General Biology (Bio107)
Chapter 13 – Populations evolve
Theories of geological gradualism helped
clear the path for evolutionary biologists,
including Charles Darwin
• Cuvier documented the succession of fossil species in the Paris
Basin; recognizes that extinction had been a common
occurrence in the history of life.
• Cuvier advocates catastrophism instead of evolution;
boundaries between strata were due to local flood or drought
that destroyed the species then present. Vacated areas are
repopulated by species immigrating from other unaffected
areas.
• Geologist J. Hutton proposes the theory of gradualism, i.e.
that the diversity of land forms (e.g., canyons) could be
explained by mechanisms currently operating.
Lamarck, Fossils & Early evolutionary ideas
• 1809: Jean Baptiste Lamarck publishes an early theory of
evolution based on his observations of fossil invertebrates in
the Natural History Museum of Paris.
• Lamarck thought that he saw what appeared to be several lines of
descent in the collected fossils and current species.
• Each was a chronological series of older to younger fossils leading to a
modern species.
• Central to Lamarck’s mechanism of evolution were the
concepts of use and disuse of parts and of inheritance of
acquired characteristics.
- There is currently no hard scientific evidence that acquired characteristics
can be inherited, but the new field of “epigenetics” and its discoveries
of gene regulation by histone modification points in that direction
• Charles Darwin (1809-1882) was born in western
England. Darwin had a consuming interest in
nature as a boy, but his father sent him to the
University of Edinburgh to study medicine.
• Darwin left Edinburgh
without a degree and
enrolled at Christ College
at Cambridge University
with the intent of becoming
a clergyman.
• At that time, most naturalists and scientists belonged
to the clergy and viewed the world in the context of
natural theology.
Field research helped Darwin frame his view of life
• The main mission of the five-year voyage of the Beagle was to
chart poorly known stretches of the South American coastline,
including the Galapagos islands.
• Darwin made several important observations:
1. Plants and animals of South America were very distinct
from those of Europe.
• Darwin noted that while most of the animal species on the Galapagos
lived nowhere else, they resembled species living on the South
American mainland.
• It seemed that the islands had been colonized by plants and animals
from the mainland that had then diversified on the different islands.
2. Most of the animal species on the Galapagos lived
nowhere else; they resembled species living on the South
American mainland.
• It seemed that the islands had been colonized by plants and animals
from the mainland and that they diversified on the different islands.
• Organisms from temperate regions of South America were more similar
to those from the tropics of South America than to those from
temperate regions of Europe.
• While on the HMS Beagle, Darwin read Lyell’s
Principles of Geology.
• Lyell’s ideas and his observations on the voyage lead
Darwin to doubt the church’s position that the Earth
was static and only a few thousand years old.
• Instead, he was coming to the conclusion that the Earth
was very old and constantly changing.
• Darwin’s views were influenced by the fossils he found and by
Cuvier’s earlier ideas, including catastrophism.
• Cuvier recognized that extinction had been a common
occurrence in the history of life.
• Fossils are relics or impressions of organisms from the past,
mineralized in sedimentary rocks. Sedimentary rocks form when
mud and sand settle to the bottom of seas, lakes, and marshes.
• New layers of sediment cover older ones, creating layers of
rock called strata.
• Fossils within layers show that a succession of organisms have
populated Earth throughout time.
• Paleontology, the study of fossils, was largely developed by
Georges Cuvier, a French anatomist. In particular, Cuvier
documented the succession of fossil species in the Paris Basin.
• After his return to Great Britain in 1836, Darwin began to
perceive that the origin of new species and adaptation of
species to the environment as closely related processes.
• For example, among the 13 types of finches that Darwin
collected in the Galapagos, clear differences in the beak
are adaptations to the foods available on their home
islands.
• By early 1840’s Darwin had developed the major features of his
theory of natural selection as the mechanism for evolution.
• 1844: writes long essay on the origin of species and natural
selection, but does not publish.
• 1858: Alfred Wallace, a young naturalist working in the East
Indies, sends Darwin a manuscript containing a theory of
natural selection identical to his own.
• 1858: Wallace’s paper and extracts of Darwin’s essay are
presented to the Linnaean Society of London.
• 1859: Ch. Darwin publishes his book On the Origin of Species
by Means of Natural Selection which becomes bestseller.
• The Origin of Species challenged a worldview that had been
accepted for centuries.
• The key classical Greek philosophers who influenced Western
culture, Plato and Aristotle, opposed any concept of evolution.
Aristotle believed that all living forms could be arranged on a ladder (scala
naturae) of increasing complexity with every rung taken with perfect,
permanent species.
• The Judae-Christian religious account of creation fortified the
idea that species were individually designed by a divine creator
and did not evolve.
