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  • Mechanisms of evolution

  • What is evolution?

  • Evolution is the development of life on Earth.

  • This is a process that began billions of years ago and is still continuing to this day.

  • Evolution tells us how it was possible for the enormous diversity of life to develop.

  • It shows us how primitive Protozoa could become the millions of different species that we see today.

  • Evolution, then, is the answer to the question that we have all asked on seeing a Daschund and a Great Dane together:

  • how is it possible for ancestors to have descendants that look so very different to them?

  • In answering this question, we want to focus on animals, excluding other forms of life such as fungi and plants.

  • The first question to ask is therefore: how can one animal develop into a whole new species of animal?

  • Ah, but just a quick question: what exactly is a species?

  • A species is a community of animals that is capable of producing offspring with one another,

  • with those offspring also being capable of reproducing in turn.

  • To understand this answer better, we need to take a closer look at the following points:

  • the uniqueness of living creatures, guaranteed through the excess production of offspring and heredity,

  • and as a second key point, selection.

  • Let's begin with uniqueness. Every creature that exists is unique, and this is essential for evolution.

  • The members of a species may strongly resemble each other in appearance;

  • however, they all have slightly different traits and characteristics.

  • They may be a bit bigger, fatter, stronger, or bolder than their fellow animals.

  • So, what is the reason for these differences? Let's take a closer look at a creature.

  • Every creature is made up of cells. These cells have a nucleus.

  • The nucleus contains the chromosomes, and the chromosomes hold the DNA.

  • DNA consists of different genes, and it's these genes that are life's information carriers.

  • They contain instructions and orders for the cells, and determine the characteristics and traits that living creatures have,

  • and it's precisely this DNA that is unique to every creature.

  • It's slightly different from individual to individual, which is why each has slightly different characteristics.

  • But how is the enormous range of DNA created?

  • One key factor is the excess production of offspring.

  • In nature, we can observe that creatures generally produce far more offspring than is necessary

  • for the survival of their species, with many offspring dying an early death as a result.

  • Often there are even more offspring than the environment in which they live is able to support.

  • This is one factor in increasing diversity within a species.

  • The more offspring that are produced, the more little differences occur,

  • and this is what nature wants: as many little differences as possible.

  • The second major cause of the uniqueness of individuals occurs in heredity itself.

  • By the way, heredity means the passing on of DNA to offspring.

  • Two very interesting factors come into play in this process: recombination and mutation.

  • Recombination is the random mixing of the DNA of two creatures.

  • When two creatures fall in love and mate, they recombine their genes twice.

  • The first time, they do this separately when they generate the gametes - that is, sperm and egg cells.

  • The gametes take half of the genes and shuffle them.

  • The second recombination occurs when a male inseminates a female.

  • The parents each provide 50% of their DNA, in other words, 50% of their unique traits and characteristics.

  • These are then recombined, or mixed, and the result is new offspring.

  • These offspring have a random mix of the DNA, and therefore the traits and characteristics of their parents.

  • This increases the diversity and differences within a species even further,

  • but mutations are also important for evolution.

  • Mutations are random changes in DNA.

  • These can also be described as copying errors within the DNA,

  • triggered by toxins or other chemical substances, or by radiation.

  • A mutation exists when part of the DNA is altered.

  • These changes are often negative, and may result in illnesses such as cancer.

  • However, they may also have neutral or positive effects,

  • such as the blue eye colour in humans, which is one such random mutation.

  • In all cases, a mutation has to affect a gamete, that is a sperm or egg cell,

  • because only the DNA in the gametes is passed on to the offspring.

  • This is also the reason why we protect our sexual organs during x-rays, whilst other parts of the body are not at risk.

  • In summary then, in the heredity process, creatures pass on their characteristics to their offspring in the form of DNA.

  • Recombination and mutation change the DNA so that each child looks different to its siblings,

  • and receives a random mix of the characteristics of its parents.

  • There's a key word here: random.

  • All of these processes are based on chance.

  • Random recombination and mutations result in individuals with random mixes of traits and characteristics,

  • which in turn mix these randomly, and pass them on.

  • But how can so much be down to chance, when all living creatures are so perfectly adapted to their environment,

  • for example, the stick insect, the hummingbird, and the frogfish?

  • The answer is provided by the second key point: selection.

  • Each individual is subjected to a process of natural selection.

  • As we have learned, each individual is somewhat different to its fellows,

  • and there is extensive variation within a species.

  • Environmental influences have an effect on living creatures. These so-called selection factors include:

  • predators, parasites, animals of the same species, toxins, changes in habitat, or the climate.

  • Selection is a process that each individual is subjected to.

  • Every creature has a unique mix of traits and characteristics.

  • This mix helps them to survive in their environment, or not, as the case may be.

  • Anyone with an unsuitable mix will be selected from the environment.

  • Those with the right mix survive, and can pass on their enhanced traits and characteristics.

  • This is why diversity is so important.

  • This is why creatures make so much effort to produce offspring that are as different as possible.

  • They increase the likelihood that at least one of their offspring passes nature's selection process.

  • They maximize their chances of survival.

  • A good example of this can be seen in a group of finches living on a remote island.

  • They are some of the most famous animals in the world of science,

  • and are known as Darwin finches, after their discoverer, Charles Darwin, and this is the story of those finches.

  • A few hundred years ago, a small group of finches was blown onto the Galapagos Islands in the middle of the Pacific, probably by a big storm.

  • The finches found themselves in an environment that was completely new to them,

  • a real finch paradise: an abundance of food and no predators.

  • They reproduced rapidly and numerously. The islands were soon heaving with finches.

  • This meant that food supplies became increasingly scarce.

  • The finch paradise was threatened with famine, and finch friends became competitors.

  • This is when selection intervened.

  • Their individuality and small differences, in this case their slightly different beaks,

  • meant that some of the birds were able to avoid competing with their fellow finches.

  • The beaks of some of the finches were more suitable for digging for worms.

  • Other finches were able to use their beaks better for cracking seeds.

  • The finches consequently sort out ecological niches. In these niches, they were safe from excessive competition.

  • They soon began to mate primarily with other finches that used the same niche.

  • Over the course of many generations, these characteristics were enhanced,

  • enabling the finches to exploit their niches successfully.

  • The differences between the worm-diggers and the seed-crackers became so large

  • that they were no longer able to mate with one another. Different species emerged as a result.

  • Today, there are 14 different species of finch living on the Galapagos Islands,

  • all of which are descended from the same group of stranded finches.

  • This is how new species are created by evolution:

  • through the interaction of unique individuals,

  • the excess production of offspring,

  • recombination and mutation in heredity,

  • and finally, through selection.

  • Why is this so important?

  • It tells us where the variety of life comes from, and why living creatures are so perfectly adapted to their habitats.

  • But it also effects us personally.

  • Every person is the result of 3.5 billion years of evolution, and that includes you.

  • Your ancestors fought and adapted in order to survive.

  • This survival was an extremely uncertain thing.

  • If we consider the fact that 99% of all the species that have ever lived are extinct,

  • then you can consider yourself part of a success story.

  • The dinosaurs have disappeared, but you are alive, watching this video,

  • because you're incredibly special, just like all the other creatures that exist today:

  • irreproducible and unique in the universe.

Mechanisms of evolution

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哦!原來「演化」是這麼回事 (How Evolution works)

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    沉豔影 發佈於 2013 年 12 月 29 日
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