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Chapter 14: The Formation of
Species and Evolutionary Change
Lecture Outline
Enger, E. D., Ross, F. C., & Bailey, D. B. (2012). Concepts in biology (14th ed.). New York: McGrawHill.
14-1
Evolutionary Patterns
at the Species Level
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Microevolution vs. macroevolution
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Microevolution involves minor differences in allele
frequency between populations of the same
species.
Macroevolution involves major differences that
have occurred over long periods that result in the
formation of new species.
A species is a group of organisms whose
members have the potential to interbreed
naturally and produce fertile offspring.
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Determining if Two Populations Belong
to the Same Species Using Gene Flow
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Gene flow is the movement of genes.
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If two or more populations exhibit gene flow,
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Then they are considered the same species
Horses and donkeys can interbreed, but do
not experience gene flow.
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From one generation to the next as a result of
reproduction
From one region to the next as a result of
migration
Their offspring, mules, are sterile.
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Hybrid Sterility
14-4
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Determining if Two Populations Belong to
the Same Species Using Genetic Similarity

Organisms belonging to the same species
have a high degree of genetic similarity.
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Similarities in DNA sequences between
individuals of two populations
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Akodon dolores and Akodon molinae
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Suggest that gene flow has occurred recently between
those populations
Presumed to be two different species of field mice
Genetic analysis indicated that they were two
populations of the same species, living in different
geographical regions.
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How New Species Originate

Speciation is the process of generating new
species.
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Speciation has occurred continuously over the
history of life on earth.
The fossil record shows that huge numbers of
new species have originated.
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There are two main mechanisms of
speciation.
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14-6
Most of these have gone extinct.
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Geographic isolation
Polyploidy
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Speciation by Geographic
Isolation
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Geographic isolation
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If it is followed by genetic divergence
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Changes in allele frequencies
Then reproductive isolation can result.
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14-7
Occurs when a portion of a population becomes
totally isolated from the rest
The isolated population becomes a new species.
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Geographically Isolated
Populations
14-8
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Mechanisms of Geographic
Isolation

Colonization of a distant area
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Appearance of a geographic barrier
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A few individuals emigrate and establish a population far
from their original home.
The distance prohibits gene flow with the original
population; the new population becomes reproductively
isolated.
Uplifting of mountains, rerouting of rivers, or formation of
deserts can subdivide a population.
This barrier prohibits gene flow between the divided
subpopulations; they can become reproductively isolated.
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Mechanisms of Geographic
Isolation

Extinction of intermediate populations
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14-10
Occurs when a population that exists between other
populations dies out
Eliminates gene flow between the remaining distant
populations
The other populations can become reproductively isolated.
Speciation will only happen if the genetic changes
accumulated during the period of reproductive
isolation generates two populations that can no
longer interbreed and make fertile offspring.
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Genetic Diversity and Reproductive
Isolation are Necessary for
Speciation

Environmental pressures and natural selection
play an important role in speciation.
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After a geographical separation, the two
subpopulations will likely experience different
environmental conditions.
Different phenotypes will be selected for in each
subpopulation.
Over time, genetic differences that accumulate may
result in structural, physiological, and behavioral
differences.
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14-11
These differences may prohibit interbreeding, thus
resulting in speciation.
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Speciation without Geographic
Barriers

Any process that leads to reproductive
isolation can lead to speciation.
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May not necessarily require geographic isolation
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14-12
Breeding or flowering at different times of year
Differences in genetically determined courtship and
mating behaviors
Genetically determined incompatibility of pollen from one
species and flowers of another
Polyploidy is the primary mechanism of speciation
in the absence of geographical isolation.
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Polyploidy

A condition of having multiple sets of chromosomes.
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More than haploid or diploid
Can result from abnormal events in mitosis or meiosis
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Can result from the mating of two different species
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Chromosomes do not separate properly
Cannot mate with its original population
Can self-fertilize and generate a new species
The hybrid ends up with a novel number of chromosomes
Cannot mate with either of the parent populations
Can self-fertilize and generate a new species
Cotton, potato, sugarcane, broccoli, wheat, etc. are
all species that resulted from polyploidy.
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Speciation without Geographic
Isolation
14-14
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Maintaining Reproductive
Isolation Between Species

New species stay reproductively isolated
from other species due to mechanisms that
prevent mating between species.
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14-15
Reproductively isolating mechanisms
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Reproductively Isolating
Mechanisms

Ecological isolating mechanisms
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Seasonal isolating mechanisms
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Two populations don’t interbreed because they
mate at different times of year.
Behavioral isolating mechanisms
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Two populations don’t interbreed because they
occupy different niches.
Two populations don’t interbreed because they
have different courtship and mating behaviors.
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Reproductively Isolating
Mechanisms

