Transcript Unit 4: Evolution

Unit 4:
Evolution
Definitions:

Evolution


Adaptation


the relative change in the characteristics of
populations that occurs over successive
generations
a particular structure, physiology or behaviour
that helps an organism survive and reproduce in
a particular environment
Variation

differences among traits occur among
members of the same species. Therefore no two
individuals are exactly alike

these variations are passed on to the next
generation
Peppered Moth P. 664-665
 Industrial


when air pollution levels are high, the trees are dark.
This “favours” the survival of dark-winged moths
when air pollution levels are low, the trees are light.
This “favours” the survival of light-winged moths
 Survival

melanism
of the “fittest”
wing colour supports the camouflage of the moth
and allows it to survive to reproduce.
Natural selection Pp 347-349
 process
in which characteristics of a population
of organisms change because individuals with
certain inheritable traits survive specific local
environmental conditions
 there
must be diversity within a species for this to
occur
 the
environment exerts a selective pressure on a
population, selecting individuals with certain
characteristics and eliminating others
Artificial selection
a
breeder selects desired characteristics
in an organism
 Eg:
Dog breeders
Charles Lyell P. 655
 developed
theory of uniformitarianism
 said that all geological processes
operated at the same rates in the past as
they do today
 important
because it indicated the world was
much older than 6000 years, and that slow
processes happening over long periods of
time could result in substantial changes
Thomas Malthus P. 656
 plant
and animal populations grew faster
than their food supply

eventually a population is reduced by
starvation, disease or war
Alfred Wallace P 657
 wrote
Darwin with essentially identical
theory of evolution
 forced
Darwin into publishing theory of
evolution
Jean Baptiste Lamarck P. 651
 presented
first theory that discussed the possibility
of evolution
 believed that organisms have an imaginary force
or desire to change themselves for the better

believed in the idea of inheritance of acquired
characteristics
 Although
this theory has been rejected Lamarck’s main
contribution was to show that evolution is adaptive, and
that the diversity of life is the result of adaptation
Georges Cuvier P. 650
 developed
the science of paleontology
 realized the Earth’s history was recorded
in the fossil record


recognized that extinction was a fairly
common occurrence
strongly opposed to the theory of evolution
Darwin


when Darwin observed living armadillos and the fossils
of ancient armadillo-like creatures in the same location,
He began to wonder why one had survived and the
other had not. He would later conclude that one form
had evolved from the other
while exploring the Galapagos Islands, he noted slight
variations among similar species of organisms from
island to island



14 species of finch that were similar to a species of finch
found on the mainland
The notable difference in finches lay in the shape of their
beaks.
different beak shapes were adaptations for eating a
certain kind of food characteristic of the various
geographic location
 Darwin
assumed that these different species had
evolved from a single ancestral mainland
species
 started to formulate ideas about evolution
 worked out the process of natural selection in
1838

Published 21 years later (pushed by Wallace)

Theory of Natural Selection
1. Overproduction


2. Struggle of existence (competition)


differences among traits occur among members of the same
species. Therefore no two individuals are exactly alike these
variations are passed on to the next generation
4. Survival of the fittest (natural selection)


because of overproduction, organisms of the same species, as
well as those of different species, must compete for limited
resources such as food, water and a place to live
3. Variation


the number of offspring produced by a species is greater than
the number that can survive, reproduce and live to maturity
those individuals in a species with traits that give them an
advantage (i.e., are well-adapted to their environment) are
better able to compete, survive and reproduce. All others die
without leaving offspring since nature selects the organisms
which survive, the process is called natural selection
5. Origin of new species (speciation)

over numerous generations, new species arise by the
accumulation of inherited variations when a type is produced
that is significantly different from the original, it becomes a new
species
Mendel - work built upon by
later scientists to reveal: P 675
 there
is MUCH genetic variation within
populations
 variations can arise through mutations
and are inheritable
 evolution, therefore, depends on both
random genetic mutations (which provide
variation) and
 mechanisms such as natural selection
Modern Evidence of Evolution
 Fossil

