Macroevolution= The evolution of taxonomic groups higher than
the species level (genera, families, etc.).
§
Macroevolution is concerned with major events
in the history of life as found in the fossil record.
§
Encompasses the origin of new designs such
as: feathers, and wings of birds,
upright posture of humans, increasing brain size in
mammals,
diversification of certain groups of organisms following
evolutionary
breakthroughs, and extinction.
§
Macroevolution extends the study of evolution
to include global environment and biological change.
Is macroevolutionary change primarily a
cumulative product of microevolution working gradually over time, or is it the
product of mechanisms other than the gradual modification of populations by
natural selection?
Biologists
reconstruct evolutionary history by studying the succession
of organisms in the fossil
record. Fossils are collected and
interpreted
by paleontologists.
A.
How
Fossils Form
Fossil= Any
preserved remnant or impression left by an organism
that lived in the
past.
Sedimentary
rocks are the richest sources of fossils.
§
These rocks form from deposits of sand and silt
which have weathered or eroded from the land and are carried by rivers to seas
and swamps.
§
Aquatic organisms and some terrestrial forms
which were swept into seas and swamps became trapped in sediments when they
died.
§
New deposits pile on and compress older sediments
below into rock. Sand is compressed
into sandstone and mud into shale.
§
A small proportion of the organisms left a
fossil record.
Fossils usually form from mineral rich hard parts of organisms (bone,
teeth, shells invertebrates) since most organic substances usually decay
rapidly.
§
Paleontologists usually find parts of skulls,
bone fragments, or teeth; although nearly complete skeletons of dinosaurs and
other forms have been found.
§
Many of the parts found have been hardened by petrification,
which
Occurs when minerals
dissolved in groundwater seep into the tissues of dead organisms and replace
organic matter.
Some
fossils, found as thin films pressed between layers of sandstone
or
shale, retain organic material.
§
Paleontologists have found leaves millions of
years old that are still green with chlorophyll and preserved well enough that
their organic
Composition and
ultrastructure could be analyzed.
§
One research team was even able to clone a very
small sample of DNA from ancient magnolia leaf.
Other
fossils found by paleontologists are replicas formed in molds left
when
corpses were covered by mud or sand.
§
Minerals from the water which filled the mold
eventually crystallized in the shape of the organism.
Trace fossils,
form in footprints, animal burrows, and other impressions
left
in sediments by animal activity. These
can provide a great deal of
information.
§
Dinosaur tracks can provide information about
the animals gait, stride length, and speed.
Rarely, an entire organism has been fossilized, which could only have
happened if the individual was buried in a medium that prevented and
bacteria fungi from decomposing the body.
A fossil represents a
sequence of improbable events:
§
An organism had to die in the right place and
at the proper time for burial conditions favoring fossilization.
§
The rock layer containing the fossil had to
escape geologic events ( erosion, pressure, extreme heat) which would have
distorted or destroyed the rock.
§
The fossil had to be exposed and not destroyed.
§
Someone who knew what they were doing had to
find the fossil.
Paleontologists thus work with an incomplete record for many reasons.
§
A large fraction of species that have lived
probably left no fossils.
§
Most fossils that were formed have probably
been destroyed.
§
Only a small number of existing fossils have
been discovered.
§
Consequently, the fossil record is comprised
primarily of species that lived a long time, were abundant and widespread, and
had shells or hard skeletons.
Even
though it is incomplete, the fossil record provides the outline of
macroevolution, but the evolutionary relationships between modern
organisms must be studied to provide the details.
Several methods are used to determine the age of fossils, which makes
them
useful in studies of macroevolution.
Sedimentation may
occur when the sea-level changes or lakes and
swamps dry and refill.
§
The rate of sedimentation and the types of
particles that sediment vary with time when a region is submerged.
§
The different periods of sedimentation resulted
in formation of rock layers called strata.
§
Younger strata are superimposed on top of older
ones.
§
The succession of fossil species chronicles
macroevolution, since fossils in each layer represent organisms present at the
time of sedimentation.
Strata from different locations can often be correlated by the
presence of similar fossils, known as index fossils.
§
The shells of widespread marine organisms are
the best index fossils for correlating strata from different areas.
§
Gaps in the sequence may appear in an area if
it was above sea level (which prevents sedimentation) or if it was subjected to
subsequent erosion.
Geologists have formulated a sequence of geological periods by
comparing many different sites.
This sequence is known as the
geological
time
scale.
§
These periods are grouped into four eras with
boundaries between the eras marking major transitions in the life forms
fossilized in the rocks.
§
Periods within each era are subdivided into
shorter intervals called epochs.
§
The divisions are not arbitrary, but are
associated with boundaries that correspond to times of change.
This record of the rocks presents a chronicle of the relative
ages of
fossils showing the order in which species groups evolved.
Absolute dating is
not errorless, but it does give the age in years rather than in relative terms
(e.g. before, after).
The most common
method for determining the age of rocks and fossils on an absolute time scales radiometric
dating.
§
Fossils contain isotopes of elements that
accumulated in the living organisms.
§
Since each radioactive isotope has a fixed half-life,
it can be used to date fossils by comparing the ratio of certain isotopes (e.g.
