and drier. Forests shrank; grassland expanded,
and horses changed. They ate grass, they grew larger,
and they ran faster because they had to escape faster
predators. Because grass wears teeth out, horses with
longer-lasting teeth had an advantage.
For most of this long period of time, there were a
number of horse types (genera). Now, however, only one
genus exists: the modern horse, Equus. It has teeth
which grow all its life, hooves on single toes, great
long legs for running, and the animal is big and strong
enough to survive in the open plain. Horses lived in
western Canada until 12,000 years ago, but all horses in
North America became extinct about 11,000 years ago. The
causes of this extinction are not yet clear. Climate
change and over-hunting by humans are suggested.
So, scientists can see that changes have happened. They
have happened slowly over a long time. How these changes
have come about is explained by the theory of evolution. |
 |
| Geographical
isolation of finches on the Galápagos Islands
produced over a dozen new species. |
Geographical distribution
This is a topic which fascinated both Charles Darwin and
Alfred Russel Wallace. When new species occur, usually
by the splitting of older species, this takes place in
one place in the world. Once it is established, a new
species may spread to some places and not others.
Australasia
Australasia has been separated from other continents for
many millions of years. In the main part of the
continent, Australia, 83% of mammals, 89% of reptiles,
90% of fish and insects and 93% of amphibians are
endemic. Its native mammals are mostly marsupials like
kangaroos, bandicoots, and quolls. By contrast,
marsupials are today totally absent from Africa and form
a small portion of the mammalian fauna of South America,
where opossums, shrew opossums, and the monito del monte
occur (see the Great American Interchange).
The only living representatives of primitive egg-laying
mammals (monotremes) are the echidnas and the platypus.
They are only found in Australasia, which includes
Tasmania, New Guinea, and Kangaroo Island. These
monotremes are totally absent in the rest of the world.
On the other hand, Australia is missing many groups of
placental mammals that are common on other continents (carnivora,
artiodactyls, shrews, squirrels, lagomorphs), although
it does have indigenous bats and rodents, which arrived
later.
The evolutionary story is that placental mammals evolved
in Eurasia, and wiped out the marsupials and monotremes
wherever they spread. They did not reach Australasia
until more recently. That is the simple reason why
Australia has most of the world's marsupials and all the
world's monotremes.
Hawaiian Drosophila
In about 6,500 sq mi (17,000 km2), the Hawaiian Islands
have the most diverse collection of Drosophila flies in
the world, living from rainforests to mountain meadows.
About 800 Hawaiian drosophilid species are known.
Genetic evidence shows that all the native drosophilid
species in Hawaiʻi have descended from a single
ancestral species that colonized the islands, about 20
million years ago. The subsequent adaptive radiation was
spurred by a lack of competition and a wide variety of
vacant niches. Although it would be possible for a
single pregnant female to colonise an island, it is more
likely to have been a group from the same species.
Distribution of Glossopteris
The combination of continental drift and evolution can
explain what is found in the fossil record. Glossopteris
is an extinct species of seed fern plants from the
Permian period on the ancient supercontinent of Gondwana.
Glossopteris fossils are found in Permian strata in
southeast South America, southeast Africa, all of
Madagascar, northern India, all of Australia, all of New
Zealand, and scattered on the southern and northern
edges of Antarctica.
During the Permian, these continents were connected as
Gondwana. This is known from magnetic striping in the
rocks, other fossil distributions, and glacial scratches
pointing away from the temperate climate of the South
Pole during the Permian. |
 |
| The hominoids are
descendants of a common ancestor. |
Common descent
When biologists look at living things, they see that
animals and plants belong to groups which have something
in common. Charles Darwin explained that this followed
naturally if "we admit the common parentage of allied
forms, together with their modification through
variation and natural selection".
For example, all insects are related. They share a basic
body plan, whose development is controlled by master
regulatory genes. They have six legs; they have hard
parts on the outside of the body (an exoskeleton); they
have eyes formed of many separate chambers, and so on.
Biologists explain this with evolution. All insects are
the descendants of a group of animals who lived a long
time ago. They still keep the basic plan (six legs and
so on) but the details change. They look different now
because they changed in different ways: this is
evolution.
It was Darwin who first suggested that all life on Earth
had a single origin, and from that beginning "endless
forms most beautiful and most wonderful have been, and
are being, evolved". Evidence from molecular biology in
recent years has supported the idea that all life is
related by common descent.
Vestigial structures
Strong evidence for common descent comes from vestigial
structures. The useless wings of flightless beetles are
sealed under fused wing covers. This can be simply
explained by their descent from ancestral beetles which
had wings that worked.
