SPECIATION -The Origin of Species

What is speciation?

Recap: Species = a population or group of

populations whose individual members can interbreed to produce viable, fertile offspring

Speciation = the formation of a new species from existing species

Sometimes called macroevolution

Process of speciation:

Parentspecies

I. Barrier

2 separate species

III. Don’t interbreedII. Diverge

Reproductive Isolation May Occur With or Without Geographic Isolation

Allopatric speciation occurs when geographic isolation creates a reproductive barrier (an extrinsic mechanism). Sympatric

speciation occurs when a reproductive barrier is created by something other than geographic isolation (intrinsic mechanisms).

Allopatric Speciation

Two species of ground squirrel are postulated to have descended from a common ancestral population that was separated by formation of the Grand Canyon.

Harris’ antelope squirrel White-tailed antelope squirrel

Intrinsic Reproductive Isolating Mechanisms Are Always Required for Speciation

Intrinsic mechanisms involve changes to organisms that prevent interbreeding.

In allopatric speciation, intrinsic mechanisms come into play once populations are physically separated.In sympatric speciation, intrinsic mechanisms are the only ones involved.

Harris’ antelope squirrel White-tailed antelope squirrel

Isolating Mechanisms

PREZYGOTIC BARRIERS

Prevents mating or egg fertilization if members of different species try to mate

POST ZYGOTIC BARRIERS

Prevents viability of offspring or their ability to reproduce

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

(different habits within an overlapping range)

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

Courtship rituals, like these, are critical for mating within a species, but ineffective for attracting members of other species.

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

A summary of reproductive barriers between closely related species

Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation

TigonResult of male tiger and female lion mating in captivity. Offspring are infertile.

Separated both geographically and ecologically.

LigerResult of male lion and female tiger mating in captivity. Offspring are infertile.

Adaptive Radiation

The diversification of a common ancestral species into a variety of differently adapted species

Studies on islands – great "laboratories" to see how a population changes in response to new environmental conditions in isolation

FOUNDER SPECIES

An example of adaptive radiation – these species all diverged from a common ancestor (founder species)

Adaptive Radiation

Types of Evolution

Divergent – species that were once similar diverge or become increasingly distinct Ex: finch beaks

Convergent – two unrelated species share similar traits because each has independently adapted to similar environmental conditions Ex: bird and bee wings

Speciation Occurs at Widely Differing Rates

A slow rate of speciation evidenced by a living horseshoe crab (13 extant species) and a 300 million year-old fossil species A rapid rate of speciation evidenced by Galapagos finches which have diversified into 13 species within the last 100,000 years.

Speciation Rates

Generalists, like the horseshoe crab, tend to remain as stable species.

Specialists, like the Galapagos finch, tend to be unstable as species.

Species Come and Go

Best estimates from the fossil record indicate that greater than 99% of species that have exited are now extinct.

A typical “lifetime” for a species is about 1 million years.

Mass Extinctions Are a Fact of Life

Speciation Dynamics - Gradualism or Punctuated Equilibrium?

Punctuated equilibrium appears to be a more accurate view of speciation dynamics.

Slow and steady change

Does Evolution Create the Perfect Organism?

No, only better organisms as evolution is constrained by history and buffeted by random events. Essentially, every organism on earth is in significant part a sum of accidents.

P. 373 #1-4, 7, 8, 11

Review Questions: text p. 387 #2, 3, 5-23, 25-27, 29-33, 43-47, 52, 53