Chapter 24 Notes
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Speciation: the origin of new species
o Focal point of evolutionary theory because the appearance of new species is the source of biological
diversity
Microevolution: changes confined to a single gene pool
Evolutionary theory must also explain how new species originate and develop through the subdivision and
subsequence divergence of gene pools
Macroevolution: refers to evolutionary change above the species level
Two basic patterns of evolutionary change:
o Anagenesis
 Accumulation of changes that gradually transform a given species into a species with different
characteristics
o Cladogenesis
 Splitting of a gene pool into two or more separate pools, which each give rise to one or more
new species
 Can promote biological diversity by increasing the number of species
24.1
The biological species concept emphasizes reproductive isolation
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Species: means “kind” or “appearance”
Morphologically distinct species are indeed discrete groups, with many differences in addition to morphological
ones
The Biological Species Concept
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Biological species concept: defines a species as a population or group of populations whose members have the
potential to interbreed in nature and produce viable, fertile offspring, but are unable to produce viable, fertile
offspring with members of other populations
o Members of a biological species are united by being reproductively compatible
Reproductive Isolation
o Reproductive isolation: the existence of biological factors the impede members of two species from
producing viable, fertile hybrids
 Although a single barrier may not block all genetic exchange between species, a combination of
several barriers can effectively isolate a species’ gene pool
o Prezygotic barriers: impede mating between species or hinder the fertilization of ova if members of
different species attempt to mate
o Postzygotic barriers: prevent the hybrid zygote from developing into a viable, fertile adult if a sperm cell
from one species does overcome prezygotic barriers and fertilizes an ovum from another species
o Prezygotic Barriers
 Habitat isolation: two species that occupy different habitats within the same area may
encounter each other rarely, if at all, even though they are not isolated by obvious physical
barriers
 Temporal isolation: species that breed during different times of the day, different seasons, or
different years cannot mix their gametes
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Behavioral isolation: courtship rituals that attract mates and other behaviors unique to a species
are effective reproductive barriers, even between closely related species
 Mechanical isolation: morphological differences can prevent successful mating
 Gamete isolation: sperm of one species may not be able to fertilize the eggs of another species;
sperm may not be able to survive in the reproductive tract of females of the other species, or
biochemical mechanism may prevent the sperm from penetrating the membrane surrounding
the other species’ eggs
o Postzygotic barriers
 Reduced hybrid viability: the genes of different parent species may interact and impair the
hybrid’s development
 Reduced hybrid fertility: may be sterile; if chromosomes of the two parent species differ in
number or structure, meiosis in the hybrids may fail to produce normal gametes
 Hybrid breakdown: some first-generation hybrids are viable and fertile, but when they mate
with one another or with either parent species, offspring of the next generation are feeble or
sterile
Limitations of the Biological Species Concept
o Number of species to which this concept can be usefully applied is limited
 Can’t apply to fossils or asexual organisms
 Also difficult to apply to the many sexual organisms about which little is known regarding their
ability to mate with different kinds of organisms
Other Definitions of Species
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Morphological species concept: characterizes a species by its body shape, size, and other structural features
o Advantages:
 Applied to sexual and asexual organisms
 Useful without information on the extent of gene flow
o Disadvantage:
 Definition relies on subjective criteria; scientists may disagree on which structural features
distinguish a species
Paleontological species concept: focuses on morphologically discrete species known only from the fossil record
o Have to distinguish this way because there is little or no information about their mating capabilities
Ecological species concept: views a species in terms of its ecological niche, its role in a biological community
o Can accommodate sexual as well as sexual species
Phylogenetic species concept: defines a species as a set of organisms with a unique genetic history; one branch
on the tree of life
o Trace the phylogenetic history of a species by comparing its physical characteristics or its molecular
sequences with those of other organisms
 Can distinguish groups of individuals that are sufficiently different to be considered separate
species
The usefulness of each of these definitions depends on the situation and the questions we are asking
o The biological species concept is particularly valuable for studying how species originate
24.2
Speciation can take place with or without geographic separation
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Speciation can occur in two main ways, depending on how gene flow between the populations is interrupted
o Allopatric speciation
o Sympatric speciation
Allopatric (“Other Country”) Speciation
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Allopatric speciation: gene flow is interrupted when a population is divided into geographically isolated
subpopulations
o Ex: river reroutes and now divides previously joined populations
Allopatric speciation can also occur without geologic remodeling, such as when individuals colonize a remote
area, and their descendants become geographically isolated from the parent population
How big does a geographic barrier have to be to keep populations apart?
