Download Evolutionary Theory

PrePre-scientific Beliefs
A Brief and Idiosyncratic
History of the Concept of
Evolution
or why we think we know how we
got here
 Understanding
of the universe was
based on the interpretation of
supernatural concepts
 There were two longlong-standing notions
that impeded the development of the
concept of evolution:
–Fixity of species
–Relatively short duration since the creation
of the earth
Ptolemy
(2nd century A.D.)
 Earth
as center of universe
and organic world fixed by
creation
 Great Scale of Being: simple to complex
(from Aristotle)
 No new species since creation
 Endorsed by Christian church
 Inorganic
Archbishop James Ussher
(1581(1581-1656)
A
scholar in the prepre-scientific tradition,
Ussher used a literal interpretation of
the bible to estimate the time of creation
–By evaluating how many years individuals
l
i
vedandhowl
ongf
r
om one“
begat
”t
ot
he
nex
t
,Ussher
’
sr
eadi
ngofGenesi
sl
edhi
m
to estimate that the earth was created in
4,004 B.C.
–I.e., the earth was less than 6,000 years
old
Scientific Revolution
 Science
provided a framework to
attempt to discover the Natural Laws or
fundamental principles that govern the
universe
 The procedure includes
–The formulation of testable hypotheses
–The application of empiricism, the
collection of data to test the hypotheses
Copernicus (1473(1473-1543)
 An
early proponent of the scientific
method, Copernicus postulated that the
sun (not the earth) was center of universe,
and the earth rotated around the sun
 He espoused the removal of a
supernatural interpretation of the cosmos
 His predictions were subject to
observation
 The planets obeyed Natural Laws
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Carolus Linnaeus (1707(1707-1778)
 Proposed
a hierarchical classification of
organisms into four levels
–C, O, G, S
 Attempted
to discover the grand design
of the Scale of Being
 Accepted fixity of species, as created
Binomial Nomenclature: Genus and
species
–e.g., Homo sapiens for man
Erasmus Darwin (1731(1731-1802)
Comte de Buffon, 1707 - 1788
 While
denying the existence of
macroevolution, believing that all
species continue as created, he
espoused microevolution
–In his view, the differences between
populations of a species were the result of
their accommodation to local
environmental conditions
Chevalier de Lamarck (1744(1744-1829)
 Evolution
 Ch
ar
l
es’Gr
andf
at
her
 Discussed
Natural Laws in his poetry
 Published several ideas that presaged
evolution in his Zoonomia
–Competition for resources between
members of a species
–He was aware that species change through
time but did not have a mechanism
Georges Cuvier (1769(1769-1832)
 Pope
of Bones-Father of Paleontology
Bones--Father
–Skills in comparative anatomy gained from
the reassembling of disarticulated
skeletons from slaughter houses made him
an expert for the reconstruction of fossils
 Believed
in the fixity of species
 Proposed Catastrophism from natural
causes as a mechanism to produce
fossils
of species according to fixed
laws
–Species adapt to changing environment by
use/disuse of organs
–Systems develop according to wants &
needs of organism
•e.g., fish in drying lake develop lungs as
needed for oxygen
–Proposed mechanism: Inheritance of
acquired characteristics
Charles Lyell (1797(1797-1875)
 Amplified
and popularized work of
earlier scholars (e.g., Hutton) proposing
uniformitarianism as an explanation for
the fossils and geologic deposits
 Uniformitarianism
–Geological processes same in past as they
are today
–Slow, gradual process of earth's formation
2
Thomas Malthus (1766(1766-1834)
I
nhi
s1798“
AnEssayont
hePr
i
nci
pl
eof
Population as It Affects the Future
I
mpr
ovementofSoci
et
y
”henot
ed:
–Population growth potentially exceeds increases
in food resources
–In nature, overpopulation checked by struggle
for existence
–Not all organisms secure adequate food every
generation
–Most die, only a few survive to perpetuate the
species
Charles Darwin (1809(1809-1882)
 Born
in a wellwell-toto-do family, father a
physician
 Had an early interest in nature,
collecting eggs, pebbles, and the like
 Did not do well in school
 Could not stomach the cruelty of
medicine when he went to the
University of Edinburgh to study
Charles Darwin (1809(1809-1882)
 Transferred
to Cambridge for a degree
in divinity
–Read Lamarck and his grandfather
Er
asmus’wr
i
t
i
ngsatt
hi
st
i
me
 As
he was about to become a
clergyman he was invited to join the
HonorExpedi
t
i
onast
heshi
p’
snat
ur
al
i
st
aboard the H.M.S. Beagle
Charles Darwin
 In
1838, after reading Malthus, Darwin
realized that through the struggle for
exi
st
ence,“
favourable variations would
tend to be preserved and unfavourable
ones to be destroyed. The result of this
woul
dbet
hef
or
mat
i
onofnewspeci
es.
