Species Concepts

This idea goes all the way back to Darwin where he used visible “gaps” in morphology to delimit species.

“I believe that species come to be tolerably well-defined objects, and do not at any one period present an inextricable chaos of varying and intermediate links.”(1859: pg 177)

“We shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations made for convenience. This may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species…”(1859: pg 282)

Based on judgments about the similarities among organisms

challenge is to make it mechanistic and testable want to accurately reflect evolutionary history of

organisms. we don’t really know whether such a thing as a

species actually exists in reality

All three assumeAll three assume two things in common 1) no gene flow-no gene flow- species form a boundary for the spread of alleles 2) species have their own evolutionary own evolutionary pathwaypathway

Proposed by Dobzhansky and Mayr, elucidated by Mayr as …

“ Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”

Definition implies:no hybridization or hybrids fail to form fertile offspring lack of gene flow

PROBLEMS:

1.1. Can not always tellCan not always tell if two groups of individuals are reproductively isolatedreproductively isolated

If two groups are separated by geographical barriersthere is no way to know if they are reproductively isolated

2. Many plantsplants hybridize freelyhybridize freely; we will discuss hybridization later in some detail

3. Cannot test it in fossil formsfossil forms 4. Irrelevant to asexual populationsasexual populations

Also called Evolutionary Species Concept This is the systematists contribution to the idea

of a species“A species is a single lineage of ancestral

descendant populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate”

Focuses on the idea of monophly (a set of species are all descended from one common ancestor)

A monophyletic group contains all of the known descendants of a single common ancestor. There are no parallel branches or interconnecting branches (such as in hybridization)

Fig 16.1 species are on the tips of the trees. Circles represent the monophyletic groups

Notice that …(1) Common ancestor does not continue on as a species and (2) every “species” divides to form only two new “sister species”

You do not see branching such as this.

or this

RATIONALE can only form separate species if the

populations have diverged from one another in isolation

The original species will always form two new species and cease to exist itself.

Appeal is that it is testable Species are identified (named) on the

basis of statistically significant differences in the traits used to estimate the phylogeny (ancestry)

Populations must have been independent long enough for diagnostic traits to emerge

Phylogenies are only available for a handful of groups

Very tiny differences, even a single DNA substitution may be used as a trait that separates groups

Could end up doubling the number of species

Very difficult to interpret when new species actually becomes a new species

Define species based on the morphological differences. Commonly used with fossils.

This definition does not demand proof of reproductive isolation or phylogenetic relationships

Used when we do not have tests for reproductive isolation or well-estimated phylogenies

Assignment to species is often arbitrary and cannot distinguish cryptic species

• ones which are strongly divergent based on non-morphological characters.

• Things such as song, temperature or drought tolerance, habitat use, or courtship displays

Today used mostly by paleontologistsFor at least some instances there is good

evidence that fossil Morphospecies may indicate real species differences

In 1930 the Red wolf appeared to be a Morphospecies being intermediate in appearance between the gray wolf and the coyote, all 3 appearing to be distinct.

Studies have shown that the red wolf is actually a hybrid between gray wolves and coyotes. Therefore its intermediate characteristics are the result of hybridization and not independent evolution. This makes the Red wolf not a distinct species for most biologists because…

Neither the BSC or the PSC allow for hybridization However, it is still considered a separate species

and the morphospecies is the only one of these 3 definitions that works.

Allopatric model- speciation occurs in populations that have been physically isolated from one another

Sympatric model Sympatric model - Populations can diverge without geographical separation, with low to moderate gene flow between them if…

1. Selection for divergence is strong 2. Mate choice is correlated with the

factor that is promoting divergence Parapatric model– Strong selection for

divergence causes the gene frequencies to diverge along a gradient

Peripatric model Peripatric model a subset of the allopatric model involving colonization

ParapatricParapatric

speciationspeciation

Involves 3 stepsInvolves 3 steps1. Isolation of members of a population from

one another2. Genetic divergence of the separated

populations3. Renewed sympatry of the populations with

reinforcement of the genetic differences which have arisen

Physical Isolation

Necessary to prevent gene flow which would keep populations homogenized

may occur when small populations become isolated at the periphery of a species’ range.

