Speciation and Extinction
Recall that the three fundamental processes of biogeograpy are:
Speciation
Extinction
Immigration
All three have geographic elements.
Speciation occurs in space as well as time, and often requires
geographic isolation.
Extinction is the end result of a large-scale process of range collapse.
In a way, speciation is the “birth” of a species and extinction is the
“death” of a species.
There are differences, however, between the “life” of a species and
that of individuals.
Whereas the birth and death of individuals are very discrete, easily
recognized events, the formation of a new species and its ultimate
extinction are not usually as easy to pinpoint.
Sexual and asexual species differ
in the way in which the individuals
are “connected” to one another.
In sexual species, the different
individuals are genetically related
through the process of
reproduction.
In asexual species, each clone
retains its genetic identity and the
different clones must be held
together by some process other
than reproduction.
Individuals in a sexual species
have a tokogenetic or reticulate
(netlike) set of relationships.
In this diagram, each circle is an individual
(different colors are different sexes). The
lines represent part-offspring reproductive
connections. (Time goes UP).
The event of speciation
takes this “network” of
interwoven individuals
and separates it into two
separate networks with
their own evolutionary
trajectory.
Thus we separate the
tokogenetic evolutionary
processes that occur
among individuals from
the cladogenetic process
that occur between
species.
Pretty nasty diagram, I realize. But it depicts the
separation of these different types of processes.
Speciation occurs at the inverted triangle (notice
that, after that), we’ve got two separate groups
without mixing. Again, time is moving UP.
Systematics is the discipline in biology that deals with the processes related to
biological diversity and diversification.
It includes two distinct subdisciplines: taxonomy and phylogenetics.
Taxonomy is, basically, involved with the naming and classification of
organisms.
Phylogenetics attempts to reconstruct their evolutionary history.
A subdiscipline of phyogenetics
is cladistics, in which we
examine the cladogenesis that
results in the splitting of
lineages..
A cladogram of the
Dinosauria
Species Concepts
It’s clear that the species is a unit of fundamental importance. But
what is it? A number of different species concepts have been
developed.
A few of these
are more widely
used than others
Morphological Species Concept – species are the smallest groups that are
consistently and persistently distinct, and distinguishable by ordinary means.
Also considered the classical species concept. This is the primary way that
species are distinguished operationally.
Biological Species Concept – a group of interbreeding natural populations
that is reproductively isolated from other such groups. Treats reproductive
isolation as fundamental.
Phylogenetic Species Concept – identifies species based on evolutionary
lineages. Species must represent a monophyletic lineage, be derived through
an evolutionary process of descent from an ancestral lineage, and be
diagnosable through examination of character state transformations.
Evolutionary Species Concept – an entity composed of organisms that
maintains its identity from other such entities through time and over space
and that has its own independent fate and historical tendencies. Probably the
basic idea that all others concepts are trying to approach.
The distribution of beak depth
in populations of Darwin’s
finches on different islands in
the Galapagos.
Note the “clumpy-gappy”
distribution. This is typical of
what most species concepts
attempt to display.
Note also the illustration of
character displacement. For
example, look at what happens
to beak depth in Geospiza fortis
when it cooccurs with other
species compared to what is
seen when it is occupies the
island alone.
It’s clear that some
populations that don’t measure
up to species level distinction
are still well differentiated from
other populations. We use a
variety of terms to describe
them:
Subspecies
Ecotypes
Phylogroups
Evolutionarily Significant Units
The black sea turtle is sometimes
considered an “ecologically significant
unit” of variation within the species of
the green sea turtle (Chelonia mydas).
The system of binomial nomenclature tha twe employ includes a
hierarchical arrangement of higher levels of classification.
Macroevolution
Refers to evolution above the species level.
Evolution rates appear to be highly variable.
George Gaylord Simpson classified evolutionary lineages into
a. bradytelic classes – those that evolve slowly.
b. horotelic classes – those evolving at typical rates
c. tachytelic classes – those evolving faster than typical rates.