• In the 1700’s, the dominant philosophy, natural theology, was
dedicated toward studying the adaptations of organisms as
evidence that the Creator had designed each species for a
purpose.
Darwin’s The Origin of Species
developed two main points:
1. The occurrence of evolutionary
change in life forms on earth
(“Descent with modification’)
2. Natural selection as its mechanism
• Darwin introduced the concept of natural
selection as a mechanism for adaptive evolution.
• Central to Darwin’s view of the evolution of
life is descent with modification.
• All present day organisms are related through
descent from unknown ancestors in the past.
• Descendants of these ancestors accumulated
diverse modifications or adaptations that fit them to
specific ways of life and habitats.
• History of life is like a tree with multiple
branches from a common trunk.
• Closely related species, the twigs of the tree,
shared the same line of descent until their
recent divergence from a common ancestor.
• Evolutionary tree of the elephant family
based on evidence from fossils.
Last common ancestor
Today
SYNAPSIDS
SAUROPSIDS
AMNIOTA
TETRAPODA
SARCOPTERYGIA
(= lobe-finned fishes)
• Darwin’s theory of natural selection.
• Ernst Mayr, an evolutionary biologist, has
dissected the logic of Darwin’s theory into three
inferences based on five observations which are:
1. Tremendous fecundity
2. Stable populations sizes
3. Limited environmental resources
4. Variation among individuals of a population
5. Heritability of some of this variation.
1. Fecundity
• All species have such great potential fertility that their
population size would increase exponentially if all individuals
that are born reproduced equally successful.
• Production of more individuals than the environment can
support leads to a struggle for
existence among the individuals
of a population, with only a
fraction of the offspring surviving
each generation.
• Strongly influenced by a 1798 essay on
human population by Thomas Malthus
• Capacity to overproduce seems to be a
characteristic of all species; only a small
fraction of eggs developing to leave offspring of their own.
2. Stable Population size
3. Limited Environmental/Nutrition Resources
4. Variation
• Individuals of a population of the same species
vary extensively in their characteristics; no two
individuals are exactly alike.
• Much of the variation observed in a species is
heritable (of genetic origin).
- some traits are modified epigenetically
• Inference: Survival in the struggle for
existence is not random, but depends in part
on the hereditary constitution of the individuals.
• Those individuals whose inherited characteristics
best fit them to their environment are likely to leave
more offspring than less fit individuals.
• Inference: This unequal ability of
individuals to survive and reproduce will
lead to a gradual change in a population, with
favorable characteristics accumulating over the
generations ( adaptation).
• Darwin’s main ideas can be summarized
in three points.
• Natural selection is differential success in
reproduction (unequal ability of individuals to
survive and reproduce).
• Natural selection occurs through an interaction
between the environment and the genetic and
morphological variability (variation) inherent
among the individual organisms making up a
population.
• The product of natural selection is the
adaptation of populations of organisms to their
environment.
• E.g. Related species of insects called mantids have
diverse shapes and colors that evolved in different
environments.
• In each generation,
environmental factors
filter heritable variations,
favoring some over
others.
• This increasing frequency
of the favored traits in a
population is evolution.
• Darwin’s views on the role of environmental factors in
the selection of heritable variation was heavily
influenced by artificial selection.
• Humans have modified a variety of domesticated
plants and animals over many generations by
selecting individuals with the desired traits as
breeding stock.
• Examples:
1. Dog varieties from wolf or coyote
2. Corn varieties from teosinthe grass
3. Cabbage varieties from wild mustard (Brassica)
- e.g. Brussels sprouts, Kohlrabi, Broccoli
Evolution of modern corn (Zea maize) by artificial selection & bottleneck effect
6,300
2,300 – 4,400
yrs. ago
yrs. ago
today
Teosinte
Primitive maize
(Zea mays
ssp. parviglumis)
(Zea mays ssp.)
Modern maize/
Corn
Gene locus
tga1  glume
hardness
su1  starch-debr.
enzyme
pbf  protein
storage
tb1  branched
 Morphological differences due to mutations in only five genomic regions
Graphic©E.Schmid/2003
(Zea mays
ssp. mays)
• If artificial selection can achieve such major
changes in a relatively short time, then natural
selection should be capable of major
modifications of species over hundreds or
thousands of generations.
• Darwin envisioned the diversity of life as evolving
by a gradual accumulation of minute changes
through the actions of natural selection operating
over vast spans of time.
• While natural selection involves interactions
between individual organisms and their
environment, it is not individuals, but populations
that evolve.
• Populations are defined as a group of successfully
interbreeding individuals of a single species that share
a common geographic area.
 no “reproductive barrier”
• Evolution is measured as the change in relative
proportions of heritable variation in a population over a
succession of generations.
