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Evolution
Charles Darwin made two major points in The Origin of Species:
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Natural selection provided a mechanism for this evolutionary change
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Natural selection results in evolutionary adaptation, an accumulation of inherited
characteristics that increase the ability of an organism to survive and reproduce in its
environment
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Evolution is a change over time in the genetic composition of a population
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In the 1700s, natural theology viewed the adaptations of organisms as evidence that the
Creator had designed each species for a purpose
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Linnaeus developed the
binomial system of naming organisms according to genus and species
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These form when mud and sand settle to the bottom of
seas
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Paleontology, the study of fossils, revealed that the older the strata, the more dissimilar the
fossils from modern life
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Catastrophism was advocated, speculating the boundaries between strata
were due to local floods or droughts that destroyed the species then present
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The primary mission was to chart
poorly known stretches of the South American coastline
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He noted that these were very distinct from those of Europe
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For example
there were clear differences in the beaks among the finches were adaptations to the specific
foods available on their home islands
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Darwin wrote a long essay on the origin of species and natural selection, but was reluctant
to publish it
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Later that year both Wallace’s paper and extracts from Darwin’s essay were presented to
the Linnaean Society of London
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Darwin developed his ideas earlier and supported the theory much more extensively
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Descendants of that common ancestor have accumulated diverse modifications, adaptations
that allow them to survive and reproduce in specific habitats
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♥ Observation 2: populations remain stable in size, except for seasonal fluctuations
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♥ Observation 4: individuals of a population vary extensively in their characteristics;
no two individuals are exactly alike
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Those with traits that are best suited for
survival and reproduction in their environment are like to leave more offspring
Inference 3: the unequal ability of individuals to survive and reproduce will lead to a
gradual change in a population, with favourable characteristics accumulation over
generations
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If artificial selection can
achieve so much change in a relatively short period of time, Darwin reasoned, and then
natural selection should be capable of considerable modification of species over thousands
of generations
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Differential success in reproduction (unequal ability of individuals to survive and
reproduce) that results from individuals that varies in heritable traits and their
environment
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The product of natural selection is the increasing adaptation of organisms to their
environment
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If the environment changes or a species moves to a new environment, natural
selection may result in adaptation to the new conditions, sometimes giving rise to a
new species in the process
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Natural selection occurs through interactions between individual organisms and
their environment but individuals do not evolve, populations evolve
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Characteristics acquired by an
organism during its lifetime may enhance its survival and reproductive success, but
there is no evidence that such characteristics can be inherited by offspring
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Environmental factors vary from place to place
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Natural selection in Action
Differential Predation in Guppy Populations
Guppies live in the wild in pools
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These were correlated
to the type of predator present in each pool
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To test whether natural selection occurred, they introduced guppies from pike-cichlid
locations to new pools that contained killifish but no guppies
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The results supported the hypothesis since poke-cichlids prey mainly on reproductively
mature adults, the chance that a guppy will survive to reproduce several times is low
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In ponds with killifish predators, guppies that survive early predation can grow slowly and
produce many broods of young
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A few drug resistance viruses may be present by
chance at the beginning of treatment
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As a result, the frequency of drug
resistance in the viral population rapidly increases
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3TC binds to the DNA chain
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instead of cytosine
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3TC- resistant varieties of HIV can discriminate between cytosine and 3TC
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However when 3TC is added, it becomes a powerful selective
agent, favouring reproduction of resistant individuals
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It can only act on
existing variation
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What is adaptive in one situation is not adaptive in another
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Homology
Show similarities which may have a different function because they have a common
ancestor
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These are
homologous structures that represent variation on the ancestral tetrapod forelimb
(pentadactyl limb)

These are evident when we look at embryonic development
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These develop into very different but homologous, adult structures
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Vestigial organs: no function, remnants of structures that were found in ancestral species,
for example the appendix, hair, skeletons in some snakes and of fossil whales retain vestiges
of the pelvis and leg bones of walking ancestors
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All vertebrate embryos have similar structures at different stages of development
