Chapter 24 ~ The Origin of Species

What is a species? Morphological species concept (Linnaeus): grouping based on how organisms look Biological species concept (Mayr): a population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring

If species are defined by reproductive ability... Group isolation leads to a stop in the exchange of genetic information Gene flow is commonly the force that causes a mixing of alleles between population Genetic divergence is a build up of differences between separated populations

Reproductive Isolation (isolation of gene pools) Reproductive Isolating Mechanism: Force that prevents successful mating between populations Prezygotic barriers: impede mating between species or hinder the fertilization of the ova Ecological/Habitat (garter snakes; water/terrestrial) Temporal (eastern spotted skunks; seasonal mating) Behavioral (blue-footed boobies; mate signaling) Mechanical (monkey flowers; pollination anatomy) Gametic (sea urchins; egg coat receptors)

Reproductive Isolation Postzygotic barriers: fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adult Reduced hybrid viability (salamanders; zygotes fail to develop or reach sexual maturity) Reduced hybrid fertility (mule; horse x donkey; cannot backbreed) Hybrid breakdown (rice and cotton; 2nd generation hybrids are sterile)

Prezygotic and postzygotic barriers Prezygotic barriers impede mating or hinder fertilization if mating does occur Individuals of different species Mating attempt Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION (b) (a) (c) (d) (e) (f) (g)

Reduce hybrid viability Reduce hybrid fertility Viable fertile offspring Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown Fertilization Gametic isolation REDUCED HYBRID FERTILITY GAMETIC ISOLATION REDUCED HYBRID VIABILITY HYBRID BREAKDOWN (k) (j) (m) (l) (i) (h)

Limitations of the Biological Species Concept The biological species concept cannot be applied to Asexual organisms Fossils Organisms about which little is known regarding their reproduction

Modes of speciation (based on how gene flow is interrupted) Allopatric: populations segregated by a geographical/ecological barrier; can result in adaptive radiation (island species- archipelago) Sympatric: reproductively isolated subpopulation within the range of its parent population; polyploidy (change in chromosome number) in wheat

Allopatric Speciation Gene flow is interrupted or reduced when a population is divided into two or more geographically isolated subpopulations One or both populations may undergo evolutionary change during the period of separation A. harrisi A. leucurus Harris’s antelope squirrel and white-tailed antelope squirrels separated by the grand canyon

Sympatric Speciation in cichlids due to sexual selection or feeding preferences Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orange light, the two species appear identical in color. The researchers then observed the mating choices of the fish in each tank. EXPERIMENT P. nyererei Normal light Monochromatic orange light P. pundamilia Under normal light, females of each species mated only with males of their own species. But under orange light, females of each species mated indiscriminately with males of both species. The resulting hybrids were viable and fertile. RESULTS CONCLUSION The researchers concluded that mate choice by females based on coloration is the main reproductive barrier that normally keeps the gene pools of these two species separate. Since the species can still interbreed when this prezygotic behavioral barrier is breached in the laboratory, the genetic divergence between the species is likely to be small. This suggests that speciation in nature has occurred relatively recently.

In allopatric speciation A new species forms while geographically isolated from its parent population In sympatric speciation The emergence of a reproductive barrier isolates a subset of a population without geographic separation from the parent species

Modes of speciation Parapatric Speciation- neighboring populations become distinct while sharing a common border. Hybrid zone- region between the two species where some hybridization still occurs Hybrid zone between two oak species on Australia

Hippidion and other genera Evolutionary Trees Evolutionary Trees- summarize information about the appearance of new species Extinctions (irreversible losses of species) are recorded and shown on these “Trees of Life” Recent (11,500 ya) Pleistocene (1.8 mya) Pliocene (5.3 mya) Miocene (23 mya) Oligocene (33.9 mya) Eocene (55.8 mya) Equus Hippidion and other genera Nannippus Pliohippus Neohipparion Hipparion Sinohippus Megahippus Callippus Archaeohippus Merychippus Parahippus Hypohippus Anchitherium Miohippus Mesohippus Epihippus Orohippus Paleotherium Propalaeotherium Pachynolophus Grazers Browsers Key Hyracotherium

Gradual vs. Punctuated equilibria Speed of speciation: gradual vs. punctuated speciation; Niles Eldredge and Stephen Jay Gould (1972); helped explain the non-gradual appearance of species in the fossil record

Adaptive radiation Is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities Pisonia seeds cling to the feathers of this black noddy tern which will soon migrate elsewhere

Argyroxiphium sandwicense The Hawaiian archipelago Is one of the world’s great showcases of adaptive radiation 5 million years ago an ancestral tarweed arrived on Hawaii and quickly diverged into many different silversword species Dubautia laxa Dubautia waialealae KAUA'I 5.1 million years O'AHU 3.7 LANAI MOLOKA'I 1.3 million years MAUI HAWAI'I 0.4 Argyroxiphium sandwicense Dubautia scabra Dubautia linearis N