Speciation

Species Definition

• Biological species: a reproductively isolated group in which all individuals potentially or actually interbreed amongst themselves but are genetically isolated from other such groups.

• Morphological species: groups of individuals showing similar phenotypes.

• Paleospecies: a chronological series of similar forms.

• Agamospecies: groups based on the amount of variation or gaps in the variation of phenotypic features.

FIGURE 9.1 Defining Species (a) Morphospecies. Viewed today, at one moment in time, species A, C, and E are clearly

distinct, demarcated by current natural discontinuities between them. (b) Paleospecies (chronospecies). Viewed historically, through time, discovered fossil intermediates (B and D) fill in the missing gaps above, giving us a more or less continuous series with no obvious discontinuities between them.

The Process of Species Formation

Four Steps:

• Single population or reproductive community.

• Development of a reproductive barrier (formation of allopatric populations).

• Differentiation of the separated populations.

• New species can no longer interbreed if barrier disappears and they become sympatric.

Four steps leading to speciation.

First, a single species is an interbreeding reproductive community.

Second, a barrier develops, dividing the species.

Third, separated into different habitats, the divided populations become differentiated through the accumulation of differences.

Fourth, so different have the separate populations become, that is when the barrier disappears and they overlap again. Interbreeding does not occur.

Allopatric speciation

The populations of Tamarin monkeys are separated on the sides of the Amazon River. Where the river tributary is wide and individuals on opposite banks do not interbreed, the populations are diverging toward separate species. Where the river tributary is narrow, the individuals still interbreed.

Allopatric speciation involves the geographic separation of the parent population into two subpopulations. It can occur if a a physical barrier separates the range of a population.

Allopatric speciation can also occur following a founder event.

Such an event may have led to the development of the thirteen species of Darwin’s finches.

Geographic isolation is usually a prerequisite to speciation. However, there are situations in which speciation can take place between populations that are sympatric. Perhaps the most common example of this is the result of polyploidy in plants.

Parapatric speciation occurs between contiguous populations. The parent population does not completely split.

Two species of crows, Corvus corone and Corvus cornix, meet along a line in central Europe. Along that line they produce hybrids. Speciation is incomplete.

The critical evidence in the case of parapatric speciation involves hybrid zones. Hybrid zones are defined as an area of contact between two different forms at which hybridization takes place.

Hybrid zones are an example of a stepped cline.

How do such clines relate to speciation?

If hybrids are disadvantageous, natural selection will work to to reinforce the reproductive isolation of the two forms. So, the process of parapatric speciation calls for:

1. The evolution of a cline.

2. Hybrids selected against in the hybrid zone.

3. Two different forms selected to mate only with others of their own type.

If hybrids are disadvantageous, natural selection will work to to reinforce the reproductive isolation of the two forms. So, the process of parapatric speciation calls for:

1. The evolution of a cline.2. Hybrids selected against in the hybrid zone.3. Two different forms selected to mate only with others of their own type.

Sympatric speciation occurs without any separation of the ancestral geographic range. Some evolutionary biologists doubt this ever happens. In all likelihood, it can only occur after the establishment of a stable polymorphism.

The green lacewings, Chrysoperla carnea and Chrysoperla downesi may illustrate an example of sympatric speciation.

C. Carnea is light green in spring and early summer, changing to brown in the fall. C. downesi is a darker green year round. Their color is adapted to their habitat. C. carnea lives in fields and meadows in the summer and moves to deciduous trees in the fall. C. downesi lives mainly on conifers. Their ranges are sympatric. The species are separated by breeding season as well as habitat. C. carnea breeds in winter and again in summer. C. downesi breeds only in the spring.

It is thought that speciation in the lacewings began with a multiple niche polymorphism. One morph was adapted to live on conifers and the other in a deciduous habitat. Once the polymorphism was established, selected would favor assortative mating, because crosses between the two morphs would not be well adapted to either habitat. Divergence in breeding season could have been the mechanism by which assortative mating was brought about.

Reproductive Isolation

Different mechanisms prevent reproduction between individuals of different species. These may occur premating or postmating, as illustrated here with two species of salamander.

How does reproductive isolation evolve?

Premating or prezygotic mechanisms:

a) Ecological or habitat isolation

b) Seasonal or temporal isolation

c) Sexual or ethological isolation

d) Mechanical isolation

e) Isolation by different pollinators

f) Gametic isolation

Postmating or zygotic mechanisms:

a) Hybrid inviability

b) Hybrid sterility

c) Hybrid breakdown

Patterns of Speciation

Cline – a gradient or gradual change in a character within a species over a geographic area.

