SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
Chapter 14: CAUSES OF EVOLUTIONARY CHANGE
- Part II -
“The only unchangeable and fixed truth we can relay on is the continued
change on planet Earth…”

To cope with the ever happening changes and continued transformation of the face
of the Earth, all forms of life have to follow these changes by a process called
adaptation

Today we know, that two major processes account for most of the evolutionary
changes observed in living organisms
1. Factors that contribute to genetic change in individual biological organisms

due to our modern understanding of molecular genetics and heredity we know that
evolutionary change of phenotypic characteristics and development of new species
requires change or new assortment of the genetic material

the 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

change or new assortment of genetic material (= DNA) within an individual living
organism is achieved by two major processes:
II.. M
Muuttaattiioonn

different forms of mutations can occur within the DNA molecule each with
different consequences for the targeted cell and the affected organisms
a. Point mutation
 one nucleotide within the DNA molecule is changed (replaced or lost);
depending which nucleotide is affected or replaced the resulting protein function
can be either left unharmed or is changed
 point mutations within the DNA can be caused by different means, most of all by
so-called mutagens
 e.g. certain chemicals, such as nitrosamines,
benz(a)pyrene,
 natural or artificial radiation (= radon, UV light, X-ray)

usually most of the point mutations either remain “silent” or are discovered by the
cell’s endogenous surveillance and repair system (see: Molecular Biology: DNA
repair mechanisms) and immediately repaired; but some mutations which change
a proteins biological function may escape these surveillance mechanisms and
lead to a novel cellular characteristics
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
b. Deletion
 a short piece of DNA gets lost or becomes actively excised out of the DNA
strand
 mutation by deletion usually has dramatic effects on the resulting protein
structure and function; it can leads to proteins with sometimes completely new
features
 DNA deletions are often caused as a consequence of the life cycle of certain
viruses which insert into the cellular genome of organisms; many viruses take a
piece of host-DNA with them (DNA thieves!)
c. 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
 many DNA insertions are caused by viruses which integrate into the genetic
material of their host cells

scientists suspect that several mutation events over long times are necessary to
change a genome in a way that its bearer profits from an improved adaptation to its
environment or from a more favorable trait

(usually asexually reproducing) organisms with very short generation spans can
adapt very quickly by means of mutations alone
 e.g. in prokaryotes, which multiply very fast, a favorable mutation
can increase its frequency within the descendant bacterial population
in a matter of hours or days
 moreover, prokaryotes have only one single allele, which means
that the altered gene can show its novel effect immediately without
being obscured by the compensatory effect of the second
(unaffected) allele

diploid eukaryotic organisms with their usually long generation times would take
long periods of times and multiple DNA events (= mutations) to bring out favorable
genetic variation
therefore, most genetic variation in higher organisms (= animals and plants) comes
from sexual recombination during the so-called crossing over event during the
formation of egg and sperm cells (see meiosis!)

IIII..
G
Geenneettiicc R
Reeccoom
mbbiinnaattiioonn

genetic recombinations are changes of the sequence of chromosomal DNA due to
rearrangement of larger stretches of DNA (= chromosomal fragments including
thousands of genes)

in cells of higher organisms, this can happen by two major cellular processes
1. by crossing over during meiotic cell division in specialized cells, so-called
germ cells located in the gonads or reproductive structures
2
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
2. by transposons (= movable genetic elements) with the help of special enzymes

these so-called 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

summarized, mutations and rearrangement of DNA by recombination and/or
transposons lead to a tremendous increase in genetic variation and diversity
amongst the individuals of a population
 the huge diversity of the human population is explained by the large
genomic material (approx. 40,000 genes and 3 billion base pairs!)
and its vast gene pool (approx. 5 billion individuals!) which both
contribute to the enormous genetic variety of the species Homo
sapiens
 some of the variations are phenotypic and anatomically visible,
e.g. skin color, eye color, hair color, ear lobes, freckles, height, etc.,
 but most of the “hidden diversity” between individuals of a
population, such as the ABO blood group, Rhesus factor, blood cell
number, etc., can only be detected and studied with the help of
sensitive biochemical techniques, such as electrophoresis, singlenucleotide polymorphism, antibodies, PCR, DNA sequencing, etc.