• Natural selection only amplifies or diminishes
heritable variations, not variations that an individual
acquires during its life.
• Natural selection is situational.
• Environmental factors vary in space and time.
• Adaptations for one set of environmental conditions may be
useless or even detrimental under other circumstances.
Populations are the units of evolution
• One obstacle to understanding evolution is the
common misconception that organisms evolve, in
a Darwinian sense, in their lifetimes.
– However, the evolutionary impact of natural
selection is only apparent in tracking how a
population of organisms changes over time.
• It is the population, not its individual, that evolve.
• Evolution on the scale of populations, called
microevolution, is defined as a change in the
allele frequencies in a population.
• The Origin of the Species convinced most
biologists that species are the products of
evolution, but acceptance of natural selection as
the main mechanism of natural selection was
more difficult.
• Darwin’s explanation lacked an under-standing
of inheritance that could explain how chance
variations arise in a population while also
accounting for the precise transmission of these
variations from parents to offspring.
• This gap was filled in by the Synthetic
Evolutionary Theory which as developed in the
1930s
The modern evolutionary synthesis integrated
Darwinian selection and Mendelian
inheritance
• When Mendel’s research was rediscovered in
the early twentieth century, many geneticists
believed that the laws of inheritance conflicted
with Darwin’s theory of natural selection.
– Darwin emphasized quantitative characters,
those that vary along a continuum.
– Mendel and later geneticists investigated
discrete “either-or” traits.
• A turning point for evolutionary theory was the
birth of population genetics, which emphasizes
the extensive genetic variation within populations
and recognizes the importance of quantitative
characters.
• In the early 1940’s, the modern synthesis of the
evolutionary theory took form.
– It integrated discoveries and ideas from paleontology, taxonomy,
biogeography, and population genetics.
• Its architects included geneticists Theodosius
Dobzhansky and Sewall Wright, biogeographer
and taxonomist Ernst Mayr, paleontologist
George Gaylord Simpson, and botanist G.
Ledyard Stebbins.
• A population is a localized group of individuals
that belong to the same species.
– Species = members of populations who are able to
successfully interbreed and produce fertile offspring in
a nature.
• Populations of a species may be isolated from
each other (= rare exchange of genetic material)
or they may intergrade with low densities in an
intermediate region.
• Members of a population are far more likely to
breed with members of the same population
than with members of other populations.
Gene pool
• Gene pool = the total aggregate of genes in a
population at any one time.
– It consists of all alleles at all gene loci in all
individuals of a population.
– Each locus is represented twice in the genome
of a diploid individual.
• Individuals can be homozygous or
heterozygous for these homologous loci.
– Often, there are two or more alleles for a gene,
each contributing a relative frequency in the
gene pool.
Gene pool & Hardy-Weinberg equation
• Hardy-Weinberg theorem describes the gene
pool of a nonevolving population.
• E.g., a wildflower population with two flower
colors. The allele for red flower color (R) is
completely dominant to the allele for white
flowers (r).
• An imaginary population of 500 plants, 20 have
white flowers (homozygous recessive - rr). The
other 480 plants have red flowers.
• Some are heterozygotes (Rr), others are
homozygous (RR).
– Suppose that 320 are RR and 160 are Rr.
• Because these plants are diploid, in our
population of 500 plants there are 1,000 copies
of the gene for flower color.
– The dominant allele (R) accounts for 800
copies (320 x 2 for RR + 160 x 1 for Rr).
– The frequency of the R allele in the gene pool
of this population is 800/1000 = 0.8, or 80%.
– The r allele must have a frequency of 1 - 0.8 =
0.2, or 20%.
• How will meiosis and sexual reproduction affect
the frequencies of the two alleles R and r in the
next generation of the imaginary wildflower
population of 500 plants ? (80% (0.8) of the flower
color alleles are R and 20% (0.2) are r).
• Assumptions:
1. Fertilization is completely random
2. All male-female mating combinations are
equally likely (random mating).
• Gamete drawn from the gene pool at random has
a 0.8 chance of bearing an R allele and a 0.2
chance of bearing an r allele.
• The processes of meiosis and random
fertilization have maintained the same allele and
genotype frequencies that existed in the
previous generation.
• For the flower-color locus, the population’s genetic
structure is in a state of equilibrium, HardyWeinberg equilibrium.
– Theoretically, the allele frequencies should remain at 0.8
for R and 0.2 for r forever.
• The Hardy-Weinberg theorem states that the
processes involved in a Mendelian system have no
tendency to alter allele frequencies from one
generation to another.
– The repeated shuffling of a population’s gene pool over
generations cannot increase the frequency of one allele
over another.
– Only true mutations do change allele frequencies
• In the wildflower example, p is frequency of red
alleles (R) and q of white alleles (r).