”
(Darwin 1892:43).
Charles Darwin
 During
the 5 year cruise of the Beagle,
Darwin collected specimens, read
Ly
el
l
’
sPrinciples of Geology,
Geology, and
became convinced of the transmutability
of species
 After returning home, a reading of
Malthus in 1838 finally triggered the
notion of natural selection as the
mechanism driving evolution
Alfred R. Wallace (1823(1823-1913)
 Came
to the same conclusions about
evolution by natural selection as a result
of his work in the Malay Archipelago
 He is supposed to have arrived at his
conclusions about natural selection
while lying delirious from a malarial
fever
3
Alfred Russell Wallace
 1858
sent essay to Darwin
–On the Tendency of Varieties To Depart
I
ndef
i
ni
t
el
yf
r
om t
heOr
i
gi
nalTy
pe”
 Wallace
became a fervent selectionist,
ending up not believing in human
evolution because he could not envision
the adaptive purpose for the human
mind
Thomas Huxley (1825 - 1895)
 Acquainted
with both Darwin and
Wallace
 Up
onr
eadi
ngDar
wi
n’
sOn
the Origin of
Species... is reported to have said,
“
Howobv
i
ous.Howst
upi
dofmenott
o
havet
houghtofi
tmy
sel
f
.
”
 Be
camet
he“
Dar
wi
n’
sBul
l
dog”
,t
he
fierce proponent of evolution by natural
selection in the public forum
Evolution by Natural Selection
 As
a result, individuals with favorable
adaptations increase in relative number
from generation to generation, so
greater and greater numbers within the
species have the adaptation
 Over long periods of time, such
successful variations (favorable
adaptations) produce differences that
result in the formation of new species
Publication
 Lyell
helped to persuade Darwin to publish
anext
r
actofhi
sbookal
ongwi
t
hWal
l
ace’
s
essay.
–Appeared in 1858 Journal of the Linnean
Society
 1859
On the Origin of Species by Means of
Natural Selection or The Preservation of
Favoured Races in the Struggle for Life
Evolution by Natural Selection
 Offspring
produced faster than food supply
increases
 All living organisms vary (especially within
species)
 Because more individuals are born than
survive, there is a struggle for existence
–individuals with favorable adaptations are most
likely to survive and leave offspring, sometimes
known as survival of the fittest
Darwin's Evidence
 Changes
bred into domesticated
organisms, especially pigeons
 Geographic distributions of species such as
the turtles of the Galapagos Islands
 The geological and paleontological record
 Comparative anatomy (e.g. the
resemblance between apes and man) and
embryology (e.g., the possession of gills by
mammalian embryos)
4
Darwin's Failures
Darwin's view of how speciation
occurs, circa 1870
 Never
understood the source of the
variation that is so key for the operation
of natural selection
 Never understood the particulate nature
of inheritance, in spite of having a
r
epr
i
ntofMendel
’
s1865publ
i
cat
i
on
Variation between individuals occurs in all
species and is caused by the variability of
the environment to which a population is
exposed.
Individual variation is inheritable since, as
my provisional hypothesis of pangenesis
explains, each part of the body emits
invisible gemmules that pass through the
bloodstream to collect in the germ cells,
carrying with them information on the exact
nature of the body part from which they
came.
This slight variation that benefits an
individual in its struggle for existence
enables it to survive longer and reproduce
more successfully than others.
Conversely, individuals with injurious
variations perish. This principle I have
called natural selection.
Due to the inheritance of beneficial
variations, natural selection also acts on
further variation in successive generations,
so that initially small beneficial traits may
in time become major through continued
descent with modification. Since the
individuals whose favorable variations
diverge most markedly from the parent
stock receive the most benefit, those most
divergent variations are preserved and
accumulated by natural selection.
After much variation has accumulated, fairly
well marked varieties and then species may be
noted, since species are only strongly marked
and well defined varieties.