If selection is strong and gene flow is low divergence could then occur rather rapidly

1. By dispersal and colonization Dispersal to novel environment such as rafting a portion of a population to an island

2. By Vicariance events

Involves Founder effect = Peripatric Speciation Peripatric Speciation (Mayr)

small group of individuals cut off from the original population colonizes a new habitat

drift and selectiondrift and selection on genes involved in mating and habitat usehabitat use leads to divergence

closely related species should be found on adjacent islands

some of the phylogenetic branching sequence should follow island formation

using mitochondrial DNA it was shown that four closely related species were found in the expected pattern

Figure 16.7 page 613

Figure 15.7 page 593

Events which split a species into two or more isolated ranges and prevents gene flow between them (or at least greatly reduces it)

Can be slow processes like rising of a mountain range, long term drying trend etc

or rapid like a lava flow that splits non-flying insect populations

A land bridge opened as the isthmus closed about 3 million years ago

Found 7 pairs of closely related morphospecies of snapping shrimp. One member of each pair on each side of the land bridge

The pairs from either side of the bridge are shown to be sister species (each other’s closest relative) believed to share the same common ancestors which split to form each pair

5

Also, interestingly, shrimp populations would have been isolated in a staggered fashion as the land bridge gradually formed in stages

Species 6 and 7 live in the deepest water and were cut off first 1-5 were in shallower water and diverged later

Figure 16.8 pg. 614

Genetic Divergence

Vicariance events and dispersal events only provide conditions for speciation

Usually you also need to have genetic driftgenetic drift and/or selection selection work on mutations in these isolated populations in order to get genetic divergence.

Sexual selectionSexual selection may also lead to genetic divergence

Secondary Contact (return to sympatry)

After return to sympatry

Possible Outcome #1 after secondary contact

Fully fertile hybrids form – no speciation has actually occurred while in allopatrywhile in allopatry.

Hybrids thrive and interbreed with both parental populations, any divergence is divergence is erasederased.

Possible Outcome #2 after secondary contact

ReinforcementReinforcement of parental forms as two recently diverged species.

The two groups are considered now to be two speciestwo species.

If populations have sufficiently diverged while in allopatry, their hybrid offspring should have markedly reduced fitness when compared to individuals in both parental populations.

Parental populations will reduce their fitness if they produce hybrid offspring, therefore this should favor assortative mating within each new species.

Selection that reduces the frequency of hybrids is called reinforcement

The final stage of speciation, that of establishing reproductive isolation by reinforcement can occur in any number of ways.

These are called pre-zygotic isolation mechanisms

Temporal isolation Temporal isolation – individuals of different species do not mate because they are active at different times of day or seasons of the year

Ecological isolation- Ecological isolation- Individuals mate in their preferred habitat, and therefore do not meet individuals of other species.

Behavioral isolation- Behavioral isolation- potential mates from incipient species meet but choose members of their own species

Mechanical isolation Mechanical isolation – copulation is attempted but transfer of sperm does not take place

Gametic incompatibility Gametic incompatibility – sperm transfer takes place but egg is not fertilized

Plot of genetic similarity versus the degree of interbreeding for various sister species of Drosophila. A value of 0 on the Y axis indicates free interbreeding, 1 indicates no interbreeding.

Figure 16.12 pg 625

Post-zygotic mechanisms may lead to hybrid offspring which are sterile or infertile

Zygotic mortality- Zygotic mortality- egg is fertilized but zygote does not develop

Hybrid inviability– Hybrid embryo forms but of reduced viability

Hybrid sterility – hybrid is viable but adult is sterile

Hybrid breakdown – F1 hybrids are viable and fertile but F2 and backcrosses to parents are inviable or sterile

Chapter 16

Possible Outcome #3 after secondary contact

Creation of a new speciesnew species through hybridization.

Formation of a new third new third speciesspecies from the hybrid formed.

Hybrid is fertilefertile but cannot back cross to either parent.

Hybridization is a common common occurrence in plantsoccurrence in plants

At least in some cases the outcome of these hybridization events determines the outcome of the speciation event

The role of Hybridization

In newly colonized areas or in new habitats, hybrids may have higher fitness than the hybrids may have higher fitness than the parentsparents

These hybrids can mate with siblings and backcross to their parents the result is a variety of hybrid variety of hybrid gene combinationsgene combinations. This is called introgressionintrogression.

If certain of these combinations is best suited to a new habitat, a third species may arisethird species may arise that is somewhere intermediate between the parental species.

