And lastly, if there were some born without legs, the first fully aquatic whales, they would have nothing to mate with, as theire ancestors would still be restricted to shore areas. or did the fully aquatic whale restrict itself to shore areas too? Why?
If evolution worked like that, it wouldn't work. I think that is a big stumbling block for a lot of people. After all, since evolution is all about reproduction - if something cannot reproduce it is an evolutionary dead end.
Evolution is not really perceptable at the level of single individuals. It happens at the level of breeding populations, and takes generations. That may sound like a cop-out but it is the only way for it to possibley work.
You take a breeding population and, over generations, benficial mutations will become dominant in the population. The beneficial mutation will have happened only once, in one individual (and may not have made a dramatic difference in body shape or behaviour - only a small change).
Then how does it get to be so prominant in the population?
Well that beneficial mutation, that occured only once out of thousands of individuals, helps to ensure that a greater percentage of offspring survive to reproduce. That means that your babies have a better chance then your neigbors babies at procreating the next genertaiton, and so forth with your grand children and great grand children.
It may all sound so improbable until you realize one thing. In many species, individuals make dozens or even hundreds of babies during their life time. But for every breeding female, on average only 2 survive long enough to reproduce. There is a huge amount of waste. That is on average, some indivduals will see thier offspring be more successful then others. . SO what if one particular female doubles the chances of her offsprings reproductive success due to a beneficial mutation? That mutation will find its way into more individuals with each passing generation.