– The probability of generating an RR offspring is
p2 (rule of multiplication).
• In our example, p = 0.8 and p2 = 0.64.
– The probability of generating an rr offspring is
q 2.
• In our example, q = 0.2 and q2 = 0.04.
– The probability of generating Rr offspring is
2pq.
• In our example, 2 x 0.8 x 0.2 = 0.32.
• The genotype frequencies should add to 1:
p2 + 2pq + q2 = 1
– For the wildflowers, 0.64 + 0.32 + 0.04 = 1.
• This general formula is the Hardy-Weinberg
equation.
• Using this formula, one can calculate:
1. Frequencies of alleles in a gene pool if one
knows the frequency of genotypes or
2. Frequency of genotypes if one knows
the frequencies of alleles.
• Populations at Hardy-Weinberg equilibrium must
satisfy five conditions.
(1) Very large population size. In small
populations, chance fluctuations in the gene
pool (= genetic drift), can cause genotype
frequencies to change over time.
(2) No migrations. Gene flow, the transfer of
alleles due to the movement of individuals or
gametes into or out of our target population
can change the proportions of alleles.
(3) No net mutations. If one allele can mutate
into another, the gene pool will be altered.
(4) Random mating. If individuals pick mates
with certain genotypes, then the mixing of
gametes will not be random and the HardyWeinberg equilibrium does not occur.
(5) No natural selection. If there is differential
survival or mating success among genotypes,
then the frequencies of alleles in the next
variation will deviate from the frequencies
predicted by the Hardy-Weinberg equation.
• Evolution results when any of these five
conditions are NOT met , i.e. when a population
experiences deviations from the stability predicted
by the Hardy-Weinberg theory.
Hardy-Weinberg is useful in public health
science
• The Hardy-Weinberg theorem can estimate the
percentage of the human population that carries
the allele for a particular inherited disease, e.g.
phenyketonuria (PKU).
About 1 in 10,000 babies born in the United States is
born with PKU, which results in mental retardation and
other problems if left untreated.
The disease is caused by a recessive allele.
• Q. What percentage of U.S. population
carries the PKU allele?
• From the epidemiological data, we know that
frequency of homozygous recessive individuals (q2
in the Hardy-Weinberg theorem) = 1 in 10,000 or
0.0001.
– The frequency of the recessive allele (q) is the
square root of 0.0001 = 0.01.
– The frequency of the dominant allele (p) is p = 1
- q or 1 - 0.01 = 0.99.
– The frequency of carriers (heterozygous
individuals) is 2pq = 2 x 0.99 x 0.01 = 0.0198 or
about 2%.
• A. About 2% of the U.S. population carries the
PKU allele.
Microevolution
• The Hardy-Weinberg theory provides a baseline
against which we can compare the allele and
genotype frequencies of an evolving
population.
• We can define microevolution as generationto-generation change in a population’s
frequencies of alleles.
– Microevolution occurs even if the frequencies
of alleles are changing for only a single
genetic locus in a population while the others
are at equilibrium.
• Four factors can alter the allele frequencies
in a population:
1. Genetic drift
2. Natural selection
3. Gene flow
4. Mutation
• All represent departures from the conditions
required for the Hardy-Weinberg equilibrium.
• Genetic drift and natural selection are the two
most important ones.
Genetic drift, gene flow & bottle neck effect
• Genetic drift occurs when changes in gene
frequencies from one generation to another
occur because of chance events (sampling
errors) that occur when populations are finite in
size (“small size phenomenon”)
– The smaller the population, the greater the
chance of deviation from an idealized result.
– Genetic drift at small population sizes often
occurs as a result of two situations: the
bottleneck effect or the founder effect.
Genetic drift
• The gene pool of small population may not be
accurately represented in next generation because
of sampling errors.
• E.g. elimination of some alleles in small populations
Bottle neck effect
• Occurs when the numbers of individuals in a
larger population are drastically reduced by a
disaster, e.g. overhunting, flooding, overfishing,
volcanism, Tsunami, etc. By chance, some
alleles may be overrepresented and others
underrepresented
among the survivors.
• Some alleles may be
eliminated altogether.
Bottle neck & Conservation biology
• Bottlenecking is an important concept in
conservation biology of endangered species.
• Populations that have suffered bottleneck
incidents have lost at least some alleles from the
gene pool.
• This reduces individual
variation and adaptability.
– E.g. the genetic variation
in the three small surviving
wild populations of
cheetahs is very low when
compared to other mammals.
Gene flow (Migration)
• Gene flow is genetic exchange due to migration of
fertile individuals or gametes between populations.
• Gene flow tends to reduce differences between
populations.
• E.g., the migration of people throughout the world
is transferring alleles between populations that
were once isolated, increasing gene flow.