Record P 659
The fossil record shows us:
 the
earliest organisms were small and simple
in structure
 over millions of years organisms became
larger and more complex
 the number of different kinds of organisms has
increased over time
 many species of organisms have disappeared
and have been replaced by new and
different species
 The
fossil record provides evidence of
constantly changing life forms.
Biogeography P 663-664
 study
of the geographical distribution of
species (continental drift)

isolation is a key factor in the evolution of
species
1) Geographic isolation

occurs when a single breeding population is divided
by a geographic boundary


barriers include:






e.g.,Canis lupus beothicus
mountain ranges
bodies of water
barriers created by humans
gene flow between the isolated group and the main
population ceases
different adaptations of populations in the separate
environments, different gene frequencies within the
separate populations and different mutations within
the population
may all allow the population to become so different
that interbreeding is impossible
2) Reproductive isolation

may occur because of geographic isolation

occurs when organism in a population can no
longer mate and produce offspring, even following
the removal of the geographic barriers
factors that contribute to reproductive isolation
include:





differences in mating habits
courtship patterns
seasonal differences in mating (very few species can
mate and reproduce at any time)
inability of the sperm to fertilize eggs
Comparative Anatomy
(homologous, analogous and
vestigial structures) P 664-665
 organisms
with similar structures evolved
from a common ancestor becomes
increasingly obvious.
 Eg:
flipper of a seal, the leg of a pig, the wing
of a bat, and the human arm all have the
same basic structure and the same pattern of
early growth.
1) Vestigial organs
 Def:
small, incomplete organs with no
apparent function
 provide evidence of ancestry
 e.g.,
snakes once had legs
2) Analogous structures
 similar
functions but different anatomically
(insect/bird wings)

good indicators that these organisms didn’t
come from common ancestors
Comparative Embryology P 665




Embryology is the study of organisms in the early
stages of development. During the late 1800s,
scientists noted a striking similarity between the
embryos of different species (see page 683, Nelson).
At a later date, biologists suggested that the similarity
of the embryos was due to their evolution from a
common ancestor. This doesn’t mean that birds
necessarily evolved from reptiles, or mammals from
birds, but rather that the young forms of these
organisms resemble the young of related species.
In a broad sense, there is a theory that every
organism repeats its evolutionary development in its
own embryology.
Scientists believe that many of the structures in an
embryo are similar to those found in common
ancestors
Heredity P 666
 Mendel’s
laws explain many variations
Since the laws of inheritance and the
science of genetics are more clearly
understood than in Darwin’s time, the
variations in organisms required for natural
selection to occur can be explained
Molecular Biology P 666-667

evolutionary relationships among species are
reflected in their DNA

the closeness of species can be determined by
comparing DNA patterns

http://www.youtube.com/watch?v=IFACrIx5SZ0&s
afety_mode=true&persist_safety_mode=1&safe=a
ctive

DNA similarity reveals a common ancestor also
shows that all life forms on earth are related, to
some extent, to the earliest organisms
How to date a fossil P 662
 the
oldest layers are the ones laid down
first and, therefore, are found at the
bottom of the site
 the younger layers, added later, are on
top since fossils form along with a given
layer of sedimentary rock, the relative
ages of the fossils can also be determined
 The oldest will be on the bottom; the
youngest will be on the top

It takes about 1000 years to form 30 cm of
sedimentary rock

Absolute dating provides a much more
accurate method of determining a fossil’s
age via radioactive dating techniques.


A radioactive isotope has an unstable nuclear
structure, and will break down, releasing
particles and energy.
The breakdown often results in a more stable
element.

