Download Punctuated Equilibrium Model of Horse Evolution

Evolution Reading
Class Set
Models of Speciation
Charles Darwin explained diversity of speciation through divergence. In divergent evolution, selection of successful
adaptations eventually leads to separation of one species into two or more species. Divergent evolution is often
depicted in a model known as a phylogenetic tree. Phylogeny proposes an ancestral evolutionary path of organisms over
millions of years. Much of the information is based on the fossil record, which is continually updated. More modern
trees are designed based on DNA nucleotide sequences and common genes for proteins like hemoglobin and
cytochrome c.
A phylogenetic tree is a graphic representation showing the evolutionary
relationships between organisms and their common ancestor. A
branching point of the tree shows a speciation event. At that point,
changes occurred in the species that result in two separate lineages
where previously there had been one.
Speciation
Common Ancestor
A phylogenetic tree shows evolutionary relationships.
A change in organisms that leads to a new species is called speciation. A species is defined as a group of organisms
capable of interbreeding and producing fertile offspring. Organisms that interbreed share genes; thus for organisms to
change over time, the distribution of genes wihin a population also changes. In order to form two different species, the
organisms must become reproductively isolated, forming two separate gene pools. Reproductive isolation can occur in a
variety of ways, which include geographic isolation, behavioral isolation, and temporal isolation.
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A single species separated by geographical
Geographic isolation occurs when physical barriers such as
continents, mountains, canyons, rivers, highways, or poplins
keep members of populations apart, preventing them from
reproducing. The organisms of the Galapagos Islands are
examples of species that developed due to the geographic
barrier of water between islands. Each population of organisms
developed adaptations over time that made them significantly
different, so that if brought together, they would no longer
reproduce.
formations eventually diverges into a separate species
Behavioral isolation can occur as a result of a geographic isolation. Organisms within a population that get separated
may develop different courtship behaviors or mating calls. These different forms of behavior lead to the separation of
gene pools because organisms within a population do not respond to the display, courtship dance, or song and thus do
not interbreed.
Temporal isolation occurs when there are differences in the timing of mating periods. For example, some species of
tomato plants bloom early while other plants bloom later, reducing the chance for cross-pollination. Differences in
timing, whether seasonal, monthly, or hourly, contribute to the maintenance of separate gene pools.
Speciation can also occur through mutations. Speciation is limited in the sense that it does not address species that
reproduce solely through asexual means. It is also difficult to apply the definition to organism that form hybrids in
nature (mules).
Speciation Occurs at Different Rates
Scientists studying evolutionary patterns discuss the occurrence of gradualism and punctuated equilibrium as two
hypotheses that explain speciation. Gradualism is the hypothesis that evolution occurs over a slow and steady rate. This
is the idea that Darwin propose in his theory of evolution. Speciation occurs through the gradual build up of new
adaptations. Gradualism is supported by the fossil record. Paleontologists have uncovered some types of fossils that
show organism in the strata have not changed much in their variations for long periods of time. This is referred to as
stasis. An example of an organism that has changed little in over 250 million years is the horseshoe crab.
A more recent idea on the rate of speciation is known as punctuated equilibrium. This hypothesis was proposed in the
1970s. Punctuated equilibrium is the hypothesis that speciation will occur rapidly in bursts with long periods of stability
in between. During stability, the populations have genetic equilibrium and remain unchanged for long periods of time.
Punctuated equilibrium occurs after sudden environmental changes or if a mutation causes a shift in the gene pool.
During this rapid change in the gene pool, speciation can occur. Examples of punctuated equilibrium occur in cases
where the gene pool of a small population is suddenly limited to survivors of a catastrophic event. The fossil record
shows that punctuated equilibrium is a form of gradualism. Paleontologists have uncovered fossils of organisms in strata
that show significant changes that have occurred in short periods of geologic time followed by fossils that have
remained unchanged for long periods of geologic time.
Gradualism and punctuated equilibrium can also be graphically represented through phylogenetic trees. Phylogenetic
trees also signify a change in speciation with regard to time. Gradualism models represent slow diversification of one
line of descent. The punctuated equilibrium model displays rapid speciation and then stability of several lines of descent.
A phylogenetic tree showing either type of branching is sometimes referred to as the tree of life.
Equus
Pliohippus
Merychippus
Equus
Pliohippus
Merychippus
Mesohippus
Mesohippus
Hyracotherium
Gradualism Model of Horse Evolution
Hyracotherium
Punctuated Equilibrium Model of Horse
Evolution
Patterns of Evolution
Biologists refer to the large-scale history of life as macroevolution. Macroevolution is characterized by six different
evolutionary patters: extinction, adaptive radiation, punctuated equilibrium, coevolution, convergent evolution, and
developmental genes.