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Molecular Biology: comparing DNA and protein structure of different organisms
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Biogeography
The original evidence used by Darwin: Organisms found in a particular area tend to be
more closely related to each other than they are to organisms found in other areas
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However the sugar glider shares more characteristics
with other Australian marsupials than with the flying squirrel
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Islands generally have endemic species (found nowhere else in the world), however these
are typically more closely related to species living on the nearest mainland
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The Fossil Record
These are found in layers of sedimentary rock, new layers cover old ones
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For example, considerable evidence suggests that prokaryotes are
the ancestors of all life and should precede all eukaryotes in the fossil record
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For
example, the origin of birds is one branch of dinosaurs
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Natural Selection as a Theory
Theory: more comprehensive than a hypothesis, accounting for many observations and data
and attempting to explain and integrate a great variety of phenomena
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Later, Gregor Mendel proposed a model of
inheritance that supported Darwin’s theory
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A
population is a localized group of individuals that belong to the same species
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Species: a group of natural populations whose individuals have the potential to interbreed
and produce fertile offspring
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It consists of all alleles
at all gene loci in all individuals of a population
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If there are two or more
alleles at the locus, then the individuals can be homozygous or heterozygous for that gene
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This theorem states that the
frequencies of alleles and genotypes in a population’s gene pool will remain constant over
generations unless acted upon by agents other than Mendelian segregation and
recombination of alleles
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This theorem states that the repeated shuffling of a population’s gene pool over generations
does not increase the frequency of one allele over another
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The combined frequencies must add up to 100%, and so p + q = 1
500 plants with two alleles (CR and CW) for flower pigment are 20 (4%) homozygous for
CW allele and have white flowers, 320 (64%) are homozygous for CR allele and have red
flowers and the rest show incomplete dominance; 160 (32%) heterozygous and produce
pink flowers
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CR accounts for 800 copies; (320 x 2) + (160 x 1)
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8 or 80%
The CW allele must have a frequency of 1- 0
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2 or 20%
The genotype frequencies must add up to 1:
𝑝" + 2𝑝𝑞 + 𝑞" = 1
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Using this we can find the frequencies of alleles in a gene pool if we know the frequency of
genotypes or the frequency of genotypes if we know the frequencies of alleles
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However, real populations do evolve, and their allele and genotype frequencies do change
over time
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In small populations, chance fluctuations in the
gene pool can cause genotype frequencies to change over time
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Gene flow, the transfer of alleles due to the migration of individuals
or gametes between populations, can change the proportions of alleles
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Introduction, loss or modification of genes will alter the gene pool
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If individuals pick mates with certain genotypes, or if inbreeding is
common, the mixing of gametes will not be random
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Differential survival or reproductive success among genotypes
will alter their frequencies
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Most mutations
occur in somatic cells and are lost when individuals die
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These can immediately change the gene pool of a population by introducing a new allele
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These can have significant impact
on phenotype, as in the case of sickle-cell disease
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Changes in regulatory regions of DNA can have profound effects
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A mutant allele may make its bearer better suited to the environment, increasing
reproductive success
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Some mutations alter gene number or sequence
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For
example, the translocation of part of one chromosome to a different chromosome could link
genes that act together to positive effect
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In microorganisms and viruses with short
generation spans, mutation rates are much higher and can rapidly generate genetic
variation
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Bacteria and viruses can also
undergo recombination, but they do so less regularly than animals and plants
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However this does not change their frequency
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Natural Selection
Individuals vary in their heritable traits
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As a result of
selection, alleles are passed on to the next generation in frequencies different from their
relative frequencies in the present population
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The smaller the sample, the greater the chance of deviation from the expected result
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Genetic drift at small population sizes may occur as a result of two situations: the bottleneck
effect or the founder effect
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By chance, some alleles may be overrepresented and
others underrepresented among the survivors
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Genetic drift will continue to change the gene pool until the population is large enough to
eliminate the effect of chance fluctuations
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Such populations have lost genetic
variation from the gene pool
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The founder effect occurs when a new population is started by only a few individuals who
do not represent the gene pool of a larger source population
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Genetic drift would continue from generation to generation until the population grew large
