The Origin of Species

• If microevolution (the genetic changes within a population over generations) were all that ever happened, the Earth would only be inhabited by a highly adapted version of the first form of life

• Fortunately for us, however, the world is full of biological diversity – many different varieties of life forms, consisting of many millions of distinct biological species

Got biodiversity?• Speciation (the

emergence of a new species) is the bridge between microevolution and macroevolution (the broader pattern of evolutionary change over long periods of time)

http://www.botany.uwc.ac.za/pssa/newsletter/images/sea-plants%20poster-A4.jpg

The Origin of Species

• A population is defined as a group of interbreeding individuals belonging to a particular species sharing a common geographic area (ex. all the hermit crabs on Fire Island National Seashore; all the squirrels on SCCC campus or Long Island – depends on how you define the area)

• A population is the smallest unit that can evolve (individuals do NOT evolve)

What defines a species?

• Biological species concept: –A species consists of a population whose

members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot interbreed with other such groups– Species are based on their ability to

interbreed; not on physical similarities!

Origin of New Species• Example: Eastern and Western

Meadowlarks; 2 different species with similar shape and coloration, but differences in song help prevent interbreeding

• Barriers to breeding can be behavioral

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Origin of New Species

• Humans have considerable diversity, but we all belong to the same species because of our ability to interbreed

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Speciation happens• It takes a reproductive barrier to keep

individuals of closely-related species from interbreeding

• Reproductive barriers may be behavioral, geographical, anatomical, or temporal

Prezygotic vs. Postzygotic Barriers

• Prezygotic – mechanisms preventing formation of a zygote; these can be ecological, behavioral, temporal, or mechanical

• Postzygotic – mechanisms preventing organisms (zygotes and beyond) from developing into a reproducing adult; these can be

Prezygotic examples• Geographical: Antelope squirrels occupying

different sides of the Grand Canyon • Ecological: Lions and Tigers inhabit different

habitats and would rarely encounter each other• Behavioral: Blue-footed boobies conduct

courtship rituals that other species would not recognize

• Temporal: Different species of wild lettuce have different blooming periods

Postzygotic examples

• Reduced hybrid viability - most hybrid offspring of different species do not survive; example – many species of salamanders are able to mate, but the offspring do not survive to reproduce themselves

• Reduced hybrid fertility – hybrid offspring reach maturity, but are sterile

Postzygotic example• Mules are the reproductive product of a horse

and a donkey mating• Mules are viable (one got me nicely around

Bryce Canyon, Utah without plunging me to my death…), but they are sterile

• Therefore, a horse and a donkey must be considered distinct species

Allopatric Speciation

• How do reproductive barriers arise?• One of the clearest forms of species is

allopatric speciation caused by a geographic barrier

• When a geographic barrier occurs, the isolated populations each become adapted to their own environment, such that over time, they may no longer interbreed ( speciation)

Allopatric Speciation• Examples of allopatric

isolation: Antelope squirrels on south and north rim of Grand Canyon (a); migration (b); volcanic activity (c)

(a)

(b)

(c)

Speciation

Sympatric Speciation• Sympatric speciation – the process by which

new species arise within the range of another species

• More controversial• In this case, a new species does not arise from

geographic isolation• Instead, a new species may arise by accident

when errors during cell division resulted in organisms with extra sets of chromosomes (very common in plants)

Sympatric Speciation

• New species formed by having extra sets of chromosomes are considered to be polyploid

• Polyploid organisms have more than 2 complete sets of chromosomes

• For example, a polyploid containing 4 sets of chromosomes will produce diploid (2n) gametes! This species would be unable to mate with normal diploid species (which produce haploid gametes)

Parent speciesDiploid

Polyploid (“tetraploid”)

1

Diploidgametes

2

Viable, fertiletetraploidspecies

Self-fertilization

3

Error in cell

division

Polyploid cells undergo

meiosis

Species A2n = 4

Gameten = 2

1 2

Species B2n = 6

Gameten = 3

Sterile hybridn = 5

Chromosomes nothomologous(cannot pair)

Viable, fertilehybrid species

2n = 10

3

3. However, ‘sterile’ hybrid can reproduce asexually (as

many plants do), and if subsequent errors in cell division occur,

chromosome duplications can resultin a fertile polyploid species!

Polyploid speciation

• Remember, polyploidy is a type of sympatric speciation

• As many as 80% of all living plants today are believed to have arisen by polyploidy!

• A polyploid contains twice (or sometimes more) the genetic diversity as its diploid predecessors, which provides an adaptive advantage!

Polyploid speciation

• Many of the plants grown for food are polyploids– Oats and Barley– Potatoes– Bananas– Peanuts– Plums and Apples– Wheat– Coffee!

Allopatric vs. Sympatric Speciation

Adaptive Evolution

• The evolution of many diverse species from a common ancestor is called adaptive radiation

• The adaptations of these species allow them to fill new habitats or roles in their communities (“niches”)

• Adaptive radiation typically occurs when a few organisms colonize new, unexploited habitats, or when environmental changes open up new opportunities for the survivors

Adaptive Evolution

• The Galapagos Islands is one of the world’s greatest showcases of adaptive radiation

• Each island arose ‘naked’ from underwater volcanoes and were gradually clothed by plants, animals and micro-organisms which strayed from the South American mainland

• Darwin’s finches are a prime example of adaptive evolution

Cactus-seed-eater(cactus finch)

Seed-eater(medium ground finch)

Tool-using insect-eater(woodpecker finch)

Macroevolution

Rise of the mammals• The extinction of the dinosaurs provided a

tremendous evolutionary opportunity to mammals, who once lived in their shadows

Mass extinctions

Suggested Readings

• The Selfish Gene, Richard Dawkins• The Blind Watchmaker, Richard Dawkins• Unweaving the Rainbow, Richard Dawkins• The Panda’s Thumb (and any other collection

of stories by), Stephen J. Gould

…some light reading for a winter night