EVOLUTION. E VOLUTION. Evolution means a gradual change over time. Since its formation about 4,5 billion years ago, the earth itself has changed continuously. This slow change is known as geological evolution.
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Evolution means a gradual change over time. Since its formation about 4,5 billion years ago, the earth itself has changed continuously. This slow change is known as geological evolution.
Many species also have changed since they first appeared. This process is known as organic evolution.
The basic idea of Theory of Evolution is that all species have a common ancestor.
Even though the fossil record is not complete (and is likely to never be complete) we can form a picture from the fossils we do have. Some animals are better respresented than others - like the horse
whale’s flipperadapted for swimming
wings of batsadapted for flying
forearm of the horseadapted for walking
They all have different functions but they are internally very similar
Apparently, whales and snakes evolved from four legged ancestors.
Embryos of closely related species show similar patterns of development.
In vertebrate embryos, there are many similarities during the early stages of development.
For ex; all of the embryos have gill slits, two chambered hearts and tails. These similarities support the idea that these organisms have a common ancestor.
According to Lamarck’s theory, evolution involved in two principles;
The more an animal uses a particular part of its body, the stronger and better developed that part becomes.
At the same time, the less a part is used, the weaker and less developed it becomes.
Ex: An athlete develops the strength of certain muscles by constant use.
2. The Inheritance of AcquiredCharacteristicsLamarck assumed that the characteristics of an organism developed through use and disuse could be passed on to its offspring.
Ex: Ancestors of modern giraffes had short necks and fed on grasses. As the supply of food near the ground decreased, the giraffes had to stretch their necks to reach leaves of the trees.
Their necks become longer from
strecthing and this trait was passed
on to their offsprings.
All organisms have some changes due to the environmental conditions. These changes which are not hereditary are called modification. Lamarck’s second theory is actually about modification.
Ex: Our skin gets darker in the sun
Darwin made several types of observations during his trips to the South America coastlines and some islands.
The most significant of Darwin’s observations were those he made on the Galapagos Islands. He found many different species of finches living on these islands. The birds were alike, yet each species was slightly different from those on the next island or in another part of the same island.
Darwin made similar observations about many plants, insects and other organisms. While species on the Galapagos Islands resembled species on the mainland, they were always different in certain charactersitics.
Darwin came to believe that these organisms originally had reached the islands from the mainland. Because of their isolation on the islands, the species had opportunities to develop special adaptations to each different region.
The six main points of Darwin’s theory are;
1.Overproduction: Most species produce far more offspring than are needed to maintain the population.
Species populations remain more or less constant because only small fraction of offspring live long enough to reproduce.
Only a small fraction can possibly survive long enough to reproduce.
3.Variation: The characteristics of the individuals in any species are not exactly alike. They may differ in exact size or shape of a body, and so on. These differences are called variations.
Some variations may not be important. Others may affect the individual’s ability to get food, to escape enemies or to find mate. These are vital importance.
4. Adaptations: Because of variations, some individuals will be better adapted to survive and reproduce than others. In the competition for existence, the individuals that have favorable adaptations to their environment will have a greater chance of living long enough to reproduce.
An adaptation is any kind of inherited trait that improves an organism’s chances of survival and reproduction in a given environment.
Structural Adaptations involve the body of an organism.
Ex: The wings of birds or insects are structural adaptations for flight.
The fins of fish and the webbed feet of ducks are structural adaptations for swimming.
Physiological adaptations involve the metabolism of organisms.
Ex: Protein web made by spiders and the poison venom made by snakes
Camouflage: The organism blends into the environment.
Warning coloration: The colors of the animal make it unpleasant it to eat and protects it from predators.
Mimicry: The organism is protected from its enemies by its resemblance to another species.
In Mullerian mimicry, the model is not defined and several unpalatable species share warning colors or patterns to evade predation. Both models and mimics are toxic. Several species from several different orders may comprise a mimicry complex. The advantage is that the predators need only encounter one form to shun the entire complex.
Batesian mimicry involves a palatable, unprotected species (the mimic) that closely resembles an unpalatable or protected species (the model). One example is this fly which looks like a bee. Birds know not to attack a bee as they will be stung.
In effect, the environment selects plants and animals with optimal traits to be the parents of next generations. Individuals with variations that make them better adapted to their environment survive and reproduce in greater numbers than those without such adaptations.
Industrial melanism is the term used for the development of dark-colored organisms in a population exposed to industrial air pollution and it is an example of observed natural selection.
Before England became industrialized, the light – colored moths blended in well with the lichens that covered the bark of the tree. As a result of this camouflage, birds that feed on the peppered moth could not easily find them. Dark – colored moths were easily eaten by the birds.
When the air pollution killed the lichens and blackened the trees, light-colored moths became easy prey for birds.
Antibiotics usually kill bacteria. However, once the use of antibiotics became common, resistant strains of bacteria began to appear.
In a large population of bacteria, there are always a few individuals with resistance to the antibiotics. In an environment containing the antibiotic, only the resistant individulas will grow and reproduce. By natural selection, the strain with resistance to the antibiotic becomes the common type.
A pure culture of bacteria can be tested for antibiotic resistance by evenly swabbing it over an agar plate and pressing discs of antibiotic into the agar. After a period of incubation, sensitivity or resistance to the antibiotic can be determined by measuring the zone that forms around the disc. Large zones mean the bacteria are sensitive to the drug, small or nonexistent zones are signs of resistance. (Image Courtesy: Centers for Disease Control and Prevention.)
DDT resistance in InsectsWhen DDT was first introduced, it was an effective killer of insects, including mosquitos. However, a small proportion of insects in various insect populations possesed a natural resistance to DDT.
When the DDT – sensitive members of a population were killed by spraying, the DDT-resistant insects survived and passed on their natural DDT-resistance to their offspring. Evetually, many insect populations were completely resistant to DDT. DDT did not create the resistance of the insects. Rather, the DDT acted as the environmetal agent for the selection of the resistant strains.
Eventually, the accumulated changes become so great that the net result is a new species.
The formation of new species is called speciation.
Isolation refers to anything that prevents two groups within a species from interbreeding. Isolating agroup of organisms seperates its gene pool from the gene pool of the rest of the species. Through mutation, genetic recombination and natural selection, a different gene pool will evolve in each group.
Mutations,genetic recombinations, natural selection
Geographic isolation occurs when a population is divided by a natural barrier, such as mountain, desert, river or other body of water, or a landslide caused by an earthquake.
As a result, the gene pool of each group becomes isolated. Over a period of time, each group will become adapted to its particular environment.
When the differences between the isolated groups become great enough, they will no longer be able to interbreed, even if they could get together.
Fruit fly experiment suggests that isolating populations in different environments (e.g., with different food sources) can lead to the beginning of reproductive isolation. These results are consistent with the idea that geographic isolation is an important step of some speciation events.
The Kaibab and Abert squirrel are formed by geographic and reproductive isolation. The Kaibab squirrel inhabits the north side of Grand canyon and the Abert squirrel south side. These two squrriel are thoght to be evolved from a common ancestor whose population was divided by Grand Canyon.
After a long period of geographical isolation, these squirrels evolved. Although they are similar in appearance but they cannot interbreed.
Differences may arise in courtship behavior, times of mating, or the structure of the sex organs.
Events after mating like, inability of sperm to fertilize eggs, the death of the embryo, or the development of offspring that are sterile.