Chapter 15: Evolution
Evolution - all the changes that have
occurred in living things since life
began.
•Many fields of biology give evidence
that evolution has occurred.
•Fossils
•Biogeography
•Anatomy
•Biochemistry
Fossil Evidence
• Fossils - the remains of past life
• Ex: shells, bones, teeth, imprints
• Tell us things such as age, habitat, diet,
& lifestyle of organisms.
• Most found embedded in sedimentary
rock
• Usually, any given layer is older than
the one above it, and younger than
those below.
Derived traits – newly evolved features,
such as feathers, that do not appear in
fossils of common ancestors
Ancestral traits – more primitive features,
such as teeth and tails, that do appear in
ancestral forms
• Relative dating –
• Location of fossils relative to one another
• Helps to determine:
• Order in which organisms have evolved
• Their approximate ages
• Absolute dating –
• Uses radioactive isotopes to measure the
amount of radiation left in a fossil
• Yields an actual age.
• Carbon 14 (14C) - only radioactive isotope
in organic matter.
• Amount of 14C in a fossil is compared with
that of a modern sample to determine the
age of a fossil.
• Half-life - length of time it takes for half of
the radioactive isotope to change into
another stable element.
• 14C changes to 14N in 5730 years
•Certain fossils serve as transitional links
between groups.
•Ex: Archaeopteryx – an intermediate
between reptiles and birds – reptile-like
jaws, teeth & tail, but had feathers & wings
•Can be used to determine the sequence in
which certain groups evolved
•Ex: fishes evolved before amphibians,
which came before reptiles, which
evolved before both birds and mammals
Transitional fossils
Geological Time Scale
• Scientists have divided earth’s history into
eras, periods, and epochs.
Biogeographical Evidence
• Biogeography - the study of the
distribution of plants and animals
throughout the world.
• The world’s six biogeographical regions
have their own distinct mix of living
things.
• Continental drift refers to the changing
positions of the continents over time.
• Two hundred twenty-five million years
ago, all the present land masses belonged
to one continent (Pangaea).
• The distribution of plants and animals is
consistent with continental drift.
• Organisms, such as certain seed plant
groups or reptiles, are widely distributed
throughout the world.
• Other groups, such as mammals that arose
after the continents broke up, have great
differences in species on different
continents.
Emu (Australia)
Ostrich (Africa)
Rhea (South America)
Similar animals from different continents
Continental drift
Anatomical Evidence
• All vertebrate forelimbs contain the same sets of
bones – this strongly suggests common they
evolved from a common ancestor.
• Homologous structures - structures that are similar
because they are inherited from a common
ancestor
• Ex: vertebrate forelimbs
• Analogous structures - used for the same purpose
but are not due to a common ancestor
• Ex: bird wing & insect wing
Bones of vertebrate forelimbs
• Vestigial structures - anatomical features that are
fully developed in one group but reduced or
nonfunctional in other, similar groups.
• Vestigial structures suggest characteristics of
organism’s ancestors.
• Ex: human appendix,
python leg bones
•Embryology - all vertebrates have a dorsal
notochord and paired pharyngeal pouches
at some point.
Significance of developmental
similarities
Biochemical Evidence
• All organisms have certain organic molecules
in common. Ex:
• All use DNA, ATP, and many identical or
nearly identical enzymes.
• Organisms use the same triplet code and the
same 20 amino acids in proteins.
• This similarity is not necessary, but can be
explained by sharing common ancestors.
• Universal genetic code is evidence for
evolution
Significance of biochemical
differences
Charles Darwin
•1831, set sail on HMS Beagle as ship’s
naturalist (observer/collector of plants,
animals, & fossils)
•Left from England, sailed around South
America, across Pacific, around Africa,
and back to England
•Most famous for observations made at
Galapagos Islands
•Darwin noticed similarities between
species seen on mainland and island
chains
•Thought similarities could be explained
by descent with modification – species
came to new environment, then changed
over time as the species adapted to its
new environment.
• Populations become modified through
natural selection
• Natural selection - the process by which
environment acts on a population,
determining which organisms are most
“fit.”
• Those organisms who are most “fit”
survive and reproduce more often than
those who are not.
• Fitness – reproductive success
•Evolution by natural selection requires:
•Variation – slight differences within the
population are passed from generation to
generation (ex: beak size)
•Struggle for existence – competition for
limited resources (ex: food, shelter)
•Differential reproduction – those best
suited to environment reproduce more often
•Differential adaptedness – each generation
contains more individuals who are better
adapted to the environment
•Darwin was influenced by:
•Charles Lyell – geologist who suggested the
Earth was several million years old and
changed over time due to geologic forces
(volcanoes, earthquakes, wind, erosion, etc.)
•People of the 1800’s believed that the Earth
was about 2000 years old and did not change.
