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Evolution
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Evolution – the process by which each type of organism
is descended from ancestors that were similar but not
identical to it
All life shares a common ancestry
Darwin (and independently, his contemporary Alfred
Wallace), proposed a mechanism for evolutionary
change
Many ideas about evolution pre-date Darwin
Jean Baptiste Lamarck (1744 – 1829) was the first to
propose a mechanism for evolution
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Inheritance of acquired characteristics – living
organisms modify their bodies through the use or
disuse of parts, and these modifications can be
inherited by their offspring
Darwin and Wallace propose that evolution
occurs by natural selection
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In 1858, Darwin and Wallace
independently described a mechanism
for evolution
Darwin published On the Origin of
Species by Means of Natural Selection
the following year
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Darwin’s theory of natural selection included
several important observations and conclusions:
Observation #1 - Natural populations ( a
population consists of all the individuals of one
species in a particular area) of all organisms
have the potential to increase rapidly –
organisms produce far more offspring than can
possibly survive
Observation #2 - Nevertheless, the sizes of most
natural populations and the resources available
to support them remain relatively constant over
time
Conclusion #1 – Therefore, there is competition
for survival and reproduction. Many individuals
die young and fail to reproduce.
• Observation #3 – Individual members of a population differ
from one another in their ability to obtain resources,
withstand environmental extremes, escape predators etc.
(Variation)
• Conclusion #2 – The most well-adapted (the “fittest”)
individuals are the ones that leave the most offspring.
Natural Selection – process by which the environment
selects those individuals whose traits best adapt them to
that particular environment.
• Observation #4 – At least some of the
variation among individuals in traits that
affect survival and reproduction is due to
genetic differences that can be passed on
from parent to offspring.
• Conclusion #3 – Over many generations,
differential, or unequal, reproduction among
individuals with different genetic makeup
changes the overall genetic composition of
the population. Evolution occurs as a result
of natural selection.
Summary of Darwin’s Theory of Natural Selection
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Overproduction – each species has the capacity to
produce more offspring than will survive to maturity
Variation – individuals in a population exhibit variation
(each has a unique combination of traits such as size,
color, and ability to tolerate harsh environmental
conditions)
Limits on population growth – there is a limited amount of
food, water, light, growing space, and other resources
available to a population, therefore, organisms must
compete for limited resources
Differential reproductive success – individuals that possess
the most favorable combinations of characteristics are
more likely to survive and reproduce
Natural selection leads to adaptation – evolutionary
modification that improves the chances of survival
and reproductive success of a population
Neo-Darwinism (Modern Synthesis)
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During the 1930’s and 40’s biologists
combined the principles of genetics with
Darwin’s theory of natural selection –
Neo-Darwinism (Modern Synthesis)
Emphasizes the genetics of populations
rather than individuals
Evidence for evolution
1. Fossil Record – remains or traces typically left
in sedimentary rock (but also in bogs, tar,
amber, and ice) by previous organisms
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shows a progression from the earliest, single-celled
organisms to the many single-celled and multicellular
organisms living today
most fossils are dated by their relative position in
sedimentary rock
Various methods are used to determine the age of fossils
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Radioactive dating – radioisotopes decay in a
characteristic and known rate
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Half-life is the amount of time required for 50% of the
radioisotope to decay – at the end of this period of time
(any where from fractions of seconds to thousands of
years, depending on isotope) radioactivity will be half of
what it was before
each isotope has its
own rate of decay
and differ in their
half-life (carbon-14,
potassium-40,
uranium-235)
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Carbon dating – excellent method of determining
the age of organic material (relatively young, tens
of thousands of years old)
uses radioactive isotope of carbon, Carbon-14
14C is unstable and spontaneously changes to 14N
all living organisms contain 14C in the same
proportion as found in the atmosphere
when organisms dies, process of incorporating
new carbon into body stops (ie, feeding)
half-life of carbon is 5730 years
Potassium-40 is used to date very old fossils
(hundreds of millions of years old)
40K decays to form 40Ar
half-life of Potassium-40 is 1300 million years
Evidence from Paleontology – based on fossil record
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fossils are difficult to explain without evolution
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examples: evolution of horse, Archaeopteryx