14C and 12 C) in a living organism to the ratio of the
same isotopes in the fossil.
half-life=
the number of years it takes for 50% of the original sample to decay.
§
The half-life of an isotope is not affected by
temperature, pressure or other environmental variables.
Carbon-14 has a half-life of 5600 years, meaning that one-half of the
Carbon-14 in a specimen will be gone in 5600 years; half of the
remaining carbon-14 would disappear from the specimen in the next
5600 years; this would continue until all of the carbon-14 had
disappeared.
§
Thus a sample beginning with 8 g of carbon-14
would have 4 g left after 5600 years and 2 g after 11,200 years.
§
Carbon-14 is useful in dating fossils less than
50,000 years old due to its relatively short half-life.
Paleontologists use other radioactive isotopes with longer half-lives to
date older fossils. For example:
§
Uranium-238 has a half-life of 4.5 billion
years and is reliable for dating rocks (and fossils within those rocks)
hundreds of millions of years old.
§
This isotope was used to place the oldest
fossil-containing rocks in the Cambrian period.
§
An error of < 10% is present with
radioactive dating.
Other
methods may also be used to date some fossils.
§
Amino acids can have either left-handed
(L-form) or right-handed(D-form) symmetry.
§
Living organisms only synthesize L-form amino
acids to incorporate into proteins.
§
After an organism dies, L-form amino acids are
slowly converted to D-form.
§
The ratio of L-form to D-form amino acids can
be measured in fossils.
§
Knowing the rate of chemical conversion (racemization)
allows this ratio to be used in determining how long the organism has been
dead.
§
This method is most reliable in environments
where the climate has not change significantly, since the conversion is
temperature sensitive.
The
dating of rocks and fossils they contain has enabled researchers to
determine the geological periods.
Evolutionary biologists interested in macroevolution try to determine
the processes involved in
large-scale evolutionary changes that can be
traced through paleontology and
taxonomy.
Some of the questions to which they are looking for answers are:
§
How do novel features which define taxonomic
groups above the species level arise?
§
What accounts for the apparently progressive
evolutionary trends found in the fossil record?
§
How has macroevolution been affected by global
geological changes?
§
How can we explain the major fluctuations in
biological diversity evident in the fossil record?
novelties. For example:
§
Birds
evolved from dinosaurs, and their wings are homologous to the forelimbs
of modern reptiles.
§
Birds are adapted to flight, yet their ancestors
were earthbound.
How
could these new designs evolve?
§
One mechanism is the gradual refinement of
existing structures for new functions.
§
Most biological structures have an evolutionary
plasticity that makes alternative functions possible.
Preadaptation
is a term applied to a structure that evolved in one
context and became co-opted for another function.
§
Natural selection can not anticipate the future
but can improve an existing structure in context of its current utility. For example:
Ø
The honeycombed bones and feathers of birds did
not evolve as adaptations for flight.
Ø
They must have been beneficial (reduction of
weight, gathering food, courtship) to the bipedal reptilian ancestors of birds
and later, through modification, become functional for flying.
§
Preadaptation can not be proven, but provides
an explanation for how novel designs can arise gradually through a series of
intermediate stages, each having some function in the organism.
§
The evolution of novelties by remodeling of old
structures for new functions reflects the Darwinian tradition of large changes
being crafted by natural selection through an accumulation of many small
changes.
The
evolution of complex structures (e.g. wings) requires such large
Modifications that changes at many gene loci are probably involved.
§
In other cases, relatively few changes in the
genome can cause major modifications in morphology (e.g. humans vs chimpanzees).
§
Thus, slight genetic divergence can become
magnified into major differences.
In
animal development, a system of regulatory genes coordinates
activities of structural genes to guide the rate and pattern of
development from zygote to adult.
§
A slight alteration of development becomes
compounded in its effect on adult allometric growth (differences in
relative rates of growth of various parts of the body) which helps to shape an
organism.
§
A slight change in these relative rates of
growth will result in a substantial change in the adult.
§
Thus, altering the parameters of allometric
growth is one way relatively small genetic differences can have major
morphological impact.
Changes in developmental dynamics, both temporal
and spatial,
have played a major role in
macroevolution.
Temporal changes in development that create
evolutionary novelties are called heterochrony.
Heterochrony= Evolutionary
changes in the timing or rate of development.
§
Genetic changes that alter the timing of
development can also produce novel organisms.
Paedomorphosis= Retention of
ancestral juvenile structures in a
sexually mature adult organism.
Ø
In some species of salamanders, sexually mature
adults retain certain larval features.
Ø
A slight change in timing that retards the
development of some organs in comparison to others produces a different kind of
animal.
§
Changes in developmental chronology may have
contributed to human evolution.
Ø
Humans and chimpanzees are closely related
through descent from a common ancestor.
Ø
They are much more similar as fetuses than as
adults.
Ø
Different allometric properties and variations
result in the human brain being proportionally larger than that in chimpanzees.
Ø
The human brain continues to grow several years
longer than the chimpanzee brain.
Ø
Thus, the genetic changes responsible for
humanness are not great, but have profound effects.
Equally
important in evolution is the alteration of the spatial pattern of development
or homeosis.