There has been much debate about the degree to which
microevolutionary processes can account for observed
macroevolutionary changes.
The fossil record reveals many evolutionary patterns that are not easily
explained as resulting from the accumulation of many, small-scale,
microevolutionary events.
Eldredge, Gould, Stanley and others suggested that macroevolution
occurs primarily as the result of two processes:
Punctuated equilibrium
Species selection
Niles Eldredge
Steven Stanley
Stephen Jay Gould
According to the fossil record,
most evolutionary lineages
consist of long periods of very
little change “punctuated” by
brief period of relatively rapid
change.
These periods of change are
often related to speciation
events.
The periods of change are
also associated with
morphological changes, range
expansions, and the
exploitation of new niches.
This pattern has come to be
known as punctuated
equilibrium.
An example of punctuated
equilibrium in the evolution of
mollusks in the Lake Turkana
Basin in eastern Africa.
Note that most taxa show long
periods of relatively little
change, “punctuated” by
periods of rapid, dramatic
change.
The “punctuations”, often, occur
simultaneously in several
different lineages. This is
suggestive of dramatic
environmental change.
In other cases, the change
occurs in association with
speciation events.
Also in the fossil record, we
see evidence of species
selection. This simply
means that evolutionary
change brings with it the
survival of species that
have particular traits.
Extinction is not random.
Usually, species with certain
traits are more likely to
persist than others.
The asteroid impact that led to the demise
of the dinosaurs some 65 million years
ago, while causing the extinction of many
types of organisms, did not impact all
groups equally. Many lineages of insects,
small mammals, and vascular plants
survived.
Microevolution and macroevolution, no doubt, both occur.
In fact, an understanding of each is necessary for an understanding of
how species are distributed on the planet and how they came to be
that way.
Speciation
All species of green land
plants share a common
ancestor in a simple green
alga that lived over 500
million years ago.
All vertebrates trace their
origins to an ancient
chordate ancestor that lived
around the same time.
The millions of species of
insects are descended from
the first ancestral species
that first invaded the land
about 400 million years ago.
How did all of this genetic
differentiation come to be?
Regardless of the geographic events involved, the divergence of an
ancestral species into two or more daughter species requires
genetic change.
Population geneticists recognize four microevolutionary processes
that can lead to genetic divergence.
Mutation
Genetic Drift
Natural Selection
Gene Flow
Mutation
• Point mutation
– Synonymous
– Transition
– Transversion
• Frame-shift mutation
• Stop mutation
• Translocation
• Fusion
Gene mutations
Normal
hemoglobin
Abnormal
hemoglobin
Leading to……
Genetic Drift
The theory of genetic drift was
developed in the 1930s by Sewall
Wright, and is sometimes referred to as
the Sewall Wright effect.
Consider a mathematical example. If two Aa frogs have
only two offspring, there is a 1/16 chance of the a gene
(or the A gene, for that matter) gene being lost after the
first generation.
Random sampling is more effective at creating variations in
small populations than in large ones. Results from the
Dobzhansky experiment with Drosophila populations.
Natural Selection
While genetic drift is typically a
rather weak force in creating
genetic differentiation (at least in
relatively large populations),
natural selection is a powerful
force.
This is the change in a population
that occurs because certain
individuals possess genetic traits
that increase their survival and
reproductive rates relative to other
individuals.
Over many generations, those
traits will tend to increase in
frequency.
Gene Flow
Migration often retards
genetic divergence,
having a homogenizing
influence. The
movement of organisms
from one population to
another works against
the establishment of
reproductive isolation.
Geographic variation
has resulted in the
subdivision of
Peromyscus
maniculatus (the deer
mouse) into 50 different
subspecies.
In the Palestine mole rat
(Nannospalax
ehrenbergi), the number
of chromosomes
increases along a
gradient of increasing
aridity from north to
south in the Middle East.