• If extensive enough, gene flow can amalgamate
neighboring populations into a single population
with a common genetic structure.
Variation is extensive in most populations
• Most populations show a great deal of variation,
both, on the anatomical and molecular level.
• Many of the variable traits
of the individuals of a
population result from
combined effect of
several genes
(“polygenic inheritance”).
• Populations are polymorphic, i.e. two or more
“morphs” are present
in noticeable numbers
• Not all variation is heritable (”epigenetics”)
Sources of variation
• Evolutionary change of phenotypic characteristics
and development of new species requires
change or new assortment of the genetic
material.
• Sequence of nucleotides on the DNA molecule
has to be changed in order bring out a new gene
product, i.e. a protein or enzyme, and (in a long
term) to lead to a new phenotype of a species.
Sources of variation
• Two important principles are responsible for
variation in biological life forms:
1. Mutations
2. Recombination
• Genetic variation due to mutation and/or
recombination, is one of the pre-conditions of
natural selection
• Natural selection acts on individuals, not their
genes
 the natural forces and principles select from the
offered arsenal of gene and allele variety within
a population
Mutations
• New alleles originate only by mutation.
– Mutations are changes in the nucleotide
sequence of DNA.
– Mutations of individual genes are rare and
random.
– Mutations in somatic cells are lost when the
individual dies.
– Only mutations in cell lines that produce
gametes can be passed along to offspring.
Mutations
1. Point mutations
• one nucleotide within the DNA molecule is changed
• can be caused by different means, mostly by mutagens
and mistakes during DNA replication (lack of DNA repair)
 e.g. certain chemicals, such as nitrosamines, benz(a)pyrene,
 e.g. natural or artificial radiation (= radon, UV light, X-ray)
• most of point mutations either remain “silent” or are
repaired
2. Deletion
• a short piece of DNA gets lost or becomes actively
excised out of the DNA strand
• has dramatic effects on protein structure and function
• often caused as a consequence of the life cycle of
certain viruses which insert and excise DNA
Mutations
3. Insertion
• a short piece of DNA integrates into the DNA
molecule
• it usually also leads to a massive stir-up of the flow of the
genetic information within a cell; altered protein shapes
• many DNA insertions are caused by viruses which
randomly integrate their DNA nto the genetic material of
their host cells
• Many insertions are also caused by “jumping genes”
(transposons, retrotransposons)
Impact of mutations
• Most point mutations, those affecting a single
base of DNA, are probably harmless.
– Most eukaryotic DNA does not code for proteins and
mutations in these areas are likely to have little
impact on phenotype.
– Even mutations in genes that code for proteins may
lead to little effect because of redundancy in the
genetic code.
– However, some single point mutations can have a
significant impact on phenotype.
• E.g. Sickle-cell disease is caused by a
single point mutation.
Impact of mutations
• Chromosomal mutations, e.g. deletions,
translocations, affect many genes and are likely to
disrupt proper development of an organism.
– However, occasionally, these dislocations link genes
together such that the phenotype is improved.
• Duplications of chromosome segments, whole
chromosomes, or sets of chromosomes are nearly
always harmful.
– However, when they are not harmful, the duplicates
provide an expanded genome.
– These extra genes can now mutate to take on new
functions.
Recombination Events
• Changes of the sequence of chromosomal DNA due to
rearrangement of larger stretches of DNA (=
chromosomal fragments including thousands of genes)
•
Can happen by twomajor cellular processes:
1. Crossing over during meiotic cell division in
germ cells located in the gonads
2. Transposons (= movable genetic elements) with the
help of enzymes called transposases
• Translocations (= movement of larger chromosomal
fragments) are in most cases harmful to the organisms,
but in rare cases they may bring out beneficial new
characteristics in the affected individual
Impact of recombination events
• In organisms with sexual reproduction, most of
the genetic differences among individuals are
due to unique recombinations of the existing
alleles from the population gene pool.
– The ultimate origin of allelic variation is past
mutations.
• Random segregation of homologous
chromosomes and random union of gametes
creates a unique assortment of alleles in each
individual.
• Sexual reproduction recombines old alleles
into fresh assortments every generation.
Mutation rate
• Gene mutations and recombinations result in
new alleles and allele combinations which are
the ultimate source of variation within
populations
Mutation Rate & Micro-Evolution
“Mutations happen all the time…this is why evolution is an
unstoppable process on planet Earth!”
• Due to DNA replication and DNA repair mechanisms,
mutation rates of individual genes are low, but since
each organism has many genes, and a population has
many individuals, new mutations arise in populations
all the time
Microevolution
• A change in the frequency of alleles in the
gene pool of a species over time is evolution in
its smallest scale or also referred to as microevolution
• The driving force of natural selection is the unequal
success in reproduction among members of a population.