Homeosis= alteration in the
placement of different body parts.
§
For example, to the arrangement of different
kinds of appendages
In animals or the
placement of flower parts on a plant.
Since each regulatory gene may influence
hundreds of structural genes,
There is a potential for evolutionary novelties
that define higher taxa to arise much faster than would occur by the
accumulation of changes in only structural genes.
macroevolution is goal-oriented
In
most cases, a correct single evolutionary progression can not be
Produced from the fossil record.
§
This is a result of divergence which may
produce organisms that seem to fit a single progression, but actually represent
related but divergent forms.
§
Modern horses (Equus) are believed to
have evolved from Hyracotherium.
§
A single evolutionary progression would have to
include:
Ø
An increase in size.
Ø
Reduction of four toes to one.
Ø
Modification of browsing teeth to grazing
teeth.
Selection of certain fossils can produce an
apparent single evolutionary progression of intermediate organisms between Hyracotherium
and modern horses (Equus).
§
This is a misrepresentation as a more complete
phylogeny shows that Equus is the only survivor of a much
more complicated evolutionary tree.
§
Examination of the complete fossil sequence
also shows that the transition from Hyracotherium to Equus
was not a smooth graduation but involved a number of transitional steps which
included a series of speciation episodes and several adaptive radiations.
Evolution has produced many genuine trends. For example:
§
Punctuated equilibrium seems to have produced a
trend toward larger size in the titanotheres.
§
The titanotheres were about as large as
elephants and the fossil records shows that they evolved from a mouse-sized
ancestral mammal.
§
Although each species in the various lineages
of titanotheres remain the same size, there was a succession of progressively
larger species.
Branching evolution (cladogenesis) can produce
a trend even if some new species counter the trend.
§
There was an overall trend in reptilian
evolution toward large size during the Mesozoic era which eventually produce
the dinosaurs.
§
This trend was sustained even though some new
species were smaller than their parental species.
Even with an equal large-to-larger and
large-to-smaller ratio, the trend would be maintained if the larger groups
speciated at a greater rate than smaller groups or lasted longer before
becoming extinct.
§
Steven Stanley originated this view of
macroevolution which holds that species are analogous to individuals with
speciation being their birth and extinction their death.
§
According to the Stanley model, an evolutionary
trend is produced by species selection which is analogous
to the production of a trend within a population by natural selection.
§
The species that live longest and generate the
greatest number of new species determine the direction of major evolutionary
trends.
§
Differential speciation thus may play a role in
macroevolution similar to the role of differential reproduction in
microevolution.
Qualities unrelated to the success of organisms
in a specific environment may be equally important in species selection.
§
The ability of a species to disperse to new
habitats may result in development of new “daughter species” as organisms adapt
to new conditions.
§
A criticism of species selection is the
argument that gradual modification of populations response to environmental
change is the most common stimulus to evolutionary trends.
No intrinsic drive toward a preordained state
of being is indicated by the presence of an evolutionary trend.
§
Evolution is a response to interactions between
organisms and their current environments.
§
An evolutionary trend may cease or reverse
itself under changing conditions. For
example, conditions of the Mesozoic era favored giant reptiles, but by the end
of that era, smaller species prevailed.
Macroevolution has dimension in space as well as time.
§
Biogeography was a major influence on Darwin
and Wallace in developing their views on evolution.
§
Drifting of continents is the major
geographical factor correlated with the spatial distribution of life.
Continental drift results from the movement of great plates of crust and
upper
mantle that float on the Earth’s molten core.
§
The relative positions of two land masses to
each other changes unless they are embedded on the same plate.
§
North America and Europe are drifting apart at
a rate of 2 cm per year.
§
Where two plates meet (boundaries), many
important geological phenomena occur:
mountain building, volcanism, and earthquakes.
Ø
Volcanism, in turn, forms volcanic islands
(e.g. Galapagos), which opens new environments for founders and adaptive
radiation.
Plate movements continually rearrange
geography, however, two occurrences had important impacts on life: the formation of Pangaea.
and the subsequent breakup of Pangeae.
At the end of the Paleozoic era (250 million
years ago), plate movements brought all land masses together into a
super-continent called Pangaea.
§
Species evolving in isolation were brought
together and competition increased.
§
Total shoreline was reduced and the ocean
basins became deeper (draining much of the remaining shallow coastal seas).
§
Marine species (which inhabit primarily the
shallow coastal areas) were greatly affected by reduction of habitat.
§
Terrestrial organisms were affected as
continental interior habitats (and their harsher environments) increased in
size.
§
Changes in ocean currents would have affected
both terrestrial and marine organisms.
§
Overall diversity was thus impacted by
extinctions and increased opportunities for surviving species.
During the early Mesozoic era ( about 180
million years ago) Pangaea began to breakup due to continuing continental
drift.
§
This isolated the fauna and flora occupying
different plates.
§
The biogeographical realms were formed and
divergence of organisms in the different realms continued.
Many biogeographical puzzles are explained by
the pattern of continental separations.
For example:
§
Matching fossils recovered from widely
separated areas.
Ø
Although Ghana and Brazil are separated by 3000
km of ocean, matching fossils of Triassic reptiles have been recovered from
both areas.