Along an elevation
gradient in California’s
Sierra Nevada
mountains, the plant
Achillea millefolium
shows dramatic
morphological
differentiation.
These plants were all
grown in the same
environment from seeds
collected at different
elevations.
What could result in this
type of selection?
Geographic Variation
There is often a geographic
component to genetic
diversity. This happens
because genetic drift and
natural selection can be
facilitated by geographic
isolation. Similarly, gene
flow back to the parent
population can be retarded.
Genetic drift can be an
important factor in small
populations, such as in an
island population.
.
Genetic drift that results from the
formation of a new population by a small,
random sample of an ancestral
population is sometimes referred to as a
founder effect
The divergence of
populations of the
monarch flycatcher
“superspecies” seem to
provide an example of the
founder effect.
The random sampling of genes present in the
parent population results in somewhat random
differences among the populations inhabiting
different islands.
A cline is a
gradual change
in features
along an
environmental
gradient. For
example, we
see a cline in
litter size with
elevation or
latitude.
We may also see a cline in a
hybrid zone. This is a narrow
geographic region separating
the range of two different
populations. Within the hybrid
zone we see rapid changes in
characteristics.
The overlap or hybrid zone between
subspecies of the California newt (Taricha
torosa). Between northern and southern
populations, a zone of gradation exists.
Allopatric Speciation
Geographic isolation is thought to be the most frequent event leading to
speciation.
Speciation that results from geographic isolation is known as allopatric
speciation.
In a heterogeneous environment, the separation of a parent population will
often result in the two subpopulations being exposed to dramatically different
environmental pressures. This will result in different selective pressures, and
force the two populations down different evolutionary pathways.
The result is the formation of two new species.
Two primary modes can be suggested by which allopatric speciation can take
place: vicariance and jump dispersal.
An illustration of allopatric
speciation through
vicariance.
Species a lives in a
continuous geographic
range (think about a
continental mass). Then,
that range is separated into
two ranges. The
populations diverge,
forming new Species b and
c.
At a later time, the range
inhabited by Species c is
subdivided. This results in
the divergence of Species c
into new Species d and e.
This is thought to be the
pattern that led to
speciation following the
breakup of the
supercontinent of
Gondwanaland. The
splitting of the large land
mass led to the formation
of subpopulations.
Geographic isolation led to
speciation.
A molecular
phylogeny of the
family Ranidae
(frogs) with the
estimated divergence
times provides good
evidence of
vicariance induced
speciation.
In short, the lower
the similarity, the
longer the period of
separation.
Allopatric speciation
may also occur as the
result of jump
dispersal, leading to
founder events. This
simply refers to the
origination of a new
population from a small
number of immigrants
from a larger parent
population.
While vicariant events
tend to occur
simultaneously in
multiple groups (since
everybody’s on the
same land mass),
founder events usually
involve only one (or at
most, a few) species.
This diagram depicting a founder event is
attempting to display that the new, “founder”
population, may not be representative of the parent
population. This is possible because it is typically a
small, somewhat random, sample.
A diagram representing
allopatric speciation by
dispersal of members of a
population to peripheral
areas. Consider a large
population of Species a on
the large land mass a.
Through jump dispersal,
small groups of individuals
reach isolated areas b, c,
and d (small islands?).
Over time, differential
selection leads to genetic
differentiation and the
development of new
Species b, c, and d. The
parent Species a still
remains.
Another, slightly different case of
allopatric speciation by jump
dispersal. In this case, there is a
sequential formation of peripheral
isolates at the edge of the home
range. First, Species b is isolated
and differentiates. Then Species
c isolates from Species b. Finally,
Species d isolates from Species
c.
The resulting cladogram shows
that c and d are the most closely
related, given their more recent
common ancestry.
The proposed
phylogeographic history
of Galapagos tortoises in
the Galapagos Islands.
Immigrants from the
mainland of South
America first colonized
the large islands of San
Cristobal and Espanola,
probably some 2-3
million years ago.
