This document discusses various concepts of what constitutes a species and mechanisms of speciation. It describes how species are defined under different species concepts, such as the biological species concept, morphological species concept, and phylogenetic species concept. The document also outlines various mechanisms that can lead to the formation of new species, including geographic isolation, changes in chromosome number, assortative mating, adaptation to different habitats, and sensory drive. Larger geographic ranges and habitat heterogeneity are associated with higher rates of diversification and speciation.

1. Evolution 17 & 18:
Species Concepts
&
Mechanisms of Speciation

3. Species Diversity
• The number of living species may be as
great as 100 million!
• Speciation may have occurred billions of
times in the last 3.8 billion years
• All cultures recognize different forms
(species) and name them
• The challenge to scientists is to name and
classify species in a testable way

4. Systematics is the
evolutionary biology
subfield that deals with
taxonomy,
understanding
speciation, describing
species, inferring
phylogenies, and
similar endeavors

5. So what is a Species???
• Smallest independently evolving unit
• Independent evolution occurs when mutation, selection, gene
flow, and drift operate independently in different populations
• Therefore, these species will follow independent evolutionary
trajectories
• The essence of speciation is lack of gene flow
• All modern species “concepts” really fall under a lineage
concept
• The different species “concepts” are really criterion for
determining when you have lineage formation
• Defining species is extraordinarily contentious
• Potentially 20+ different species concepts

6. What is a Species???
https://www.youtube.com/watch?v=9fOfFlMe6ek
Another video if you need it:
https://www.youtube.com/watch?v=dnfaiJJnzdE

7. Remember:
A species is a hypothesis!!!!!!!!!
Species will have a set of characteristics and
certain kinds of characteristics are what are used to
define what is a species under each concept---e.g.,
if reproductive isolation is the requirement and
then we find out that these two species breed
freely, we would reassess our hypothesis
What is and isn’t a species is different under strict
concepts…

8. Biological Species Concept
(BSC)
(as proposed by E. Mayr)
• "Species are groups of actually or potentially
interbreeding natural populations that are
reproductively isolated from other such
groups."
• The Biological Species Concept is the first
modern attempt to define species, but
numerous problems with this concept, on both
theoretical and practical grounds

9. Problems with Biological Species
• Practical problems
 asexual species?
 allopatric species? (most borderline cases don't occur
sympatrically)
 geological time?
 level of reproductive isolation? (hybrids occur naturally!)
• Theoretical problems
 reproductive compatibility is a primitive character;
reproductive isolation is derived
 many cases, sister lineages are reproductively isolated, but
distantly related lineages are not
• The BSC doesn't apply to allopatric species, parthenogenetic species, or
fossil species; highly limited (long held by ornithologists)

10. Problems with Biological Species

11. Morphological Species Concept
Based on morphological differences between individuals

12. Morphological Species Concept
Problems (hint, each image shows a single species!)
Morphological Polymorphism

13. Ecological Species Concept
Based on ecological (habitat use, diets, overall niche)
differences between individuals

14. Ecological Species Concept
Based on ecological (habitat use, diets, overall niche)
differences between individuals

15. Evolutionary Species Concept (ESC)
• Closer to defining species as individuals and as lineages which
have beginnings and ends
• Evolutionary species concept (as proposed by Simpson, modified
by Wiley)
"A single lineage of ancestor-descendent populations which
maintains its identity from other such lineages and which has
its own evolutionary tendencies and historical fate"
• Lineages may separate, become species, and come back together
in the future (at which time they are no longer different species).
This a very mutable concept.

16. More ideas about the
Evolutionary Species Concepts
• All organisms, past and present, belong to some evolutionary
species
• Species must be isolated from each other to the extent that this is
required to maintain their separate identities, tendencies, and fate.
• Therefore, reproductive isolation in the BSC sense is not required
• Moreover, Evolutionary species may or may not exhibit
recognizable phenetic differences (# of species may be over or
under-estimated, usually the latter)
• No separate, single evolutionary lineage may be subdivided into a
series of ancestral and descendent species

17. ESC: Benefits and Problems
• Benefits: clear conceptually; applies to asexual
species, through time, and allopatric species
• Problems: application (difficult to know the
future), asexual species (too many independent
lineages),
• The Evolutionary Species Concept separates the
idea of what species are from how they are
recognized. This leads us to: How are species
recognized?

18. Phylogenetic Species Concept
• The practical approach of dealing with the more philosophically
grounded ESC
• Phylogenetic species concept uses a two-step process to define
species
• 1. Grouping taxa (regardless of rank) using the criterion of
monophyly, or at least potential monophyly…build a tree?
• 2. Ranking groups by the criterion of finding the smallest
diagnosable monophyletic groups
• A problem is that almost any population can be diagnosed
• Also, when species initially split, most characters will not show
reciprocal monophyly. That is, the gene trees may not follow the
species trees!!