enough for sampling errors to be minimal
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Gene Flow
Gene flow is genetic exchange due to migration of fertile individuals or gametes between
populations
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If extensive enough, gene
flow can amalgamate neighbouring populations into a single population with a common
gene pool
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Genetic Variation
Only the genetic component of variation can have evolutionary consequences as a result of
natural selection
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Variation within a population
This includes both quantitative and discrete characters
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For
example, plant height rages from short to tall
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Discrete characters are usually determined by a single locus with different alleles that
produce distinct phenotypes
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Phenotypic polymorphism occurs when two or more discrete phenotypes are represented in
high enough frequencies to be noticeable in a population
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Human populations are polymorphic for a variety of physical (freckles) and
biochemical (blood types) characters
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A Closer Look at Natural Selection
Only natural selection leads to adaptation of an organism to its environment
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Evolutionary Fitness
Reproductive success is generally subtler and depends on factors other than battle for
mates
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Wildflowers attract more pollinators
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Fitness is defined as the contribution an individual makes to the gene pool of the next
generation
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Relative fitness is zero for a
sterile organism, even if it is robust and long-lived
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Individuals that mature quickly, become fertile at an early age, and live for a short time
have greater relative fitness than individuals that live longer but mature later
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Directional Selection is most common during environmental change or when individuals
migrate to a new habitat with different environmental conditions
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For example, fossil evidence indicates that the average size of black beats in Europe
increased during each glacial period, and decrease again during the warmer interglacial
periods
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This is
important in early stages of speciation
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It
reduces variation and maintains the status quo for a trait
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The Preservation of Genetic Variation
The tendency for natural selection to reduce variation is countered by mechanisms that
preserve or restore variation, including diploidy and balanced polymorphisms
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Even those that are unfavourable can persist in a population
through their propagation by heterozygous individuals
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The offspring is homozygous recessive on very rare occasions
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Balancing Selection
This occurs when natural selection maintains stable frequencies of two or more phenotypes
in a population, a state called balanced polymorphism
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Individuals who are heterozygous at a particular locus have greater fitness than
homozygotes and natural selection will maintain multiple alleles in that locus
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Homozygous recessive suffer from sickle-cell disease, homozygous dominant
are vulnerable to malaria and heterozygotes are resistant to malaria
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Selection can be result of direct competition among individuals of one sex for mates of the
opposite sex
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Selection also occurs when one sex is choosy in selecting their mates from individuals of the
opposite sex since females invest more in eggs and parental care
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For example, bright plumage may make male birds
more visible to predators but they help an individual gain a mate
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- The offspring of asexual females will be reproductive daughters
- Half the offspring of sexual females will be daughters; the other half will necessarily
be males

Maintenance of sex: the process of meiosis and fertilisation generate genetic variation on
which natural selection can act
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Natural selection acts in the present, favouring individuals that best fit the current,
local environment
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Evolution is limited by historical constraints
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For example, birds might be better off with 4
limbs but they descended from reptiles, therefore they only have 2
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Adaptations are often compromises
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Similarly,
human limbs are flexible and allow versatile movements, but are prone to injuries
(sprains, torn ligaments and dislocations)
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Chance events affect the subsequent
evolutionary history of populations
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Natural selection favours only the fittest
variations from those phenotypes that are available
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The many imperfections of living organisms are evidence of evolution
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Microevolution: the study of adaptive change in a population
Macroevolution: evolutionary changes above the species level

Speciation addresses the question of how new species originate and develop through the
subdivision and subsequent divergence of gene pools
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Ernst Mayr proposed the biological species concept
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A biological species is the largest set of populations in which genetic exchange is possible
and that is genetically isolated from other populations
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Reproductive Isolation
Distinction between species depends on reproductive incompatibility, the concept hinges on
reproductive isolation, the existence of biological barriers that prevent members of two
species from producing viable, fertile hybrids
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These barriers are intrinsic to the
organisms, not due to simple geographic separation
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Reproductive barriers can be categorized as prezygotic or postzygotic, depending on
whether they function before or after the formation of zygotes
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These barriers include habitat isolation, behavioural