• Natural selection leads to gradual change in the
characteristics of a population of organisms which gets
the favored characteristics of its most reproductive
members.
• In most cases the physical environment (e.g. climate,
seasonal changes, predators, etc.) screens for the most
favored traits within a certain species.
• Natural selection tends to reduce the phenotypic
variability in a population over time; but the individuals of
a population do not become genetically uniform due to the
existence of so-called recessive alleles
Diploidy and balancing selection preserve
variation in populations
• The tendency for natural selection to reduce
variation is countered by mechanisms that
preserve or restore variation, including diploidy
and balanced polymorphisms.
• Diploidy in eukaryotes prevents the elimination
of recessive alleles via selection because they
do not impact the phenotype in heterozygotes.
– Even recessive alleles that are unfavorable
can persist in a population through their
propagation by heterozygous individuals.
• Balanced polymorphism maintains genetic
diversity in a population via natural selection.
• One mechanism in balance polymorphism is
heterozygote advantage.
• In some situations individuals that are
heterozygous at a particular locus have greater
survivorship and reproductive success than
homozygotes.
• E.g. heterozygous advantage maintains
genetic diversity at the human locus for one
chain of hemoglobin.
– A recessive allele causes sickle-cell disease in
homozygous individuals, but heterozygous individuals
are resistant to malaria and pass on their traits.
• The frequency of the recessive sickle-cell allele
is highest in areas where the malarial parasite is
common.
• The advantages of heterozygotes over
homozygous recessive
individuals who suffer
sickle-cell disease and
homozygous dominant
individuals who suffer
malaria are greatest here.
– Sickle-cell allele
may reach 20% of
gene pool, with 32%
heterozygotes resistant
to malaria and 4% with sickle-cell disease.
Neutral variation
• Some genetic variations, neutral variation,
have negligible impact on reproductive success.
– E.g. diversity of human fingerprints seems to
confer no selective
advantage to some
individuals over others.
– Much of the protein and
DNA variation detectable
may be neutral in their
adaptive qualities.
• The relative frequencies of
neural variations will not be
affected by natural selection;
some will increase and others will decrease by
chance effects of genetic drift.
Variation & Natural Selection
• Natural selection acts on the phenotypic variations of
individuals of a population by different means.
• Important examples are:
1. Competition for resources and mates
2. Vulnerability to diseases, pathogens, etc.
3. Resistance to repellants, poisons or
environmental toxins
4. Resistance to periods of malnutrition
5. Camouflage protection or other successful
escape strategies from predators
• Some individuals of a population always turn out to be
better adapted to their environment than other members
• Better adapted individuals have a higher chance to
survive and are more likely to reproduce
3 different ways are known by which natural selection can
alter the phenotypic variation of an ideal population
1. Stabilizing selection
- natural selection pressure favors the evolution of
intermediate variants
- mostly observed in stable environments
2. Directional selection
- selection pressure acts against individuals at one of the
phenotypic extremes (bell-shaped curve)
- frequency of rare phenotypic variants increases,
while the originally dominating phenotype disappears
from due to on-going “selective pressure”
- bell-shaped population curve is shifted to the right or left
- e.g. formation of DDT-resistant insects over time
3. Diversifying selection
- selection pressure favors the reproduction/survival of
individuals at both phenotypic extremes
- individuals with intermediate phenotype feel most
selection pressure and decline in the population
- causes a discontinuity of the variations, causing two or
more morphs or distinct phenotypes
- e.g. the African swallowtail butterfly (Papilo dardanus)
produces two distinct morphs, both of which resemble
brightly colored but distasteful butterflies of other species
- although obviously eatable, both morphs gain protection
from predation
3. Diversifying Selection
Speciation
The total number of species in the world changed over long
periods of time and phenotypes of species do not remain the
same on Earth.
Early biologists of the 19th century suspected some common
mechanism that is responsible for the appearance of new
species over time; C. Darwin introduced the term speciation
to explain the appearance of new species on Earth
Definition: Speciation
“Process by which new species of biological organisms
evolve from a single population of parental species over
long periods of time.”
Speciation = responsible for the evolution of new species.
Definition: Species
A group of successfully interbreeding members of a
population that are reproductively isolated from members of
an original “parent population”; different species are
biochemical and geographically separated from each other
by a reproductive barrier.
Members of a “parent population” acquire and pass on new
isolating traits while isolated from its parent isolation and
ceasing to exchange genetic material with the “left-behind”
population.