In this way, transmutation of species occurs and
new species originate.
The process resulting in new species, repeated
all over the earth and over great spans of
time, has produced the present diversity of
life, the clear relationships between species
alive and extinct, and the descent of all life
from some primordial species.
5
Evolution
“
Not
hi
ngi
nbi
ol
ogymakes
sense except in the light of
ev
ol
ut
i
on”
The Modern Synthesis
“
Theal
t
er
nat
i
v
et
ot
hi
nki
ngi
n
evolutionary terms is not to
t
hi
nkatal
l
”
 Sir
 Theodosius
Dobzhansky
Simultaneous Rediscovery of
Mendel (1900)
 Hugo
de Vries,
Vries, Holland
 Erich von Tschermak,
Tschermak, Austria
 Carl Correns,
Correns, Germany
Peter Medawar
Basis of population genetics-genetics-equilibrium model
E. Castle, 1903, American
animal breeder
 Godfrey H. Hardy, 1908, British
mathematician
 Wilhelm Weinberg, 1908, German
physician
Development of Population
Genetics from HH-W Model
 William
 R.A.
Fisher, 1930, The Genetical
Theory of Natural Selection
 J.B.S. Haldane,
Haldane, 1932, The Causes of
Evolution
 Sewall Wright, 1931, Evolution in
Mendelian Populations
6
Synthesis of genetics and
Darwinism
 Theodosius
Dobzhansky, 1937,
Genetics and the Origin of Species
 Julian S. Huxley, 1942, Evolution. The
Modern Synthesis
 Ernst Mayr, 1942, Systematics and the
Origin of Species
 George Gaylord Simpson, 1944, Tempo
and Mode in Evolution
Species Concepts
 Typological
species
–Discreteness of species
–Fixity of species
–One member of each species was all that
was necessary to serve as a type
specimen representing the morphology of
the entire species
–Think of a dog show where the judge
evaluates the dog against a notion of the
perfect type for the breed
Species Concepts
 Species
is derived from the Latin
specere,
t
ol
ookat
”
,r
ef
er
r
i
ngt
ot
he
specere,“
outward appearance as the basis of
species
 Aristotle conceived of species as
unstable and highly changeable
–believed in spontaneous generation and all
kinds of crosses between different species
Dar
wi
n’
sSpeci
esConc
ept
 On
the view that species are only strongly
marked and permanent varieties, and that
each species first existed as a variety, we can
see why it is that no line of demarcation can
be drawn between species, commonly
supposed to have been produced by special
acts of creation, and varieties which are
acknowledged to have been produced by
secondary laws (Darwin, 1872:171 sixth
edi
t
i
onof“
Or
i
gi
nofSpeci
es”
)
.
Biological Species Concept
 Species
are groups of actually or
potentially interbreeding natural
populations that are reproductively
isolated from other such groups (Mayr,
1963:19)
–Defines species in terms of a gene pool,
isolated from other similar gene pools
Limitations
 Restricted
to sexually reproducing
organisms, so does not apply to single
cell organisms that reproduce by simple
cell division (mitosis)
 Also does not fit many plants species
 No clear application to the fossil record,
since reproductive isolation does not
show up in fossilized materials
7
Where does evolution take place?
 Biological
Hierarchy
 Biosphere
–Community
•Population
–Individual
•Organs, tissues, cells, molecules
 Where
is the species?
–Ourt
ax
onomydoesn’
tr
ef
l
ec
tec
ol
ogi
cal
reality
PrePre-mating Mechanisms
 Mechanisms
causing a failure to meet
potential mates in space or time
–Habitat isolation
•Species occupying different segments of the
habitat, e.g., smallsmall-mouthed salamander,
Ambystoma texanum,
texanum, breeds in ponds, but A.