Extinction
Extinction is the complete loss of a species from Earth. Extinction of organisms is caused by habitat destruction,
pollution, invasive species, or catastrophic events. Extinction occurs when organisms are unable to adapt to the changes
in the environment. Scientists classify extinctions into two types: background extinctions and mass extinctions.
Background extinctions are those that occur at a continuous slow, steady rate. This type of extinction usually only affects
a small number of species in a small area. For, example, the effect of global climate change on the polar ice caps is
diminishing the polar bear population. If their habitat continues to be destroyed, the number of polar bears that survive
annually is reduced. Eventually, they may become extinct.
Mass extinctions are those that affect large numbers of species over a large area for thousands or millions of years.
These are thought to be caused by catastrophic events such as an asteroid impact or a severe climatic change. The fossil
record shows five periods of time where mass destruction of multiple species drastically altered the gene pool.
Adaptive Radiation
Evolution of one species into several different species through natural selection is known as adaptive radiation.
Adaptive radiation typically occurs after mass extinction when species move into new, unoccupied regions. After a mass
extinction, there is a large amount of ecological space for a species to migrate with little to no competitors. As
populations increase in size, competition within the population become will also increase. As a result, variations within a
population become specialized and groups become smaller and more selective. This ultimately forms a new species.
Darwin observed adaptive radiation with the birds of the Galapagos Islands.
When Darwin observed the birds on the Galapagos
Islands, he recognized what he thought was a
relative of birds called grosbeaks that have large,
seed-eating beaks. He collected and preserved
specimens of each type and sent them to scientists
in England for classification. His misidentified
grosbeaks turned out to be finches. Darwin was
amazed by the variety of finches living on the
islands. Each type had a different type of beak
specially adapted to eat a specific type of food.
Darwin wondered how a family of finches could be
so different yet live so close together. Eventually, he
surmised the original founders of the Galapagos
finch species gradually diversified through natural
selection and evolved into the species he observed.
Coevolution
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Darwin hypothesized that the founding finches on the Galapagos
Islands had originally come from the mainland of South America
and had diverged into different species due to natural selection.
Coevolution is the evolution of two different species according to their interaction with
one another. Each species places selective pressure on the other, so they evolve
together. Coevolution is often represented by the adaptations of organism in feeding
relationships. For example, birds like vultures do not have feathers on their heads
because they feed on carrion (dead animals). The waste from the rotting carcass or
bacteria from decomposing flesh would cling to feathers, eventually causing disease.
Without head feather, vultures have less chance of contracting diseases. The
relationships between pollinators and flowers are also examples of coevolution. Some
flowers are red with long tubular shapes and provide high concentrations of nectar.
These flowers are most likely pollinated by hummingbirds that can see the color red
very well, have a high need for energy and have tubular-shaped beaks. Other symbiotic
relationships such as predator-prey, parasitism, commensalism, and mutualism
influence coevolution.
Convergent Evolution
Sometimes organisms face similar environmental
demands, such as moving through air or water, and
develop structures through natural selection that have
similar functions yet are structurally different.
Structures that have the same function but are
physically different are called analogous structures.
When species are not related yet have similar body
morphology or shape, they represent convergent
evolution. For example, bats have wings and can fly like
birds, yet they are mammals and not closely related to
birds. Animals that live in water, such as dolphins, seals,
and sharks, share similar torpedo-shaped body styles
that enable them to swim rapidly through the water.
Each of these animals has body parts that allow it to
move through the water rapidly, but the internal
structure of each body part is very different among
organisms. In fact, the internal structure is so different
there is no evidence of a common evolutionary history.
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Organisms with analogous structures or comparable body styles
enable them to function in similar ways.
Developmental Genes
Most animals have a specific sequence of nucleotides that are approximately 180 units long, called a homeobox, or Hox
genes. Hox genes are also known as developmental or master control genes because they influence body plans of
organism during embryological development by turning certain genes “on” and “off.” Basically, they control the head-totail pattern of development. For example, Hox genes in humans control the location of arm and leg buds. Hox genes in
insects control wing placement and the number of wings. They are the genes that determine the head, thorax, and
abdomen body style of insects. Embryos of dogs, fish, turtle, pigs, and humans show similarities in body plans due to
similarities in the activation of developmental genes. Scientists believe Hox genes play an important role in evolutionary
history because mutations that occur in a Hox gene lead to the development of a body structure in a different position,
leading to diversity within animal species.