§
Why Australia has such a unique fauna and
flora.
Ø
Australian marsupials are very diverse and
occupy the same ecological roles as placental mammals on other continents.
Ø
Marsupials probably evolved on the portion of
Pangaea that is now North America and migrated into the area that would become
Australia.
Ø
The breakup of Pangaea isolated Australia (and
its marsupial populations) 50 million years ago, while placental mammals
evolved and diversified on the other continents.
adaptive radiations of survivors
The evolution of modern life has included long, relatively quiescent
periods punctuated by briefer intervals of more extensive turnover
In species composition.
§
These intervals of extensive turnover included
explosive adaptive radiations of major taxa as well as mass extinctions.
A.
Examples of Major Adaptive Radiations
adaptive zones
allowing many taxa to diversify greatly their
early
History. For example:
§
Evolution of wings allowed insects to enter an
adaptive zone with abundant new food sources and adaptive radiation resulted in
hundreds of thousands of variations on the basic insect body plan.
§
A large increase in the diversity of sea
animals occurred at the boundary between the Precambrian and Paleozoic
eras. This was a result, in part, of
the origin of shells and skeletons in a few key taxa.
Ø
Precambrian rock contains the oldest
animals(700 million years old) which were shell-less invertebrates that
differed significantly from their successors found in Paleozoic rock.
Ø
Nearly all the extant animal phyla and many
extinct phyla evolved in less than 10 million years during the mid-Cambrian
(early Paleozoic era).
Ø
Shells and skeletons opened a new adaptive zone
by making many new complex body designs possible and altering the basis of
predator-prey relationships.
Ø
It is possible that genes controlling
development evolved during this time, resulting in a potential for increased
morphological complexity and diversity.
An empty adaptive zone can be exploited only if the appropriate
evolutionary novelties arise.
For example:
§
Flying insects existed for 100 million years
before the appearances of flying reptiles and birds that fed on them.
Conversely, an evolutionary
novelty can not enable organisms to
exploit adaptive zones that are occupied or that do not exist.
§
Mass extinctions have often opened adaptive
zones and allowed new adaptive radiations.
§
For example, mammals existed 75 million years
before their first large adaptive radiation in the early Cenozoic. This may have resulted from the ecological
void created with the extinction of the dinosaurs.
B.
Examples of Mass Extinctions
Extinction is inevitable in a changing world. The average rate of
extinction has been between 2.0 and 4.6 families (each family
may include many species) per million years.
Extinctions may be caused by habitat destruction or by unfavorable
environmental changes.
§
Many very well adapted marine species would
become extinct if the ocean’s temperature fell only a few degrees.
§
Changes in biological factors may cause
extinctions even if physical factors remain stable.
§
Since many species coexist in each community,
an evolutionary change in one species will probably impact other species. For example:
Ø
The evolution of shells by some Cambrian
animals may have contributed to the extinction of some shell-less forms.
There have been periods of global environmental
change which greatly disrupted life and resulted in mass extinctions.
§
During these periods, the rate of extinction
escalated to as high as 19.3 families per million years.
§
These mass extinctions are recognized primarily
from the decimation of hard-bodied animals in shallow seas which have the most
complete fossil record.
§
Two (of about a dozen) mass extinction episodes
have been studied extensively by paleontologists.
The Permian extinctions (the boundary between
the Paleozoic and Mesozoic eras) eliminated over 90% of the species of marine
animals about 250 million years ago.
§
Terrestrial life was probably also affected
greatly. For example, 8 of the 27
orders of Permian insects did not survive into the Triassic.
§
This mass extinction took place in less than 5
million years and probably resulted from several factors.
Ø
Occurred about the time Pangaea was formed by
the merging of continents which disturbed many habitats and altered the
climate.
Ø
A period of extreme vulcanism and resulting
volcanic debris in the atmosphere may have altered the global temperature.
The Cretaceous extinction (the boundary between
the Mesozoic and Cenozoic eras eras) occurred about 65 million years ago.
§
More than 50% of the marine species and many
terrestrial plants and animals (including dinosaurs) were eliminated.
§
During this time the climate was cooling and
many shallow seas receded from continental lowlands.
§
Increased volcanic activity during this time
may have contributed to the cooling by releasing materials into the atmosphere
and blocking the sunlight.
Evidence also indicates that an asteroid or
comet struck the Earth (impact hypothesis) while the
Cretaceous extinctions were in progress.
§
Iridium, an element rare on earth but common in
meteorites, is found in large quantities in the clay layer separating Mesozoic
and Cenozoic sediments.
§
Walter and Luis Alvarez (and colleagues), after
studying this clay layer, proposed that it is fallout from a huge cloud of dust
ejected into the atmosphere when an asteroid collided with the Earth.
§
This cloud would have both blocked the sunlight
and severely disturbed the climate for several months.
§
Although the asteroid hit the earth during this
time, some researchers feel it did not cause the mass extinction of this
period.
The impact hypothesis consisted of two
parts: a large asteroid or comet
collided with the Earth and the collision caused the Cretaceous extinctions.
§
Many forms of evidence support the idea that a
large comet or small asteroid collided with the Earth 66 million years ago.