19. Examining a
gene of four
individuals
Sp.1
Sp.2
Ind.1
Ind.2
Ind.3
Ind.4
Ind.5
Initial Barrier to gene flow

20. Sp.1
Sp.2
Ind.1
Ind.2
Ind.3
Ind.4
Ind.5
If we looked at the genes after initial split. It would appear that
members of sp. 1 are more closely related to sp. 2 (ind. 4 related to 3)

21. Sp.1
Sp.2
Ind.1
Ind.2
Ind.3
Ind.4
Ind.5
Therefore, only after a certain period of time through continued isolation
from members of other species and continued genetic contact among
members of the same species, will the species appear reciprocally
monophyletic
This is not a problem
with the concept, but
with our ability to
detect lack of gene
flow

25. Cryptic species in marine phytoplankton
Historically two species id’d under microscope using morphology, but
is this meaningful variation?
Tested under a
variety of concepts
including genetic
data (PSC) and
reproduction
between species
(BSC)
Confirms, plus
additional species
id’d! 8 total

26. Evolutionary significant units (ESU) = important populations for
genetic diversity in a species, important to conservation efforts
Almost extinct species of ray (skate); previously classified as one
species, then two, then one again, new data (genetic) indicates two

27. The “CLASSIC” process of speciation

28. While physical separation is typically the first step in speciation,
anything that causes a reduction in gene flow between two potential
species (populations) can be an isolating factor (e.g., a genetic event
that changes number of chromosomes, different breeding times,
etc.).
Mechanisms of Isolation

29. Gene flow tends to homogenize population frequencies and reduce
the differentiation of populations
Recall the banded and unbanded watersnakes in Lake Erie-
migration homogenizes the island populations
What would happen if lake currents around islands made it
extremely difficult for mainland snakes to swim to islands?
Genetic processes of the island snakes (their evolutionary
trajectory) is now independent of the mainland snakes
This is the beginning of allopatric speciation-physical (geographic)
isolation resulting in speciation
Physical Isolation

30. Allopatric speciation
Allopatry can be due to dispersal (movement of individuals) or
vicariance (an encroaching physical feature)

31. 3 Hawaiian Drosophila
(diverse morphology)
Hawaii=archipelago of
islands that emerged over
time
Phylogeny shows the
dispersal patterns of the
species of Drosophila on
the islands
Geographic isolation via dispersal and
colonization

32. Vicariance can be a fast or slow event (lava spewing through a habitat
or the rise of mountain ranges)
Snapping shrimp are marine and found on the Pacific and Atlantic side
of Central America
Did the rise of the isthmus of Panama cause speciation?
Geographic isolation via vicariance

33. A DNA phylogeny (b) of the shrimp sister
species shows that generally each pair has
an Atlantic (Carribean) and a Pacific
species
The slow rise of the land bride is reflected
in the estimated timing of the species pair
divergences
Geographic isolation via vicariance

34. For some organisms (especially plants) changes in chromosome
number can reproductively isolate species instantly
Chromosome numbers need complement each other for
reproduction to happen
Changes in chromosome number due to mutation during meiosis
can result in a new species without any physical isolation
(sympatric speciation)
These are differences ploidy (diploid organisms are what we
typically think of, an extra set of chromosomes ups the ploidy to a
tetraploid, which can only breed with other tetraploids)
2% to 4% of plant species are due to this! Also seen in some lizards,
frogs, salamanders, etc.
Mutation can cause Isolation

35. Polyploidy

36. Timing isolation-can be
different times for
reproduction (flowering
time in plants, pupation
time in insects)
Japanese winter moths
are genetically isolated
due to the timing of
their reproductive cycle
But only in northern
regions, in south timing
is the same
Temporal Isolation

37. Japanese land snail synestration (the direction
the shell curves in)
Mutation that causes shell to curve in other
direction---each direction can only mate with
those that are also in that direction because
genitals do not line up properly for
reproduction
Anatomical incompatibility
Other examples sympatric

38. Sympatric speciation and other “rare” speciation events
https://www.youtube.com/watch?v=IdcEgJazOHc
But…“True” Sympatric speciation
is still considered rare

39. What causes the phenotypic differences we observe between species
(not just the absence of gene flow, but what we “see”)?
Even in similar species, a close exam can show some morphological
differences between species
Ecological factors (differing habitats) are important, acted on by
natural selection
Genetic drift may also contribute as well (but difficult to document)
Mechanisms of Divergence

40. Biotic factors (competition, predation, prey) and abiotic factors
(climate)
When populations of a single species occupy different habitats with
different selection pressures, a frequent outcome is phenotypic and
genetic divergence, which can result in speciation if gene flow is
limited
Example: Mimulus plants
Adaptation to different habitats