Rudimentary body parts, those that are smaller and
simpler in structure than corresponding parts in
ancestral species, are called vestigial organs. Those
organs are functional in the ancestral species but are
now either nonfunctional or re-adapted to a new
function. Examples are the pelvic girdles of whales,
halteres (hind wings) of flies, wings of flightless
birds, and the leaves of some xerophytes (e.g. cactus)
and parasitic plants (e.g. dodder).
However, vestigial structures may have their original
function replaced with another. For example, the
halteres in flies help balance the insect while in
flight, and the wings of ostriches are used in mating
rituals, and in aggressive display. The ear ossicles in
mammals are former bones of the lower jaw.
"Rudimentary organs plainly declare their origin and
meaning..." (p262). "Rudimentary organs... are the
record of a former state of things, and have been
retained solely though the powers of inheritance... far
from being a difficulty, as they assuredly do on the old
doctrine of creation, might even have been anticipated
in accordance with the views here explained" (p402).
Charles Darwin
In 1893, Robert Wiedersheim published a book on human
anatomy and its relevance to man's evolutionary history.
This book contained a list of 86 human organs that he
considered vestigial. This list included examples such
as the appendix and the 3rd molar teeth (wisdom teeth).
The strong grip of a baby is another example. It is a
vestigial reflex, a remnant of the past when pre-human
babies clung to their mothers' hair as the mothers swung
through the trees. This is borne out by the babies'
feet, which curl up when it is sitting down (primate
babies grip with the feet as well). All primates except
modern man have thick body hair to which an infant can
cling, unlike modern humans. The grasp reflex allows the
mother to escape danger by climbing a tree using both
hands and feet.
Vestigial organs often have some selection against them.
The original organs took resources, sometimes huge
resources. If they no longer have a function, reducing
their size improves fitness. And there is direct
evidence of selection. Some cave crustacea reproduce
more successfully with smaller eyes than do those with
larger eyes. This may be because the nervous tissue
dealing with sight now becomes available to handle other
sensory input.
Embryology
From the eighteenth century it was known that embryos of
different species were much more similar than the
adults. In particular, some parts of embryos reflect
their evolutionary past. For example, the embryos of
land vertebrates develop gill slits like fish embryos.
Of course, this is only a temporary stage, which gives
rise to many structures in the neck of reptiles, birds
and mammals. The proto-gill slits are part of a
complicated system of development: that is why they
persisted.
Another example are the embryonic teeth of baleen
whales. They are later lost. The baleen filter is
developed from different tissue, called keratin. Early
fossil baleen whales did actually have teeth as well as
the baleen.
A good example is the barnacle. It took many centuries
before natural historians discovered that barnacles were
crustacea. Their adults look so unlike other crustacea,
but their larvae are very similar to those of other
crustacea.
Artificial selection
Charles Darwin lived in a world where animal husbandry
and domesticated crops were vitally important. In both
cases farmers selected for breeding individuals with
special properties, and prevented the breeding of
individuals with less desirable characteristics. The
eighteenth and early nineteenth century saw a growth in
scientific agriculture, and artificial breeding was part
of this.
Darwin discussed artificial selection as a model for
natural selection in the 1859 first edition of his work
On the Origin of Species, in Chapter IV: Natural
selection:
"Slow though the process of selection may be, if feeble
man can do much by his powers of artificial selection, I
can see no limit to the amount of change... which may be
effected in the long course of time by nature's power of
selection".
Nikolai Vavilov showed that rye, originally a weed, came
to be a crop plant by unintentional selection. Rye is a
tougher plant than wheat: it survives in harsher
conditions. Having become a crop like the wheat, rye was
able to become a crop plant in harsh areas, such as
hills and mountains.
There is no real difference in the genetic processes
underlying artificial and natural selection, and the
concept of artificial selection was used by Charles
Darwin as an illustration of the wider process of
natural selection. There are practical differences.
Experimental studies of artificial selection show that
"the rate of evolution in selection experiments is at
least two orders of magnitude (that is 100 times)
greater than any rate seen in nature or the fossil
record".
Artificial new species
Some have thought that artificial selection could not
produce new species. It now seems that it can.
New species have been created by domesticated animal
husbandry, but the details are not known or not clear.
For example, domestic sheep were created by
hybridisation, and no longer produce viable offspring
with Ovis orientalis, one species from which they are
descended. Domestic cattle, on the other hand, can be
considered the same species as several varieties of wild
ox, gaur, yak, etc., as they readily produce fertile
offspring with them.
