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CONCEPTS IN BIOLOGY
TWELFTH EDITION
Enger • Ross • Bailey
CHAPTER 14
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Speciation
A species is a group of organisms whose
members have the potential to interbreed
naturally and produce fertile and viable
offspring.
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Determining if two populations belong to
the same species using gene flow

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.
Hybrid sterility
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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.
1.
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Most of these have gone extinct.
2.
Reproductive isolation
Genetic isolation - Polyploidy
Speciation by reproductive isolation

Geographic isolation
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Occurs when a portion of a
population becomes totally
isolated from the rest

If it is followed by genetic
divergence.
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Then reproductive isolation can
result.
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Changes in allele frequencies
The isolated population
becomes a new species
Geographically isolated populations
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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.
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.
Over time, genetic differences that accumulate
may result in structural, physiological and
behavioral differences.

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These differences may prohibit interbreeding, thus
resulting in speciation.
Speciation without geographic
barriers

Any process that leads to reproductive
isolation can lead to speciation.
–
May not necessarily require geographic isolation
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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.
Polyploidy

A condition of having multiple
sets of chromosomes.
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
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More than haploid or diploid
Can results from abnormal events
in mitosis or meiosis
Cotton, potato, sugarcane,
broccoli, wheat, etc. are all
species that resulted from
polyploidy.
Speciation without geographic
isolation
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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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reproductively isolating mechanisms
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 types 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.
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.
Evolutionary patterns above the
species level

The development of new species is the
smallest irreversible unit of evolution.
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After a speciation event, the new species
continues to diverge from the original species.
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.
Divergent Evolution
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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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This is the basic pattern of evolution.
Other special patterns also exist.
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
Adaptive radiation

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
Adaptive radiation in the
Galapagos finches
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Adaptive radiation in terrestrial
vertebrates
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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
Convergent evolution
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Homologous and analogous
structures

Determining if a characteristic that is similar
in two different species is a result of
convergent or from 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
Convergent Evolution –
Analogous structures
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Homologous structures
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Rates of evolution

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.
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
Gradualism vs. punctuated
equilibrium
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An evolutionary diagram
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Where are we on the tree?
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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
common ancestors.
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The course of human evolution
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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 allowing 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.
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Had larger brains and bodies
Able to use tools for a more diverse diet
The origin of Homo sapienstwo points of view

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.
Sociobiology

Study of evolution of
behaviors
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Altruism
Inclusive fitness