Transcript Evolution - ISGROeducation

Evolution
Theories of Evolution
Geological Time
What is evolution?
Evolution is the permanent genetic in a
population of individuals.
It does not refer to changes that occur to an
individual within its own lifetime – individuals do
not evolve, but populations can.
The modern theory of evolution states that all
living organisms share a common origin, dating
back more than 4 billion years.
Theories of Evolution:
17th Century Views
View from the seventeenth century of the origin of birds and fish is summarised in the
following quotation:
There is a tree, the tree of life — not, it is true, common in France, but commonly observed in
Scotland. From this tree leaves fall; upon one side they strike the water and slowly turn into
fishes, upon the other they strike the land and turn into birds.
Benoit De Maillet (1656–1738)
De Maillet proposed that new kinds of organism could evolve as a result of changes to
the structures of an existing organism, such as a fin transforming to a wing.
Explaining the appearance of birds on Earth, De Maillet stated that flying fish that were
chased out of the water:
… might have fallen some distance from shore among plants which, while supplying them
with food, prevented them from returning to the water. Here, under the influence of the air,
their anterior fins with their raised membranes transformed into wings … the ventral fi ns
became limbs, the body was remodelled, the neck and beak became elongated and the fish
discovered itself a bird.
Theories of Evolution:
19th Century Views
Erasmus Darwin (1731–1802)
British physician and leading intellectual
In Zoonomia, published in 1794, Erasmus Darwin argued that all living
organisms originated from a single common ancestor. He wrote:
Would it be too bold to imagine that, in the great length of time since the earth
began to exist, perhaps millions of ages before the commencement of the history
of mankind that all warm-blooded animals have arisen from one living filament …
with the power of acquiring new parts … and thus possessing the faculty of
continuing to improve … and of delivering these improvements by generation to its
posterity, world without end!
While Erasmus Darwin accepted that species could be transformed to produce
new species, he gave no mechanism for this process.
Theories of Evolution:
19th Century Views
Jean Baptiste Lamarck (1744-1829)
French naturalist
Published in 1809 his views that:
structures in individual organisms could change in response to
environmental conditions and to physiological need
these acquired structural changes would be transmitted to the next
generation.
In simple terms, a giraffe has a long neck because it stretched to
reach high leaves, and this long neck was then inherited by it’s
children.
Lamarck believed that organs appeared or disappeared
according to the use made of them. He thought that use
strengthened or enlarged an organ permanently and habitual
disuse led to permanent loss.
Theories of Evolution:
19th Century Views
Robert Chambers (1802-1871)
Supported the idea that species could evolve.
In his best-selling book, Vestiges of the Natural History of
Creation, published anonymously in 1844 Chambers wrote:
The idea, then, which I form of the progress of organic life on the globe …
is that the simplest and most primitive type … gave birth to the type next
above it, that this again produced the next higher, and so on to the very
highest, the stages of advance being in all cases very small — namely,
from one species only to another …
The cause of evolution identified by Chambers was that it
occurred ‘under a law to which that of like-production is
subordinate’.
Since this is not a testable hypothesis, and able to be disproved,
it has no scientific status.
Theories of Evolution:
19th Century Views
The views of De Maillet, Erasmus Darwin, Lamarck and Chambers were based on an
assumption that species could change.
This view, known as the transmutation of species, recognised that species could
change over time to produce new species and were not fixed and unchanging.
In contrast, the general view held by many scientists in the early 1800s, particularly in
Britain, was that species were unchanging and that each species was fixed in its
structure and characteristics for all time.
According to this view, each species was the result of an act of creation — a view
known as special creation of species. If this were the case, then each of the different
kinds of fossil organisms from the past and each modern species would have been
specially and individually created.
These contrasting views — transmutation of species versus special creation of species
— are not compatible.
The view that species were fixed and each was specially created was finally dismissed
as a result of the theory of evolution by natural selection developed by Charles Darwin
and Alfred Wallace.
Evolution
The concept that species can change and give rise over time to
new forms is known as evolution.
Evolution is also defined as descent with modification.
Evolution can account for the diversity of species, past and
present.
Evidence from the fossil record and from many other fields of
study supports the view that species can change and that
evolution has occurred.
Evolution provides an explanatory framework for many
observations.
Evolution means that past and modern species are related and
that different kinds of organism living today are descended from
various kinds of organism that lived in the geological past.
The Darwin-Wallace View
Various naturalists in the seventeenth and early eighteenth centuries proposed
that species could evolve or be transformed to produce new species, but none
of these naturalists identified a testable mechanism to explain how evolution
could occur.