The best-documented new species came from laboratory
experiments in the late 1980s. William Rice and G.W.
Salt bred fruit flies, Drosophila melanogaster, using a
maze with three different choices of habitat such as
light/dark and wet/dry. Each generation was put into the
maze, and the groups of flies that came out of two of
the eight exits were set apart to breed with each other
in their respective groups.
After thirty-five generations, the two groups and their
offspring were isolated reproductively because of their
strong habitat preferences: they mated only within the
areas they preferred, and so did not mate with flies
that preferred the other areas.
Diane Dodd was also able to show how reproductive
isolation can develop from mating preferences in
Drosophila pseudoobscura fruit flies after only eight
generations using different food types, starch and
maltose.
Dodd's experiment has been easy for others to repeat. It
has also been done with other fruit flies and foods.
Observable changes
Some biologists say that evolution has happened when a
trait that is caused by genetics becomes more or less
common in a group of organisms. Others call it evolution
when new species appear.
Changes can happen quickly in the smaller, simpler
organisms. For example, many bacteria that cause disease
can no longer be killed with some of the antibiotic
medicines. These medicines have only been in use about
eighty years, and at first worked extremely well. The
bacteria have evolved so that they are no longer
affected by antibiotics anymore. The drugs killed off
all the bacteria except a few which had some resistance.
These few resistant bacteria produced the next
generation.
The Colorado beetle is famous for its ability to resist
pesticides. Over the last 50 years it has become
resistant to 52 chemical compounds used in insecticides,
including cyanide. This is natural selection speeded up
by the artificial conditions. However, not every
population is resistant to every chemical. The
populations only become resistant to chemicals used in
their area. |
|
 |
| In 1842, Charles
Darwin penned his first sketch of On the Origin
of Species. |
History
Although there were a number of natural historians in
the 18th century who had some idea of evolution, the
first well-formed ideas came in the 19th century. Three
biologists are most important.
Lamarck
Jean-Baptiste de Lamarck (1744–1829), a French
biologist, claimed that animals changed according to
natural laws. He said that animals could pass on traits
they had acquired during their lifetime to their
offspring, using inheritance. Today, his theory is known
as Lamarckism. Its main purpose is to explain
adaptations by natural means. He proposed a tendency for
organisms to become more complex, moving up a ladder of
progress, plus use and disuse.
Lamarck's idea was that a giraffe's neck grew longer
because it tried to reach higher up. This idea failed
because it cannot be reconciled with heredity (Mendel's
work). Mendel made his discoveries about half a century
after Lamarck's work.
Darwin
Charles Darwin (1809–1882) wrote his On the Origin of
Species in 1859. In this book, he put forward much
evidence that evolution had occurred. He also proposed
natural selection as the way evolution had taken place.
But Darwin did not understand about genetics and how
traits were actually passed on. He could not accurately
explain what made children look like their parents.
Nevertheless, Darwin's explanation of evolution was
fundamentally correct. In contrast to Lamarck, Darwin's
idea was that the giraffe's neck became longer because
those with longer necks survived better. These survivors
passed their genes on, and in time the whole race got
longer necks.
Mendel
An Austrian monk called Gregor Mendel (1822–1884) bred
plants. In the mid-19th century, he discovered how
traits were passed on from one generation to the next.
He used peas for his experiments: some peas have white
flowers and others have red ones. Some peas have green
seeds and others have yellow seeds. Mendel used
artificial pollination to breed the peas. His results
are discussed further in Mendelian inheritance. Darwin
thought that the inheritance from both parents blended
together. Mendel proved that the genes from the two
parents stay separate, and may be passed on unchanged to
later generations.
Mendel published his results in a journal that was not
well-known, and his discoveries were overlooked. Around
1900, his work was rediscovered. Genes are bits of
information made of DNA which work like a set of
instructions. A set of genes are in every living cell.
Together, genes organise the way an egg develops into an
adult. With mammals, and many other living things, a
copy of each gene comes from the father and another copy
from the mother. Some living organisms, including some
plants, only have one parent, so get all their genes
from them. These genes produce the genetic differences
which evolution acts on. |
 |
| Mutation followed by
natural selection results in a population with
darker colouration. |
Darwin's theory
Darwin's On the Origin of Species has two themes: the
evidence for evolution, and his ideas on how evolution
took place. This section deals with the second issue.
Variation
The first two chapters of the Origin deal with variation
in domesticated plants and animals, and variation in
nature.
