Natural Selection & Speciation Science & Society Picture Library
Journal of Researches 1836 5 – year voyage of the Beagle On the Origin of Species by Means of Natural Selection(1859)
The phenotype is the external, observable expression of an organisms genetic make up. The phenotype, is the expression of an organisms genotype.The genotype is the sum of heritable information (genes) carried by an organism
Alternative forms of a gene are called alleles.So…phenotypic variation within a population is related to allele frequency (the frequency of alternate forms of a gene).
-Natural Selectionis the differential success of individuals within a population such that traits are eliminated or emphasized over (evolutionary) time. This is trait-based, acting on individuals… but the effects accumulate in populations over generations. (1)Heritable Variation (2) This Variation Results in Differences in (Evolutionary) Fitness
For any given trait, there is generally a “normal” distribution…natural selection sometimes applies pressure along this distrubution
Sexual Selection Fitness is not specifically related to competition and access to resources… …it also is related to mate acquisition
Sometime this “sexual selection” drives morphological change Lion’s mane is thought to be an example of sexual selection…and it is not useful in the African heat!
Sexual selection can cause the accumulation of apparently useless, and bizarre, traits.
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:Gene flow Selection
Gene flow is the movement of genetic material within, and among, populations of a given species • For (most) animals this involves the movement of individuals (offspring, mostly) over geographic distances. For gene flow to be accomplished that individual would then reproduce.
Gene flow is the movement of genetic material within, and among, populations of a given species • For (most) animals this involves the movement of individuals (offspring, mostly) over geographic distances. For gene flow to be accomplished, then, that individual would then reproduce. • In plants, gene flow is accomplished through pollen movement, and through seed (or clone) dispersal.
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 (i.e., evolutionary fitness).
Flowering dogwood (Cornus florida) http://www.plantbio.ohiou.edu/trees/Cornus%20florida.htm
Gene flow is holding the populations together. Selection is driving the populations apart.
Differentiation, the development of ecotypes... maybe, later, speciation
Modes of speciation:(1) Allopatric speciation (classical model).- Founder effect speciation.- Peripheral isolates speciation.(2) Sympatric speciation- Genetic hiccups.
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.
- 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.
- 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 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)
Sympatric Speciation • Sympatric speciation occurs when new species evolve from a single ancestral species while inhabiting the same geographic region – there is no geographic constraint to interbreeding • Driven by multiple mechanisms: • Genetic Hiccup: occurs when organism carries one or more extra sets of chromosomes than parents; common in plants, usually causes death in animals • Differing habitats: hunt different prey or utilize different resources • Sexual selection: different mating calls or behavior
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. Generally does not work with animals! - In plants, “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, but being self-fertile can produce offspring by itself.
Sympatric Speciation Sexual Selection Polyploidy Behavior