This changed when Charles Darwin (1809–1882) and Alfred Russel Wallace
(1823–1913) proposed their theory of evolution by natural selection.
The power of the Darwin–Wallace theory of evolution by natural selection was
that:
the theory identified a mechanism or a cause of evolution
this mechanism was testable by observation and experimentation.
The theory also asserted that individual organisms do not evolve in their lifetimes
but that evolutionary change occurs over several generations in populations.
In developing their thinking about evolution, both Darwin and Wallace were
influenced by observations made on independent and separate voyages to
remote islands
Darwin’s Voyage
In 1831, Charles Darwin was offered the unpaid position as naturalist on HMS
Beagle which undertook a five year voyage around the world.
Upon visiting the Galapagos Islands, a cluster of more than one dozen
volcanic islands located in the Pacific Ocean, nearly 1000 kilometres west of
Equador, Darwin realised that, while the islands had similar plants and
animals, each island had particular and distinctive varieties.
He observed that the Galapagos Islands were home to more than a dozen
species of finches. These finches showed some similarities to finches he had
seen in Chile in South America.
After his return to England in 1836, Darwin spent many years thinking about
how species could change.
Darwin came to realise that each species of finch was not specially created.
Instead, he recognised that each kind of finch on the Galapagos Islands had
evolved from a few ancestral finches that reached there from South America.
In 1844, he began to put on paper an outline of his ideas of evolution by
natural selection.
Wallace’s Voyage
Alfred Wallace also travelled widely and visited many islands.
In 1858, Wallace was in the Moluccas thinking about the problem of how species are
formed.
Wallace wrote:
. . . checks must also act upon animals, and keep down their numbers . . . While vaguely
thinking how this would affect any species, there suddenly flashed upon me the idea of the
survival of the fittest — that individuals removed by these checks must be, on the whole,
inferior to those that survived. Then considering the variations continually occurring in every
fresh generation of animals and plants, and the changes of climate, of food, of enemies
always in progress, the whole method of species modification became clear to me.
So, Wallace concluded that natural selection was a plausible mechanism for evolution
of species and he wrote a manuscript outlining his ideas and ‘sent it by the next post to
Mr Darwin’.
Darwin realised that Wallace had independently reached the same conclusion as he
had, namely, that new species could arise as a result of the action of natural selection
acting over many generations on populations.
They agreed to present their proposal jointly to the scientific community.
Neo-Darwinsim
During the first half of the twentieth century, biologists combined Darwin’s
ideas with the concepts of Mendelian genetics to produce a synthesis of ideas
that is known as neo-Darwinism (neo = new).
Darwin and Wallace identified one mechanism to explain how species evolve
— the slow and gradual accumulation over long periods of time of inherited
differences through natural selection.
However, this is not the only process.
During the late 1900s, it was recognised that new species could also be
produced by other means including:
random or chance events, known as genetic drift
changes in the number of sets of chromosomes (which is a significant and very
rapid means of production of new plant species).
Intelligent Design
Is the assertion that some features of living things are best explained as the work of a designer
rather than as the result of a random process like natural selection.
It points to complex structures in living organisms such as the eye and systems like the
mechanisms for blood clotting as evidence against natural selection, suggesting they could not
have arisen through the gradual fits and starts of evolution.
Gaps in the fossil record, particularly during the Cambrian period where there was an explosion of
new species, are also used as evidence for intelligent design.
The term "intelligent design" originated in response to a 1987 United States Supreme Court ruling
involving constitutional separation of church and state – specifically its ruling that teaching
creationism was unconstitutional in public school science curricula.
The difference between intelligent design and creationism is that intelligent design's advocates do
not try to identify the “designer”. By not naming the Judo-Christian creator God and accepting the
story of Genesis as a parable, supporters of ID claim it is a scientific theory, and seek to
fundamentally redefine science to accept supernatural explanations – i.e. the external “designer”.
Many American high schools boards are pushing for the incorporation of intelligent design
alongside or in place of traditional evolutionary theories within the science curriculum.
Comparison of excerpts from two
text books for high school biology
Biology
(Kenneth Miller and Joseph Levine)
Darwin made bold assumptions
about heritable variation, the age of
Earth and relationships among
organisms. New data from
genetics, physics and biochemistry
could have proved him wrong on
many counts. They didn’t.
Scientific evidence supports the
theory that living species
descended with modification from
common ancestors that lived in the
ancient past.