All living things show variation. Every population which
has been studied shows that animal and plants vary as
much as humans do. This is a great fact of nature, and
without it evolution would not occur. Darwin said that,
just as man selects what he wants in his farm animals,
so in nature the variations allow natural selection to
work.
The features of an individual are influenced by two
things, heredity and environment. First, development is
controlled by genes inherited from the parents. Second,
living brings its own influences. Some things are
entirely inherited, others partly, and some not
inherited at all.
The colour of eyes is entirely inherited; they are a
genetic trait. Height or weight is only partly
inherited, and the language is not at all inherited.
Just to be clear: the fact that humans can speak is
inherited, but what language is spoken depends on where
a person lives and what they are taught. Another
example: a person inherits a brain of somewhat variable
capacity. What happens after birth depends on many
things such as home environment, education and other
experiences. When a person is adult, their brain is what
their inheritance and life experience have made it.
Evolution only concerns the traits which can be
inherited, wholly or partly. The hereditary traits are
passed on from one generation to the next through the
genes. A person's genes contain all the traits which
they inherit from their parents. The accidents of life
are not passed on. Also, of course, each person lives a
somewhat different life: that increases the differences.
Organisms in any population vary in reproductive
success. From the point of view of evolution,
'reproductive success' means the total number of
offspring which live to breed and leave offspring
themselves.
Inherited variation
Variation can only affect future generations if it is
inherited. Because of the work of Gregor Mendel, we know
that much variation is inherited. Mendel's 'factors' are
now called genes. Research has shown that almost every
individual in a sexually reproducing species is
genetically unique.
Genetic variation is increased by gene mutations. DNA
does not always reproduce exactly. Rare changes occur,
and these changes can be inherited. Many changes in DNA
cause faults; some are neutral or even advantageous.
This gives rise to genetic variation, which is the
seed-corn of evolution. Sexual reproduction, by the
crossing over of chromosomes during meiosis, spreads
variation through the population. Other events, like
natural selection and drift, reduce variation. So a
population in the wild always has variation, but the
details are always changing.
Natural selection
Evolution mainly works by natural selection. What does
this mean? Animals and plants which are best suited to
their environment will, on average, survive better.
There is a struggle for existence. Those who survive
will produce the next generation. Their genes will be
passed on, and the genes of those who did not reproduce
will not. This is the basic mechanism which changes a
population and causes evolution.
Natural selection explains why living organisms change
over time to have the anatomy, the functions and
behaviour that they have. It works like this: |
- All living things have such
fertility that their population size could increase
rapidly for ever.
- We see that the size of populations
does not increase to this extent. Mostly, numbers remain
about the same.
- The food and other resources are
limited. Therefore, there is competition for food and
resources.
- No two individuals are alike.
Therefore, they will not have the same chances to live
and reproduce.
- Much of this variation can be
inherited. Parents pass such traits to the children
through their genes.
- The next generation can only come
from those that survive and reproduce. After many
generations of this, the population will have more
helpful genetic differences, and fewer harmful ones.
Natural selection is really a process of elimination.
The elimination is being caused by the relative fit
between individuals and the environment they live in.
|
Selection in natural populations
There are now many cases where natural selection has been
proved to occur in wild populations. Almost every case
investigated of camouflage, mimicry and polymorphism has
shown strong effects of selection.
The force of selection can be much stronger than was thought
by the early population geneticists. The resistance to
pesticides has grown quickly. Resistance to warfarin in
Norway rats (Rattus norvegicus) grew rapidly because those
that survived made up more and more of the population.
Research showed that, in the absence of warfarin, the
resistant homozygote was at a 54% disadvantage to the normal
wild type homozygote. This great disadvantage was quickly
overcome by the selection for warfarin resistance.
Mammals normally cannot drink milk as adults, but humans are
an exception. Milk is digested by the enzyme lactase, which
switches off as mammals stop taking milk from their mothers.
The human ability to drink milk during adult life is
supported by a lactase mutation which prevents this
switch-off. Human populations have a high proportion of this
mutation wherever milk is important in the diet. The spread
of this 'milk tolerance' is promoted by natural selection,
because it helps people survive where milk is available.
Genetic studies suggest that the oldest mutations causing
lactase persistence only reached high levels in human
populations in the last ten thousand years. Therefore,
lactase persistence is often cited as an example of recent
human evolution. As lactase persistence is genetic, but
animal husbandry a cultural trait, this is gene–culture
coevolution. |
 |
| Homologous bones in
the limbs of tetrapods. The bones of these
animals have the same basic structure, but have
been adapted for specific uses. |
Adaptation
Adaptation is one of the basic phenomena of biology. Through
the process of adaptation, an organism becomes better suited
to its habitat.