Ø
Many craters have been found and indicate that
a large number of objects have fallen to the Earth’s surface.
Ø
A large crater ( ≈ 300 km in
diameter) located beneath sediments on the Yucatan coast of Mexico has been
located.
§
Questions about the impact hypothesis are now
concentrated on the second part: the
collision caused the Cretaceous extinctions.
§
Advocates of the impact hypothesis point to
several items in their support:
Ø
The large size of the impact would darken the
Earth for several years and the reduction in photosynthesis output would be
sufficient to cause food chains to collapse.
Ø
Severe acid precipitation would result from the
increased mineral content of the atmosphere.
Ø
The content of sediments at the upper
Cretaceous boundary indicated global fires were occurring and smoke from these
fires would increase the atmospheric effects of the impact.
§
Opponents of the impact hypothesis hold that
the impact occurred within the period of mass extinction, but that the two
occurrences are not a cause and effect event.
Ø
Many paleontologists and geologists believe
that the climatic changes which occurred were due to continental drift,
increased vulcanism, and other processes.
Ø
They also feel these events wee sufficient to
cause the mass extinction.
Many paleontologists are now trying to
determine how sudden and uniform the Cretaceous extinctions were (on a
geological time scale).
§
Disappearance of diverse groups (from
microscopic marine plankton to dinosaurs) during a short time span would
support the impact hypothesis.
§
A gradual decline with different groups
disappearing at different rates, would support hypothesis emphasizing
terrestrial causes.
§
It is possible that the impact was a final,
sudden event in environmental changes that were affecting the biota of the late
Cretaceous.
Mass extinctions, whatever the cause,
profoundly affect biological diversity.
§
Not only are many species eliminated, but those
that survive are able to undergo new adaptive radiations into the vacated
adaptive zones and produce new diversity.
Phylogeny=
The evolutionary history of a species or group of related
Species.
§
Phylogeny is usually diagrammed as phylogenetic
trees that trace inferred evolutionary relationships.
Systematics= The study of biological diversity in an
evolutionary context.
§
Biological diversity reflects past episodes of
speciation and macroevolution.
§
Encompasses taxonomy in its search
for evolutionary relationships.
Taxonomy= Identification and
classification of species,; is a component of systematics.
The taxonomic system used today was developed by Linnaeus in the
eighteenth century. This system
had two main features: the
assignment of a binomial to each species and a filing system for
grouping species.
The binomial (two part Latin name) assigned to each species is
Unique to that species.
§
The first word of the binomial is the genus
(pl. genera); the second word is the specific epithet of the species.
§
The scientific name of a species combines the
genus and specific epithet.
§
Each genus can include many species of related
organisms. For example: Felis silvestris is the domestic
cat; Felis
lynx is the lynx.
§
Use of the scientific name defines the organism
referred to and removes ambiguity.
The filing system for grouping species into a
hierarchy of increasingly general categories formalizes the grouping of
organisms.
§
Binomial nomenclature is the first step in
grouping: similar species are grouped
in the same genus.
§
The system then progresses into broader
categories.
Ø
Similar genera are grouped into the same family
Ø
Families are grouped into orders.
Ø
Orders are grouped into classes.
Ø
Classes are grouped into phyla
Ø
Phyla are grouped into kingdoms.
§
Each taxonomic level is more inclusive than the
one below. The more closely related two
species are, the more levels they share.
The two main
objectives of taxonomy are to sort out and identify closely related species and
to order species into the broader taxonomic categories.
§
In sorting, closely related organisms are
assigned to separate species (with the proper binomial) and described using the
diagnostic characteristics which distinguish the species from one another.
§
In categorizing, the species are grouped into
broader categories from genera to kingdoms.
§
In some cases, intermediate categories (i.e.
subclasses; between orders and classes) are also used.
§
The named taxonomic unit at any level is called
a taxon
(pl. taxa).
§
Rules of nomenclature have been
established: the genus name and
specific epithet are italicized, all taxa from the genus level and higher are
capitalized.
The goal of systematics is to have classification reflect the
evolutionary affinities of species.
§
Groups subordinate to other groups in the
taxonomic hierarchy should represent finer and finer branching of phylogenetic
trees.
§
A monophyletic taxon is one where a
single ancestor gave rise to all species in that taxon and to no species placed
in any other taxon. For example, Family
Ursidae evolved from a common ancestor.
(Taxon 1)
§
A pilyphyletic taxon is one whose
members are derived from two or more ancestral forms not common to all
members. For example, Kingdom Plantae
includes both vascular plants and mosses which evolved from different algal
ancestors. (Taxon 2)
§
A paraphyletic taxon is one that
excludes species that share a common ancestor that gave rise to the species
included in the taxon. For example,
lass Reptilia excludes the Class Aves although a reptilian ancestor common to
all reptiles is shared. (Taxon 3)
The ideal in systematics is for each taxon to
be monophyletic, creating a classification reflecting the evolutionary history
of the organisms.
Systematists classify species into higher taxa based on the extent of
similarities in morphology and other characteristics.
Homology= Likeness attributed
to shared ancestry.
§
The forelimbs of mammals are homologous, they
share a similarity
In the skeletal
support that has a genealogical basis.
§
Homology must be distinguished from analogy in
evolutionary trees.
