What is a species

1
Macroevolution and Speciation
Evolution creates (and destroys) new species, but

What is a species?
Its not as straightforward a question as most
believe.
2
What is a Species?
There is only one extant (existing) human
species.
3
What is a Species?
And these are all members of a single species.
4
Determining What Is and What Isnt a Distinct
Species Can Have Economic Consequences
Northern spotted owl (left) and barred owl
(right).
5
What is a Species?
The definition well 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.
6
One Problem in the Biological Species Concept
For asexually-reproducing organisms, like these
bacteria, what constitutes a species?
7
How Many Species Are There?
We dont 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).
8
Two Patterns of Speciation
9
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.
10
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).
11
Allopatric Speciation
White-tailed antelope squirrel
Harris 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.
12
Intrinsic Reproductive Isolating Mechanisms Are
Always Required for Speciation
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.
13
Many Intrinsic Reproductive Isolating Mechanisms
Drive Speciation
14
Many Intrinsic Reproductive Isolating Mechanisms
Drive Speciation
15
Behavioral Isolation Mechanisms
Courtship rituals, like these, are critical for
mating within a species, but ineffective for
attracting members of other species.
16
Many Intrinsic Reproductive Isolating Mechanisms
Drive Speciation
17
Many Intrinsic Reproductive Isolating Mechanisms
Drive Speciation
18
Hybrid Infertility Was the Impetus for Cloning a
Mule
19
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.
20
Speciation Rates
Speciation also becomes rapid when, as occurred
with Galapagos finches, new niches become
available.
21
Speciation Dynamics - Gradualism or Punctuated
Equilibrium?
Punctuated equilibrium appears to be a more
accurate view of speciation dynamics.
22
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.
23
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.
24
Mass Extinctions Are a Fact of Life
25
The Cretaceous/Tertiary Mass Extinction
26
Are We Now Causing a Mass Extinction?
27
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.
28
Cladistics
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.
29
Its 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.
30
Distinguishing Homology from Analogy
Analogy is similarity due to convergent evolution.
Analogy mistaken for homology confuses
phylogenies.
31
Another Set of Analogies Created by Convergent
Evolution
32
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.