Speciation and Mass Extinction
April 19 and 21, 2004
Readings: Ch 17, p 260-265, 268-269, Ch 19, p 306
A few definitions:
Species: a group of organisms that can breed with each other and produce progeny still of the same kind. More modern definition: a group of interbreeding populations that share a gene pool and are reproductively isolated from other organisms.
- If geographic range is small, a species will typically have few phenotypic (physical) variations
- If geographic range is large, a species will typically have many phenotypic variations (humans, dogs)
Population: group of interbreeding individuals in a particular geographic area
Speciation: the process by which new species evolve from a parent species
Gene Pool: the sum of all genes in a population; all the genes of all the members of a population
Adaptive radiation: The evolution of many new species from a single species, with each new species being adapted to new habitat or resource.
I. How do new species form from existing species???
Allopatric ("other country") speciation: Speciation is frequently a two-step process:
(1) geographic isolation followed by (2) reproductive isolation
The crucial event happens when members of a population are severed from each other by geographic isolation. Examples:
- a pond dries up to make two ponds
- a river is re-routed through a field
- a new road goes up
- a few seeds stuck on a bird's feather fall on a new island
- a flood washes a few lizards or insects to an island
- and with enough time...continental drift (see below) this gets back to Lyell's gradualism
- PS. and on the subject of TIME...Twenty Billion Years. check this out!
With the members (and their gene pools) isolated from each other, they then become two unique populations - each varying somewhat among members - by chance.
Once geographic isolation occurs, the two populations can over time change relative to each other, because each population has
Given enough time, even if the geographic barrier is removed, reproductive isolation may occur (inability to breed). The result is the formation of two species from one species.
Continental drift - the formation and movement of the continents: a makor factor in both speciation AND mass extinction over time!
- The earth's crust is split into 7 enormous tectonic plates, moving in different directions in relation to each other, causing continental drift at a rate of about 2 cm / year, continually rearranging continents, thrusting up mountains, causing earthquakes and volcanos...
- Alfred Wegener proposed (in 1915) that 200 MYA (in the early Jurassic), all the continents were fused into one "supercontinent" - Pangaea (note: his hypothesis was not accepted for over 50 years!)
- As continents slowly drifted apart, geographic isolation would occur (explainsand "matching fossils" (as well as matching layers of rock) found on two different continents).
- Here is a simple math problem about continental drift: We know that the continents are moving apart at the rate of an inch per year. We also know that Africa and South America are currently 4000 miles apart. Question: How many years has it been since Africa and South America were touching (as in the figure above)? ________________years. Hint: Use the Google calculator - a new 'way cool' feature of Google that EVERYONE just has to try! (Whee!)
- Going along with continental drift, though, was habitat destruction for some organisms - and mass extinction - change in temperature, loss of shallow water, or land... more on this in a few!
II. Once species have formed, how do they stay separate??
Reproductive Isolation: Reproductive barriers prevent closely related species from ever merging their gene pools ever again...
Pre-mating isolation (loss of ability to mate with closely related species)
Post-mating isolation: (mating occurs but hybrid offspring die early or are sterile)
- Zygotic mortality
- Hybrid inviability
- Hybrid sterility
Examples of hybrid sterility:
- A mule is a sterile hybrid of a donkey (stallion) and a horse (mare).
- A cama is a hybrid between a camel and a llama
- A tiglon is a sterile hybrid of a tiger and a lion
- A zebroid is a sterile hybrid a zebra and a horse
You may be wondering about the evolution and speciation of than none other than Homo sapiens
"Do I know you?"
No Homo neandertalensis (Neanderthal) genes in the Homo sapiens gene pool!
The 'Out of Africa' Hypothesis: One of the going theories, but fresh debates still arise...
III. How quickly do new species arise?
Gradualism - evolution occurs gradually resulting in many transitional or intermediate stages,leading to a gradual divergence of species.
Punctuated equilibrium - evolution occurs in spurts of relatively rapid rate, in between which there are long periods of little evolutionary change. Steven Jay Gould and Niles Eldredge (1972)
The fossil record indicate that both mechanisms occur - most likely many gradual changes are occuring at the level of the genotype (unseen) before evidence of change is seen at the level of the phenotype.