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fossil record provides direct evidence for evolution
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fossils range from bone, teeth, shells, to actual body parts
preserved in bogs, tar, amber or ice
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Fossils most commonly form in aquatic environments and
typically form in sedimentary rock
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Comparative Anatomy – related species demonstrate
similarities in their structure
– Homologous structures – body parts that have similar
structure but may have different functions and
appearance
– probably were derived from the same structure in a
common ancestor (example: pentadactyl limb of
vertebrates - human arm, cat forelimb, whale front
flipper, and bat wing all have a similar arrangement of
bones, muscles, and nerves)
– Vestigial structures – structures that have no
apparent purpose
- often are homologous
to structures that are
found in and used by
other organisms (ex:
whales evolved from
four-legged ancestral
mammals – whales do
not have hind legs yet
they have small pelvic
and leg bones
embedded in their
sides)
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Analogous structures – body parts that
have the same function but very different
internal anatomy (insect wing and bird
wing)
– may have developed as a result of convergent
evolution – unrelated species share the same
environment (similar environmental pressures)
and independently evolve similar structures
3.Embryological development – all
vertebrates have similar patterns of
embryological development indicating a
common ancestor
4. Molecular comparisons – similarities and
differences in biochemistry and molecular biology
of various organisms provides evidence for
evolution
- genetic code is
universal – same
bases make up
DNA, same amino
acids make up
proteins, same use
of mRNA, codons all
code for amino acids
the same way in all
organisms
- closely related
species have
similarities in DNA
sequences
Artificial Selection
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line of evidence that supports evolution by
natural selection
the breeding of domestic plants and
animals to produce specific desirable
traits (ex. different breeds of dogs)
people are doing the “selecting” rather
than the environment
people have bred very different dogs, all
descendants of the wolf
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Evidence for Evolution
Evidence from geographical distributions (biogeography)
if evolution did not occur, we would expect to find a given
species everywhere that it could survive – in reality this
does not occur
example – Australia (a separate land mass for millions of
years) has distinctive animals (marsupials and
monotremes) not found anywhere else
200 million years ago, Australia and other continents
were joined together (Pangea)
Australian continent gradually separated from others
monotremes and marsupials continued to thrive and
diversify in Australia
isolation of Australia prevented placental mammals
(which arose elsewhere at a leter time) from competing
with monotremes and marsupials
in other areas of the world, placental mammals
outcompeted the marsupials and monotremes (which
were eliminated)
Evolutionary Change in Populations
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Evolution occurs in populations, not
individuals
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Individuals do not evolve in their lifetime
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Evolutionary changes are those that occur
from generation to generation
 Population – consists of all the
individuals of the same species that live
in a particular place at the same time
Population Genetics
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branch of genetics dealing with the frequency,
distribution, and inheritance of alleles in populations
gene pool – sum of all the genes of all the individuals in
a population including all the alleles for all the genes
present in the population
allele frequency – the percentage of a specific allele of a
given gene locus in the population
 evolution of populations is best understood in terms
of allele frequencies
 if the allele frequencies remain constant from
generation to generation, then the population is not
undergoing any evolutionary change and is in genetic
equilibrium
evolution can be defined as changes in gene
frequencies that occur in a gene pool over time (change
in the genetic makeup of populations over time)
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Natural Selection
mechanism of evolution in which members of a
population that possess more successful
adaptations to the environment are more likely
to survive and reproduce
causes differential reproduction among
organisms with different alleles – fitness of an
organism is measured by its reproductive
success (not just whether or not it survives)
acts on the phenotype (although phenotype is
an expression of genotype)
Three major types of Natural Selection
1. Directional Selection – favors phenotypes at one
of the extremes of the normal distribution
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selects against both the average individuals and
individuals at the opposite extreme
over successive generations, one phenotype
gradually replaces the others
most common during
periods of
environmental
change or when
members of a
population migrate to
a new habitat with
different
environmental
conditions
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Stabilizing Selection – selection against
the phenotypic extremes, average is
