Science Society Picture Library Natural Selection Speciation Speciation

Science & Society Picture Library Natural Selection & Speciation

Speciation

First, what is a species?

Second: Cladistics, cladograms, clades

Speciation is the process of one species diverging (evolving) into two (or more) species. At the most basic level, the process is dictated by two processes: 1) Gene flow 2) Selection (Drift)

1) Gene flow is the movement of genetic material within, and among, populations of a given species • In plants, gene flow is accomplished through pollen movement, and through seed (or clone) dispersal.

Populations of Populus nigra.

Gene flow binds together the populations that make up a species.

Speciation is the process of one species diverging (evolving) into two (or more) species. At the most basic level, the process is dictated by two processes: 1) Gene flow 2) Selection

2) (Natural) Selection is the differential success of individuals within a population such that traits are eliminated or emphasized over (evolutionary) time. - variation among individuals in a heritable trait. - variation results in difference in reproductive success/survival.

Think about the range of conditions this species experiences given its distribution. Flowering dogwood (Cornus florida) http: //www. plantbio. ohiou. edu/trees/Cornus%20 florida. htm

Role of Selection? (eg. habitats different)

Gene flow is holding the populations together. Selection is driving the populations apart.

what if gene flow is prevented?

Differentiation, the development of ecotypes. . . maybe, later, speciation

Ok…so how do you get from here, to speciation? ?

Four (broadly accepted) modes of speciation: 1) Allopatric speciation (classical model). 2) Founder effect speciation. 3) Peripheral isolates speciation. 4) Genetic hiccups.

1) Allopatric speciation (classical model). - A species range is divide geographically by some event, dividing the species into sub-groups and blocking gene flow. - Selection pressures within the sub-groups drive them toward dissimilarity, differentiation occurs, then eventually the two become reproductively isolated.

Populus nigra.

- Gene flow is blocked

- Gene flow is blocked Adaptation, differentiation, ecotypes, reproductive isolation…speciation

2) Founder effect model of speciation. - Some event leads to a very small population- or with plants a single individual, arriving in some new location, beyond the range of gene flow. - Usually intense selection pressure (and inbreeding) commence- the founder population diverges quickly and becomes a new species.

Populus nigra.

- Gene flow is blocked (& inbreeding & intense selection pressure) Adaptation, differentiation, ecotypes, reproductive isolation…speciation

3) Peripheral isolates speciation. - Perhaps most common (or constant) - Individuals at the edge of a species range are isolated from the main body of the range. - Gene flow is restricted- Selection intense. - Ecotypes (varieties) develop, then reproductive isolation, then speciation.

Puma concolor

Puma concolor Gene flow from the south to the north of this range is highly restricted

Puma concolor Populations on the periphery of this very large range are differentiating into varieties, moving (perhaps) toward speciation

Argentine puma Costa Rican Cougar Eastern South American cougar North American Cougar (Puma concolor couguar) includes the previous subspecies and synonyms arundivaga, aztecus, browni, californica, coryi, floridana, hippolestes, improcera, kaibabensis, mayensis, missoulensis, olympus, oregonensis, schorgeri, stanleyana, vancouverensis and youngi; Northern South American cougar Southern South American puma

Puma concolor Some populations are already completely isolated (e. g. , Florida)

4) Genetic hiccup. - Something goes wrong during the reproductive process. - Some mutation occurs, or new portion of the genome is turned on, that cause a wildly different morphology, or other reproductive isolating mechanism. - In plants, another “hiccup” involves a change in the number of chromosomes. E. g. , reproduction involving two diploid organisms produces a triploid offspring. (which then cannot interbreed with the parent population. - Less well understood, the subject of much current research.

Speciation