barbouri breeds in streams
–Seasonal isolation
•Plant species which release pollen at different
times
PostPost-mating Mechanisms
 Mechanisms
reducing the viability of
hybrid offspring
–Gametic mortality
•Sperm transfer takes place but the egg is not
fertilized, e.g., Sonoran topminnow and
nematodes
–Zygotic mortality
•Egg is fertilized but zygote dies. Goat by sheep
cross results in early zygotic death, as do some
leopard frog hybrids (Rana
(Rana pipiens)
pipiens)
Reproductive Isolation
 PrePre-mating
–Mechanisms that prevent the transfer of
gametes between members of different
species
 PostPost-mating
–Mechanisms that reduce the viability or
fertility of hybrid offspring
PrePre-mating Mechanisms
 Mechanisms
causing a failure to mate
between individuals who meet
–Mechanical isolation
•Copulation is attempted but no transfer of
sperm take place. Many insect species require
a lock and key link for sperm transfer
•Behavioral or ethological isolation
•Potential mates meet but do not mate
–Coloration, movement, song, or odor may not elicit
readiness for sperm transfer
PostPost-mating Mechanisms
 Mechanisms
reducing the fertility of
hybrid offspring
–Hybrid inviability
•Zygote produces offspring of reduced viability,
e.g., some frogs in the genus Pseudophryne
–Reduced fertility
•Horse by donkey cross produces a sterile mule
•Papio hamadryas male by Papio anubis female
results in fertile female and infertile males
8
Paleospecies
A
paleospecies or chronospecies is a
morphologically or anatomically defined
fossil species
–Since interbreeding is not determinable for
extinct species, species boundaries are
determined by similarity of form
Speciation
 The
process by which new species are
produced from earlier ones. This is the
central process of macroevolution, or
evolution at the level of the species or
above
 Microevolution involves evolutionary
change at the level of the population,
and is defined by changes in allele
frequencies within the population
Origin of Species
 Speciation
occurs when populations of
a species become reproductively
isolated so that they can no longer
interbreed with full fertility
 Geographic isolation of populations
usually initiates the process
–Selection operates to produce isolating
mechanisms of anatomy, behavior, genes
9
Synthetic Evolution, circa 1950s
New variants of genes are continually being added
to populations through mutation. In addition, as
explained by Mendel's two laws, recombination
of genes increases the variation initially
produced by mutation, since segregation and
independent assortment of genes result in
almost endless novelty in gene combinations and
thus an almost infinite variety of possible
genotypes in a population.
Dar
wi
n’
sFi
nc
hes
Because of the particulate nature of
inheritance, no loss of variation occurs
from parent to progeny. Thus, each
population has an enormous store of
variation on which natural selection can
act. The fittest individuals, defined as those
which leave the most viable progeny that
mature and reproduce, contribute the most
genes to the next generation, and thus their
genes become most common in the gene
pool of the population.
If a population should become divided into two
by a geographic barrier, evolution of each
new population continues independently.
Differences between the two, including
differences in reproductive processes,
gradually accumulate such that reproductive
isolating mechanisms become more and
more effective over prolonged periods of time.
New mutations and new combinations of
genes continually produce some individuals
that are better adapted than others; natural
selection favors these and selects out less fit
types. Gradually, the population evolves
through the differential perpetuation of
genes.
Ultimately, on secondary contact, the two
populations are reproductively isolated and
two new species have evolved by the
process of geographic speciation.
Therefore, species can be defined as groups
of actually or potentially interbreeding
populations that are reproductively isolated
from other such groups.
10
Modes of Evolution
Speciation events lead to multiplication and
diversification of species into higher taxa
that retain clear phylogenetic links with
parental stock, so that all species can be
traced to the origin of life itself.
 Cladogenesis
is branching evolution,
the production of two or more daughter
species from a single ancestral species
 Anagenesis is linear evolution, the
production of a single daughter species
from a single ancestral species
Tempo of Evolution
 Gradualism
views evolutionary change
as occurring at a slow, steady rate over
time
 Punctuated Equilibrium views the tempo
of evolution as consisting of long
periods of stability (equilibrium),
followed by relatively short periods of
rapid evolutionary change
(punctuations)
Tempo of Evolution
Sources
 Darwin,
a. Gradual change through time
b. Increasing rate of change through time
c. Punctuated equilibrium: stasis and bursts of
change
C. 1872 On the Origin of
Species by Means of Natural Selection,
orThePr
eser
vat
i
onof“
Favour
edRaces
in the Struggle for Life. 6th Edition.
London: Murray.
 Mayr, E. 1963 Animal Species and
Evolution. Cambridge, MA: Belknap
Press
11
Sources
 Price,
P.W. 1996 Biological Evolution.
Philadelphia: Saunders.
 Relethford, J.H. 1997 The Human
Species: An Introduction to Biological
Anthropology. Third Edition. Mountain
View, CA: Mayfield Press.
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