Analogy= Similarities due to
convergent
evolution, not common ancestry.
Convergent evolution=
Acquisition of similar characteristics in species from different evolutionary
branches due to sharing similar ecological roles with natural selection shaping
analogous adaptations.
The distinction between homology and analogy is
some times relative. For example:
§
The wings of birds and bats are modifications
of the vertebrate forelimb.
Ø
The appendages are thus homologous.
Ø
As wings they are analogous since they evolved
independently from the forelimbs of different flightless ancestors.
§
Insect wings and bird wings are analogous.
Ø
They evolved independently and are constructed
from entirely different structures.
§
Convergent evolution has produced analogous
similarities between Australian marsupials and placental mammals on other
continents.
Homology must be sorted from analogy to
reconstruct phylogenetic trees on the basis of homologous similarities.
§
Generally, the greater the amount of homology,
the more closely related the species and this should be reflected in their
classification.
§
Adaptation and convergence often obscure
homologies, although studies of embryonic development can expose homology that
is not apparent in nature structures.
§
Additionally, the more complex two similar
structures are, the less likely it is that they have evolved
independently. For example:
Ø
The skulls of humans and chimpanzees are
composed of many bones which are fused together and match almost
perfectly. It is unlikely such a
complex structure would have evolved independently in separate groups.
Molecular comparisons
of proteins and DNA have added another useful method for studying the
evolutionary relationships between species.
§
Inherited nucleotide sequences in DNA program
the corresponding sequences of amino acids in proteins.
§
Examination of these macromolecules provide
much information about evolutionary relationships.
The
primary structure of proteins is genetically programmed and a
similarity in the amino acid sequence of two proteins from different
species indicates that the genes
for those proteins evolved from a
common gene present in a shared ancestor.
The advantages of molecular comparison are:
§
They are objective and quantitative.
§
They can be used to assess relationships
between species so distantly relate that no morphological similarities exist.
Studies of cytochrome c (an ancient protein common to all aerobic
organisms) have been used to compare many diverse species.
§
The amino acid sequence has been determined for
species ranging from bacteria to complex plants and animals.
§
The sequence in cytochrome c is identical in chimpanzees
and humans.
§
The sequence in humans and chimpanzees differs
at only one of the 104 amino acid positions in the rhesus monkeys.
§
Chimpanzees, humans and rhesus monkeys belong
to the Order of Primates.
Ø
Comparing these sequences with non primate species
shows greater differences (13 with the dog and 20 with the rattlesnake).
§
Phylogeonetic trees based on cytochrome c are
consistent with evidence from comparative anatomy and the fossil record.
The most direct
measure of common inheritance from shared ancestors is a comparison of the
genes or genomes of two species.
Three methods can be
used for DNA comparison: DNA-DNA hybridization,
restriction mapping, and DNA sequencing.
DNA-DNA hybridization
can compare whole genomes by measuring the degree of hydrogen bonding between
single-stranded DNA obtained from two sources.
§
DNA is extracted from different species and the
complementary strands separated by heating.
§
The single-stranded DNA from the two species is
mixed and cooled to allow double-stranded DNA reformation which results from
hydrogen bonding.
§
The hybrid DNA is then reheated to separate the
double strands.
§
The temperature necessary to separate the
hybrid DNA is indicative of the similarity in the DNA from the two species.
Ø
The temperature correlation is based on the
degree of bonding between the strands of the two species with more bonding
occurring with greater similarity.
Ø
The more extensive the pairing, the more heat
is needed to separate the hybrid strand.
§
Evolutionary trees constructed through DNA-DNA
hybridization usually agree with those based on other methods, however, this
technique is very beneficial in settling taxonomic debates that have not been
finalized by other methods.
Restriction mapping provides
precise information about the match-up of specific DNA nucleotide sequences.
§
Restriction enzymes are used to cut DNA into
fragments which can be separated by electrophoresis and compared to restriction
fragments of other species.
§
Two samples of DNA with similar maps for the
locations of restriction sites will produce similar collections of fragments.
§
The greater the divergence of two species from
the common ancestor, the greater the differences in restriction sites and less
similarity of the restriction fragments.
§
This method works best when comparing small
fragments of DNA.
§
Mitochondrial DNA (mtDNA) is best suited for
this type of comparison since it is smaller than nuclear DNA (produces smaller fragments) and mutates
about ten times faster than nuclear DNA.
§
The faster mutation rate of mtDNA allows it to be used to determine
phylogenetic relationships between not only closely related species, but also
populations of the same species.
Ø
mtDNA was used to establish the close
relationship among the Pima, Mayan, and Yanomami groups of Native Americans.
Ø
The results supported linguistic evidence that
these groups descended from the first wave of immigrants to cross the Bering
land bridge from Asia during the late Pleistocene.
DNA sequencing is the most precise
method of comparing DNA as it determines the actual nucleotide sequence of a
DNA segment.
§
Uses polymerase chain reaction (PCR) technology
to clone traces of DNA>
§
PCR is coupled with automated sequencing to
provide a simpler and faster method of collecting sequence data.
§
DNA sequencing and comparisons show exactly how
much divergence there has been in the evolution of two genes derived from the
same ancestral gene.