•Darwin reasoned that if the Earth could
change, so could its inhabitants.
•Thomas Malthus – an economist who
wrote an essay on overpopulation of
humans. Said that the human population
was kept in check by the limited of
supply of food and living space.
•Darwin reasoned that the same limits
could apply to organisms in nature.
•Darwin’s ideas were controversial because
people in the 1800’s had very religious views of
the Earth’s creation and the origin of the various
species of life.
•Darwin was not the first to suggest that life
forms could change – about 50 years earlier,
Jean-Baptiste de Lamarck suggested that
organisms acquired traits during their lifetime to
adapt to their environment and passed those
acquired traits onto their offspring.
•Darwin published his findings in 1859 in a book
entitled The Origin of Species by Means of
Natural Selection.
•He was motivated to publish his book in 1859
because Alfred Wallace had independently come
up with the same conclusions and was ready to
publish his findings.
•Darwin is credited with the theory of natural
selection due to the volume of evidence to
support his theory.
Adaptations vs. Variations
•Variations – differences that exist within a
population that may have no effect on
fitness
•Ex: length of your thumb
•Adaptations – a variation that all members
of a population have inherited because that
trait improves fitness
•Ex: an opposable thumb
•Three types of adaptations:
•Structural – physical features of an organism
•Ex: long tongue to get food, sharp teeth
•Behavioral – actions an organism takes
•Ex: migration, tracking prey, storing nuts,
growing towards light
•Physiological – functioning/biochemical
processes
•Ex: venom, ink of octopus, protein in web,
respiration rate, digestive enzyme, blood
clotting
•Sources of variation:
•Mutations – individual genes change
•Ex: A  a or ATC  AGC
•Events during meiosis – crossing over
between homologous chromosomes and
independent assortment of chromosomes
•Random fusion of gametes – which sperm
fertilizes which egg - chance
• Three types of natural selection are known:
• Stabilizing selection – an intermediate
phenotype is favored.
• Directional selection – one extreme
phenotype is favored.
• Disruptive selection – both extreme
phenotypes are favored over an
intermediate phenotype.
http://nortonbooks.com/college/biology/animations/ch17a03.htm
Stabilizing selection
Directional selection
Disruptive selection
Population genetics
•Gene pool – total of all the genes of all the
individuals in a population
•If the frequency of genes in a population changes,
evolution has occurred
•Gene frequencies will change to confer survival
and reproductive success
•Microevolution v. macroevolution
•Microevolution - change in gene frequencies (one
or two traits) within a population over time
•Macroevolution – change in gene frequencies of
many traits, resulting in a new species
Macroevolution – changes in the
organisms (over time) are significant
enough to result in the new organisms to
be considered an entirely new species
•(vs. microevolution which result in
relatively few changes in the
organisms and would NOT cause them
to be considered a new species –
changes might be in color or size only)
•In 1908, G. H. Hardy and W. Weinberg created
a law to explain why allele frequencies stay the
same from one generation to the next.
•Hardy-Weinberg Law – p2 + 2pq + q2
•Explains why recessive alleles don’t just
disappear in a population
http://nortonbooks.com/college/biology/animations/ch17a02.htm
http://nortonbooks.com/college/biology/animations/ch17p01.htm
• Gene frequencies will stay the same in a
large population over time provided:
1) There are no mutations or mutations are
balanced.
2) There is no genetic drift; changes in allele
frequencies due to chance alone are
insignificant.
Ex: Founder
effect and
bottleneck
effect
http://nortonbooks.com/college/biology/animations/ch16a01.htm
3) There is no gene flow – no migration of
individuals in or out of the population.
http://nortonbooks.com/college/biology/animations/ch17a01.htm
4) Mating is random – individuals pair by
chance and not by choice (selective breeding
WILL change the gene pool).
5) There is no selection – no selective force
favors one genotype over another.
http://nortonbooks.com/college/biology/animations/ch16a02.htm
In real life, these conditions are rarely met,
and microevolution occurs
Ex. of microevolution – peppered moth
population changed as trees became covered in
soot due to Industrial Revolution in England
Founder effect – a few
individuals found a
colony, and only a
fraction of the original
gene pool is
represented
- Ex: founding member
of Amish population
had recessive allele for
rare kind of dwarfism % of Amish with this
allele higher than in
general population
Founder effects
A founder effect occurs when a new colony is started by a few
members of the original population. This small population size
means that the colony may have:
reduced genetic variation from the original population.
a non-random sample of the genes in the original population.
For example, the Afrikaner population of Dutch settlers in South
Africa is descended mainly from a few colonists. Today, the
Afrikaner population has an unusually high frequency of the
gene that causes Huntington’s disease, because those original
Dutch colonists just happened to carry that gene with unusually
high frequency. This effect is easy to recognize in genetic
diseases, but of course, the frequencies of all sorts of genes are
affected by founder events.