Mechanical isolating mechanisms
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Biochemical isolating mechanisms
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Two populations don’t interbreed because their
gametes are chemically incompatible.
Hybrid infertility/inviability
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Two populations don’t interbreed because they
have incompatible genitalia.
Two populations that can interbreed, but their
offspring are sterile or die before reproductive
maturity.
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Evolutionary Patterns Above
the Species Level

The development of new species is the
smallest irreversible unit of evolution.
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14-18
After a speciation event, the new species
continues to diverge from the original species.
Several different evolutionary patterns follow
speciation.
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Divergent Evolution
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An evolutionary
pattern in which
individual speciation
events cause
successive branches
in the evolution in a
group of organisms.
The evolution of
horses.
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Extinction
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The loss of a species.
Most species that have ever existed are now
extinct.
Ever-changing environments leads to the
generation of new species and the
elimination of others.
Divergence is accompanied by a great deal
of extinction.
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14-20
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This is the basic pattern of evolution.
Other special patterns also exist.
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Adaptive Radiation
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A special evolutionary pattern
Involves a rapid increase in the number of
kinds of closely related species
A kind of evolutionary explosion of new
species in a short amount of time
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Adaptive Radiation
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Thought to occur because
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A particular organism invades a previously
unexploited environment.
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A particular type of organism evolves a new set of
characteristics that allows it to displace previously
successful organisms.
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Animals moving to land
Galapagos finches
Reptiles replacing amphibians
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Adaptive Radiation in the
Galapagos Finches
14-23
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Adaptive Radiation in Terrestrial
Vertebrates
14-24
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Convergent Evolution

A special evolutionary pattern in which
similar characteristics develop in unrelated
groups of organisms
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The characteristics serve a similar purpose in the
particular environment, but have very different
ancestors.
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Spines in desert plants
Eating while flying in bats, dragonflies, and swallows
Body shape of whales, sharks, and tuna
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Convergent Evolution
14-26
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Homologous and Analogous
Structures
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Determining if a characteristic that is similar
in two different species is a result of
convergent or common ancestry is important.
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Homologous structures
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Analogous structures
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Have different appearances and functions that arose
from a common ancestor
Result from divergent evolution
Have similar structures and functions but arose from
different ancestors
Result from convergent evolution
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Maintaining Traits through Time
14-28
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Rates of Evolution
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Vary greatly
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From thousands to millions of years
When the environment changes rapidly
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Organisms change rapidly as a result of natural
selection.
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When the environment is stable
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High rate of speciation
High rate of extinction
Organisms change very little
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Theories About the Rate
of Evolution
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Gradualism
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Punctuated equilibrium
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The fossil record shows gradual changes in
physical features of organisms over time.
Darwin’s view of evolution by natural selection
implied gradualism.
The fossil record also shows long periods of
stasis.
Argues that evolution happens in spurts of
change, followed by long periods of equilibrium
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Gradualism vs. Punctuated
Equilibrium
14-31
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The Tentative Nature of the
Evolutionary History of Organisms
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Tracking the evolutionary history of any given
organism is difficult because most ancestral
forms are now extinct.
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Fossils are helpful, but the fossil record is
incomplete and provides limited amount of
information about each specimen.
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The likelihood of fossilization is low.
Only certain types of organisms can be fossilized.
Evolutionary biologists use the information
that they have to build evolutionary diagrams
for groups of organisms.
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An Evolutionary Diagram
14-33
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Where are we on the tree?
14-34
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Human Evolution

Our understanding of human evolution is
based mainly on information from the fossil
record.
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Humans are mammals, primates, specifically
anthropoids.
 Hominins
are humans and their human-like
ancestors.
 Hominids refers to hominins and African great
apes.
14-35
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The Course of Human Evolution
14-36
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An Overview of Human Evolution
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Early primates were adapted to living in forests.
As the climate became drier, grasslands replaced forests.
Early hominins (Australopithecus) were adapted to living in
grasslands.
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Stood upright to allow for
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Later hominins (Homo) had larger brains and used tools.
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More rapid movement over long distances
Ability to see over longer distances
Freed arms for using tools, etc.
Had larger brains and bodies
Able to use tools for a more diverse diet
Believed to have evolved at least 2.2 to 2.5 million years ago
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The Origin of Hom o Sapiens —
Two Points of View
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Out-of-Africa hypothesis
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Multiregional hypothesis
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Modern humans originated in Africa from other hominin
species.
Migrated to Asia and Europe and displaced other hominin
species that had colonized those areas earlier.
Homo erectus migrated and then evolved into H. sapiens.
Various subgroups of H. erectus existed throughout Africa,
Asia, and Europe and interbred to give rise to the races we
know today.
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