o Depends on the ability of the organisms to move about
Once geographic separation has occurred, the separated gene pools diverge through:
o Different mutations arise
o Sexual selection takes a different course in the respective populations
o Other selective pressures ac differently on separated organisms
o Genetic drift alters allele frequencies
Because small, isolated populations are more likely than large populations to undergo a significant change in
their gene pool in a relatively short time due to selection and drift, they are also more likely to experience
allopatric speciation
To confirm a case of allopatric speciation, you have to determine whether the allopatric populations have
changed enough that they no longer have the potential to interbreed and produce fertile offspring
o They can figure this out by bringing together members of separated populations in a lab setting
 Biologists can also assess allopatric speciation in the wild
o Ex: testing mating calls and reactions to them
Sympatric (“Same Country”) Speciation
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Sympatric speciation: takes place in geographically overlapping populations
o How can reproductive barriers between sympatric populations evolve when the members remain nin
contact?
 Mechanisms of sympatric speciation include chromosomal changes and nonrandom mating that
reduces gene flow
Polyploidy
o Polyploidy: extra set of chromosomes
o Autopolyploid: individual that has more than two chromosome sets, all derived from a single species
 Tetraploid (has 4 sets of chromosomes) can’t interbreed with other plants of the original
population (2n) but can produce fertile 4n offspring
 Shows how autopolyploidy can generate reproductive isolation without any geographic
separation
o Common form of polyploidy can occur when two different species interbreed and produce a hybrid
 Interspecific hybrids are often sterile because the two sets of chromosomes can’t pair during
meiosis
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But over time ,various mechanism can change a sterile hybrid into a fertile polyploidy
known as an allopolyploid
o Fertile with each other but cannot interbreed; so they make a new species
o Many important agricultural crops (oats, cotton, potatoes, tobacco, wheat) are polyploids
Habitat Differentiation and Sexual Selection
o Polyploidy speciation also occurs in animals, although it is less common
 Other mechanisms can also lead to sympatric speciation in both animals and plans
o Mate choice based on coloration is the main reproductive barrier that normally keeps the gene pools
separate
Allopatric and Sympatric Speciation: A Summary
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Allopatric speciation: new species forms while geographically isolated from its parent population
o As the isolated population evolves by natural selection and genetic drift, reproductive isolation from the
ancestral species may arise
 These reproductive barriers prevent interbreeding with the parent population, even if the
populations come back into contact
Sympatric speciation: requires the emergence of a reproductive barrier that isolates a subset of a population
without geographic separation from the parent population
o Common mechanism is allopolyploidy (hybridization between closely related species coupled with errors
during cell division that lead to fertile polyploidy individuals)
o Can result when a subset of a population becomes reproductively isolated because of a switch to a
habitat, food source, or other resource not used by the parent population
o In animals, speciation can also result from sexual selection in a polymorphic population
Adaptive Radiation
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Adaptive radiation: evolution of many diversely adapted species from a common ancestor upon introduction to
various new environmental opportunities and challenges
o Occurs when a few organisms make their way to new, distant areas or when environmental changes
cause numerous extinctions
Studying the Genetics of Speciation
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Rapid strides in genetics are enabling researchers to pinpoint genes that play a key role in particular cases of
speciation
The Tempo of Speciation
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The fossil record includes many episodes in which new species appear suddenly in a geologic stratum, persist
essentially unchanged through several strata, and then apparently disappear
Punctuated equilibrium: describes periods of apparent stasis punctuated by sudden change
Some scientists suggest that these patterns require an explanation outside the Darwinian model of gradual
descent with modification
o Not the case
 Such punctuations may have been less abrupt than they appear from the fossil record
 Even though the emergence of this species actually took tens of thousands of years, this
period of change left no fossil record
o Darwin noticed this pattern in the fossil record
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Stasis can be explained
 All species continue to adapt alter they come into existence, but often in ways that
cannot be detected from fossils, such as small modifications in their biochemistry