Arrows show the likely
dispersal events (solid
representing natural
events with dashed
arrows indicating
probable relocations by
humans).
There are 15 recognized races of tortoise, all generally considered to be
members of the single species Geochelone elephantopus..
On the other hand, Darwin’s finches in the
Galapagos have reached the final stages
of speciation. All are probably derived
from a single ancestral population, also
probably arriving at the Galapagos some
2-3 million years ago.
Sympatric and Parapatric Speciation
While most speciation events are
certainly allopatric in nature, it is
clear that geographic isolation is
not always necessary for
speciation to take place.
We refer to this as sympatric
speciation if the overlap is
extensive (i.e., a new species
develops within the range of the
parent species) and parapatric
speciation if the overlap is only
along the margin of the parent
population.
An illustration of sympatric
speciation (A) and parapatric
speciation (B).
Clearly, for sympatric speciation to occur we must propose some
method by which we can achieve reproductive isolation without
geographic isolation.
In other words, we have to propose a way to form two different
breeding groups within a relatively continuous population.
Two mechanisms have been proposed:
Disruptive selection
Chromosomal changes
Disruptive Selection
Sometimes selective pressures within
a population can select for two
extremes of a phenotype, effectively
“pushing” the population in two
different directions.
It is thought that this may be common
in herbivorous insects that are
specialized for particular host
species.
Matings between “host-specific races”
would produce offspring that have
lower fitness in either of the two
parent niches.
This would favor the development of
a prezygotic isolating mechanism, like
preferential mate selection.
Note: Prezygotic isolating mechanisms
create reproductive isolation prior to
mating. Things like mate selection.
Postzygotic isolating mechanisms act to
prevent the development of the embryo.
Sympatric speciation can also
occur as the result of
chromosomal changes.
Sometimes, random changes in
the genetic material during
meiosis or during early
development can result in
changes in the number of
chromosomes or the gene
sequence in chromosomes.
Mutant individuals with new
chromosome numbers have
impaired fertility when they mate
with individuals with the normal
chromosome number.
This can lead to sympatric
speciation.
Recent years has seen increased evidence for sympatric speciation.
In isolated lakes, such as the Rift Valley Lakes of central Africa,
The cichlids of Lake Tanganyika show a great diversity in body form.
The cichlids of Lake
Malawi show great
diversity of head
shape, mouthparts,
and feeding habits.
The pupfishes of Lago Chichancanab in Yucatan
have been isolated in small lakes for only a few
thousand years. The lake contains five species of
the pupfish Cyprinodon which have diverged
sympatrically within the last few thousand years.
Studies suggest that the divergence is not
complete.
Adaptive Radiation
Adaptive radiation is the
diversification of species
to fill a wide variety of
ecological niches. We see
it when a single ancestral
species gives rise to many
types of descendants in
the same region.
The different descendants
fill a variety of available
niches.
One of the best examples of
adaptive radiation can be seen in the
cichlid fish of the east African lakes.
They have developed very different
morphologies in order to take
advantage of a variety of different
ecological niches.
Lake Malawi alone contains over 300
species in a group called the
“mbuna”.
Island groups also provide
opportunities to observe
adaptive radiation.
The Hawaiian
honeycreepers show a
dramatic variation of bill
shapes.
This is similar to the
adaptive radiation showed
by the finches of the
Galapagos.
We can see a radiation in
the marine realm in the
anglerfishes of the order
Lophiformes.
Histrio sp.
Antennarius sp.
Ogcocephalus sp.
Lophius sp.
Melanocetus sp.
In plants, the phlox family has
undergone a dramatic radiation
on the North American continent
over the last 20 million years.
Extinction
The ultimate fate of every species.
Consider that 100
million years ago
there, there was a
variety of life on the
planet similar to that
which we see today.
Virtually all of those
species are gone
now.
100 million years from
now, virtually all the
species that are here
now will be gone too.
Just like the ultimate outcome of life is death,
the ultimate outcome of speciation is extinction.
