Speciation

Diversity

Question

It is estimated that of all species that have ever lived on earth are now extinct.

A. 20 %

B. 45 %

C. 80 %

D. 90 %

E. 99 %

Mass Extinctions Are a Fact of Life

It is estimated that 99% of all species that have ever lived on earth are now extinct.

Did the creation end in the Garden?

• Before Charles Darwin, many naturalist believed the creation of new species ended in the Garden of Eden.

• By the 18th century scientist began to understand that extinctions were a common phenomena.

• Was the Diversity of life winding down?

Charles DarwinThere is grandeur in this view of life, with its several powers, having been originally breathed by the creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.

From the Origin of Species by Charles Darwin

Darwin’s Tree of Life

• The ascent of life through time.

• Each new branch or species arise from the modification of an ancestral species.

• The process by which new species arise is called Speciation.

What is a species?

Its not as straightforward a question as most believe.

Evolution creates (and destroys) new species, but …

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.

Are these the Same Species?

What is a Species?

• It is the smallest evolutionarily independent unit– Different species follow different evolutionary trajectories.– The essence of speciation is lack of gene flow

It is difficult to determine when populations are actually evolving independently.

species form a boundary for the spread of alleles

Proposed species concepts

• There are three different species concepts currently in use.

1. The Biological species concept

2. The Phylogenetic species concept

3. Morphospecies concept

The Biological Species Concept (BSC)

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

Ernst Mayr 1942

• Definition implies:– A population is reproductively isolated if it possesses genetic

differences that prevent it from inbreeding with other biological species.

– There is no hybridization or hybrids fail to form fertile offspring. – Lack of gene flow ensures independent evolutionary trajectories

between populations.

The BSC does not include geographical isolation.

Two populations that cannot interbreed because they are separated by some physical barrier such as a river or a mountain range are not defined as separate species. Why?

This makes sense, because geographic isolation may be temporary (rivers change their course mountains erode), whereas reproductive isolation caused by genetic differences is usually irreversible.

Problems associated with the BCS

1. Many plantsplants hybridize freelyhybridize freely

2. Cannot test fossil formsfossil forms

3. Irrelevant to asexual populationsasexual populations

The Phylogenetic Species Concept (PSC)

Species are identified by estimating the phylogeny of closely related populations and finding the smallest monophyletic

groups.

PSC (cont)• Taxa labeled A-G represent

populations. • They re the smallest

monophletic groups on the tree and represent distinct species.

• If populations cannot be clearly distinguished in a phylogeny by unique, derived characters, then they will form clusters like the populations designated B, E, and G.

• Clusters B, E and G would be considered part of the same species

Explain the Figure below

In this example, salamander lineages A and B are separate species. Each has a common ancestor that individuals of other species do not. Even though it has diversified a lot, Lineage C is a single species, according to the phylogenetic species concept. None of the subspecies of Lineage C has a single common ancestor separate from the other subspecies.

PSC Rational • Species are formed when a single interbreeding

population splits into two lineages that do not exchange genetic material. – Form Sister species (Lineages that diverged from

the same ancestral node on a phylogenetic tree)– Identified from diagnostic traits called

synapomorphies (shared derived characteristics)

• Species are identified on the basis of statistically significant differences in the traits used to estimate the phylogeny (ancestry)

PROBLEMS with PSC(cont)• 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

Morphospecies• Define species based on the morphological

differences. Commonly used with fossils.

• 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

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

Application of the 3 species definitions to red wolf

• Red wolf appeared in 1930 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.

How do you miss a whole Elephant Species?

• Up until recently, scientists believed there were two species of elephant: the African elephant and the Asian elephant.

• Geneticists conducting a comprehensive DNA sampling of elephants from across Africa recently found that there are in fact two species of African elephants.

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SpeciationSpeciation

Speciation

• Classically, speciation has been hypothesized to be a three-stage process– 1 Population becomes reproductively isolated.– 2 divergence in traits– 3 Development of isolating mechanisms

(Secondary contact)

Allopatric Speciation

• Allopatric (Allo= different, Patric=homeland)

• Population are separated due to geographical barriers

Geographic isolation

1. By dispersal and colonization

Dispersal to novel environment such as rafting a portion of a population to an island

2. By Vicariance events

Events that physically divide a population into two or more isolated ranges like rising of a mountain range, long term drying trend, or lava flow.

Example of Dispersal and Colonization

• Question Why do each of the islands of Hawaii have their own drosophilid species?

• Hypothesis: It is due to the Founder effect. – small group of individuals cut off from the

original population colonizes a new habitat – Drift, mutations and selectionDrift, mutations and selection leads to

divergence

Predictions

• Closely related species should be found on adjacent islands

• The phylogenetic branching sequence should follow island formation

• Figure 15.7b

The analysis of mitochondrial DNA showed that closely related species were found in the predicted pattern .

Vicariance events

• 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 or rapid like a lava flow.

Case study of Mexican Cavefish• In Mexico, researchers

identify a river running through rocks to a cave system below.

• In the cave they find fish that are like the population on the outside, however with some important differences

Case study continued• The eyes of the cavefish

have become vestigial (no longer functional)

• The fish have also lost their pigmentation.

• The two populations of fish are no longer able to mate

• Speciation has occurred.

Example from isthmus of Panama

• 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

SECOND STAGE OF SPECIATION

Genetic Divergence

Mechanisms of Genetic Divergence.

• If the environments are different between isolated populations then natural selection, mutations, and genetic drift will work independently on each group. This will speed up genetic divergence and speciation.