Speciation is completed after establishment of a
reproductive barrier between members of the separated
populations
Evidence for Evolution
• Even though evolution is called a theory it does
not mean that evolutionary theory is a mere
guesswork. Its ideas are supported by a vast
amount of evidences coming from many
different scientific disciplines, such as:
1. Direct observation
2. Fossil evidence
3. Comparative anatomy
4. Comparative embryology
5. Biogeography
6. Atavism
7. Molecular biology
1. Direct observation
• The evolution of resistance to insecticides in
hundreds of insect species and the rapid
evolution of antibiotic resistance in bacteria
are classic examples of natural selection in
action.
- Insecticides are poisons that kill insects that are pests
in crops, swamps, backyards, and homes.
• The results of application of new insecticide are
typically encouraging, killing 99% of the insects.
• However, the effectiveness of the insecticide
becomes less effective in subsequent
applications.
In wide areas of Asia
where the insecticide
DDT was intensively used
in the past decades,
some insects, i.e.
Anopheles, developed
resistance to this
environmentally very
stable and ecologically
critical chemical.
2. Fossil evidence
• Evidence of evolution by natural selection
in the much grander changes in biological
diversity is strongly documented by fossil
records.
– Evidence that the diversity of life is a produce
of evolution pervades every research field of
biology.
– As biology progresses, new fossil discoveries,
including the revelations of molecular biology,
continue to validate the Darwinian view of life.
• Fossils of possibly first animals (650 – 544 Mio years ago), Vendian period
indicate first diversification of soft-bodied organisms
Original fossil
Name
Dickinsonia
- phylum ??
Tribrachidium
-Cnidarian?
- Echinoderm?
Fossils of the Chengjiang site (China)  570 Mio years old
Naroia
- soft-bodied trilobite
Leanchoilia
illecebrosus
Reconstruction
Original fossil
Name
Archaeopteryx lithographica
Feathered reptilian (theropod-like) of the late Jurassic
(200 – 144 Mio years ago)
- bony tail with reduced tail vertebrae (short for theropods)
- reptilian teeth
- clawed fingers
- feathers
• Evolutionary transitions in life forms have left
signs in the fossil record.
– For example, a series of fossils documents
the changes in skull shape and size that
occurred as mammals evolved from reptiles.
– Recent discoveries
include fossilized
whales that link
these aquatic
mammals to
their terrestrial
ancestors.
Rodhocetus kasrani
 Fossil find: Pakistan, Age: 47 Mio yrs. old
 extinct whale ancestor of the Eocene sea
I. Reconstructed body features (Science 293(5538): 2239ff (2001)
II. Restoration of the found fossilized skeleton
Sahelanthropus tschadensis
(6 – 7 Mio years ago)
 almost complete cranium (skull) of “Toumai”
- Discovered in 2001 in today’s Chad (S. Sahara, Africa)
- Small brain case (320 – 380 ccm)
- the oldest known hominid after the evolutionary important
split from chimpanzees (= Chimp- hominid split)
- erect stance ?, bipedalism ?
• The succession of fossil records is compatible
with what is known from other types of evidence
about the major branches of descent in the tree
of life.
– For example, fossil fishes predate all other
vertebrates, with amphibians next, followed by
reptiles, then mammals and birds; this is
consistent many other types of evidence.
– In contrast, the idea that all species were
individually created at about the same time
predicts that all vertebrate classes would
make their first appearance in the fossil
record in rocks of the same age.
• This is not what paleontologists actually
observe.
3. Comparative anatomy
• Descent with modification is evident in anatomical
similarities between species grouped in the same
taxonomic category.
• E.g., the forelimbs of human, cats, whales, and
bats share the same skeletal elements, but
different functions because they diverged
from the ancestral
tetrapod forelimb.
• They are
homologous
structures.
• Comparative anatomy confirms that evolution is
a remodeling process via alteration of existing
structures, rather than uniquely engineered for
their existing function.
– Historical constraints on this retrofitting are
evident in anatomical imperfections.
– For example, the back and knee problems of
bipedal humans are an unsurprising outcome
of adapting structures originally evolved to
support four-legged mammals.
Homologous structures
• Another prominent example
of homologous structures in
animals is the evolution of
jaws by modification/
re-modeling of gill arches
• Some of the most interesting homologous structures
are vestigial organs, structures of marginal, if any
importance, to a current organism, but which had
important functions in ancestors.
• Prominent examples are:
1. Vestiges of the pelvis and leg bones in skeletons of
some snakes and of fossil whales
2. Presence of wing bones but reduced wings and
incapability to fly in some bird species, e.g. Emu,
ostrich, penguins
3. Presence of tail bone (coccyx) but no tail in adult
humans. Tail is transiently visible during early
embryonic development.
4. Presence of nearly complete eyes but no optical
nerves in sight-less, cave-dwelling species, e.g. the
cave crayfish
Vestigal Structures
Coccyx
(‘tail bone”)
Blind
Cave crayfish
4. Comparative embryology
• Homologies that are not obvious in adult
organisms become evident when we look at
embryonic development.