isolation, temporal isolation, mechanical isolation and gametic isolation
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For
example two species of garter snakes occur in the same areas, one lives mainly in
water and the other is terrestrial, therefore they rarely encounter each other
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These displays identify potential mates of the correct
species and synchronize gonadal maturation
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For example, the western spotted skunk and the eastern
spotted skunk overlap but they do not interbreed since the former mates in late
summer and the latter in late winter
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For
example, mechanical barriers contribute to the reproductive isolation of flowering
plants that are pollinated by insects or other animals
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For example, in species with internal
fertilization, the environment of the female reproductive tract may not be
conductive to the survival of sperm from other species
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If a sperm does fertilize the ovum, postzygotic barriers may prevent the hybrid zygote from
developing into a viable, fertile adult
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♥ Reduced hybrid viability: genetic incompatibility between the two species may abort
the development of the hybrid at some embryonic stage or produce frail offspring
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♥ Reduced hybrid fertility: even if the hybrid offspring are vigorous, the hybrids may
be infertile, and the hybrid cannot backbreed with either parental species
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For example, a mule (hybrid product between a horse and a
donkey) is a robust organism but it cannot mate with either horses or donkeys
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Limitations of the Biological Species Concept
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It is limited when applied to species in nature, for example one cannot test the reproductive
isolation of morphologically similar fossils which are separated into species based on
morphology
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In addition, many species reproduce entirely asexually and are assigned to species
based mainly on structural and biochemical characteristics
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For
example, a species that is a parasite may be defined in part by its adaptations to a specific
organism
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Modes of Speciation
Allopatric Speciation: geographic separation of populations restrict gene flow
Sympatric Speciation: speciation occurs in geographically overlapping populations when
biological factors, such as chromosomal changes and non-random mating, reduce gene
flow
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Alternatively, some individuals may colonize a new, geographically remote area and
become isolated from the parent population
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The significance of a barrier depends on the ability of the organism to move around
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For example, birds can fly across certain barriers
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However, very small, isolated
populations develop into new species; most simply persist or perish in their new
environment
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Sympatric Speciation
New species arise within the range of the parent populations
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In plants, sympatric speciation can result from
accidents during cell division that result in extra sets of chromosomes, a mutant condition
known as polyploidy
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An autopolyploid mutant is an individual that has more than two chromosome sets, all
derived from a single species
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This can self-pollinate, but it cannot mate with diploids form the original population,
because of abnormal meiosis by the triploid hybrid offspring
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Hybrids are usually sterile,
but they are quite vigorous and propagate asexually
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These are only fertile with each other; they cannot breed with either parent species
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Reproductive isolation can result when genetic factors cause individuals to exploit
resources not used by the parent
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As it accumulates genetic differences due to natural selection and genetic drift,
reproductive isolation from the ancestral species may arise as a by-product of the genetic
change
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Sympatric speciation requires the emergence of some reproductive barrier that isolates a
subset of the population without geographic separation from the parent population
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Adaptive Radiation
It is the evolution of many diversely adapted species from a common ancestor when new
environmental opportunities arise
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The Timing of Speciation
In the fossil record many species appear as new forms rather suddenly, persist essentially
unchanged and then disappear from the fossil record
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For example, suppose that a species survived for 5 million years, but most of its
morphological alterations occurred in the first 50,000 years which is only 1% of its total
lifetime
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During stasis all species continue to adapt but often by changes that do not leave a fossil
record, such as small biochemical modifications, behaviour, internal anatomy and
physiology
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- Gradualism model: Species descended from a common ancestor gradually diverge
more and more in their morphology as they acquire unique adaptations
- Punctuated equilibrium model: A new species changes most as it buds from a parent
species and then changes little for the rest of its existence

Macroevolution
Speciation is at the boundary between microevolution and macroevolution
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Speciation occurs when a population’s genetic divergence from its ancestral population
results in reproductive isolation
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For example, the simplest eyes are just clusters of photoreceptors, light-sensitive pigmented
cells
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These allow the animal to
distinguish light from dark
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The most
complex evolved by incremental adaptation of organs that benefitted their owners at each
stage
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Evolutionary Trends
For example, the evolution of the modern horse was a series of changes from a small,
browsing ancestor with four toes to modern horses (Equus) with only one toe per foot and
teeth modified for grazing on grass
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