Of Pandas and People
(Percival Davis and Dean Kenyon)
Intelligent design means that
various forms of life began abruptly
through an intelligent agency, with
their distinctive features already
intact – fish with fins and scales,
birds with feathers, beaks and
wings, etc. Some scientists have
arrived at this view since fossil
forms first appear in the rock
record with their distinctive features
intact, rather than gradually
developing.
Can you believe in God and Evolution?
That’s up to you!!!!!
Many scientists, including the director of the Human
Genome Project do.
“I do not find the wording of Genesis to suggest a scientific
textbook but a powerful and poetic description of God’s
intentions in creating the universe. The mechanism of
creation is left unspecified. If God, who is all powerful and
who is not limited by space and time, chose to use the
mechanism of evolution to create you and me, who are we to
say that wasn’t an absolutely elegant plan? And if God has
now given us the intelligence and the opportunity to discover
his methods, that is something to celebrate”.
Time scales in evolution
Time is a critical element in evolution.
Estimating the age of rocks and of fossils that
they contain is important in reconstructing the
evolution of life on Earth.
How old is the Earth?
James Ussher (1581–1656)
Anglican Archbishop of Armagh
Published his calculation of the age of the Earth based on the Bible.
By adding the ages of Adam and Eve and their descendants as given in the ‘Genesis’
chapter, Ussher concluded that the Earth was created on the evening of Sunday 23
October 4004 BC, making it about 6000 years old.
This view prevailed until the late eighteenth century
James Hutton (1726–1797)
Scottish farmer and amateur scientist
Published his conclusions about the age of the Earth in his Theory of the Earth and
identified that ‘countless ages are required to form mountains, rock and soil’.
Hutton recognised that repeated cycles of sedimentation, compression into rock, uplift
and erosion had occurred and concluded that the Earth was millions of years old.
He recognised that the processes of building layers of sedimentary rock were slow and
yet, in spite of this, some of these layers were several kilometres thick.
How old is the Earth?
Charles Lyell (1797–1875)
Published the three-volume Principles of Geology n 1830–33
Lyell recognised that present-day cycles of sedimentation, compression into rock, uplift and
erosion must have been repeated many times in the past and that the same geological forces
acting today would also have acted in the past.
Lord Kelvin (1824–1907)
Calculated the age of the Earth based on an estimated rate of cooling of Earth from an original
molten state
His first estimate (in 1862) was 98 million years old. (In 1897, he amended this estimate to 20 to
40 million years.)
Kelvin’s estimates were too low because at that time, no-one knew about radioactive elements that
are a source of internal heat in the Earth’s crust and so affect the rate of cooling.
John Joly (1899)
Irish scientist
Estimated the age of the Earth from salt content in the oceans.
He calculated that it would have taken about 90 million years for the oceans to accumulate the
present-day salt levels from inflow of river waters.
This method was an underestimate of Earth’s age because it failed to account for salt in other
locations, such as salt deposits.
How old is the Earth?
Other attempts to date the Earth were based on estimating rates at which sediments
were deposited under the sea and then measuring the total thickness of sedimentary
rock strata.
Based on average sedimentation rates of 0.3 metres per 1000 years, one estimate of
the Earth’s age as about 1600 million years was obtained in 1910.
From about 1910 onwards, estimates using simple radiometric methods were made of
the age of the Earth.
By the mid-twentieth century, modern radiometric dating techniques had been
developed.
In 1956, Clair C. Patterson (1922–1995) used these techniques to calculate an
accurate and reliable estimate of the age of the Earth based on the age of an iron
meteorite. This meteorite had an estimated age of about 4500 million years.
Since meteorites came into existence at the time of formation of the solar system, their
age also identifies the age of the Earth.
Since then, the age of many more meteorites has been determined and all have been
found to have ages between 4400 and 4500 million years.
This value has been confirmed by the dating of some moon rocks at 4500 million years
old.
Geologic Time
Our understanding of geologic time or deep time is recent.
Based on modern techniques of radioactive dating, we now know that the
Earth is about 4500 million years old — an interval that provides sufficient time
for evolutionary and geological processes to occur.
The geologic history of the Earth is divided into various time intervals as a
hierarchy that includes eons, eras and periods.
This geologic time scale developed in the eighteenth and nineteenth centuries
when scientists organised sedimentary rock strata in the same region into
groups of similar relative ages and also recognised similarities in rock strata
in different regions because they contained identical fossils.
By about 1840, the major divisions of the time scale based on relative ages
were established but the absolute ages were not identified until during the
twentieth century.