Radioactive dating involves measurements of
the decay of radioactive isotopes such as:



potassium-40, which becomes argon-40
uranium-238, which changes to lead-206
carbon-14, which becomes nitrogen-14
 For
example, if a rock contains thorium
232 and lead 208 in equal amounts, then
one half of the original thorium 232 has
decayed;
 one half-life has passed and the rock must
be 14 billion years old
Nucleic Acid evidence for
evolution Pp 666-667

Cytochrome C





protein found in mitochondria
amino acid sequence is so similar among organisms that it
can be used to indicate relatedness
e.g., chimps and rhesus monkeys differ by one amino acid;
chimps and horses by 11
the longer the period of time since an organism evolved
from a simple ancestor, the greater the number of
differences in the nucleotide sequences for the
cytochrome c gene
http://www.youtube.com/watch?v=Wpc_M2qI74&feature=related&safety_mode=true&persist_
safety_mode=1
Hardy-Weinberg law Pp 681686

Gene pool - the entire genetic content of a
population

If all other factors remain constant, the gene
pool will have the same composition
generation after generation.
This stability is called genetic equilibrium.


Only if that equilibrium is upset can the
population evolve.



The principle can be expressed mathematically by
the formulae:
p2 + 2pq + q2 = 1
and p + q = 1

where

p = frequency of dominant allele

q = frequency of recessive allele


If the values for p and q are known, this equation can be
used to calculate the frequency of all three genotypes,
PP, Pq, and qq.
If the frequencies of the three genotypes are known,
the frequencies of the alleles can be calculated.
The conditions under which no
change in the gene pool will
occur are:
 1)large
populations. This condition is
necessary to ensure that changes in gene
frequencies are not the result of chance
alone
 2) random mating.
 3) no mutations
 4) no migration. No new genes enter or
leave the population
 5) equal viability, fertility and mating
ability of all genotypes (i.e., no selection
advantage)
Example:


Consider a simple situation - one gene with two
alleles, A and a.
The genotypes that might be found in a large
population will be AA, Aa and aa.



In mathematical terms, the frequencies with which
the alleles will occur must add up to one (and so
must the frequencies of the genotypes)
if the dominant allele, A, is found in 70% of the
population (i.e., has a frequency of 0.7),
the recessive allele will have a frequency of 1 - 0.7
= 0.3, or 30%.
The expected frequencies of the 3 possible
genotypes can be calculated with a Punet
square, or with the Hardy-Weinberg equation:


 A(0.7)
a
(0.3)
 Eggs
Sperm______________
A (0.7)
A (0.3)________
AA (0.49)
Aa (0.21)______
Aa (0.21)
aa (0.09)______


The equation predicts that the frequencies of the 3
genotypes possible in the next generation will be:

p2 + 2pq + q2 = 1

(0.7)2 + 2(0.7 x 0.3) + (0.3) = 1

Genotypes: 49% AA; 42% Aa; 9% aa
Given this distribution of genotypes, it’s possible to
predict the frequency of the A and a alleles in the
population:




F1 generation 0.49 AA ; 0.42 Aa; 0.09 aa
potential gametes A A ; A a; a a
A = 0.49 + 0.21 = 0.7
a = 0.21 + 0.09 = 0.3
Problem: Suppose a recessive genetic disorder
occurs in 9% of the population. What
percentage of the population is heterozygous, or
carriers, of the disorder?

a = 0.09 = q2 ; q = 0.3

AA = ? = p2; 1 - q = p; 1 = 0.3 = 0.7

p2 + 2pq + q2 = 1

(0.7)2 + 2(0.7 x 0.3) + (0.3) = 1

0.49 + 0.42 + 0.9 = 1
The Hardy-Weinberg law:


compares natural populations with an ideal
situation; such comparisons are a measure of
change
In nature, allele frequencies are not constant
and populations do change over time, or
evolve



it shows that meiosis and sexual reproduction by
themselves do not cause populations to
change
Merely recombining genes does not change
allele frequencies in a gene pool
Other factors must be at work
Mutations P 688
 may
provide new alleles in a population
and, as a result, may provide the variation
required for evolution to occur

if a mutation provides a selective
advantage it may result in certain
individuals producing a disproportionate
number of offspring as a result of natural
selection
Genetic Drift P 689
 in
small populations the frequencies of
particular alleles can be changed by
chance alone

the smaller the population the less likely the
parent gene pool will be reflected in the
next generation
Bottleneck effect p. 690

as a result of chance certain alleles are over
represented and others are under
represented in the reduced population

genetic drift then follows and the genetic
variation in the surviving population is
reduced
eg


