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Macroevolution and Speciation
Evolution creates (and destroys) new species, but …
What is a species?
Its not as straightforward a question as most believe.
These are members of different species - eastern (left) and western (right) meadowlark.
What is a Species?
There is only one extant (existing) human species.
What is a Species?
And these are all members of a single species.
Determining What Is and What Isn’t a Distinct Species Can Have
Economic Consequences
Northern spotted owl (left) and barred owl (right).
What is a Species?
The definition we’ll use is this: A species is a group of individuals capable of
interbreeding to produce fertile offspring.
This is the biological species
concept. Like all attempts to
define a species, it has many
problems.
One Problem in the Biological Species Concept
For asexually-reproducing organisms, like these bacteria, what constitutes a species?
How Many Species Are There?
We don’t know.
About 2 million species have been described.
Estimates of existing species number range from 4 million to 100 million
(with 10-15 million being a more commonly considered upper estimate).
Two Patterns of Speciation
How Do
Species Arise?
The key to speciation
is reproductive
isolation of
populations.
There are extrinsic
and intrinsic
reproductive isolating
mechanisms.
Geographic isolation is
the primary extrinsic
reproductive isolating
mechanism.
Reproductive Isolation May Occur With or Without
Geographic Isolation
Allopatric speciation occurs
when geographic isolation
creates a reproductive
barrier (an extrinsic
mechanism).
Sympatric speciation occurs
when a reproductive barrier
is created by something
other than geographic
isolation (intrinsic
mechanisms).
Allopatric Speciation
Harris’ antelope squirrel
White-tailed antelope squirrel
Two species of ground squirrel are postulated to have descended from a common
ancestral population that was separated by formation of the Grand Canyon.
Intrinsic Reproductive Isolating Mechanisms Are Always
Required for Speciation
Harris’ antelope squirrel
White-tailed antelope squirrel
Intrinsic mechanisms involve changes to organisms that prevent interbreeding.
In allopatric speciation, intrinsic mechanisms come into play once populations
are physically separated.
In sympatric speciation, intrinsic mechanisms are the only ones involved.
Many Intrinsic Reproductive Isolating Mechanisms Drive
Speciation
(different habits within an overlapping range)
Many Intrinsic Reproductive Isolating Mechanisms Drive
Speciation
Courtship rituals, like
these, are critical for
mating within a species,
but ineffective for
attracting members of
other species.
Behavioral Isolation Mechanisms
Courtship rituals, like these, are critical for mating within a species, but
ineffective for attracting members of other species.
Many Intrinsic Reproductive Isolating Mechanisms Drive
Speciation
Many Intrinsic Reproductive Isolating Mechanisms Drive
Speciation
Hybrid Infertility Was
the Impetus for
Cloning a Mule
Speciation Occurs at Widely Differing Rates
A slow rate of speciation evidenced by a living horseshoe crab (13 extant
species) and a 300 million year-old fossil species
A rapid rate of speciation evidenced by Galapagos finches which have diversified
into 13 species within the last 100,000 years.
Speciation Rates
Generalists, like the horseshoe crab, tend to remain as stable species.
Specialists, like the Galapagos finch, tend to be
unstable as species.
Speciation also becomes rapid when, as occurred with
Galapagos finches, new niches become available.
Speciation Dynamics - Gradualism or Punctuated Equilibrium?
Punctuated equilibrium appears
to be a more accurate view of
speciation dynamics.
Does Evolution Create the Perfect Organism?
No, only better organisms as evolution is
constrained by history and buffeted by random
events.
Essentially, every organism on earth is in
significant part a sum of accidents.
Species Come and Go
Best estimates from the fossil record indicate that greater than 99% of species that
have exited are now extinct.
A typical “lifetime” for a species is
about 1 million years.
Mass Extinctions Are a Fact of Life
The Cretaceous/Tertiary Mass Extinction
Gary Larson
Are We Now Causing a Mass Extinction?
Classifying Life’s Rich Diversity
Why bother?
An intrinsic reason is that modern classification systems tell who’s related to
whom and how we all came to be.
Classifying Life’s Rich Diversity
Why bother?
A practical reason is that if we
want to preserve an environment
compatible with human life, we’d
better know what’s out there.
The Linnaean
Hierarchical
Classification System
How Do We Classify Organisms?
Ideally, classification is based on establishing the evolutionary relationships
between organisms.
The evolutionary relationship between organisms is their phylogeny.
Cladistics is the method
of classification based
on establishing
phylogenies (i.e. getting
at evolutionary
relationships.
Cladistics proceeds by
comparing shared
ancestral and shared
derived characters
between sets of
organisms.
Cladistics
A phylogeny
(cladogram) for
vertebrates.
each node
indicates a
common
ancestor
The greater the number of derived characters shared by a pair of organisms, the
closer their degree of relationship.
The closer the degree of relationship, the closer the most recent common
ancestor.
It’s Critical (and often difficult) To Distinguish Homology
from Analogy
Homologous structures, like the bat wing and gorilla arm, are similar because
they are derived by modification of a shared ancestral structure.
Homology is the key to establishing phylogenies.
Distinguishing Homology from Analogy
Analogy is similarity due to convergent evolution.
Analogy mistaken for homology confuses phylogenies.
Another Set of Analogies Created by Convergent Evolution
Ocotillo of the
US southwest
Allauidia of
Madagascar
Results of Cladistic Analyses Sometimes Run Counter to Classical
Classification Schemes
Which pair is more closely related? A lizard/crocodile or bird/crocodile?
Cladistic analysis indicates that the bird/crocodile pair is more closely related.