DNA adaptation in human life
Lewontin's recipe

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:


There is variation for this trait in the population


This variation is caused by mutations

Part (or the totality) of this variation is heritable


Heritability is the part of the variance in a trait that is explained by genes. It can be measured as a parent-offspring correlation.

The variation of this trait is correlated with fitness


loosely speaking, fitness is an index of both fecundity and survival



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:


Variation


different pencils have different colors

Heritability


simulate that your pencils reproduce (asexual reproduction is conceptually easier to understand) and offsprings have similar (or even the same) color than parents

Correlation between trait and fitness


Pencils of different colors have different fitness.
For example each red pencils create two red pencils the next generation while blue pencils do not manage to replicate at all.



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.


While most mutations occurs during reproduction, some mutations does also occurs during body growth or stated differently during cell reproduction (Mitosis). In consequence, some pairs of your cells share exactly the same genome while other pairs of cells have a sligthly different genome.
Moreover, once some mutations had occurred within the development of an individual, these mutations might influence the fitness of the cells and therefore some alleles (variant of a gene) might raise in frequency while others decrease in frequency (Natural selection). It is important to understand that this process of natural selection select for cells that have a higher fitness and therefore do not necessarily select for the cells that allows the individual to increase its own fitness.




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!



Note:

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.