• For example, all vertebrate embryos have
structures called pharyngeal pouches in their
throat and a notochord at some stage in their
development.
– These embryonic structures develop into very
different, but still homologous, adult
structures, such as the gills of fish or the
Eustacean tubes that connect the middle ear
with the throat in mammals.
5. Biogeography
• Biogeography = geographical distribution of
species.
- first suggested as evidence for evolution by Ch. Darwin.
• Species tend to be more closely related to other
species from the same area than to other species
with the same way of life (environment), but living
in different areas/continents.
Prominent examples for biogeography evidence
are:
1. Australian marsupial sugar gliders vs. the
North American eutherian flying squirrels.
Marsupial animals only diversified in Australia, while all
other continents are populated by dominating placental
(eutherian) mammalians
 Marsupials evolved on Earth when South America, Antarctica, and
Australia were still joined, but Australia separated from the other
continents before the placental mammals arose.
2. Galápagos Islands are home to many more finch
species than are found on the South American mainland
3. Cacti are restricted to North American deserts, while
drought-resistant Euphorbia plants dominate in Africa.
• Sugar glider and flying squirrel have adapted to
the same mode of life, but they are not closely
related.
• Instead, the sugar glider from Australia is more
closely related to other marsupial mammals from
Australia than to
the flying squirrel, a
placental mammal
from North America.
• Resemblance between
them is an example of
convergent evolution.
Divergence of the Galapagos finches from ancestral
colonizers from the South American main land
• In island chains, or archipelagos, individual
islands may have different, but related species as
the first mainland invaders reached one island and
then evolved into several new species as they
colonized other islands in the archipelago.
• A well-investigated example are the fruit flies
(Drosophila) on the Hawaiian Archipelago.
• All of the 500 or so endemic species of Drosophila
in the Hawaiian archipelago descended from a
common ancestor that reached Kauai over 5
million years ago.
6. Atavism
• The very rare, sudden appearance of
morphologies and phenotypic traits in living
organisms that are usually found in distant still
living or extinct life forms.
• Not completely understood what leads to the
reappearance of these unusual body features in
different life forms.
• Scientists suspect mutations of critical gene
segments (i.e. gene promoters?, transcription
factors?) that are crucial for the regulation of the
genes that are responsible for the expression of
morphology (“morphogenes?, Hox genes?”).
• Examples of atavism are:
1. Extra fleshy hind flippers in dolphins
- observed occasionally in modern dolphins
- seen in fossil records of early dolphin
ancestors which roamed the seas some 40
million years ago;
2. Lack of amphibian metamorphosis in
Californian and Mexican mole salamanders
(Axolotl)
- the dramatic morphological changes
(metamorphosis) is not observed in
- reason(s) for this “stopped metamorphosis”
is/are not known;
3. Extra nipples or even breast in humans
- humans usually have one pair of nipples on
their chest which develop along the milk line; - in other mammalian animal life forms the milk
line gives rise to up to 5 pairs of nipples to
suckle the offspring;
- 1/20 human males
develop at least one
extra pair of nipples;
some even more;
7. Molecular & biochemical evidence
• Further evidence for the process of evolution and
descent from a common ancestor comes form
biochemical analysis of proteins and analysis of
sequence homologies of RNA/DNA molecules.
• Biological organisms use:
1. the same basic biochemical molecules
 e.g. DNA, RNA, pyruvate, acetyl-CoA and ATP
2. the same DNA triplet code
 to encode the genetic information of their enzymes
and proteins
3. many identical or nearly identical proteins
and enzymes
• Common ancestry is experimentally substantiated
by analysis of degree of similarity in:
1. Amino acids
e.g. cytochrome c, superoxide dismutase
2. Nucleotide sequences
e.g. ribosomal RNA (16S rRNA) or mtDNA
Species are not fixed but undergo changes in their
inherited characteristics (variation) and gradually
develop over long periods of time into new
species (speciation)
The understanding that organisms are not fixed
but evolve culminated in the theory of evolution;
evolution is the great unifying theme of modern
Biology
“Nothing in Biology makes sense except in the
light of evolution.”
T. Dobzhansky (1973)
• Darwin’s main ideas can be summarized in
three points.
• Natural selection is differential success in
reproduction (unequal ability of individuals to
survive and reproduce).
• Natural selection occurs through an interaction
between the environment and the variability
inherent among the individual organisms making
up a population.
• The product of natural selection is the adaptation
of populations of organisms to their environment.
• By attributing the diversity of life to natural
causes rather than
to supernatural
creation, Darwin
gave biology a
sound, scientific
basis.
• As Darwin said,
“There is grandeur
in this view of life.”