Chapter 14 Notes


Chapter 14

– The Origin of Species


I. What is a species?

A. Taxonomy

– branch of biology concerned with naming and classifying the diverse forms of life

B. Appearance alone does not always define a species

Fig 14.1A and Fig. 14.1B

C. The biological species concept

1. How do biologists define species? a) A population or group of populations whose members have the potential to interbreed and produce fertile offspring b) Reproductive isolation of species prevents gene flow and maintains the gap b/w species – what if it didn’t?

D. The biological species concept and ring species

1. Ring species

– a species whose distribution forms a ring as it extends its range around some geographic barrier

– Fig. 14.1C a) Ex. Northern American Salamander species

(Ensatina eschscholtzii) b) Monterey and Large-blotch salamander populations overlap, but no interbreeding – different species.

E. Other species concepts

1. Biological species concept does not work for all situations:

a) Classification of extinct forms of life cannot rely on interbreeding. Relies on the appearance and chemical analysis of fossils. b) What about asexual organisms? (prokaryotes and most single-celled protists)

(1) Some organisms can exchange genes

(conjugation), but otherwise there is no gene flow.

(2) Asexual organisms are assigned to species based on structural and biochemical characteristics.

c) Even with most sexually reproducing species we lack enough information about interbreeding… so we need another way to determine species

2. Morphological species concept

– a) Classification based mainly on observable and measurable phenotypic traits

3. Genealogical species concept

– proposed by evolutionary biologists - defines a species as a set of organisms with a unique genetic history (tip of the branches of the tree of life)

4. Ecological species concept

– identifies species in terms of their ecological niches

II. Reproductive barriers keep species separate

A. A human and a fern are obviously different species that cannot interbreed, what what stops closely related organisms like the California salamanders?

B. Reproductive barrier - a biological feature of the organisms themselves to prevent populations belonging to closely related species from interbreeding even when ranges overlap

1. Prezygotic barriers

– 5 main types a) Temporal isolation – species breed at different times (different seasons, time of day, different years)

(1) Ex. Western and Eastern spotted skunk geographic ranges overlap – western breeds in fall and eastern breeds in late winter

b) Habitat isolation – two species live in the same general area, but not in the same kind of places

(1) Two garter snakes found in western North

America – on lives mainly in water, the other on land.

(2) Two species of parasites living in different hosts.

c) Behavioral isolation – little or no sexual attraction b/w females and males of different species

(1) Ex. Special signals that attract mates and elaborate mating behaviors unique to a species is the

MOST important reproductive barrier among closely related organisms

(2) Ex. Male fireflies signal females with a very specific pattern of blinking – females respond only to signals of their species.

(3) Many species recognize mates by odor – pheromones – signals opposite sex of same species to alter behavior.

d) Mechanical isolation – female and male sex organs are not compatible

(1) Ex. Male copulatory organs of many insects species have complex structures that will only fit female parts of their own species.

(2) Flowering structures in plants specifically adapted to certain insects or animals.

e) Gametic isolation – male and female from two species may copulate, but the gametes do not unite.

(1) In many mammals, sperm cannot survive in female of another species

(2) What about sea urchins, which release their gametes into the sea?

2. Postzygotic barriers

– operate after hybrid zygotes are formed a) Hybrid inviability – genes of two parent species are not compatible – hybrids don’t survive b) Hybrid sterility – hybrids reach maturity, but are sterile. Ex. Mule – hybrid offspring of female horse and male donkey. Fig. 14.2C c) Hybrid breakdown – 1 st

generation hybrids are viable, but when they mate, offspring are feeble or sterile – different cotton plant species.

C. Next question: What are the situations that make reproductive isolation?


III. Geographic isolation can lead to speciation

A. Separation of a population is a key event in the origin of many species

– gene flow blocked.

1. Each population can now follow its own path

B. Allopatric speciation

– Initial block of gene flow to many species seems to have been a geographic barrier

1. Emergence of a mountain range

2. Large lake subsides into smaller lakes

3. A land bridge may form, isolating marine species

(Isthmus of Panama)

4. How large must a geographic barrier be?


Geographic isolation creates opportunities, but doesn’t guarantee new species

– only occurs if reproductive barriers arise.

D. More likely to occur with small populations

– greater chance of altering gene pools through genetic drift or natural selection.

E. Ex. In < 2 million years small populations of animals and plants colonized the Galapagos and speciated.

1. lets look at some hypothesis concerning how this might have happened…

IV. Islands are living laboratories of speciation

A. Evolutionary laboratories

– when oceanic islands are:

1. Far enough apart to isolate populations,

2. Close enough to allow occasional dispersion.

B. The Galapagos

1. Most species on Galapagos occur nowhere else

2. Islands born naked from underwater volcanoes

3. Gradually covered with life from organisms that rode the wind and the ocean currents (or road animals that road the wind

– seeds) from other islands or the South

American mainland.


Has 14 species of closely related finches (Darwin’s finches) a) Many similarities, but differ in feeding habits and beak type – correlated to what they eat. (Fig. 14.4A) b) All 14 species appear to have come from a single species from the mainland – go over Fig. 14.4B c) Today, each island contains multiple finch species with some have as many as ten. d) In contrast, island of Cocos – 700 km north of the

Galapagos – is so isolated that the finches have not

had opportunity to colonize other islands. How many species would you predict to be on this island?

C. Adaptive radiation

– emergence of numerous species from a common ancestor introduced to new and diverse environments.

V. New species can also arise within the same geographic area as the parent species

A. Not all spe cies are the result of geographic isolation…

B. Sympatric speciation -