Species Concept Chapter 16

DAILYWORKDAILYWORK Write down your best definition of a “species.” Include any characteristics that scientists might use to determine if two populations of organisms that look similar are the same species or are different.

What is a species? Defining “species” is more difficult that many people imagine. The “Barnyard taxonomy” mentality makes it look easy: a cow is a cow, a horse is a horse.

Natural species In nature, defining species gets much harder. A species may live over a wide geographic range and vary slightly from one region to another, so that the individuals at one end of the range look different from those at the other end of the range.

Can hybridize Do not hybridize

One species or two? Cordillerian FlycatcherPacific Flycatcher Reproductively isolated species

One species or two? Myrtle WarblerAudubon’s Warbler Varieties within the same species

One species or two? Silene douglasii var. oraria Silene douglasii var. douglasii Two subspecies of the same species.

Defining “species” Among animals and for the most part with plants, a species is generally considered to be a group of interbreeding populations whose offspring are fertile and produce offspring similar to the parents. (i.e. “breed true”) Defining “species” among the fungi and the one-celled organisms is much more difficult.

DAILYWORKDAILYWORK If two populations of organisms that look similar are different species, what keeps the two species separate? Think of behaviors, location, other factors that keep the two populations from mixing.

Reproductive isolation While species may be hard to define, we do know that reproductive isolation is how animal and plant species are maintained. Two types of isolation: Premating isolation Postmating isolation

Premating Isolation Premating isolation prevents organisms from interbreeding: Geographic isolation Temporal isolation Behavioral isolation Mechanical incompatibilities

Geographic isolation Populations of the same species which become separated from one another may be subject to different selective pressures that favor slightly different traits. Without interbreeding between the two populations, genetic differences between the two groups are maintained.

Geographic Isolation Kaibab SquirrelAbert Squirrel Geographically-isolated sub-species.

Geographic Isolation Colonization of islands can lead to strong geographic isolation. Islands far from the mainland have few migration events, and strong isolation.

Ecological Isolation Two populations may live in the same geographical region, but occupy different habitats within the region. Ecological isolation slows, but does not eliminate, gene flow between populations.

Ecological Isolation White-crowned Sparrow: Occupies fields, meadows, forest edges. White-throated sparrow: Lives and feeds within dense thickets. Both are found in the same regions.

Temporal isolation Two species may live in the same habitat, but reproduce at different times. Different breeding seasons reduce gene flow between populations.

Temporal isolation

DAILYWORKDAILYWORK Would isolation have a bigger effect on the genetics of small populations or large populations over several generations? Why?

Behavioral isolation Two populations may share a habitat and breed at similar times, but animals especially may have different courtship rituals. An animal that does not recognize another animal’s courtship ritual will be unlikely to breed with that animal.

Behavioral isolation The dark-phase and light-phase Western Grebes occupy the same habitat and breed at similar times, yet maintain their color forms by differing sets of courtship behaviors (probably learned).

Mechanical isolation Mechanical isolation occurs when individuals from two populations attempt to interbreed, but incompatible body shapes, pollen types, etc. may foil the attempt.

Mechanical isolation Salvia mellifera and Salvia apiana are similar species of sage that occupy the same desert regions. S. mellifera takes pollen off of the backs of bees, while S. apiana takes pollen from the sides of bees.

Mechanical isolation Chihuahuas and Great Danes are the same species, shaped by human selection. However, mechanical isolation prevents them from interbreeding.

DAILYWORKDAILYWORK Suppose no one had seen Chihuauas and Great Danes before. Suppose you are a naturalist who comes across an island where both Chihuahuas and Great Danes live. Would you describe them as the same species or different species? Why?

Postmating Isolation Sometimes two species can breed. Two mechanisms can maintain reproductive isolation: Gametic incompatibility Hybrid inviability

Gametic Incompatibility Even if mating or pollination takes place, sperm may fail to fertilize an egg. Chemical defenses may exist to ward off sperm from another species. In plants that rely on pollinators that visit many flowers (such as honeybees), strong gametic incompatibility systems often exist.

Hybrid Inviability Hybrid embryos may die early in development. If born, some hybrid animals are weak. Viable hybrid animals (mules, ligers) are often infertile. Hybrid birds, even if fertile, may have “hybrid” courtship rituals that interfere with successful mating.

Effects of Isolation Geographic isolation can lead to allopatric speciation if the separation is maintained for many generations. Ecological, temporal, and behavioral isolation can lead to sympatric speciation over many generations.

Allopatric Speciation Allopatric speciation occurs when populations are completely separated from one another by geographical barriers: Formation of a river Rise of a mountain range Accidental migration to an island

Allopatric speciation requires strong geographic separation that prevents gene flow between the main population and the smaller, new population.

Sympatric speciation occurs within or on the margins of a larger population. Behaviorial, ecological, or temporal isolation reduces gene flow between populations, allowing the populations to diverge. Sympatric Speciation

The stronger the isolating mechanism, the more likely it is that the two populations will diverge. Populations on the margins of the range are subject to different selection pressures than the main population.

If environmental conditions change rapidly and dramatically, some organisms will go extinct. The more specialized an organism is to its habitat, the more vulnerable it is during a rapid climate change, even though that specialization was an advantage before the change. Extinction

The earth’s history is punctuated with several major extinction events. For some of these events, we have clues about the cause, such as an asteroid crash or a global climate change. We are living in the middle of a major extinction event -- and we are the cause. Major Extinction Events

Recap “Species” and other taxa are human- defined categories. Not all organisms fit into our neat taxonomic boxes! Divergence that leads to speciation is the result of isolation, followed by different selection pressures. Isolation may be geographic, ecological, behavioral, or physiological.