Northern elephant seal; Hunting reduced
population to as few as 20 individuals. The
population today has reduced genetic
variation as a result.
Founder effect p. 691
 when
a small number of individuals
colonize a new area the chances are
high that they do not contain all the
genes represented in the parent
population
 eg
 NL
moose: since these founders are in a new
environment, they will experience different
selection pressure
Gene Flow p 692
 the
movement of new alleles into a gene
pool

can reduce genetic differences between
populations
Non-random Mating p. 692
 1)
inbreeding
 2) self-fertilization
 3) assortative mating (choosing mates
with a similar phenotype)

This is the basis for artificial selection e.g.,
breeding dogs
Natural Selection p. 693

some individuals in a population will leave more
offspring than others due to selective pressures
1) Stabilizing selection favours an intermediate
phenotype and acts against extreme variants e.g.,
baby weights are between 3 and 4 kg

2) Directional selection favours the phenotypes at
one extreme over another.



Common during periods of environmental change e.g.,
in the wild budgies are usually green
3) Disruptive (diversifying) selection takes place when
extremes of a phenotypic range are favoured relative
to intermediate phenotypes.

As a result, intermediates will be eliminated from the
population
Sexual Selection p. 695
 characteristics
used in sexual selection
may not be adaptive in the sense that
they help an individual survive.

E.g., peacock tail However, they may
increase the chances of being chosen as a
mate and therefore of passing genes along
to the next generation
Biological barriers to reproduction
may contribute to speciation
 Biological

1) Pre-zygotic barriers p.709


differences in times for mating (season, year, time of day)
5) Mechanical isolation


because of differing habitats, species may not encounter each other
4) Temporal isolation p 710


bird song, courtship rituals, pheromones ... species-specific signals
3) Habitat isolation


either impede mating between species or prevent fertilization of the
ova if individuals from different species attempt to mate
2) Behavioural isolationism


barriers:
anatomically so different that mating is impossible
6) Gametic isolation

gametes of different species will not fuse
 Post-zygotic


hybrid is sterile e.g., donkey + horse = mule, which is
usually sterile
Hybrid breakdown


genetic incompatibility of the interbred species may
stop development of the hybrid zygote
Hybrid sterility p 711


when the sperm of one species successfully fertilizes the
ovum of another and a zygote is produced, these
barriers prevent the hybrid from developing into normal,
fertile individuals
Hybrid inviability


barriers P 710
1st hybrid generation is viable and fertile. Subsequent
offspring of hybrids are sterile or weak
http://www.youtube.com/watch?v=vJFo3trMuD8
Convergent and divergent
evolution P 721

Divergent evolution


Convergent evolution




is adaptive radiation (homologous structures will be
present between species)
occurs when the environment selects similar
adaptations in unrelated species
If the environments are similar, it is logical to assume that
some of the same kinds of traits would be favored in the
different populations
(Analagous structures will be present among species)
E.g.,



Wings - birds, bats, bees
fins/streamlined shape - dolphins and sharks
eye structure - humans and octopus
The process of coevolution
P 722
 This
process of joint evolution of two or
more species is called coevolution.
Flowering plants and insects, predator
prey relationships, parasites and their
hosts
 http://www.youtube.com/watch?v=Weu
QfToa254&feature=PlayList&p=6362D7918
29F413A&playnext_from=PL&index=54&pl
aynext=3