favored – associated with a population
that is well adapted to its environment,
tends to reduce variation
3. Disruptive Selection (Diversifying) – favors two
or more different phenotypes at the expense of
the mean
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special type of directional selection – trend is in
several directions instead of just one
results in divergence of groups of individuals within
a population
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Natural selection works on genetic
variations
genetic variation originates with
mutations – sexual reproduction also
greatly contributes to genetic variation
(crossing over, independent assortment,
random union of gametes etc)
Coevolution
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occurs when two species interact extensively and exert
strong selection pressures on each other
examples include predators and prey and organisms
that live in symbiotic relationships (mutualism,
commensalism, parasitism)
as one evolves a new feature or modifies an old one,
the other typically evolves new adaptations in
response
Natural Selection may lead to Extinction
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the death of all the members of a species
occurs as a result of environmental change
some species may be predisposed to extinction as a
result of localized distribution and overspecialization
Three major environmental changes may drive a
species to extinction:
– competition for limited resources with other species
– introduction of new predators or parasites
– habitat change and destruction
• greatest cause of extinctions
• humans are rapidly destroying habitats
• climate changes have caused many extinctions
(as a result of plate tectonics)
Mass extinctions
• disappearances of many varied species in
a relatively short period of time over a
wide area
• fossil record reveals episodes of
extensive worldwide extinctions
– may have resulted from enormous meteorites
hitting earth and kicking up enough dust to
block out most of the sun’s rays
– most recent catastrophe occurred about 65
million years ago and coincided with the
disappearance of dinosaurs
Origin of Species and Speciation
• Species – definition for “species” has
changed over time
– biological species concept – current definition:
groups of actually or potentially interbreeding
natural populations which are reproductively
isolated from other such groups and capable of
producing fertile offspring
– each species has a gene pool that is isolated
from that of other species and each is restricted
by reproductive barriers from interbreeding with
other species
Speciation
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process by which new species arise –
depends on two factors:
– Isolation of populations – a population
becomes reproductively isolated from other
members of the species – there is little gene
flow between the different groups
– Genetic Divergence – during the period of
isolation, the gene pools of the separated
populations begin to diverge in genetic
composition
– they evolve sufficiently large genetic
differences so they can no longer interbreed
and produce vigorous, fertile offspring
Two possible situations lead to speciation:
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allopatric speciation – speciation that occurs when
one population becomes geographically isolated from
the rest of the species
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population evolves as a result of natural selection
and/or genetic drift
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most common method of speciation
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often results from physical isolation due to the
constant changing of the earth’s surface
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river shifting courses, glaciers migrating, mountain
ranges forming, land bridges forming, etc.
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may also result when a small population migrates and
colonizes a new area away from the range of the
original species
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overall effect is to stop gene flow
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speciation is more likely to occur if the isolated
population is small – genetic drift (including founder
effect) has more effect on smaller populations
2. sympatric speciation – a new species
develops within the same geographical
region as the parental species – more
common in plants than animals – usually
results from:
 ecological isolation
 chromosomal aberrations
1. ecological isolation –
the same geographical
area may contain two
distinct types of habitats
(distinct food sources,
nesting places etc)
– different members of a
single species may begin
to specialize in one
habitat or the other
– natural selection in the
different habitats may
result in speciation
2. changes in chromosome number may cause
instantaneous speciation
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polyploidy – common speciation mechanism in plants –
possession of more than two sets of chromosomes
may occur when a fertilized egg duplicates its
chromosomes but does not divide into two daughter
cells – all subsequent divisions may be normal and all
cells are now tetraploid
most tetraploid plants are healthy and vigorous and can
go through meiosis
gametes produced can only fuse with other gametes
from tetraploid plants – cannot fuse with gametes from
original parents
occurs in plants because plants can self-fertilize or
reproduce asexually
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allopolyploidy – occurs when plants of two different
