1. Chapter 24 ~ The Origin of Species

2. 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

3. 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

4. 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)

5. 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)

7. 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)

8. 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)

9. 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

10. 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

11. 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

12. 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.

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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