§
Ribosomal RNA (rRNA) sequencing is a similar
technique which can provide information about some of the earliest branching in
phylogenetic relationships since DNA coding for rRNA changes very slowly.
§
RRNA sequencing has been very useful in
examining the relationships among bacteria.
The
nucleotide sequences in DNA traces recovered from fossils that retain organic
material can be analyzed by using PCR.
§
Fossilized DNA was first successfully analyzed
in 1990. Using PCR, researchers
amplified DNA extracted from 17 million-year-old magnolia leaves.
§
A short piece of DNA from this sample was
compared to homologous DNA from modern magnolias.
§
Since 1990, DNA fragments have been sequenced
from a frozen mammoth (40,000 years old), an insect fossilized in amber (40
million years old), and a frozen Stone Age man (5000 years old).
§
Mary Schweitzer found DNA traces in the bone
marrow of a 65-million-year-old Tyrannosaurus rex from Montana.
Ø
Analysis is currently underway to eliminate the
possibility that the trace is a result of contamination from a fungus or other
organism.
It is obvious that the techniques from
molecular biology will make even more important contributions to the fields of
paleontology, systematics, and anthropology.
§
This is consistent with fossil evidence showing
that tuna and sharks have been separated much longer than bats and dolphins.
DNA comparisons may be even more reliable than
protein comparisons.
§
Phylogenetic branching based on nucleotide
substitutions in DNA generally approximates dates determined from the fossil
record.
§
The difference in DNA between two taxa is more
closely correlated with the time since divergence than is morphological
difference.
Molecular clocks (DNA and protein) are
calibrated by graphing the number of nucleotide or amino acid differences
against the times for a series of evolutionary branch points known from the
fossil record.
§
The graph can then be used to determine the
time of divergence between taxa for
which no substantial fossil record is available.
The assumption that mutation rates for genes
(and their protein products) are relatively constant is the basis for using
molecular clocks in evolutionary biology.
§
This assumption is relatively solid when
comparing groups of closely related species.
§
Molecular clocks are less reliable when
comparing more distantly related groups since differences in generation times
and metabolice rates affect mutation rates.
Among
closely related species, the constant mutation rate for specific genes implies
that the accumulation of selectively neutral mutations, changes the genome more
than adaptive mutations.
§
Many evolutionary biologists doubt the
prevalence of neutral variation, so they also question the use of molecular
clocks to accurately date the time of divergence.
§
There is less skepticism about the value of
molecular clocks for determining the relative sequence of branch points in
phylogeny.
§
Modern systematists use available molecular
data along with all other evidence to reconstruct phylogeny.
branch points along the tree and the degree of divergence between
branches.
§
The locations of branch points along the tree
symbolize the relative times of origin for different taxa.
§
The degree of divergence between branches
represents how different two taxa have become since branching from a common
ancestor.
The question of which property
of phylogenetic trees should be most
Important in grouping species into taxa has divided taxonomy into
three schools: phenetics,
cladistics, and classical evolutionary
taxonomy.
Phenetics makes no evolutionary assumptions and decides taxonomic
affinities entirely on the basis of measurable similarities and
differences.
§
A comparison is made of as many characters
(anatomical characteristics) as possible without attempting to sort homology
from analogy.
§
Pheneticists feel that the contribution of
analogy to overall similarity will be overridden by the degree of homology if
enough characters are compared.
§
Critics of phenetics argue that overall
phenotypic similarity is not a reliable index of phylogenetic proximity.
§
While supported by few systematists, the
emphasis of phenetics on multiple quantitative comparisons has made important
contributions to systematics.
Ø
Especially useful for analyzing DNA sequence
data and other molecular comparisons between species.
Cladistics classifies
organisms according to the order in time that branches arise along a
phylogenetic tree, without considering the degree of divergence.
§
This produces a cladogram, a dichotomous tree
that branches repeatedly.
Ø
Each branch point is defined by novel
homologies unique to the various species on that branch.
Each species in the cladogram has a mixture of
primitive characters that existed in the common ancestor and characters that
evolved more recently.
§
The sharing of primitive characters indicates
nothing about the pattern of evolutionary branching from a common ancestor.
Ø
The presence of five separate toes can not be
used to divide these vertebrates among evolutionary branches.
§
Fossil evidence indicates the distant common
ancestor to all of these species had five toes, thus this homology is a shared
primitve character.
Ø
Seals and horses lost this trait independently
while the other species retained the trait.
§
A major difficulty in cladistics is finding
characters that are appropriate for each branch point.
§
The cladogram is based on synapomorphies which are
shared derived characters
Ø
These are homologies which evolved in an
ancestor common to all species on one branch of the cladogram but not common to
the other branch.
§
Hair and mammary glands are synapomorphies for
the branching point leading to the mammalian limb and the lizard on the other
limb.
§
Branching points along the mammalian limb can
also be based on synapomorphies.
Ø
Dental and skeletall modifications can be used
to define the horse branch from the cat, lion, and seal.
Ø
Retractable claws and other synapomorphic characters
separate the seal from the cat and lion branch.
Ø
The ability to purr is used to define the final
branching between the cat and lion.