Adaptation is one of the two main processes that explain the
diverse species we see in biology. The other is speciation
(species-splitting or cladogenesis). A favourite example
used today to study the interplay of adaptation and
speciation is the evolution of cichlid fish in African
rivers and lakes.
When people speak about adaptation they often mean something
which helps an animal or plant survive. One of the most
widespread adaptations in animals is the evolution of the
eye. Another example is the adaptation of horses' teeth to
grinding grass. Camouflage is another adaptation; so is
mimicry. The better adapted animals are the most likely to
survive, and to reproduce successfully (natural selection).
An internal parasite (such as a fluke) is a good example: it
has a very simple bodily structure, but still the organism
is highly adapted to its particular environment. From this
we see that adaptation is not just a matter of visible
traits: in such parasites critical adaptations take place in
the life cycle, which is often quite complex.
Limitations
Not all features of an organism are adaptations. Adaptations
tend to reflect the past life of a species. If a species has
recently changed its life style, a once valuable adaptation
may become useless, and eventually become a dwindling
vestige.
Adaptations are never perfect. There are always tradeoffs
between the various functions and structures in a body. It
is the organism as a whole which lives and reproduces,
therefore it is the complete set of adaptations which gets
passed on to future generations.
Genetic drift and its effect
In populations, there are forces which add variation to the
population (such as mutation), and forces which remove it.
Genetic drift is the name given to random changes which
remove variation from a population. Genetic drift gets rid
of variation at the rate of 1/(2N) where N = population
size. It is therefore "a very weak evolutionary force in
large populations".
Genetic drift explains how random chance can affect
evolution in surprisingly big ways, but only when
populations are quite small. Overall, its action is to make
the individuals more similar to each other, and hence more
vulnerable to disease or to chance events in their
environment. |
- Drift reduces genetic variation in
populations, potentially reducing a population’s ability
to survive new selective pressures.
- Genetic drift acts faster and has
more drastic results in smaller populations. Small
populations usually become extinct.
- Genetic drift may contribute to
speciation, if the small group does survive.
- Bottleneck events: when a large
population is suddenly and drastically reduced in size
by some event, the genetic variety will be very much
reduced. Infections and extreme climate events are
frequent causes. Occasionally, invasions by more
competitive species can be devastating.
- In the 1880/90s, hunting reduced the
Northern elephant seal to only about 20 individuals.
Although the population has rebounded, its genetic
variability is much less than that of the Southern
elephant seal.
- Cheetahs have very little variation.
We think the species was reduced to a small number at
some recent time. Because it lacks genetic variation, it
is in danger from infectious diseases.
- Founder events: these occur when a
small group buds off from a larger population. The small
group then lives separately from the main population.
The human species is often quoted as having been through
such stages. For example, when groups left Africa to set
up elsewhere (see human evolution). Apparently, we have
less variation than would be expected from our worldwide
distribution.
- Groups that arrive on islands far
from the mainland are also good examples. These groups,
by virtue of their small size, cannot carry the full
range of alleles to be found in the parent population.
|
Species
How species form is a major part of evolutionary biology.
Darwin interpreted 'evolution' (a word he did not use at
first) as being about speciation. That is why he called his
famous book On the Origin of Species.
Darwin thought most species arose directly from pre-existing
species. This is called anagenesis: new species by older
species changing. Now we think most species arise by
previous species splitting: cladogenesis.
Species splitting
Two groups that start the same can also become very
different if they live in different places. When a species
gets split into two geographical regions, a process starts.
Each adapts to its own situation. After a while, individuals
from one group can no longer reproduce with the other group.
Two good species have evolved from one.
A German explorer, Moritz Wagner, during his three years in
Algeria in the 1830s, studied flightless beetles. Each
species is confined to a stretch of the north coast between
rivers which descend from the Atlas mountains to the
Mediterranean. As soon as one crosses a river, a different
but closely related species appears. He wrote later:
"... a [new] species will only [arise] when a few
individuals [cross] the limiting borders of their range...
the formation of a new race will never succeed... without a
long continued separation of the colonists from the other
members of their species".
This was an early account of the importance of geographical
separation. Another biologist who thought geographical
separation was critical was Ernst Mayr.
