The Origin of Species

The Origin of Species

Patterns of Speciation • Fossil record indicates two patterns: • anagenesis (phyletic evolution) - an unbranched lineage • cladogenesis -branching evolution • most commot form • promotes greater diversity by increasing the number of species

Two Concepts of Species • Morphospecies - defined by their anatomical features • Biological Species - population whose members have the potential to interbreed in nature to produce fertile offspring but who cannot successfully interbreed with other species

Limitations of the Concept of Biological Species • Some species do not have sexual reproduction • some species do not contribute 1/2 of genetic makeup (bacteria) • species definition doesn’t work for extinct forms of life • geographical separation doesn’t allow us to determine if they are the same species

Case of Peromyscus maniculatus • There are 4 populations, phenotypically distinct (called subspecies due to geographical separation) • overlapping of territories does occur and some interbreeding takes place • therefore, according to definition of “biological species” they should be considered the same species.

Permomyscus maniculatus artemisiae and Peromyscus maniculatus nebrascensis • their ranges overlap but they don’t interbreed • they do interbreed with the other two subspecies where they overlap, and thus there is gene flow • if the corridors for gene flow were eliminated by extinction of the other two species, P. m. artemisiae and P. m. nebrascensis would be considered separate species.

So What Is A Species? • Probably no single definition can be used to cover all cases • morphospecies concept is most practical for taxonomic purposes

Reproductive Barriers • Prezygotic - prevent mating or fertilization • habitat isolation • temporal isolation • behavioral isolation • mechanical isolation • gametic isolation

Habitat Isolation • Not necessarily geographic isolation • aquatic vs terrestrial, ie. Aquatic vs terrestrial garter snakes • host specific, ie. Two parasites living on separate hosts

Temporal Isolation • Different times of the day, ie diurnal vs nocturnal • different seasons, ie western and eastern spotted skunk • western breed in late summer • eastern breed in late winter • different years

Behavioral Isolation • Different signals • songs • flashes of light • courtship ritual

Mechanical Isolation • Anatomical incompatibility • penis structure in primates • length of pollen tubes or styles in flowers

Gametic Isolation • Chemical incompatibility • improper receptors on surface of sperm and egg • different pollen, different pollen tube lengths, different growths

Reproductive Barriers • Postzygotic - prevents the development of viable, fertile adults • hybrid invariability • hybrid sterility • hybrid breakdown

Hybrid Invariability • Once the egg is fertilized, offspring either is not brought to term or is too weak or too frail

Hybrid Sterility • Exampe is the mule • cross between horse and donkey • mules cannot backbreed with either

Hybrid Breakdown • Hybrid is fertile, but when hybrids mate with each other or their parents, offspring are feeble or sterile

Introgression • When alleles do slip by all reproductive barriers, the transplantation of alleles between species is called introgression.

How Does Speciation Occur? • Geographical Speciation • allopatric speciation • sympatric speciation • Genetic Speciation • divergence • peak shifts

Allopatric Speciation • Geographic barrier physically isolates the population • mountain ranges • creeping glaciers • land bridges • lakes and rivers • canyons

Found on South rim of Canyon Found on North rim of Canyon

Conditions Favoring Allopatric Speciation • Gene pool of the peripheral isolate differs from that of parent population from the beginning • until the peripheral isolate becomes a large population, genetic drift will continue to change the gene pool at random • evolution by natural selection may take a different direction in the peripheral isolate rather than in parent population

There is evidence allopatric speciation is faster in small popluations rather than large ones • Adaptive Radiation - the emergence of numerous species from a common ancestor that spreads to new environments, ie Darwin’s finches.

Adaptive Radiation

Sympatric Speciation • New species arise within parent population; reproductive isolation occurs with geographical isolation • many plant species have origins due to accidents in cell division • autopolyploid • allopolyploid

Autopolyploids • By accident, a species doubles its chromosomes from (2n) to tetraploid (4n) • tetrapoloids can mate with other tetraploids or themselves • cannot breed with original diploids

Allopolyploids • Two different species form a polyploid plant • the resultant hybrid is infertile but may be more vigorous than parents and propagate asexually • from these may arise fertile hybrids

Speciation by Peak Shifts • Adaptive landscapes by Sewell Wright • adaptive peaks - equilibrium state where gene pool allele frequencies maximize average fitness • to change to a different gene pool allele that maximizes average fitness, population must go into a valley • then population has to work itself to a new adaptive peak

Peak shfits are not triggered by new physical environment but by nonadaptive changes in the genetic system • can be caused by found effect or bottleneck effect • genetic drift can cause peak shifts

Models for Tempo of Speciation • Gradualism - diverge more and more over time • punctuated equilibrium - new species changes most as it buds from parent species and then changes little after that

Speciation by Divergence • Two populations, over time, adapt to disparate environments and accumulate differences in frequencies of genotypes and phenotypes • from this adaptive divergence of two gene pools, reproductive barriers between the populations may evolve coincidentally, differentiating the two as two species.

The Origin of Species