 Palenotologists base hypotheses about descent almost entirely on external anatomy
and skeletons
24.3
Macroevolutionary changes can accumulate through many speciation events
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As species diverge and speciate again and again, seemingly small differences can accumulate and become more
pronounced
o So, speciation constitutes the beginning of macroevolutionary change
 Macroevolutionary transformations accumulate through natural selection, mutation, genetic
drift, and gene flow
 It is the cumulative change during thousands of small speciation episodes that accounts for
sweeping evolutionary changes
Evolutionary Novelties
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The Darwinian concept of descent with modification can be extended to account for even major morphological
transformations
o In most cases, complex structures have evolved in increments from much simpler versions that
performed the same basic function
 Ex: evolution of eyes from simple to complex
 Throughout their evolutionary history, eyes retained their basic function of vision
o But evolutionary novelty can also arise when structures that originally played
one role gradually acquire a different one
 Structures that evolve in one context but become co-opted for another
function are sometimes called exaptations
 Natural selection cannot predict the future; it can only improve
a structure in the context of its current utility; can’t evolve in
anticipation for future use
 The concept of exaptation offers one explanation for how novel
features can arise gradually through a series of intermediate
stages, each of which has some function in the organism’s
current context
o “Evolution is like modifying a machine while it’s
running”
Evolution of the Genes That Control Development
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Genes that program development control the rate, timing, and spatial pattern of changes in an organism’s form
as it develops from a zygote into an adult
Changes in Rate and Timing
o Heterochrony: an evolutionary change in the rate or timing of developmental events
 Ex: organism’s shape depends in part on the relative growth rates of different body parts during
development
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Allometric growth: proportioning that helps give a body its specific form
o Changing relative rates changes the adult form substantially
 Heterochrony can also alter the timing of reproductive development relative to the
development of somatic organs
 If reproductive development accelerates compared to somatic development, the
sexually mature stage of a species may retain body features that were juvenile
structures in an ancestral species
o Called paedomorphosis
 Such an en evolutionary alteration of developmental timing can produce
animals that appear very different from their ancestors, even though
the overall genetic change may be small
o In summary, heterochrony affects the evolution of morphology by altering the rates at which various
body parts develop or by changing the timing of onset or completion of a particular part’s development
Changes in Spatial Pattern
o Substantial evolutionary changes can also result from alterations in genes that control the placement
and spatial organization of body parts
 Homeotic genes: determine basic features of an organism
 Ex: how legs will develop or how plant’s flower parts are arranged
o Products of one class of homeotic genes called Hox genes provide positional information in an animal
embryo
 Prompts cells to develop into structures appropriate for a particular location
 Changes in Hox genes can have a profound impact on morphology
Evolution is Not Goal Oriented
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Extracting a single evolutionary progression from the fossil record can be misleading
o Ex: by selecting certain species from the available fossils, it is possible to arrange a succession of animals
intermediate between Hyracotherium and living horses that show trends toward increased size, reduced
number of toes, and modification of teeth for grazing
 However, if all fossil horses are included, this trend vanishes
Branching evolution can result in an evolutionary trend even if some new species counter the trend
o Ex: one model of long-term trends considers species to be analogous to individuals: speciation at their
birth, extinction is their death, and new species that diverge from them are their offspring
 Species selection: a theory maintaining that species living the longest and generating the
greatest number of species determine the direction of major evolutionary trends
 Suggests that “differential speciation success” plays a role in macroevolution similar to
the role of differential reproductive success in microevolution
Whether or not the species selection model is valid, there are other possible sources of trends observed in the
fossil record
The appearance of an evolutionary trend does not imply that there is some intrinsic drive toward a particular
phenotype
o Evolution is the result of the interactions between organisms and their current environments
 If environmental conditions change, an apparent evolutionary trend may cease or reverse itself