What are the odds that another organism mutates at around the same time/when the first one hasn’t died yet and that that mutation changes the second organism into the exact same species as the first one (changes the same dna bases) AND that those 2 organisms find each other and have sex?
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“Species” just means “group of individuals that can mostly breed successfully with each other” and it’s really just a helpful descriptive term rather than any kind of fundamental natural law. We all have different genes from everyone around us but most of us do mange to pass on our genes. This mix of everyone mating keeps the population more or less similar.
Mutations happen in every generation. Most of these have no obvious effects, and it is only when many, many mutations have happened in a population over time that we start to see some changes in the group of individuals with those mutations. Because the individual changes are very small and each in isolation brings no particular advantage or disadvantage, they can spread throughout that population over time, meaning that the species itself slowly becomes another species (ie different from and unable to mate with its distant ancestors even if they invent a Time Machine). In other words, evolution happens at the group level, not at the individual level.
If some groups of a species are isolated from each other, this “drift” can happen differently in the different groups, leading to what was one species becoming two species that, even if brought back together, will not breed, either because they are physically unable to, or because they just don’t recognise each others’ mating behaviours.
Sometimes if different species do come back together before they have diverged too far, they can still breed, leading to “cross” species, such as https://en.wikipedia.org/wiki/Grizzly–polar_bear_hybrid
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