One example of natural speciation is the three-spined
stickleback, a sea fish that, after the last ice age,
invaded freshwater, and set up colonies in isolated lakes
and streams. Over about 10,000 generations, the sticklebacks
show great differences, including variations in fins,
changes in the number or size of their bony plates, variable
jaw structure, and color differences.
The wombats of Australia fall into two main groups, Common
wombats and Hairy-nosed wombats. The two types look very
similar, apart from the hairiness of their noses. However,
they are adapted to different environments. Common wombats
live in forested areas and eat mostly green food with lots
of moisture. They often feed in the daytime. Hairy-nosed
wombats live on hot dry plains where they eat dry grass with
very little water or goodness in it. Their metabolic system
is slow and they sleep most of the day underground.
When two groups that started the same become different
enough, then they become two different species. Part of the
theory of evolution is that all living things started off
the same, but then split off into different groups over
billions of years. |
|
Modern evolutionary
synthesis
This was an important movement in evolutionary biology,
which started in the 1930s and finished in the 1950s. It
has been updated regularly ever since. The synthesis
explains how the ideas of Charles Darwin fit with the
discoveries of Gregor Mendel, who found out how we
inherit our genes. The modern synthesis brought Darwin's
idea up to date. It bridged the gap between different
types of biologists: geneticists, naturalists, and
palaeontologists.
When the theory of evolution was developed, it was not
clear that natural selection and genetics worked
together. But Ronald Fisher showed that natural
selection would work to change species. Sewall Wright
explained genetic drift in 1931. |
- Evolution and genetics: evolution
can be explained by what we know about genetics, and
what we see of animals and plants living in the wild.
- Thinking in terms of populations,
rather than individuals, is important. The genetic
variety existing in natural populations is a key factor
in evolution.
- Evolution and fossils: the same
factors which act today also acted in the past.
- Gradualism: evolution is gradual,
and usually takes place by small steps. There are some
exceptions to this, notably polyploidy, especially in
plants.
- Natural selection: the struggle for
existence of animals and plant in the wild causes
natural selection. The strength of natural selection in
the wild was greater than even Darwin expected.
- Genetic drift can be important in
small populations.
- The rate of evolution can vary.
There is very good evidence from fossils that different
groups can evolve at different rates, and that different
parts of an animal can evolve at different rates.
|
 |
| Common garter snake
(Thamnophis sirtalis sirtalis) has evolved
resistance to the defensive substance
tetrodotoxin in its amphibian prey. |
Some areas of research
Co-evolution
Co-evolution is where the existence of one species is
tightly bound up with the life of one or more other species.
New or 'improved' adaptations which occur in one species are
often followed by the appearance and spread of related
features in the other species. The life and death of living
things is intimately connected, not just with the physical
environment, but with the life of other species.
These relationships may continue for millions of years, as
it has in the pollination of flowering plants by insects.
The gut contents, wing structures, and mouthparts of
fossilized beetles and flies suggest that they acted as
early pollinators. The association between beetles and
angiosperms during the Lower Cretaceous period led to
parallel radiations of angiosperms and insects into the late
Cretaceous. The evolution of nectaries in Upper Cretaceous
flowers signals the beginning of the mutualism between
hymenoptera and angiosperms.
Tree of life
Charles Darwin was the first to use this metaphor in
biology. The evolutionary tree shows the relationships among
various biological groups. It includes data from DNA, RNA
and protein analysis. Tree of life work is a product of
traditional comparative anatomy, and modern molecular
evolution and molecular clock research.
The major figure in this work is Carl Woese, who defined the
Archaea, the third domain (or kingdom) of life. Below is a
simplified version of present-day understanding.
Macroevolution
Macroevolution: the study of changes above the species
level, and how they take place. The basic data for such a
study are fossils (palaeontology) and the reconstruction of
ancient environments. Some subjects whose study falls within
the realm of macroevolution: |
- Adaptive radiation, such as the
Cambrian Explosion.
- Changes in biodiversity through
time.
- Mass extinctions.
- Speciation and extinction rates.
- The debate between punctuated
equilibrium and gradualism.
- The role of development in shaping
evolution: heterochrony; hox genes.
- Origin of major categories: cleidoic
egg; origin of birds.
|
It is a term of convenience: for most biologists it does not
suggest any change in the process of evolution. For some
palaeontologists, what they see in the fossil record cannot
be explained just by the gradualist evolutionary synthesis.
They are in the minority.