A popular analogy is
presented as “The
Red Queen
Hypothesis”. The
idea is that a
species must
continually evolve in
order to keep up
with (or stay slightly
ahead of) an
environment (both
biotic and abiotic)
that is also
constantly changing.
Like the Red Queen in Lewis Carroll’s Through the Looking
Glass, it “takes all the running you can do just to keep in the
same place.”
Some taxonomic or ecological
groups have higher rates of
extinction than others.
Small, herbivorous mammals have
lower extinction rates than large,
carnivorous ones.
Tasmanian “tiger” – now extinct
Mathematical models
have been developed to
use demographic factors
to predict the
susceptibility of a
species to extinction.
One thing is clear. When
populations become
small, the chance of
extinction resulting from
purely random factors
goes up.
It appears that the
chance of extinction
increases non-linearly as
population size
decreases.
Results of a study employing
demographic factors to predict time to
extinction.
Recent extinctions
Over the last 200 years, humans
have directly or indirectly caused
the extinction of thousands of
species…. that we know of.
Case in point – the passener
pigeon (Ectopistes migratorius).
Read the discussion on page 216
in your text.
Others….
Carolina parakeet
Stellar’s sea cow
Great auk
The American chestnut (Castanea
dentata) has been virtually wiped out
by an introduced pathogenic fungus
(Endothia parasitica) which reached
the U.S. accidentally in 1904. The
resulting blight destroyed most existing
trees, and threatens the species with
extinction.
The damming of the
Chagres River during the
building of the Panama
Canal led to the formation
of Barro Colorado Island.
Since the formation of the
island, perhaps as many
as 50 species of birds
have become extinct on
the island.
It appears that the small
populations that were
present on the island died
out and were not
replaced by colonists
from the mainland.
Some species have shown a remarkable
ability to come back from the brink of
extinction.
The northern elephant seal (Mirouga
angustirostris) and the sea otter (Enhydra
lutris) were both hunted to near extinction
in the early part of the 20th Century.
Once protected, both have recovered to
large, healthy populations.
The gray whale (Eschrichtius robustus) is
also recovering under protection.
A number of bird species (sandhill
cranes, trumpeter swans, snowy egrets,
and others) have also made significant
recoveries.
Extinction in the Fossil Record
In some cases, the
fossil record is
complete enough to
show that many
species became
extinct over a relatively
short time period.
These events have
come to be known as
mass extinctions.
They apparently result
from some drastic,
widespeard
environmental change.
The six largest mass
extinctions have been
particularly significant.
Not only did they
eliminate large portions
of the existing biological
diversity.
They also seem to have
been precursors to
significant adapative
radiations as organisms
rapidly filled the newly
available niches.
One of the most recent extinction events
was the disappearance of the Pleistocene
“megafauna” (primarily large mammals,
between ~ 15,000 and ~ 8,000 years ago.
This led to the disappearance of a large
number of mammalian species, particularly
in North America.
A variety of causes have
been suggested. A number
of lines of evidence are
now suggesting that
humans may have played
a large role.
One major mass
extinction occurred about
250 million years ago at
the boundary between
the Permian and Triassic
Periods.
A number of possible
causes have been
suggested, focusing
primarily on periods of
global warming or
cooling.
A reconstruction of the
ancient seabed in
southern China before
and after the PermoTriassic mass
extinction event.
The cause of other mass
extinctions has also been
widely debated.
Maybe the most significant
was the extinction at the
“K-T bondary”, the
transition from the
Cretaceous Period of the
Mesozoic Era to the
Tertiary Period of the
Cenozoic Era about 65
million years ago.
At this time, major groups
of plants and animals from
many groups became
extinct.
Among the groups that
disappeared were the
dinosaurs.
Tyrannosaurus
It appears to have
resulted from the impact
of an asteroid….
…which struck in the region of
the Yucatan peninsula of
Mexico.
Read the account on page 219
of your text.
Chicxulub Crater