gene mutations and recombinations result in new alleles, and are the ultimate
source of variation within populations
M
Muuttaattiioonn R
Raattee &
&M
Miiccrroo--E
Evvoolluuttiioonn
“ … 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
 by studying the mitochondrial DNA of many different species,
scientists have estimated that it takes approximately 500,000 years
for 1% of DNA to be changed
 this estimated mutation rate is currently questioned and future
studies will have to show whether mutation rates are constant and the
same for different nucleic acids, e.g. mtDNA, genomic DNA or rRNA,
on our planet

mutations are relatively common and the ultimate source of new alleles; high
levels of molecular variation are common in natural populations, although many
mutations (usually recessive) are hidden.
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

the mutation rate varies greatly among species and even among genes of an
individual; large scale effects of mutation result only when mutations, e.g. caused
by errors in DNA replication, chemicals, or radiation, are combined with other
factors, e.g. viruses, that reshuffle the gene pool

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 micro-evolution
N
NA
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ALL S
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“… natural selection is the only driving force by which adaptation of
biological organisms to its environment occurs … and ultimately new
biological organisms arise over time!”
“Natural selection is the creative force of evolution, not just the executioner
of the unfit…”
(S.J. Gould)

genetic variation due to mutation and/or recombination, is one of the pre-conditions
of the process of natural selection, which, however, acts on individuals, not their
genes
 the natural forces and principles select from the offered arsenal of
gene and allele variety within a population

natural selection acts on the phenotypic variations of individuals of a population by
different means, e.g.:
1.
2.
3.
4.
5.
Competition for resources and mates
Vulnerability to diseases, pathogens, etc.
Resistance to repellants, poisons or environmental toxins
Resistance to periods of malnutrition
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 the other members

usually the better adapted individuals have a higher chance to survive and are
more likely to reproduce

as a consequence, they more likely pass on their (favorable) adaptations to the next
generation;

the alleles of the favorable trait will be in greater frequency among the individuals of
the next generation, than those traits of the less "fit" members of the population
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

natural selection filters out certain genotypes from the “gene pool” of a
population by targeting the whole organisms (= phenotypes) of its members
 not all variations within a population are heritable and environmentally
induced phenotypic changes are not passed on to the off-spring (see
Lamarckism!)
 only genetic components of variations laid down in the DNA of sperm
or egg cells is passed over and can lead to evolution as a result of
selection

members of a population usually have an unequal chance of surviving and
reproducing

in other words, natural selection is differential survival and reproduction of
individuals carrying alternative, inherited traits; it involves differences in the relative
contributions of various genotypes to the next generation.

variation in the competence of genotypes can come from many causes/sources:
1. Selective agents
 environmental factors, including competitors, predators, parasites and
environmental conditions of the physical environment, such as
repellants, toxins, etc.
2. Fertility/fecundity differences among genotypes
3. Differences in frequency of reproduction among genotypes
4. Differences in viability/longetivity among genotypes
 physical expression of certain “disease-related genes”, e.g.
oncogenes, before reaching the reproductive age lowers
survival chances and the reproductive fitness

differential increases in genotypes(= alleles) within a population are ultimately
due to:
1. Differential survival &
2. Differential reproduction

a consequence of natural selection is the change in frequencies of diploid genotypes
(= alleles) within a population
 ““bbiioollooggiiccaall ffiittnneessss”” is a measure of an individuals ability to survive, reproduce
and to make a greater contribution to the gene pool of the next generation
 a biological organism that does not reproduce has a biological fitness
of “zero”
“Biological fitness in a Darwinian sense refers to the propensity of
individuals of a population to survive and to successfully reproduce in their
environments…”
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

