Unit 4: Evolution
Definitions: Evolution
the relative change in the characteristics of populations that occurs over successive generations
Adaptation 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 melanism 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 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 the number of offspring produced by a species is greater than the
number that can survive, reproduce and live to maturity 2. Struggle of existence (competition)
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 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) 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 e.g.,Canis lupus beothicus
barriers include: 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&safety_mode=true&persist_safety_mode=1&safe=active
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=W-pc_M2qI74&feature=related&safety_mode=true&persist_safety_mode=1
Hardy-Weinberg law Pp 681-686 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 Punetsquare, or with the Hardy-Weinberg equation:
Sperm______________
A (0.7) A (0.3)________
A(0.7) AA (0.49) Aa (0.21)______
a (0.3) Aa (0.21) aa (0.09)______
Eggs
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. Whatpercentage 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 speciationBiological barriers:
1) Pre-zygotic barriers p.709 either impede mating between species or prevent fertilization of the ova if
individuals from different species attempt to mate 2) Behavioural isolationism
bird song, courtship rituals, pheromones ... species-specific signals 3) Habitat isolation
because of differing habitats, species may not encounter each other 4) Temporal isolation p 710
differences in times for mating (season, year, time of day) 5) Mechanical isolation
anatomically so different that mating is impossible 6) Gametic isolation
gametes of different species will not fuse
Post-zygotic barriers P 710 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 genetic incompatibility of the interbred species may stop
development of the hybrid zygote Hybrid sterility p 711
hybrid is sterile e.g., donkey + horse = mule, which is usually sterile
Hybrid breakdown 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
is adaptive radiation (homologous structures will be present between species)
Convergent evolution 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=WeuQfToa254&feature=PlayList&p=6362D791829F413A&playnext_from=PL&index=54&playnext=3
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