Working with theBiological Species Concept

• Speciation is a two-part process– 1. Identical populations must diverge– 2. Reproductive isolation must evolve to maintain these

differences

• Speciation occurs much more readily in the absence of gene flow– This much more likely in geographically isolated

populations

• Populations can become geographically isolated for several reasons

Consider how the amount of genetic divergence (change) forms

a continuum:

Microevolution Macroevolution small changes large changes

Microevolution = adaptationMacroevolution = speciation

The Evolution of Species

• Significant changes in the gene pool could lead to the evolution of a new species over time.

• The evolution of new species, a process called speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.

The Evolution of Species

• Geographic isolation occurs whenever a physical barrier divides a population.

• A new species can evolve when a population has been geographically isolated.

Reproductive isolation can result in speciation

• Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring

• A change in chromosome numbers- polyploidy can lead to speciation

Isolating Mechanisms

• Reproductive isolating mechanisms are the barriers that prevent genetic exchange between species

– Prezygotic isolating mechanisms• Prevent the formation of zygotes

– Postzygotic isolating mechanisms• Prevent the proper functioning of zygotes after

they have formed

Prezygotic barriers: Geographic isolation• Species occur in different areas

– physical barrier– allopatric speciation

• “other country”Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel

Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel

Ammospermophilus spp

Prezygotic barriers: Ecological isolation

• Species occur in same region, but occupy different habitats so rarely encounter each other– reproductively isolated

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest

lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest

Prezygotic barriers: Temporal isolation

• Species that breed during different times of day, different seasons, or different years cannot mix gametes– reproductive isolation– sympatric speciation

• “same country”

Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

Prezygotic barriers: Behavioral isolation

• Unique behavioral patterns & rituals isolate species– identifies members of species – attract mates of same species �

• courtship rituals, mating calls• reproductive isolation

Blue footed boobies mate only after a courtship display unique to their species

Blue footed boobies mate only after a courtship display unique to their species

Prezygotic barriers: Mechanical isolation

• Morphological differences can prevent successful mating– reproductive isolation

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

Plants

Mechanical isolation

• For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer– lack of “fit” between sexual organs:

hard to imagine for us… but a big issue for insects with different shaped genitals!

Damsel fly penises

Animals

Prezygotic barriers: Gametic isolation

• Sperm of one species may not be able to fertilize eggs of another species– mechanisms

• biochemical barrier so sperm cannot penetrate egg– receptor recognition: lock & key between egg &

sperm • chemical incompatibility

– sperm cannot survive in female reproductive tract

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Postzygotic barriers:

• Prevent hybrid offspring from developing into a viable, fertile adult– reduced hybrid viability– reduced hybrid fertility– hybrid breakdown

zebroid

Forms of Selection• Selection is a statistical concept

– One cannot predict the fate of any single individual– But it is possible to predict which kind of individual will tend

to become more common in a population

• Three types of natural selection have been identified– Stabilizing selection

• Acts to eliminate both extreme phenotypes

– Disruptive selection• Acts to eliminate intermediate phenotypes

– Directional selection• Acts to eliminate a single extreme phenotype

Three kinds of natural selection

Natural selection acts on variations• Stabilizing selection is a natural selection that

favors average individuals in a population.

Selection for

average size

spiders

Normal variation

Natural selection acts on variations

• Directional selection occurs when natural selection favors one of the extreme

variations of a trait.

Normal

variation

Natural selection acts on variations

• In disruptive selection, individuals with either extreme of a trait’s variation are selected for.

Selection for

light limpets

Normal variation Selection for

dark limpets