usually a population includes two or more contrasting forms (= morphs) of a
phenotypic characteristic or trait (e.g. blue, green, brown eye color); we say a
population is polymorphic regarding this characteristic
 e.g. pattern polymorphism of the California King snake, of lady beetles or the
ABO blood groups in humans

graphical presentation of the frequency of phenotypic variants of an ideal
polymorphic population resembles a bell-shaped curve (= “Poisson distribution”)
 this bell-shaped distribution of the phenotypic variants is the result of
polygenic inheritance patterns

the idea of natural selection was developed Darwin as one of his explanations for
the observed evolutionary changes of species
 his idea was strongly influenced by an assay of the British economist
T. Malthus on human population dynamics
 since natural resources on Earth are limited, Darwin deduced that the
production of more individuals than the environment can support
causes struggle for existence among the individuals of a population
 he concluded further that as a consequence only individuals with
inherited characteristics that adapt them best to their environment are
most likely to survive

the driving force of natural selection is therefore the uunneeqquuaall ssuucccceessss iinn
rreepprroodduuccttiioonn 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
 Darwin also reasoned that it is in most cases the physical
environment (e.g. climate, seasonal changes, predators, etc.) which
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
 the recessive allele remains hidden in the gene pool of a population
and becomes only subject to natural selection in the case of
phenotypical expression in homozygous individuals (= individuals
which bear both recessive alleles)

not all genetic variations are subject to natural selection, they are called neutral
genetic variations;
 some genetic traits, e.g. human finger print, provide no selective
advantage for the individual carrier

evolutionary biologists are controversial about how much of the genetic variation is
neutral and about how many alleles confer no selective advantage to its carriers
 some argue that certain genetic variations only appear to be neutral
and influence the reproductive success of its carrier in subtle,
for us humans (currently) not visible or measurable ways
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

3 different ways (= modes) are known by which natural selection can alter the
phenotypic variation of an ideal, polymorph population
1. Stabilizing selection



the natural sseelleeccttiioonn pprreessssuurree favors the evolution of intermediate variants
stabilizing selection is mostly observed in stable environments
individuals of the population have an intermediate phenotype which is the best
adapted one
2. Directional selection



the sseelleeccttiioonn pprreessssuurree acts against individuals at one of the phenotypic
extremes (= left or right of the bell-shaped curve)
the frequency of rare phenotypic variants increases, while the originally
dominating phenotype disappears from the population
due to on-going “selective pressure”, the bell-shaped population curve is shifted
to the right or left, respectively
- e.g. the formation of DDT-resistant insects over time; only those insects
resistant to DDT survived and reproduced, leading over time to populations
largely resistant to DDT
- e.g. resistance of many bacterial species to antibiotics is another example of
- directional selection; today, increasing numbers of bacterial strains are
reported to show some degree of resistance against the most commonly used
antibiotics, such as penicillin or tetracycline
- this unwanted development necessitates the development and more prudent
use of new generations of antibiotic medicines
Stabilizing selection in stable environments
 favoring of intermediate morph
S
Seelleeccttiioonn P
Prreessssuurree


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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
Directional selection
 favoring of phenotypic extremes
S
Seelleeccttiioonn P
Prreessssuurree


3. Diversifying selection


the sseelleeccttiioonn pprreessssuurree favors the reproduction/survival of individuals at
both phenotypic extremes

individuals with intermediate phenotype feel most selection pressure and decline
in the population

