Does our DNA adapt by human lifetime? Or do we have the same genetic information from birth to death? I mean: What is usually called "evolution" means "natural selection" like this: http://www.youtube.com/watch?v=hOfRN0KihOU&list=UUsXVk37bltHxD1rDPwtNM8Q -stronger animals have more descendants, so they make bigger percent of strong animals. But does evolution work by some primitive genetics-engineering too? Thank you.
A very nice way to consider natural selection is through the lense of Lewontin's recipe. Evolution of a given trait (tail length for example) through natural selection occurs whenever the three following conditions are met:
You can take two minutes to think about how logical it is that changes in frequency of variants (natural selection) occurs whenever these three conditions are met. You can imagine any objects you want (pencils of colors for example) and simulate these three steps:
You can quickly see how the red pencils become more frequent in your pencil population through time. If you wait long enough your pencil population will be totally composed of red pencils. And you'll need to simulate a mutation in order to create a blue pencil. I think you understand the how different are the roles of mutations and natural selection in evolution with this example.
Genetic Adaptation During Lifetime of a multicellular organism
For a beginner in evolutionary biology, saying that a multicellular individual's genome does not change during its lifetime might be considered satisfying. In reality it is slightly more complicated. Two elements that are mostly influent very early in the lifetime of an individual have to be considered.
I don't fully understand what you mean when saying: "But does evolution work by some primitive genetics-engineering too?" Could you try to rephrase this question?
Please, let me know if I answered your question!
Here is a good way to understand the difference between "gene" and "allele". A gene might be a called the "eyes color" gene for example, while the three alleles of the "eyes color" gene might be callsed "blue eyes", "brown eyes" and "green eyes". In this sense, the alleles are the different variants of a gene. Mutations increase the number of alleles and natural selection reduces the number of alleles by selecting for the allele that cause its holder to have the greatest fitness. If you understood that, it is already good!
Note: Natural selection does not necessarily reduce the number of alleles, several alleles might be kept (polymorphism) in certain "types" of natural selection (frequency dependent selection and overdominance (heterozygote advantage), fitness varies in space and/or time, selection acts on different levels). The "type" of Natural selection that reduces the number of alleles after some time is called directional selection.
The concept that the environment directly changes DNA and alters the characteristics of the offspring falls under the heading of Lamarkism. In the context of DNA the Lamarkian view would be that the giraffe got its long neck by stretching its neck cells reaching up to tall trees for leaves, which then (somehow) changed the DNA in its gametes and thus produced offspring with longer necks. There's no compelling evidence for Lamarkism and Remi.b 's nice answer describes DNA and evolution/selection/adaptation in its standard form and how giraffes would evolve such features.
OP initially asked 'does our DNA adapt during our lifetime'. Certain regions of the DNA in each of our immune T-cells and B-cells change (recombine) to code for specific protein molecules that bind to antigens. These are the genes for the T-cell receptor in T-cells and antibodies in the case of B-cells. So the DNA in a T-cell or B-cell is different compared to other cells in the body including the gametes. These regions of altered DNA help us fight infection.
You can easily imagine that an individuals ability to fight infection may affect whether they can survive to reproductive age. Genes involved in immune system function might be selected for during deadly epidemics. And paradoxically, some genes responsible for debilitating or deadly diseases can persist in human society due to their protective effect against other diseases.
For completeness there is also some emerging data that neurons have some alterations in their DNA but its not clear what, if anything, that means and whether its of functional significance. Unless similar types of changes are made in gametes, DNA altered by this mechanism will not be passed on to offspring.
Finally, DNA in cells can be modified by a process called epigenetics. Epi- means 'around'. In epigenetics various regions of the DNA strand (and some related proteins that keep DNA coiled up called histones) are chemically modified by a set of enzymes. Epigenetics affects how certain genes are switched on and off and is perhaps best known for inactivating one of the two X chromosomes in female cells. In epigenetic modifications the DNA sequence remains the same but the epigenetic 'decorations' around the DNA can vary and this process can be influenced by the cells environment.