Bottleneck effect – population decline results in
the majority of a population being prevented
from reproducing due to a natural disaster or
human interference
•Ex: cheetahs are so similar genetically that
they appear inbred, which decreases fertility
and could lead to extinction
http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15
-6-OSU.swf
http://www.biology.arizona.edu/evolution/act/drift/about.html
Bottlenecks occur when a population’s size is reduced for at least one generation.
Because genetic drift acts more quickly to reduce genetic variation in small
populations, undergoing a bottleneck can reduce a population’s genetic variation by
a lot, even if the bottleneck doesn’ t last for very many generations. This is
illustrated by the bags of marbles shown below, where, in generation 2, an
unusually small draw creates a bottleneck.
Reduced genetic variation means that the population may not be able to adapt to
new selection pressures, such as climatic change or a shift in available resources,
because the genetic variation that selection would act on may have already drifted
out of the population.
An example of a
bottleneck:
Northern elephant seals have reduced genetic variation probably
because of a population bottleneck humans inflicted on them in the
1890s. Hunting reduced their population size to as few as 20
individuals at the end of the 19th century. Their population has since
rebounded to over 30,000—but their genes still carry the marks of
this bottleneck: they have much less genetic variation than a
population of southern elephant seals that was not so intensely
hunted.
Genetic Drift
Genetic drift—along with natural selection, mutation, and migration—is one of
the basic mechanisms of evolution.
In each generation, some individuals may, just by chance, leave behind a few
more descendents (and genes, of course!) than other individuals. The genes of
the next generation will be the genes of the “lucky” individuals, not necessarily
the healthier or “better” individuals. That, in a nutshell, is genetic drift. It
happens to ALL populations—there’s no avoiding the vagaries of chance.
Look at this cartoon. Genetic drift affects the genetic makeup of the population
but, unlike natural selection, through an entirely random process. So although
genetic drift is a mechanism of evolution, it doesn ’ t work to produce
adaptations.
Gene Flow
Gene flow—also called migration—is any movement of genes from one population to
another. Gene flow includes lots of different kinds of events, such as pollen being blown
to a new destination or people moving to new cities or countries. If genes are carried to
a population where those genes previously did not exist, gene flow can be a very
important source of genetic variation. In the graphic below, the gene for brown coloration
moves from one population to another.
Gene flow has several important effects on evolution:
Within
a
population:
It can introduce or reintroduce genes to a population, increasing the genetic variation of
that population.
Across
populations:
By moving genes around, it can make distant populations genetically similar to one
another, hence reducing the chance of speciation. The less gene flow between two
populations, the more likely that two populations will evolve into two species.
Mechanisms of macroevolution
•Coevolution – change of two or more species in
close association with each other
•Ex: predator & prey, parasite & host, insects
and plants they pollinate
•Convergent evolution – unrelated species evolve
similar traits because they are adapting to similar
environments
•Ex: shark & dolphin fins; insect, bird & bat
wings
Ex. of convergent evolution:
•Adaptive radiation/divergent evolution – 2
or more related species become unalike as
they adapt to different environments
•Ex: various species of finches on
Galapagos
Speciation
• Species - group of interbreeding
subpopulations that share a gene pool and
are isolated reproductively from other
species.
• Reproductive isolation can occur due to:
– Pre-mating isolating mechanism reproduction is not attempted
– Post-mating isolating mechanisms - do
not produce fertile offspring
• Hybrids - Offspring produced by the
interbreeding between two different
species
• Hybrid organisms are produced through
artificial selection (selective breeding) –
humans breed organisms for desired
traits
Process of Speciation
• Speciation – development of a new
species
– Occurs whenever reproductive isolation
develops
– Various ways it can happen:
• Allopatric speciation
• Sympatric speciation
•Speciation occurs when one population
is isolated from another population
•Isolation can be geological,
reproductive, or filling different
ecological niches to reduce
competition
•With isolation comes changing
environmental factors exerting
selective pressure on mutations and
adaptations.
•Allopatric speciation occurs when a
geographic barrier isolates two
subpopulations from each other; when
the barrier is removed, the two groups
are no longer able to reproduce.
•Ex: humans building a new canal,
earthquake lifts up section of land
Allopatric speciation
•Sympatric speciation occurs when a
single population suddenly becomes two
reproductively isolated groups without
geographic separation.
•Ex: some plants can spontaneously
double the number of chromosomes,
thus making them unable to reproduce
with others of their own kind
The Pace of Speciation
• Two hypotheses:
• Phyletic gradualism – suggests that
change is slow and steady within a
lineage before and after a divergence; few
transitional links would exist.
• Punctuated equilibrium – suggests that a
period of no change is punctuated by
period of rapid speciation; would explain
few transitional links because evolution
would have occurred too rapidly.
Phyletic gradualism versus punctuated
equilibrium