it causes a discontinuity of the variations, causing two or more morphs or distinct
phenotypes
Paappiilloo ddaarrddaannuuss) produces two distinct
- e.g. the African swallowtail butterfly (P
morphs, both of which resemble brightly colored but distasteful butterflies of
other species
- although obviously eatable, both morphs gains protection from predation
more than 100 examples are known, which outcomes can be clearly attributed to
the principle of natural selection
- e.g. the European land snail C
Ceeppaaeeaa nneem
moorraalliiss changes its shell color dependent
on the conditions of its habitat
- e.g. the peppered moth B
Biissttoonn bbeettuullaarriiaa appears in two color variety in Great
Britain; a bright and a dark-colored form before the “Industrial Revolution” the
light-colored forms dominated in the Biston population and dark-peppered moths
were very rare;
 the light-colored moths were better camouflaged to the bright
background of the tree bark;
8
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
 after the onset of industrialization in the 1900s and its resulting
air-pollution due to massive coal-burning, the dark-colored moth
became more abundant within the Biston population
 they turned out to be better camouflaged to tree bark which was
blacked by industrial soot
 by the early 1900s the Biston population in British industrial areas
consisted almost entirely of black moth; the light-colored forms of
Biston became the easy prey of birds on the dark-colored tree
 bark; they produced less off-spring and declined in numbers within
the Biston population
Diversifying selection
 favoring of two phenotypic extremes
S
Seelleeccttiioonn P
Prreessssuurree

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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
Factors that contribute to the rate of genetic change in a population

scientists unraveled many factors and events that lead to an increased rate of
genetic change and a change in allelic distribution in a population
1. Genetic drift

evolutionary change of biological organisms can arise as a consequence of the
random change of the relative frequencies of alleles in a population over a
number of generations

the phenomenon of by-chance-alone variation of the frequency of alleles
observed in small gene pools is also known as ggeenneettiicc ddrriifftt
2. Limited fertilization

if parents of diploid organisms produce only a limited number of offspring, some
of their alleles may not be passed on to their offspring
- most genes in diploid organisms occur in two versions (= alleles) which are
located on two different chromosomes (= diploid chromosomal set)
- since the two alleles are random-distributed during meiosis, sperm and egg
cells receive only one random set of (haploid) chromosomes;
- therefore in limited fertilization of one individual some of its alleles may not be
passed over anymore and not be represented within its population anymore

due to this random process of distribution of the parental alleles, the relative
frequencies of certain alleles in a population changes or drift over time in a
process

due to only a random change of the frequencies of traits (=alleles) but not the traits
itself, genetic drift does not enable the individual species to evolve a better
adaptation to its environment!
 genetic drift rather leads to a better adaptation of the
population due to gradual changes of its genetic make-up
3. Migration

the frequency of alleles in a certain region may change due to the migration of
interbreeding organisms with different traits from a different area into that region

migration usually leads to a rapid micro-evolution, means a rapid arise of new
allele frequencies within a population
4. Small group phenomenon

a special form of rapid change of allelic frequencies in a population is known to
scientists as “small group phenomenon”
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

a small group of surviving, separated or emigrating individuals starts a new
population which new gene pool will only represent a small percent of the alleles of
the original population
5. Catastrophes, Disasters & Luck

other major factors which (ever since the beginning of life on Earth) powerfully
influenced the speed of micro-evolution were natural catastrophes and disasters,
such as meteorite impacts, earthquakes, volcano eruptions, fires, etc.

under these conditions the survival of a certain, not necessarily the “best
adapted”, allele combination is a mere situation of “good luck”

the surviving individuals with its (limited) gene pool will eventually start a new
population with changed allelic frequencies
S
Sppeecciiaattiioonn
“Evolution of one species into two species requires separation events”

to total number of different forms of life on planet Earth is not fixed; it is an agreed on
conception amongst biologists that the total number of species in the world changed
over long periods of time and that different Earth periods were populated by different
species

since the total number of species does not remain the same on Earth, the early
biologists of the 19th century suspected some common mechanism that is
responsible for the appearance of new species over time

in the 1860s, the English scientist Charles Darwin introduced the term speciation to
explain the multiplication of species on Earth
Definition: Speciation
Speciation is the process by which several new species of biological organisms are
produced from a single population of parental species