species interbreed to form a hybrid
following fertilization, if polyploidy occurs (doubling of
chromosomes because fertilized egg does not separate
into two cells) then proper synapsis and segregation of
chromosomes can occur
Reproductive isolating mechanisms
• structural and/or behavioral modifications
that prevent interbreeding between two
different species
• prevents gene flow and preserves genetic
integrity of each species
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Premating isolating mechanisms (prezygotic
barriers) – prevent mating between species
1. temporal isolation – genetic exchange is prevented
between two groups because they reproduce at
different times of the day, season, or year
2. behavioral isolation – elaborate courtship behaviors
are recognized only by a member of the same species
– common in birds – courtship behaviors are only
understood by females of the same species
3. mechanical incompatibility (mechanical isolation) –
occurs when structural differences in male and female
genital organs prevent successful mating – in animals,
male and female sexual organs may not fit together –
in plants, flower size or structure may prevent pollen
transfer between species
4. gametic isolation – if mating takes place between two
species, the gametes may fail to combine – may be
due to molecular and chemical differences
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Postmating isolation mechanisms
(postzygotic barriers)
1. hybrid inviability – if fertilization does occur,
resulting hybrid may be weak or unable to
survive (most spontaneous abort during
embryonic development)
2. hybrid sterility – if the hybrid does survive, it
will often be sterile (often because
chromosomes fail to pair properly during
meiosis) – ex. mules, ligers
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Speciation may occur rapidly or gradually – two
models have been developed to explain evolution
as observed in the fossil record:
1. Punctuated equilibrium – suggests that the
fossil record accurately reflects evolution as it
actually occurs in the history of a species
• evolution occurs with long periods of stasis
(no evolutionary change) interrupted
(“punctuated”) by short periods of rapid
speciation
• speciation occurs in “spurts”
• few transitional forms exist in the fossil
record because few transitional forms occur
during speciation
2. Gradualism – more traditional view of
evolution – evolution proceeds
continuously over long periods of time
 this model says that the fossil record is
missing transitional forms because the
fossil record in incomplete
Adaptive Radiation
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when a species gives rise to many new
species in a relatively short period of time
typically occurs when populations of a single
species invade a variety of new habitats and
evolve in response to the differing
environmental selection pressures
Darwin’s finches are a good example –
encountered a wide variety of unoccupied
habitats in the Galapagos Islands
may occur after mass extinctions when the
survivors fill empty habitats (ex – adaptive
radiation of mammals after extinction of
dinosaurs)
Modern examples of observed evolution
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Darwin’s finches – finches of the Galapagos islands
all finches range in size, 10-20 cm long and are of
brownish or black coloration
shapes of beaks varied greatly from island to island
based on different feeding habits resulting from different
selective pressures
Rosemary and Peter Grant studied two species of
finches on the Galapagos – Geospiza fortis and
Geospiza scandens
G. fortis had a short, wide beak for feeding on large, hard
seeds - during 1982-83 there was a severe El Nino
bringing heavy rain and changing the vegetation – by
1991, only 37% of the original population remained and
those on average had longer more narrow beaks
(compared to the original population) to eat the new
smaller softer seeds being produced
the population of G. scandens on the same island did not
change over that time since its food and diet remained
the same
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Transient Polymorphism
a population may have more than one
common form – polymorphism
Example – Peppered moth
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Two forms – grey and black
Industrial melanism – natural selection resulting
from industrial processes
Proportions of black and grey have changed over
time (changes are temporary) – transient
polymorphism
Balanced Polymorphism
• Occurs when two or more alleles within a population
are not transient and changing but are stabilized by
natural selection
• example – Sickle cell anemia
– Condition is more common in areas where malaria is a
problem
– Homozygous (HbSHbS) condition is lethal
– Heterozygous condition (HbAHbS) gives person slight
resistance to malaria
– Homozygous (HbA HbA) more likely to die from malaria
– Allele for HbS remains relatively stable in population due to
heterozygote advantage
• Another example of natural selection:
– Antibiotic resistance in bacteria
– Read about in text
Patterns of Evolution
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Divergent evolution – related organisms
become less alike – may result in speciation
– adaptive radiation – members of a
species adapt to a variety of habitats – ex.
Darwin’s finches
• Convergent evolution – two unrelated
groups develop similar characteristics
– occurs when different species share the
same environment and are subjected to
similar selection pressures – ex. fish and
dolphins