However, it is also possible that, if the hybrids have equal or greater fitness than either parental population, complete introgression may occur. The result is one speciesone species somewhat like the one that like the one that existed prior to geographic separationexisted prior to geographic separation

Possible Outcome #4 after secondary contact

1.1. When parents and hybrid are equally fit…When parents and hybrid are equally fit…The zone is wide.

Hybrid traitsHybrid traits are found with highest frequency at the center of the zone. Gene frequency changes are dominated by drift.Width of the zone is determined by

a) distance of dispersal in each generation

b) How long zone has existed

2.2. When hybrids are When hybrids are less fit than less fit than purebred individualspurebred individuals (parents)… FateFate of the hybrid zone depends on depends on the strength of selectionthe strength of selection against them.(a) Strong selectionStrong selection leads to reinforcementreinforcement, with a very narrow and short-lived hybrid zone. (b) If selection is weakselection is weak, the hybrid zone is wider and longer lived wider and longer lived

A balance develops between formation of the hybrids and the selection pressure against the hybrids.

3.3. WhenWhen Hybrids are more fitHybrids are more fit than purebredsthan purebreds

Depends onDepends on the extent of the the extent of the environmentenvironment in which the hybrids are at an advantage advantage New speciesNew species results if hybrids are more fitif hybrids are more fit in areas outside the range of the parental outside the range of the parental speciesspecies If the advantage is advantage is at the boundariesat the boundaries between the two parental species then will form a stable hybrid zoneform a stable hybrid zone (Parapatric speciation)Often found in areas called ecotones where markedly different plants and animals meet

Between the basin and mountain subspecies Hybrids shown to be more fit than parents in

transitional zones Showed that the hybrid zone is maintained

because the hybrids have superior fitness in the transitional zone

Potential practical problems Some crop species are closely related to

weed species Herbicide resistance genetically engineered

into a crop plant could be transferred to weed plants through hybridization

How much genetic variation is necessary to produce a new species?

The BSC requires that no hybridization whatsoever occur, however.....

Fertile hybrid offspring can be found even when the parental populations are markedly different from one another.

Current research focuses on the number, location or nature of the genes which distinguish closely related species in an attempt to uncover past speciation through hybridization

Requires development of reproductive isolation while individuals are still in contact and gene flow is still possible

1. Polyploidy2. Genetic divergence (drift, natural selection

etc.)3. Sexual selection

Polyploidy, or the condition of having extra sets of chromosomes can lead to genetic isolation of populations.

Polyploids have 3n, 4n, 5n, 6n etc. numbers of chromosomes rather than the normal diploid number.

This condition is detected by looking for two factors

1. In plants at least, chromosome numbers greater than n=14 are considered to be of polyploid origin

2. Related individuals will have chromosome numbers which are multiples of some basic number, for example, in Chrysanthemum different species have 2n numbers of 18,36,54,72, and 90

Autopolyploids

Allopolyploids

The union of unreduced gametes from genetically and chromosomally compatible individuals, that may be thought of as being from the same species.

During meiosis In the autoploids the chromosomes pair up into quadrivalents and mostly get uneven segregation of chromosomes. Called aneuploid gametes (have either too many or too few chromosomes).

This leads to reduced fertility or sterility

Unreduced gametes

Polyploids are derived from a hybrid between unreduced gametes of two different diploid species.

Genetic or chromosomal incompatibility arises.

In meiosis, there is a natural formation of homologous pairs with twice as many pairs as either parent.

Get balanced segregation and normal gametes

Typically these hybrid polyploids have near normal fertility.

Unreduced Unreduced gametesgametes

Two different Two different speciesspecies

Because cannot back cross to either parent the resulting gametes are sterile

Limits the individuals they can cross with to other allopolyploids

If species cannot self fertilize then...May have difficulty finding each other and

cannot out-compete the original parent populations. But…

If they dodo interbreed with each other, or are self-compatible, they are chromosomally isolated and can begin to diverge from parent populations immediately.

Sexual selection promotes divergence efficiently because it affects gene flow directly

There is an example in the book of Drosophila flies that may have diverged due to sexual selection

In the Beak of the Finch we read about cases of sexual selection which can help prevent hybridization in the finches.

Recent studies are suggesting that sexual selection is an important and necessary factor for sympatric speciation