Altruism and group selection
Altruism – the willingness of some to sacrifice themselves
for others – is widespread in social animals. As explained
above, the next generation can only come from those who
survive and reproduce. Some biologists have thought that
this meant altruism could not evolve by the normal process
of selection. Instead a process called "group selection" was
proposed. Group selection refers to the idea that alleles
can become fixed or spread in a population because of the
benefits they bestow on groups, regardless of the alleles'
effect on the fitness of individuals within that group.
For several decades, critiques cast serious doubt on group
selection as a major mechanism of evolution.
In simple cases it can be seen at once that traditional
selection suffices. For example, if one sibling sacrifices
itself for three siblings, the genetic disposition for the
act will be increased. This is because siblings share on
average 50% of their genetic inheritance, and the
sacrificial act has led to greater representation of the
genes in the next generation.
Altruism is now generally seen as emerging from standard
selection. The warning note from Ernst Mayr, and the work of
William Hamilton are both important to this discussion.
Hamilton's equation
Hamilton's equation describes whether or not a gene for
altruistic behaviour will spread in a population. The gene
will spread if rxb is greater than c:
rb > c
where: |
- c \ is the reproductive cost to the
altruist,
- b \ is the reproductive benefit to
the recipient of the altruistic behavior, and
- r \ is the probability, above the
population average, of the individuals sharing an
altruistic gene – the "degree of relatedness".
|
 |
| This diagram
illustrates the twofold cost of sex. If each
individual were to contribute to the same number
of offspring (two), (a) the sexual population
remains the same size each generation, where the
(b) Asexual reproduction population doubles in
size each generation. |
Sexual reproduction
At first, sexual reproduction might seem to be at a
disadvantage compared with asexual reproduction. In order to
be advantageous, sexual reproduction (cross-fertilisation)
has to overcome a two-fold disadvantage (takes two to
reproduce) plus the difficulty of finding a mate. Why, then,
is sex so nearly universal among eukaryotes? This is one of
the oldest questions in biology.
The answer has been given since Darwin's time: because the
sexual populations adapt better to changing circumstances. A
recent laboratory experiment suggests this is indeed the
correct explanation.
"When populations are outcrossed genetic recombination
occurs between different parental genomes. This allows
beneficial mutations to escape deleterious alleles on its
original background, and to combine with other beneficial
alleles that arise elsewhere in the population. In selfing
populations, individuals are largely homozygous and
recombination has no effect".
In the main experiment, nematode worms were divided into two
groups. One group was entirely outcrossing, the other was
entirely selfing. The groups were subjected to a rugged
terrain and repeatedly subjected to a mutagen. After 50
generations, the selfing population showed a substantial
decline in fitness (= survival), whereas the outcrossing
population showed no decline. This is one of a number of
studies that show sexuality to have real advantages over
non-sexual types of reproduction.
What evolution is used for today
An important activity is artificial selection for
domestication. This is when people choose which animals to
breed from, based on their traits. Humans have used this for
thousands of years to domesticate plants and animals.
More recently, it has become possible to use genetic
engineering. New techniques such as 'gene targeting' are now
available. The purpose of this is to insert new genes or
knock out old genes from the genome of a plant or animal. A
number of Nobel Prizes have already been awarded for this
work.
However, the real purpose of studying evolution is to
explain and help our understanding of biology. After all, it
is the first good explanation of how living things came to
be the way they are. That is a big achievement. The
practical things come mostly from genetics, the science
started by Gregor Mendel, and from molecular and cell
biology.
Evolution gems
In 2010 the journal Nature selected 15 topics as 'Evolution
gems'. These were:
Gems from the fossil record |
- Land-living ancestors of whales
- From water to land (see tetrapod)
- The origin of feathers (see origin
of birds)
- The evolutionary history of teeth
- The origin of vertebrate skeleton
|
|
Gems from habitats |
- Natural selection in speciation
- Natural selection in lizards
- A case of co-adaptation
- Differential dispersal in wild birds
- Selective survival in wild guppies
- Evolutionary history matters
|
|
Gems from molecular processes |
- Darwin's Galapagos finches
- Microevolution meets macroevolution
- Toxin resistance in snakes and clams
- Variation versus stability
|
 |
| As evolution became
widely accepted in the 1870s, caricatures of
Charles Darwin with an ape or monkey body
symbolised evolution. |
Responses to the idea of
evolution
Debates about the fact of
evolution
The idea that all life evolved had been proposed before
Charles Darwin published On the Origin of species. Even
today, some people still discuss the concept of evolution
and what it means to them, their philosophy, and their
religion. Evolution does explain some things about our human
nature. People also talk about the social implications of
evolution, for example in sociobiology.