Some taxonomic surprises are produced by
cladistic systematics:
§
The branch point between crocodiles and birds
is more recent than between crocodiles and other reptiles (a fact also supported by the fossil record).
Ø
Crocodiles and birds have synapomorphies not
present in lizards and snakes.
§
In a strict cladistic analysis, the Class Aves
and the Class Reptilia, as we know them now, would eliminated.
Ø
The birds would be included in a cladogram of
the animals we know as reptiles.
§
Birds are deemed superficially different
because of the morphological changes associated with flight which have
developed since their divergence from reptilian ancestors.
Classical
evolutionary systematics attempts to balance the extent of divergence and the
branching sequence.
§
Predates both phenetics and cladistics, but now
incorporates some ideas from both.
§
In cases of systematic conflict, a subjective
judgment is made about which type of information receives the highest priority.
In
reference to crocodiles and birds, classical systematists recognize a
closer genealogical relationships between crocodiles and birds than
between crocodiles and lizards.
§
However, they combine crocodiles and lizards in
a taxon (Class Reptilia) that excludes birds because the ability to fly was an
evolutionary advancement which allowed birds to enter a new adaptive zone.
§
The resulting divergence of birds was so
extensive that they are placed in a separate class (Aves).
No biological theory has produced as much
debate and controversy as
the Theory of Evolution proposed by Charles
Darwin.
§
The debate began when Darwin published The
Origin of Species
In 1859 and continues
today.
§
The closest thing to a consensus has been the
modern synthesis which has dominated evolutionary theory for the past 50 years.
§
The view of evolution is called a synthesis
since the ideas presented were drawn from several disciplines (paleontology,
biogeography, systematics, population gentics) anmd continues to incorporate
discoveries from new fields such as molecular biology.
Darwin’s view of life was reaffirmed by the modern synthesis and
updated by applying the principles of genetics.
§
Its view is distinctly gradualistic in that it
holds large-scale evolutionary changes are the accumulation of many minute
changes occurring over vast spans of time.
§
Microevolution (changes in allele frequencies
in populations) is extrapolated to explain most macroevolution.
The
classical version of the modern synthesis holds that natural selection is the
major cause of evolution at all levels.
§
Populations adapt by natural selection.
§
New species arise when isolated populations
diverge as different adaptations evolve.
§
Continued divergence due to natural selection
differentiates the higher taxa.
Ø
That mechanisms such as genetic drift and
chromosomal mutations can cause rapid,
nonadaptive evolution is recognized by the
modern synthesis; but major emphasis of the synthesis is on gradualism and
natural selection.
Many evolutionary biologists disagree with the
view that the evolution recorded in the fossil record can be explained by
extrapolating the process of microevolution.
The debate focuses in part on the pace of evolution.
§
Many transitions in the fossil record are
punctual, not gradual.
§
Gradualists hold that the apparent abruptness
is in part derived from the imperfection of the fossil record and in part is a
semantic issue confused by the vastness of geological time (e.g. is a change
occurring over 10,000 years sudden or gradual?).
§
Punctuationalists state that the imperfection
of the fossil record is not enough to account for the rarity of transitional
forms if speciation and the origin of higher taxa were primarily gradual
extensions of microevolution.
§
The debate also includes differing opinions
about the degree to which microevolution compounded over time is sufficiently
to explain macroevolution.
The hierarchical theory favored by many
evolutionary biologists gives mechanisms different levels of importance at
different levels of evolution.
§
IN this theory, natural selection is the key to
adaptive evolution of populations, plays a smaller role in speciation, and
plays an even smaller role in macroevolution.
§
Most new species begin as small populations
isolated from their parent populations (geographical or genetic isolation).
§
These isolates can evolve relatively rapidly,
divergence from the parent population being due to genetic drift as much as to
selection.
§
Chance may cause speciation to begin before
selection has produced new adaptations.
Also, some major new adaptations may evolve with minimal genetic change
if regulatory genes are involved.
§
Continental drift and mass extinctions have
probably had at least as much effect on the history of biological diversity as
gradual adaptation caused by selection influencing gene pools at the population
level. The historical contingency is
gaining favor with researchers holding this view.
Historical contingency=
The occurrence of unforeseen events.
§
In a hierarchical theory of evolution, most
evolutionary trends progress by species selection not by phyletic transition
due to an accumulation of microevolutionary changes.
§
Species selection being the differential
survival and branching of separate species that change little after they come
into existence.
All groups in this debate recognize that
natural selection is the mechanism of adaptation and should be the centerpiece
of evolutionary biology.
§
Selection adapts a population to its
environment with generation-to-generation changes in the gene pool that are
adaptive.
§
It is also natural selection that refines unique
adaptations when a new species or higher taxon comes into existence.
§
Although events leading to speciation and
episodes of macroevolution may have little influence on adaptation, new species
only persist long enough to be entered into the fossil record only if they have
adapted to their environment through natural selection.
The modern synthesis has never proposed that
evolution is always smooth and gradual or that processes other than changes in
gene pools due to selection are unimportant.
§
The questions are not so much about the nature
of evolutionary mechanisms as about their relative importance.
§
Debate about how life evolved is indicative
that evolutionary biology is an active science and the debate will continue.