Tuesday, September 25, 2007

The Origin of Species

Species: a species is a population whose members are able to interbreed wih each other and create viable(survivable) and fertile offspring. The members of a species must also be reproductively compatible. Although some organisms may be a part of the same species, they may not look alike. And some members of the same species may be prevented from breeding due to certain factors.

The images above show an eastern and a western meadowlark, which although they are in the same species, do not mate due to differences in songs and behavior.

Species originate by many different evolutionary processes. Populations can become geographically and reproductively isolated. If populations are isolated, they evolve independently from one another. In order for a new species to originate, a population has to be separated from other populations. This isolation can occur in different countries (allopatric isolation) or in the same country (sympatric isolation).

There are many pre-reproduction barriers that can prevent mating or fertilization in a species. These six barriers are geographic isolation, ecological isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation.

-Geographic Isolation: When there is a physical barrier that causes the species to occur in seperate areas. This is an allopatric or "different country" isolation.
-Ecological Isolation: When species occur in the same area but live in different environments, so they rarely come into contact and get the chance to reproduce. For example, lions and tigers are biologically able to reproduce and create viable, fertile offspring, but since lions live in the grasslands and tigers live in rainforests, they rarely come into contact with each other and do not get the chance to mate.

-Temporal (time based) Isolation: When species breed at different times of the day, different seasons, or different years and do not mate. This is a sympatric or "same country" isolation. For example, in certain species of cicadas, some cicadas reproduce every 13 years while others reproduce every 17 years, so the two groups never cross paths and reproduce.

-Behavioral Isolation: When unique behavioral patterns and rituals isolate species. These unique behaviors help identify members of a species and attract mates of the same species. For example, Ms.Foglia's favorite birds, the blue-footed boobies, have a courtship display that is unique to their species, and they will not mate until that courtship display is completed.

-Mechanical Isolation: When structural differences prevent successful mating. For example, in many closely related species of plants, there are certain differences, such as differences in color, that help attract different kinds of pollinators.

-Gametic Isolation: When the sperm of one species may not be able to fertilize the eggs of another species. This can be due to chemical incompatibility or a biochemical barrier that prevents the sperm from entering the egg. For example, some sea urchins may release their sperm and eggs at the same time, but the gametes of different species may be unable to fuse.

Post-Reproduction Barriers: These barriers can prevent hybrid offspring from developing into a fertile, survivable adult.

-Reduced Hybrid Viability: Genes of different parent species could interact and impair the hybrid offspring's development.

-Reduced Hybrid Fertility: No matter how strong a hybrid offspring is, they may still be unable to reproduce due to differences in structure or number of chromosomes of parents that may impair the offspring's ability to produce normal gametes. For example, mules, which are a hybrid of a horse and a donkey, are unable to reproduce because they have 63 chromosomes and cannot make normal gametes.

-Hybrid Breakdown: Although hybrids may be fertile in the first generation, when they mate their offspring can become weak or sterile.

Rate of Speciation

There is a debate about whether speciation happens gradually or rapidly over time. In the theory of gradualism by Charles Darwin and Charles Lyell, they believe that new traits emerge gradually over long spans of time, and that big changes are due to the accumulation of many small changes over a long period of time. They believe that the rate of change is constant. In the theory of punctuated equilibrium by Stephen Jay Gould and Niles Eldredge, they believe that the rate of speciation is not constant and that changes take place over hundreds of thousands of years rather than millions. They also believe that there are long periods of time with little or no change.

That basically sums up the origin of species that we learned about today in class. If you have any questions, just ask and I'll try to help.

1 comment:

fletch3836 said...

This is amazing! You sherpas are leading the way!