speciation is the process that is responsible for the evolution of new species; i.e.
new groups 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 due to
existence of a reproductive barrier
during speciation members of a “parent population” acquire and propagate 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 full-fledged reproductive barrier
between members of the different and separated populations
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

today we differentiate between 4 different forms of speciation to explain the
evolution of new species each satisfying different observations in nature:
1. Sympatric or ecological speciation
- idea already introduced by Charles Darwin in the 1850s because of his
observations of different finch species on the Galapagos islands
- it states that different species arise from a founder population because of
different preferences for different ecological niches in an area
- no geographical separation is necessary
- it is a rare form of speciation, which is rarely observed in mammals, birds,
butterflies and birds; it is frequently observed with fishes and insects
2. Allopatric or geographical speciation
- the isolating process which triggers this form of speciation is of geographical
nature, e.g. the separation of continents due to continental drift or the
formation of new islands due to volcanic activity
- it is the exclusive mode of speciation among birds and mammals
- biologists discriminate between two forms of allopatric speciation:
2.1.
Dichopatric (secondary) speciation
- caused by the “sudden” appearance of a geographical barrier which
separates a large interbreeding population into two (unequal)
halves (see Graphic below)
- e.g. formation of the Bering strait between Siberia and Alaska
which separated the large land mammals of the northern
hemisphere during the Pleistocene
- rise of new animal species in Siberia and North America
2.2.
Peripatric (primary) speciation
- means the establishment of (small) founder populations beyond the
periphery of the present range of species due to the appearance of
impassable barriers and terrain, such as valleys, rifts, freeways,
artificial ports, dams (see Graphic below)
- forced “evolutionary departure” of individuals of a parent population
into new ecological niches
- form of speciation which is very vulnerable to extinctions!
3. Instantaneous speciation
- describes the sudden appearance of reproductive isolation due to various
chromosomal variations, e.g. polyploidy, gene duplications, deletions
- rather rare form of speciation which is frequently observed with plant hybrids,
fishes, amphibians and reptiles
- often leads to massive polyploidy and parthenogenesis of affected species;
widely considered leading to “evolutionary dead-end forms of life”
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
Graphic: The two forms of allopatric speciation
A. Dichopatric (secondary) speciation
Land mass/
Habitat
Time
Time
Time
Individual
Species C
Parent population
(Species A)
Species B
New geographical
barrier
Residual
Interbreeding zone
B. Peripatric (primary) speciation
Separated
“founder population”
Extinct
“founder population”
 eventually merges
with parent population
Parent population
(Species A)
Eventually becomes
“New species”
Graphic©E.Schmid/2004
4. Speciation by hybridization
- two different polyploid hybrids of a species give rise to a non-polyploid
species
- rare form of speciation with only 8 cases known to biologists
- occurs mostly in smaller or peripheral populations and in “fringe habitats”
which have been drastically reduced in size by human activity

if there is a genetic background explaining the different mechanisms of speciation (=
existence of “speciation genes”), it has to be looked at in the genes responsible
for the successful fertilization between a sperm and egg cell
- mutations in these genes might be responsible for the successful
establishment of the reproductive barrier which is crucial for the rise of a new
species over time
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

as we heard in the previous sections, genetic change is created by random
mutations, viruses, events during sexual reproduction and many factors that
increase or decrease the frequencies of alleles in future generations

but so far the genetics of speciation is the “genetics of isolating mechanisms”
between members of a population; the key to understand the evolution of one
species into (eventually many) new species over long periods of time is indeed the
detailed knowledge about separation events

if two usually interbreeding groups become separated, each will (unavoidably)
change over time, BUT (due to the random character of the genetic change
events) each in a different way at the different loci
 since separated groups usually don’t mix their genes or gained
mutations anymore, each group will begin to accumulate different
mutations
 moreover, since they usually stop interbreeding with each other
again, they will begin to look different from each other