Can evolution that results in formation of a new species be seen in a (human) lifetime? YES
Many plant species self-fertilize. If a major change happens at the genetic level (like a doubling or fusing of chromosomes), the eggs and sperm are still compatible because they are from the same parent. New species of sunflowers, alfalfa, apples, bananas, roses, wheat, rye, barley, potatoes, and peanuts have occurred in the last 100 years or so (most of these were created by agricultural breeding and probably happened much more quickly because of human intervention)
IV. Mass extinctions and the formation of new species:
One of the most predominant features of the history of life are the periodic occurances of mass extinctions
Five major mass extinctions - plus lots of known smaller extinctions - PLUS a special mention of the 6th Major Mass Extinction on Planet Earth?!?!?
For those organisms that survive the extinction (and there are always some), there are then many geographical, physiological and ecological opportunities for expansion (adaptive radiation), giving rise to the dominant species of the new period.
The five worst mass extinctions in Earths history and their possible causes, according to paleobiologist Doug Erwin of the Smithsonian Institutions National Museum of Natural History. [John Sepkoski and David Raup (University of Chicago 1983) determined that medium-sized extinction events occur regularly every 26 - 28 million years. Lucky for us, the last one of these was 11 million years ago....]
1. Cretaceous-Tertiary extinction (the 'K-T'), about 65 million years ago, probably caused or aggravated by impact of several-mile-wide asteroid that created the Chicxulub crater now hidden on the Yucatan Peninsula and beneath the Gulf of Mexico. The extinction killed 16 percent of marine families, 47 percent of marine genera, and 18 percent of land vertebrate families, including the dinosaurs, over a time period of 100,000 years..
2. End Triassic extinction, roughly 205 million years ago, most likely caused by massive floods of lava erupting from the central Atlantic magmatic province -- an event that triggered the opening of the Atlantic Ocean. The volcanism may have led to deadly global warming. Rocks from the eruptions now are found in the eastern United States, eastern Brazil, North Africa and Spain. The death toll: 22 percent of marine families, 52 percent of marine genera. Vertebrate deaths are unclear.
3. Permian-Triassic extinction, about 240 million years ago. Many scientists suspect a comet or asteroid impact, although direct evidence has not been found. Others believe the cause was flood volcanism from the Siberian Traps and related loss of oxygen in the seas. Still others believe the impact triggered the volcanism and also may have done so during the Cretaceous-Tertiary extinction. The Permian-Triassic catastrophe was Earths worst mass extinction, killing 95 percent of all species, 53 percent of marine families, 84 percent of marine genera and an estimated 70 percent of land species such as plants, insects and vertebrate animals.
4. Late Devonian extinction, about 370 million years ago, cause unknown. It killed 22 percent of marine families and 57 percent of marine genera. Erwin said little is known about land organisms at the time.
5. Ordovician-Silurian extinction, about 435 million years ago, caused by a drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. The toll: 25 percent of marine families and 60 percent of marine genera.
V. How do mass extinctions occur??
Example: why did the dinosaurs become extinct??? Impact hypothesis
An impact of this size definately struck the Earth 65 MYA, but - did this collision cause the Cretaceous extinctions?? No one knows, but in support of the hypothesis:
The GOOD news: the species that manage to survive a mass extinction become the "stock" for the new adaptive radiations that fill the many gaps left by extinctions. Lucky for us one primate survived the Cretacious extinctions......
(PS. No human or hominid existed 65 MYA to thumb its nose at the dinosaurs...!)
Deep down at Chicxulub
VI. Not to end on a sad note, BUT...Is human activity pushing the creatures on our dear Mother Earth into a 6th mass extinction? Sadly, many scientists are finding evidence that this may be so....we currently are causing species extinction at a rate equivlent to a mass extinction.
Will humans ever become extinct?
Next up...human population growth
1. Define a biological species
2. Describe allopatric speciation and the two main steps involved. Be able to relate this to continental drift.
3. Distinguish between gradual speciation and punctuated equilibrium
4. Distinguish between pre- and post-zygotic reproductive barriers
5. Explain why many hybrids are sterile
6. Explain what a mass extinction is and descrivbe the most recent (do not memorize the 5 mass extinctions!)
7. Explain the evidence for the impact hypothesis.
8. Explain the evidence for human impact on a possible "6th mass extinction"...