41. Mimulus ecotypes
Moving the coastal, perennial
Mimulus to inland habitat and
vice versa results in plants from
the transplanted region to do not
well (less fit to that environ)
Genetic manipulation of a certain
region related to life-history
phenotypes making the plant
more or less coastal/inland
showed similar results (light
colored lines for each)

42. Mimulus ecotypes (plus Heliconius)
The inland, annual ecotype of Mimulus guttatus
occurs in seasonally dry habitats and flowers early in
the spring, whereas the sympatric coastal, perennial
form is found in wetter areas and is dormant in the
early spring and flowers later. Hybridization between
these ecotypes would produce offspring that are less
fit in either habitat. Traits that confer local adaptation
to these distinct environments are located on an
inversion (shown as a long rectangle) that preserves
these phenotypic combinations81. e |
Heliconius butterflies are a classic example
of Mullerian mimicry. Many species of the
genus Heliconius (for example, Heliconius numata
silvana and Heliconius numata aurora) mimic the
wing patterns of Melinae spp. to avoid predators.
Each of these wing patterns requires a distinct
combination of alleles that influence colour and
shape, and recombinants between these distinct types
are maladapted. The different Heliconius mimics are
closely related and occur sympatrically, yet hybrids
are rarely found in nature. It has been shown that two
phenotypically distinct mimics have an inversion that
harbours at least two colour-pattern loci97.

43. Traits the influence
how species choose
mates, where
individuals choose
partners with traits
most similar to their
own
Strong assortative
mating can lead to
isolation and
speciation
Female crickets
choose specific
songs
Assortative Mating

44. How does this lead to speciation?
Created hybrid families of these crickets
which had wide variation in songs and genetic
makeup from each species
Within each hybrid family, the pulse rate of
the male song matches the female preference
Could not separate male song and female
preference despite how mixed up the genes
had become!
Genetic linkage
Assortative Mating

45. Habitats influence traits that are naturally selected for, these traits
can also be involved in mate choice and cause divergence in mating
preferences as well
Cichlid fish are a well studied group for understanding speciation
Multiple factors can act together

46. Some places have very different looking species (red and blue),
others have more intermediate phenotypes (low water clarity locales)
The water depth and clarity affects how the fish see (LWS gene)
The colors also reflect female preference to mates
Multiple factors can act together

47. Speciation via Sensory Drive
In cichlids, water depth, female preference, the alleles for seeing
red, and water clarity are all acting together
Speciation by sensory drive
Predictions
-higher frequnct of red LWS allele and more red males
found in deeper waters
-female preference for red will be associated with having
the red LWS alleles (females are NOT red though)
-strongest associations between color, water depth, LWS
allele frequencies, female preference will be high clarity water

48. Water
clarity
increases
from left to
right…
Male color
and depth
distribution
increases
As does the
LWS allele
freq.
Females
have
stronger
pref.

49. Hybridization and gene flow
Even separate species may hybridize
Can be during secondary contact (when two sister species come
back into geographic contact after previous divergence)
Very common in plants
What can keep species separate?
Prezygotic isolation-zygotes never form because of no mating
Postzygotic isolation-mating occurs but offspring less fit than
either parental species

50. Possible outcomes of hybridization
A shows speciation
B, C, D show possible
outcomes of hybridization
Does not show something
that can also happen---total
breakdown of species
barriers and merge into a
single species

51. Sympatric and allopatric populations of two
species of Drosophila occur on islands
Females produce more eggs when mating with
allopatric crosses (which would never occur)
compared to sympatric crosses (which need an
isolating mechanism)
If mated to both species, sympatric females lay
more eggs from their own species

52. A new hybrid
species may also
form
The audoboni
species of warbler
has mtDNA from
coronata, but
nuclear genes
from nigrifrons
This species
resulted from
hybridization

53. What drives diversification?
Do rates of speciation change over time? How does
habitat heterogeneity influence speciation rates?
How does introduction to a new/novel environ
influence speciation and result adaptive
radiation?

54. Anolis lizards in Caribbean-arrive on an island and diversify into
different morphotypes. The morphotypes across the islands look
similar, but the lizards on each island are more closely related to each
other than they are to the similar morphotypes on other islands
Speciation occurs for longer on larger islands,
plateaus sooner on small islands

55. What drives diversification? Larger range size, more opportunity to
get into new habitats
Took characteristics from toads with large ranges and small ranges
Created a numerical value summarizing this that would be correlated
with range size expansion abilities
Higher p =
greater
expansion
associated
characters,
allows
toads to
diversify

56. Review this if you need more help!
https://www.youtube.com/watch?v=te--qLowH2s