many events are known to scientists that can cause separation of biological
species
1. Geographical separation
 Due to Earthquakes and formation of new landscapes
 Due to Drifting continents (= Plate Tectonics)
 e.g. separation of the North American plate from the Eurasian
plate created new species on both continents
 e.g. plate tectonics created a huge rift valley and high mountain
ranges, that separated East Africa from west Africa about 8
million years ago
 the much drier East African climate lead to the evolution of
different animal groups, which showed clear adaptations to a
drier environment
 it obviously also triggered the early hominid evolution
 fossil records of early hominids, e.g. Australopithecines, could
be only found in East Africa, while genus Pan (including the
chimpanzees) and other apes evidently evolved in West Africa

Due to Glaciation & Changes in sea levels
2. Anthropogenic (= human-caused) separation
 Due to building of freeways or artificial water ways, e.g. channels
 Due to urban development, which may lead to a separation of a valley,
canyon, etc.

The speed of speciation (= speciation rate) is primarily determined by ecological
factors, such as nutrition, competition, temperature, water supply, light intensity, etc.
- little speciation is observed on large uniform continents, e.g. Australia,
Pangeae (?)
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
-
fast and frequent speciation ( high speciation rate) is usually observed
when the range of species is dissected by geographical and ecological
barriers
E
Evvoolluuttiioonn ooff nneew
w ssppeecciieess iiss aacccceelleerraatteedd bbyy m
maassss eexxttiinnccttiioonn ooff ddoom
miinnaattiinngg ““oolldd
ssppeecciieess””

the abrupt disappearance of vast numbers of biological organisms, so-called
mass extinctions, has happened several-fold on planet Earth and carefully
documented by many scientists in the past decades
Example:
The famous Permian-Triassic (or P-T) mass extinction, which is characterized by the
extinction of 50% of all shallow water marine life forms (biota), is explained by the
formation of the supercontinent Pangaea; the formation of this supercontinent by
tectonic movement of the Earth’s crust lead to less miles of shallow water shoreline;
since habitat area determines species diversity, less habitable shorelines lead to the
dramatic decrease in numbers of shallow water species during that time

mass extinctions can and have been be triggered by many factors, most of all
by:
1. Tectonic events
 Due to tectonic drift of the continental plates over long periods of time
 E.g. see P-T mass extinction theory
2. Extraterrestrial events
 due to changing solar activities
 connection between frequency and intensity of solar spots and
the Earth’s climate?
 due to changes in Earth rotation and the Earth’s inclination angle
 due to impacting asteroids or meteorites
3. Biological activities
 the metabolic or habitual activities of newly evolved organisms destroy
previously existing species
4. Human activities
 the vast and global destruction of habitats of many plant and animal species
by human civilization activities, such as tropical rain forest deforestation,
creation of new agricultural areas, urbanization
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
TThhee H
Haarrddyy--W
Weeiinnbbeerrgg--LLaaw
w

although there are usually dominant and recessive alleles present within a
population, nature does not end up in the existence of only dominant
phenotypes after many generations and long periods of times

scientists discovered that nature rather keeps an equilibrium state of both alleles
in diploid biological organisms

the frequency and distribution of both (= dominant and recessive) alleles within
a population can be mathematically calculated and described with the help of the socalled ‘‘H
Haarrddyy--W
Weeiinnbbeerrgg--llaaw
w’’ (named after its two discoverers)
Hardy-Weinberg-law:
(1)
pp222 ++ 22ppqq ++ qq222 == 11
 p and q = the allele frequency within a population
 pp is the frequency of the ddoom
miinnaanntt aalllleellee
e.g. 78% of all individuals of a given population have the
W ) = 0.78
dominant allele W
W; that means p(W
 qq is the frequency of the rreecceessssiivvee aalllleellee
w) =
e.g. 22% of all individuals have the recessive allele w
w; that means p(w
0.22
 pp222//qq222 is the frequency of the homozygous ddoom
miinnaanntt
//rreecceessssiivvee genotype

by knowing the frequency of only one of the genotypes within a given population,
usually of the less frequent homozygous recessive (= ww) trait, one can calculate
the frequency for the other allele combinations with the help of equation (1)

the frequency of two alleles in the population of gametes of individuals of one
generation is the same as it is in the gamete population of the parental generation
BUT: the Hardy-Weinberg law is only valid when 5 conditions are full-filled within an
(ideal) population