Genetic Drift

• This sampling process leads to the random fixation of alleles with the concurrent loss of other alleles

• Effect most predominant in small populations and founder effect populations. Especially with bottlenecking

• Because peripheral populations in colonization events are almost always small, genetic drift is hypothesized to be the key to the second stage of speciation

The role of genetic drift is controversial

• Hundreds of small populations have been introduced to new habitats around the world in the last 150 years, but few dramatic changes in genotypes have occurred.

• Now a more balanced approach is taken and the role of natural selection is also brought into the equation.

Natural selection

If one of the split populations occupies a novelnovel environment, Natural selection can quickly lead to divergence

Secondary Contact

Reinforcement • 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

• Selection that reduces the frequency of hybrids is called reinforcement

Reinforcement finalizes the speciation process by completing reproductive isolation

• The final stage of speciation, that of establishing reproductive isolation occurs as Isolating mechanisms develop between groups.

• These are mechanisms that prevent interbreeding between two groups and assure continued genetic divergence.

Isolating mechanisms

• Mechanisms that lead to, or cause genetic divergence by preventing interbreeding between two groups.– As long as two groups do not interbreed their

gene pools will continue to drift further and further apart.

– The longer two species are genetically isolated from each other the more different they become from each other.

Isolating mechanisms

• Isolating mechanisms can be divide into two different types. – Isolating mechanisms that occur before

fertilization are called Prezygotic mechanisms

– Isolating mechanisms that occur after fertilization are called postzygotic mechanisms

Types of Prezygotic Isolating mechanisms

• Behavioral isolation: Potential mates meet but cannot figure out what to do about it because patterns of courtship may be altered to the extent that sexual union is not achieved

• Temporal isolation: (Time) Different groups overlap in range but may not be reproductively mature in the same season.

• Mechanical isolation: Potential mates attempt engagement but sperm cannot be successfully transferred . This may be due to differences in reproductive organs.

Types of Prezygotic Isolating mechanisms cont.

• Gametic isolation: Sperm is transferred but sperm and egg are incompatible.

• Ecological isolation: potential mates never meet because they live in different habitats

Types of Postzygotic isolating mechanisms

• Zygotic mortality: Egg is fertilized but zygote does not develop properly dies before birth because parents are genetically incompatible.

• Hybrid inviability: Hybrid very weak and can’t live outside the uterus.

• Hybrid offspring: Hybrid is sterile.

Sympatric Speciation

A new species can arise in a single generation if a genetic change produces a reproductive barrier

between mutants and the parents.

Sympatric speciation

• Sympatric (Sym = same, Patric=homeland) speciation occurs in the same geographical region without physical isolation.

• A new species can arise in a single generation if a genetic change produces a reproductive barrier between mutants and the parent population.

• Example: Accidents during cell division that result in extra sets of chromosomes (Polyploidy). Self-fertilization can give rise to new individuals that are unable to mate and form fertile off-spring with the parent species.

Sympatric Speciation

• Accidents during cell division may results in extra sets of chromosomes in the progeny, a condition called polyploidy.

• The differences in chromosome number act as a reproductive barrier between parent and offspring.

• This is type of speciation is quite common in plants that are able to self-fertilize.

Sympatric Speciation

• Botanist estimate that up to 50% of all plant species are polyploids.

The evolution of Wheat

Parapatric Speciation

Parapatric Speciation

• Parapatric (Para = near, Patric=homeland) neighboring populations become distinct species while maintaining contact through Hybrid zones

Parapatric

speciation

Hybridization

Creation of new species through hybridization

There are 3 possible outcomes when hybridization occurs

1. Reinforcement of parental forms as two recently diverged species

2. Formation of a new third species

3. Formation of secondarysecondary hybrid zones

New Species through Hybridization

• Three species of Sunflower– Helianthus annuus, H. petiolaris and H. anomalous.

• Rieseberg crossed individuals of H. annuus and H. petiolaris to produce three hybrid populations.– The three experimental hybrids were genetically almost

identical to the natural H. anomalous.

• Only certain alleles work in combination and that other hybrids are inviable and have reduced fitness.

Speciation through Hybridization

• Hybrids can mate with siblings and backcross to their parents the result is a variety of hybrid gene variety of hybrid gene combinationscombinations.

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

• Secondary contact and gene flow between recently diverged species can result in the formation of a new third species.

Size, shape and longevity of hybrid zones are determined by 3 possible outcomes of hybridization

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.

Width of the zone is determined by

a) distance of dispersal in each generation

b) How long zone has existed

Factors (cont)2.2. When hybrids are When hybrids are less fit than purebred individualsless fit than purebred individuals

(parents)… FateFate of the hybrid zone depends on the strength of depends on the strength of selectionselection 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 wider and longer lived longer lived

A balance develops between formation of the hybrids and the selection pressure against the hybrids. (similar to the banded and non-banded snakes we used as an example of selection- migration balance)

Factors (cont)

3.3. When When Hybrids are more fit than purebredsHybrids are more fit than purebreds Depends on the extent of the environmentDepends on the extent of the environment 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 at the boundariesadvantage is at the boundaries between the two parental species then will form a form a stable hybrid zone stable hybrid zone Often found in areas called ecotones where markedly different plants and animals meet

Example from Big sagebrush in Utah

• 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

Hybridization Issues

• 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

Genetic differentiation and isolation • How much genetic variation is necessary to produce a

new species? • The BSC requires that no hybridization whatsoever

occur, so it predicts that a virtual “ genetic revolution” or radical reorganization of the genome would be required in order for hybrids to survive.

• Large scale changes in the genome are unlikely • They have also been shown to be unnecessary.• 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

Do we see speciation occurring today?

Sockeye Salmon Sea run vs. lake dwelling

Skipper butterflies Different host plant

Blueberry and apple maggot flies

Different host plant

Warbler finches Habitat selection (high vs low elevation)

Heliconius butterflies Different habitats

Lake White fish Dwarf versus normal sized individuals

THE END