Some people have the religious belief that life on Earth was
created by a god. In order to fit in the idea of evolution
with that belief, people have used ideas like guided
evolution or theistic evolution. They say that evolution is
real, but is being guided in some way.
There are many different concepts of theistic evolution.
Many creationists believe that the creation myth found in
their religion goes against the idea of evolution. As Darwin
realised, the most controversial part of the evolutionary
thought is what it means for human origins.
In some countries, especially in the United States, there is
tension between people who accept the idea of evolution and
those who do not accept it. The debate is mostly about
whether evolution should be taught in schools, and in what
way this should be done.
Other fields, like cosmology and earth science also do not
match with the original writings of many religious texts.
These ideas were once also fiercely opposed. Death for
heresy was threatened to those who wrote against the idea
that Earth was the center of the universe.
Evolutionary biology is a more recent idea. Certain
religious groups oppose the idea of evolution more than
other religious groups do. For instance, the Roman Catholic
Church now has the following position on evolution: Pope
Pius XII said in his encyclical Humani Generis published in
the 1950s:
"The Church does not forbid that (...) research and
discussions (..) take place with regard to the doctrine of
evolution, in as far as it inquires into the origin of the
human body as coming from pre-existent and living matter."
Pope Pius XII Humani Generis
Pope John Paul II updated this position in 1996. He said
that Evolution was "more than a hypothesis":
"In his encyclical Humani Generis, my predecessor Pius XII
has already [said] that there is no conflict between
evolution and the doctrine of the faith regarding man and
his vocation. (...) Today, more than a half-century after
(..) that encyclical, some new findings lead us toward the
recognition of evolution as more than an hypothesis. In fact
it is remarkable that this theory has had progressively
greater influence on the spirit of researchers, following a
series of discoveries in different scholarly disciplines."
Pope John Paul II speaking to the Pontifical Academy of
Science
The Anglican Communion also does not oppose the scientific
account of evolution.
Using evolution for other
purposes
Many of those who accepted evolution were not much
interested in biology. They were interested in using the
theory to support their own ideas on society.
Racism
Some people have tried to use evolution to support racism.
People wanting to justify racism claimed that certain
groups, such as black people, were inferior. In nature, some
animals do survive better than others, and it does lead to
animals better adapted to their circumstances. With humans
groups from different parts of the world, all evolution can
say is that each group is probably well suited to its
original situation. Evolution makes no judgements about
better or worse. It does not say that any human group is
superior to any other.
Eugenics
This amazing idea of eugenics was rather different. Two
things had been noticed as far back as the 18th century. One
was the great success of farmers in breeding cattle and crop
plants. They did this by selecting which animals or plants
would produce the next generation (artificial selection).
The other observation was that lower class people had more
children than upper-class people. If (and it's a big if) the
higher classes were there on merit, then their lack of
children was the exact reverse of what should be happening.
Faster breeding in the lower classes would lead to the
society getting worse.
The idea to improve the human species by selective breeding
is called eugenics. The name was proposed by Francis Galton,
a bright scientist who meant to do good. He said that the
human stock (gene pool) should be improved by selective
breeding policies. This would mean that those who were
considered "good stock" would receive a reward if they
reproduced. However, other people suggested that those
considered "bad stock" should not have babies. The German
Nazi government (1933–1945) used eugenics as a cover for
their extreme racial policies, with dreadful results.
The problem with Galton's idea is how to decide which
features to select. There are so many different skills
people could have, you could not agree who was "good stock"
and who was "bad stock".
Algorithm design
Some equations can be solved using algorithms that simulate
evolution. Evolutionary algorithms work like that.
Social Darwinism
Another example of using ideas about evolution to support
social action is Social Darwinism. Social Darwinism is a
term given to the ideas of the 19th century social
philosopher Herbert Spencer. Spencer believed the survival
of the fittest could and should be applied to commerce and
human societies as a whole.
Again, some people used these ideas to claim that racism,
and ruthless economic policies were justified. Today, most
biologists and philosophers say that the theory of evolution
should not be applied to social policy.
Controversy
Some people disagree with the idea of evolution. They
disagree with it for a number of reasons. Most often these
reasons are influenced by or based on their religious
beliefs. People who do not agree with evolution usually
believe in creationism or intelligent design.
Despite this, evolution is one of the most successful
theories in science. People have discovered it to be useful
for different kinds of research. None of the other
suggestions explain things, such as fossil records, as well.
So, for almost all scientists, evolution is not in doubt. |
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 Kiddle: Evolution
Wikipedia: Evolution |
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