Geologic Time
Geologic ages almost defy comprehension.
We can gain some understanding by representing the age of Earth as a 100-metre
track, with the starting line being the present time and the finishing line being the time
of formation of Earth more than 4500 million years (Myr) ago.
A human life of 72 years on this scale would be an undetectable 0.002 millimetres.
Just the tip of a fingernail (one millimetre) over the starting line would take us back 45
000 years.
Two steps (two metres) would take us to about 90 million years ago, to the time when
Australia was part of the Gondwana super-continent and dinosaurs dominated life on
Earth.
Perhaps the most remarkable fact about this geologic time scale is that the first indirect
evidence of life on Earth appeared about 3850 million years ago, but it was not until the
Ediacaran period about 620 million years ago that the first multicellular animals
appeared in the fossil record.
The first members of the genus Homo appeared only 2.4 million years ago and the first
modern human (H. sapiens, our species) first appeared about 190 000 years ago.
Geological Eras
The boundaries of eras are marked by major
evolutionary events, such as mass extinctions.
The four eras are (from oldest to youngest):
Precambrian
Palaeozoic
Mesozoic
Cenozoic
Precambrian Era
4560 – 570 million years ago
Extends from the origin of Earth and its oceans and atmosphere,
to the origin of life, with first the evolution of prokaryotic cells and
then single-celled and multicellular organisms.
Multicellular organism only appeared 680 mya towards the end
of the Precambrian era. Their fossils are referred to as
Ediacaran fauna.
Fossils of Edicaran fauna are nearly all small (~3cm in diameter.
Some were small and worm-like and probably burrowed through
soft sand and mud on the bottom of the sea. Others are like
jellyfishes and sea anemones as well as some extinct species.
Palaezoic Era
570 – 245 million years ago
Beginning of this era is marked by the appearance of a diversity of animals
(the Cambrian explosion) and the its end is marked by the great Permian
extinction.
Palaezoic era is divided into six periods:
Cambrian – marked by appearance of invertebrates, trilobites were most common
marine multicellular animals of the early Cambrian
Ordovician – widespread shallow seas, diverse marine life, first vertebrates
(jawed fish)
Silurian – first invasion of land by plants, fungi and scorpion like arthropods
Devonian - first trees and land vertebrates, radiation of fish
Carboniferous – ferns and amphibians dominant, first reptiles
Permian – reptiles dominant, Pangaea formed and mass extinction occurred as a
result of reduced rainfall and extremes of temperature.
Mesozoic Era
245 – 65 million years ago
Often described as the “Age of Reptiles”
Divided into three periods:
Triassic – marine and dinosaurs, including flying
pterosaurs
Jurassic – dinosaurs dominant, first birds
Cretaceous – flowering plants originate, first
marsupials and placental mammals
Cenozoic Era
65 million years ago to present
The Cretaceous extinction (included dinosaurs)
marked the start of the Cenozoic era, leading to
modern organisms we are familiar with today.
Divided into two periods:
Tertiary – flowering plants and mammals increase,
Homo genus appears
Quaternary – major ice ages, human range
increases
Relative age versus absolute age
The age of a person can be stated in two ways — by relative age
or by absolute age.
For example:
the statement ‘Kym is older than Tran who is younger than Shane’ gives
the relative ages of these persons but does not give their chronological
ages.
Relative age allows us to place the persons in order of age: Kym then
Shane then Tran.
the statement ‘Kym is 34 years, Shane is 18 years and Tran is 16 years
old’ gives the absolute ages of these people.
Likewise, the ages of geological structures, such as a deposit of
mudstone or a layer of solidified volcanic ash (tuff), can be given
in either relative or absolute terms.
Relative age
The capacity to estimate the age of rocks enables the age of fossils to be inferred
when they are embedded in a rock layer of known age or are located between layers of
solidified volcanic ash that can be dated.
The stratigraphic method of dating rocks gives the relative age of rock strata and uses
the principle of superposition. This principle simply states that, for rock layers or
strata (singular: stratum), the oldest stratum is at the bottom and progressively
younger layers lie above it.
A problem with this method is that sometimes layers of rocks are eroded away,
buckled, moved or reburied and the original geological sequence is destroyed.
Relative age
Another method for dating rocks
uses the principle of correlation
and is based on identifying particular
fossils, known as index fossils.
These are geologically short-lived
species whose fossils have a limited
occurrence in the fossil record and
so are found only in a restricted
depth of sedimentary rock strata.