Represent an early step in the evolutionary process.

Yarrow

Tadpoles of the leopard frog show clinal variation in their ability to survive in waters of varying temperatures.

This reflects adaptation to different climates in populations found at different latitudes.

In 1949, Moore fertilized leopard frog eggs from females in the north with sperm from males progressively farther to the south.

The degree of embryo or tadpole abnormalities was scored, from A (normal young) through progressively more abnormalities to F (high death rate).

Today, this study and others prompt biologists to actually divide leopard frogs into subspecies or even different species.

The four groups of leopard frogs resemble one another closely in their external appearance. But early tests of interbreeding produced defective embryos in some combinations, leading biologists to suspect that these might be different subspecies or even different species.

Research on males’ mating calls indicates that the various groups differ substantially, and that such prezygotic behavior separates and reproductively isolates members of each group, producing four species: (1) Rana pipiens; (2) Rana blairi; (3) Rana utricularia; (4) Rana berlandieri.

Ecotype

A local population genetically adapted to a local environment.

Within a species, ecotypes do not typically overlap geographically.

An ecotype that may be visually detected may be considered a subspecies.

A cline is a geographic series of ecotypes.

Yarrow – Achillea sp.

A transect, cross section of the yarrow’s distribution from sea shore to high mountains is shown across California. Note the clinal variation in height.

Three garden plots were selected at three different locations along the transect- sea level (Stanford), 4,600 feet (Mather), and at 10,000 feet (Timberline).

Yarrow seeds collected from five locations along this transect were planted, grown into young plants, and then divided into equivalent tufts, clones, planted at the three garden sites-Stanford, Mather, Timberline.

Note especially that sea-level clones (from San Gregorio) at high elevations did poorly (died), and high-elevation clones (from Big Horn Lake) at low elevations still did not grow to large heights.

Clearly, ecotypes have developed within yarrow.

Ring Species - Herring Gulls

As glaciers retreated, herring gulls (Larus argentatus) were released out of a north Pacific refugia spreading one way across North America and into western Europe; and spreading in the other direction across Alaska into Siberia.

From Siberia, as the herring gull now extended its range further across Asia, it tended to differentiate, producing a subspecies (or species by some ornithologists) such as the vega gull (Larus vegae) and farther west the lesser blackbacked gull (Larus fuscus).

Eventually its current circumpolar distribution became established (dashed lines). Adjacent subspecies interbreed (solid arrows), but where the ends of the circular range of the herring gull meet and overlap in Europe, there is very little interbreeding.

Ring species—salamanders

The ensatina salamander (Ensatina eschscholtzii) occurs from Canada to Southern California with interbreeding between adjacent populations through this range.

The Central Valley—a dry, hot lowland area—is divided into a coastal arm and inland arm. However, where these two arms of the species meet again in Southern California, interbreeding does not occur.

Parallelism and Convergence

Just as different habitats can produce different selective pressures and result in ecotypes, similar habitats can produce similar selective pressures and lead to organisms with similar characters.

Parallelism – the situation in which closely related organisms, in similar habitats, take on similar traits.

Convergence – the situation in which distantly related organisms take on similar traits as a result of living in similar habitats.

Australian marsupials resemble placental mammals in the rest of the world.

Within the relative isolation of Australia, the marsupials entered similar habitats as counterparts among the placentals elsewhere.

Under similar selective pressures, similar features and ecological lifestyles evolved, but upon a marsupial theme.

Convergent Evolution-Cacti

Different families of desert plants have evolved similar adaptations to the desert’s dry, hot conditions - namely, succulent shoots with spines. Two such plant species are shown from Africa. The third, from the New World, is the endemic member of the true cactus family (Cactaceae).

Latitudinal Gradients

Within taxonomic groups, species are not distributed evenly across the planet. There are distinct gradients for many groups, with the number of species decreasing as one moves away from the equator.

A number of explanations have been suggested.

Latitudinal Gradient for Mammal Species

Suggested Explanations for Latitudinal Gradients

Evolutionary Time

Climate Stability

Productivity

Ecological Effects

An interactive exercise

“Every population of a species differs from all others. The degree of difference between populations of a species ranges from almost complete identity to distinctness almost of species level”

Ernst Mayr