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I feel there is a part of the theory of evolution I'm not understanding correctly or maybe I'm missing it, but would like cleared up.

From what I understand, over time random mutations occur which in some cases provide an advantage allowing that animal to survive and reproduce. Its children will have that mutation allowing them to survive and so on over many many years until it evolves into a new species and / or the old ones become extinct.

For example the rock pocket mouse.

The part I'm getting stuck on is that unless these mutations happen instantly and in a large number, given its over such a big time scale how do animals who are not well adapted get enough time to evolve to survive?

Another example is fish that evolve to be able to "walk" out of the sea to escape predators.

If moving out allowed them to survive how did they get enough time to evolve and not be made extint in the time they didn't not have these limbs.

Hopefully I have explained myself clearly :)

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Could not fit in a comment..

Welcome to Biology.SE! Here are a bunch of comments.

  • The sentence until it evolves into a new species and / or the old ones become extinct sounds weird and there seems to be some confusion between evolution and speciation. Also it seems that you have a very static view of what is a species, as if species were predetermined categories into which evolution came to show itself!

  • I am not sure but I think your question has to do with "valley crossing", that is "how can a population evolve toward some phenotypic trait if intermediate phenotypic traits are not fit?" Is that your question? You may be interested in "shifting balance theory" but it might be a bit too advanced.

  • It is common to overconsider the valley to cross (the deleterious impact of interemediate traits) and those traits are not necessarily so deleterious

  • Mutations do not always have small effects on traits. One mutation may have some very important impact and therefore, one mutation alone might be enough to cross a whole valley! You amy be interested in Fisher's geometric model. Fisher's geometric model is just some very arbitrary model and is not much inspired by any biological observation but it helps getting the concept of how mutations impact fitness (=function of survival and reproductive fitness).

  • When you say how do animals who are not well adapted get enough time to evolve to survive? I may use the word animals to refer to populations or to individuals in a population (extinction of genotype is a population and extinction of populations are two different things). If you referred to individuals then, your interest seems to be into "valley crossing" and "mutational effect"(effect of a mutation on phenotype or fitness). If you referred to populations, then you may be interested in "evolutionary rescue" or more broadly speaking in the so-called "theory of extinction".

  • There are probably other posts (or part of answers) on this site that will help you. A very good answer would be very very long. But maybe a moderately good answer may be relatively short!

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From what I understand, over time random mutations occur which in some cases provide an advantage allowing that animal to survive and reproduce. Its children will have that mutation allowing them to survive and so on over many many years until it evolves into a new species and / or the old ones become extinct.

The part I'm getting stuck on is that unless these mutations happen instantly and in a large number, given its over such a big time scale how do animals who are not well adapted get enough time to evolve to survive?

You are quite confused about what is evolution! The idea is not at all that all the individuals of a population will acquire the same change and become another species.

One individual gets a beneficial mutation and thus reproduces faster than others. So this mutation spreads little by little in the population because eventually only descendants of the first individual bearing the mutation will remain.

This is what is called a selective sweep. But it has nothing to do with the concept of species, which involves much more complex processes.

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