HONORS LIVING ENVIRONMENT
TOPIC 22: EVOLUTION AND BIODIVERSITY
MS. ETRI
PART 5: SPECIATION AND PATTERNS OF EVOLUTION
HOW DO WE DEFINE SPECIES?

***BIOLOGICAL SPECIES CONCEPT: a group of populations whose members have the potential to
interbreed in nature and produce fertile offspring (offspring that themselves can reproduce).
o
Focuses more on how discrete groups of organisms arise and are maintained by reproductive
isolation.

PHYLOGENETIC SPECIES CONCEPT: Defines a species as the smallest group of individuals that
share a common ancestor and thus form one branch of the tree of life.
o
Agreeing on the amount of difference required to establish separate species remains a
challenge.

ECOLOGICAL SPECIES CONCEPT: Identifies species in terms of their ecological niches, focusing on
unique adaptations to particular roles in a biological community.
o
EXAMPLE: Two species of fish may be similar in appearance by distinguishable based on what
they eat or the depth of water in which they are usually found.
DARWIN’S RESEARCH

Most of Darwin’s theory of evolution focused on the role of natural selection in the gradual adaptation of
a population to its environment.

We call this process microevolution—changes in the gene pool of a population from one generation to
the next.

However, microevolution is not the only force at play. SPECIATION—the process by which one species
splits into two or more species, has a lot to do with evolutionary change.
o
Every time speciation occurs, the diversity
of life increases.
o
Over the course of 3.5 billion years, an
ancestral species first gave rise to two or
more different species, which then branched
to new lineages, which branched again, until
we arrive at the millions of species that live,
or once lived, on Earth.
o
This origin of species explains both the
diversity and unity of life.
WHAT CAUSES SPECIATION?
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1. GEOGRAPHIC ISOLATION

MS. ETRI
A key event in the origin of a new
species is the separation of a population
from other populations of the same
species.

ALLOPATRIC SPECIATION: the
formation of new species in populations
that are geographically isolated from
one another.
o
Several geologic processes can
isolate populations:

A mountain range may
emerge and gradually
split a population of
organisms that can inhabit only lowlands.

A large lake may subside until there are several smaller lakes, isolating certain
fish populations.

Continents can split and move apart.

Can also occur when individuals colonize a remote area and become
geographically isolated from the parent population.
2. BEHAVIORAL AND REPRODUCTIVE ISOLATION

A fly will not mate with a frog or a fern. But what prevents species that are closely related from
interbreeding?

Reproductive isolation depends on one or more types of reproductive barriers—biological
features of the organism that prevent individuals of different species from interbreeding.
o
TYPES OF REPRODUCTIVE BARRIERS:

PREZYGOTIC—prevent mating or fertilization between species.



Happens before zygotes form.
POSTZYGOTIC—operate after hybrid zygotes are formed.
WHAT MIGHT CAUSE REPRODUCTIVE BARRIERS TO ARISE?
o
The environment of an isolated population may include different food sources, different
types of pollinators, and different predators.
o
As a result of natural selection acting on preexisting variations, a population’s traits
may change in ways that also establish reproductive barriers.
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PATTERNS OF EVOLUTION
1. EXTINCTION

Mass extinctions have detrimental
effects on biological diversity.

By removing large numbers of species,
a mass extinction can decimate a
thriving and complex ecological
community.

Mass extinctions are random events that
act on species indiscriminately.
o
They can permanently remove
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species with highly advantageous features and change the course of evolution
forever.

Most organisms which appeared on this planet through its evolutionary history are now extinct.

The earth and environment are constantly changing, therefore new species are constantly
being created and destroyed.

CAUSES: lava releasing large amounts of carbon dioxide into the atmosphere, extraterrestrial
objects.
2. ADAPTIVE RADIATION—the evolution of many diverse species from a common ancestor.

Isolated island chains are often inhabited by unique collections of species.

Islands that have physically diverse habitats and that are far enough apart to permit populations
to evolve in isolation, but close enough to allow occasional dispersions to occur are often the
site of multiple speciation events.

HOW DID DARWIN’S FINCH SPECIES EVOLVE FROM A SMALL POPULATION OF
ANCESTRAL BIRDS THAT COLONIZED ONE OF THE ISLANDS?
o
Completely isolated on the
island, the founder population
may have changed significantly
as natural selection adapted it
to the new environment, and
thus became a new species.

Later, a few individuals
of this species may
have migrated to a
neighboring island,
where, under different
conditions, this new
founder population
was changed enough
through natural
selection to become
another new species.

Some of these birds may then have recolonized the first island and coexisted
there with the original ancestral species if reproductive barriers kept the
species distinct.

Multiple colonizations and speciation on the many separate islands of the
Galapagos probably followed.
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3. CONVERGENT EVOLUTION
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Species from different evolutionary branches may come to resemble one another if they live in
similar environments and natural selection has favored similar adaptations.
o
In such cases, body structures and even whole organisms may resemble each other.
o
Similarity due to convergent evolution (rather than due to descent from a common
ancestor) is called analogy.
4. COEVOLUTION

Occurs when a change in one species acts as a
new selective force on another species, and the
resulting adaptations of the second species in turn
affect the selection of individuals in the first
species.

EXAMPLE: herbivore-plant interactions.
TWO THEORIES ABOUT THE PACE OF EVOLUTION
1. GRADUALISM

Other fossil species appear to have diverged gradually over long periods of time.

Differences accumulate, and new species evolve gradually from the ancestral population.
2. PUNCTUATED EQUILIBRIUM

Characterized by long periods of little apparent morphological change (equilibria) interrupted
(punctuated) by relatively brief periods of sudden change.

Many fossil species appear suddenly in a layer of rock and persist essentially unchanged
through several layers (strata) until disappearing just as suddenly.

KEEP IN MIND:
o
Even when fossil evidence points to punctuated equilibrium, species may not have
originated as rapidly as it appears.
o
Suppose that a species survived for 5 million years, but that most of the changes in its
features occurred during the first 50,000 years of its existence.
o
Time periods this short often cannot be distinguished in fossil strata.
o
And should a new species originate from a small, isolated population, the chances of
fossils being found are low.
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