The presence of these fossils in rock
strata, even in widely separated
regions of the world, can be used to
identify these rocks as having the
same age.
These two methods give the relative
ages of rock layers throughout the
world.
Rocks of the same age are identified
by the name of a geological period,
such as Devonian or Jurassic.
Absolute age
During the twentieth century, techniques for identifying the absolute age of
rocks were developed.
The most important method of estimating the absolute age of rocks is the
radiometric dating technique that is based on the decay of certain radioactive
elements.
The radioactive elements concerned are those present in minerals in igneous
rocks and each element decays at a particular rate.
Igneous rocks form when molten rock solidifies, either below the Earth’s
surface (for example, granite) or on the Earth’s surface (for example, basalt,
tuff).
This form of dating cannot be applied to sedimentary rocks that are derived
from the erosion of pre-existing rocks because the minerals that they contain
were formed prior to the rocks themselves.
The principle of radiometric dating depends on the fact that various elements,
known as parents, contain radioactive isotopes that spontaneously decay or
break down to form stable daughter products.
Absolute age
The time for the decay is specific for each
radioactive isotope and the half life is the time
taken for half the original radioactive isotope to
decay.
Each technique can be used over a particular
age range that depends on the half life of the
parent radioactive isotope.
Idealised decay of a radioactive parent
element over a time period of five half lives.
Example: Potassium-Argon Dating
K40 → Ar40
When lava erupts at the
surface, it loses all its argon.
With time K40 decays to Ar40.
The longer the time, the
more Ar40.
Thus by precisely measuring
the amount of Ar40 and K40 in
a rock, geoscientists can
make precise estimates of its
age.
← K40
←
Ar40
Example: carbon 14 dating
C14 → N14
All living organisms are built of carbon-containing organic matter, such as proteins (for
example, the keratin of hair, nails, hooves, claws and the collagen of bones) and
structural carbohydrates (for example, cellulose of plant tissues).
When an organism is alive, the carbon in its organic matter is a mixture of two
isotopes:
the stable isotope, carbon-12 (C-12)
the radioactive isotope, carbon-14 (C-14) which decays to N-14.
In life, the proportion of these two isotopes is constant and matches that in the carbon
dioxide in the atmosphere. This proportion remains constant during the life of an
organism.
After one half life of 5730 years, half the original amount of C-14 present in the organic
material at the time of death will have disappeared, and so on for each successive halflife period.
By measuring the ratio of C-14 to C-12 in a sample of organic material, an estimate of
the time since death of the organism that produced this material can be obtained.
C-14 dating can date bones and also artefacts, such as the wooden handles of tools or
localised collections of ash and fragments of burnt wood.
C-14 dating can estimate the absolute age of this material provided it is not older than
about 60000 years.
Thermoluminescence
Thermoluminescence is the emission of light from a mineral
when it is heated.
It can be used to date objects such as pottery, cooking hearths
and fire treated tools up to 500000 years old.
It measures the accumulated radiation dose of material
containing crystalline minerals since it was either heated (lava,
ceramics) or exposed to sunlight (sediments). The older the
object the more radiation it will have accumulated.
The material is heated during measurements, and a weak light
signal is produced that is proportional to the radiation dose.
Electron-spin resonance (ESR)
Electron-spin resonance (ESR) is a useful dating technique for ages from
about 50 000 years ago to 500 000 years old.
This period encompasses much of the evolutionary history of the genus Homo.
The ESR dating technique depends on the fact that when objects are buried
they are bombarded by natural radiation from the soil.
For objects are composed of minerals (e.g. flint tools and fossil teeth) this
bombardment causes some of the electrons in the minerals to move from the
ground state to a higher energy level and some remain trapped at higher
energy levels.
The rate at which high energy electrons become trapped is determined by the
background radiation; the longer the material has been buried, the greater the
accumulation of higher-energy electrons.
Electron-spin resonance (ESR)
When material is exposed to heat, fire or bright sunlight, all the electrons
return to the ground state so that the ‘electron clock’ is re-set to zero.
As a result, ESR can be used to estimate the time since material under
investigation was last heated, such as when a flint instrument was burned in a
fire or when a tooth last lay exposed on the ground in sunlight.
In ESR, scientists make direct measurements of:
the number of high-energy electrons trapped in the material under investigation
the radiation owing to any unstable isotopes in the material itself
the background radiation in the soil in which the sample was buried.
Using these measures, it is possible to calculate a date that indicates the time
that has passed since the ‘electron clock’ was last set to zero.