1. the population has to be very large
2. the population has to be isolated and no migration of individuals in or out of the
population takes place
3. no mutations take place which may alter the gene pool
4. the mating among the individuals of the population is random (= random mating)
5. all individuals have equal reproductive success
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

the preconditions of the Hardy-Weinberg law are rarely if not full-filled at all in nature;
the Hardy-Weinberg law rather describes the allelic situation within a theoretical (=
ideal) population

in the “real natural world” on our planet we usually see populations that differ from
that picture:
1. Many populations in nature are small

the population number of many endangered species today, e.g. the Siberian tiger or
the Chinese Panda bears is small
 e.g. today only about 31,000 individuals of the
Northern Elephant seals are counted, after they got almost extinct in
the 1890s by hunters;
 in these seals only one allele was found for each examined 24 gene
loci;

this obvious loss of genetic variety within a small population of a species which
suffered a high evolution pressure is also described as the bboottttllee--nneecckk eeffffeecctt

the bottle-neck effect leaves only a limited number of “survival alleles” back in a
population of a species which faced strong evolutionary pressure, such as hunting,
earthquake, brush fires, ice ages, epidemical diseases, etc.

the bottle-neck effect leads to a genetic drift

decreased genetic variety and genetic drift also occurs after colonization of a new
territory, e.g. island by a small group of individuals (e.g. Galapagos islands); this
scenario is also called the so-called founder effect
2. For many populations the migration or movement of fertile individuals into or
out of it is the routine

e.g. the transfer of sperm or pollen of plants to other plant populations by wind,
ocean streams or animals occurs
 this so-called gene flow also leads to a genetic drift within
the population

gene flow usually reduces the genetic difference between populations of the same
species, while reproductive isolation increases the risk of bringing out unfavorable
recessive genes or alleles

today due to our sophisticated and world-bridging, modern transport systems there
is more gene flow in the human population than ever before in human history
17
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
3. Mutations and chromosomal recombinations occur regularly but infrequently
during the cellular life cycle

after a gene has changed by (a) mutation(s), it duplicates itself in its changed (=
mutated) form

if these mutant genes are present in the egg or sperm cells of an organisms they
may alter some heritable characteristics

if multiple mutations become manifested over long periods of time within the germ
line of individuals of a certain population new species may arise

alterations of the DNA sequence are caused by many factors, e.g. chemicals (=
mutagens), radiation (UV light, X-ray) or certain viruses

most mutations lead to unfavorable traits; the affected genes are usually not
propagated into the next generation because the affected individuals die before birth
or don’t survive long enough to successfully reproduce themselves

some mutations are immediately repaired or remain without obvious effect on the
organisms phenotype during his life-time

very rare mutation events, however, at some time and under certain environmental
conditions may turn out to give its carrier or its descendants (which inherited the
mutation) an evolutionary advantage
“… mutations are indeed the only source which lead to the formation of
new genetic variation within a gene pool; mutations lead to genetic drift and
are the ultimate driving force of evolutionary change on planet Earth.”
4. Usually non-random mating is the rule in most populations

“Hardy-Weinberg random mating” is rarely the case and individuals of many plant or
animal populations mate with their immediate neighbors

even in the human population we observe similar patterns and human males and
females with similar phenotypic traits tend to mate more frequently;
 e.g. over-average-sized, tall women tend to marry taller men
 non-random mating also leads to a genetic drift

in real populations all five preconditions are violated to some extent
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