Microevolution is defined as changes within a species that aren't drastic enough to create an entirely new species. Changes that result in a new species are part of macroevolution. Often microevolution can lead to macroevolution as changes become more pronounced and two distinct species emerge. Both are caused by mutation, genetic drift, gene flow or natural selection.
When describing or discussing evolution sometimes it's easier to narrowing your discussion at the small scale level or the large scale level. When scientists are talking about small scale evolutionary changes that might be described as microevolution and that's when you're talking about evolution at the individual species scale. What's going on in this particular group of squirrels in this particular park for example. And microevolution can be caused by 4 factors, often a population maybe under all 4 of these factors influence. One would be mutation, obviously if you've got a group of squirrels in a park and some of them undergo some mutations and they start developing say lighter spots in amongst their hair that would be a genetic changed i.e. evolution within that squirrel population.
Gene flow which is a term that means essentially migration if all the spotted squirrels then leave to go to some other park, well that's reduced the genetic diversity within that population. Therefore, it's undergoing genetic migration. On the other hand if some belong squirrels come into the park that's adding yet another genetic change into the mix. Genetic drift, if you've only got say 3 of our spotted squirrels in your population of 100 squirrels and all of a sudden a tree branch falls on one of them you just destroyed 5% of your population you went from 15% population being spotted to only 10% of your population. That's a random genetic drift it wasn't caused by the spots attracting the branch.
And then there's natural selection if it's easier to see those squirrels when you're hunting those squirrels because you can say "look I spotted one," that would reduce the number of spotted squirrels therefore there would be microevolution within this population. A lot of times though you'll also be talking about the macro evolutionary scale. Now that's when you're talking about how did this particular group of organisms that includes squirrels and ground hogs or whatever things that belong to the group that squirrels belong to. How do they branch off of the rest of them mammals because mammals include things like bears and dolphins and squirrels but if you've ever looked at a bear you're unlikely to say "wow that looks really closely related to a squirrel how did that come about?"
There's two hypothesis or theories to explain how groups diverge from each other and one of them is called phyletic gradualism and that's the idea that if you start with some common ancestor you have slow accumulation of changes over a long periods of time. If this one ancestral group led to species a, b, c, d, e, f and g with d and g being extinct species how do they come about? Well you have these slow accumulations you start getting differences in the two populations of the universe ancestor here and those slowly accumulated over a long geologic periods. With lots of transition species but why don't we find those fossils? Well obviously becoming a fossil is kind of a rare event, the chances of any one of you watching this video becoming a fossil are almost nil.
You have to land in the right conditions not being eaten by scavengers that kind of stuff. I mean look at all the KFC you've eaten, it's very unlikely that every drumstick bone is going to become a fossil. So the explanations for why you don't find so many transitional fossils here is because the fossil evidence is incomplete. People are finding new fossils everyday and maybe we'll find kind of a transition species queue here alright. That's different from the idea punctuated equilibrium, which says hey species don't slowly change like that all the time, instead a lot of times if the environment stays the same why mess with success? So a species will stay similar and it'll only undergo change often due do drastic changes in the environment like "oh my goodness there's a flood" and it wipes things out.
This would be something that a scientist might predict would be happening due to some of the oil spills in the Golf of Mexico for example. Numbers of species are being affected by the oil and who knows what effects it'll have and it may cause some new species to arise pretty quickly and suddenly, because only the weird mutants may have an advantage in those conditions. Earlier if you were a weird mutant that could eat oil the chances of you surviving were very low because there wasn't a lot of oil in the water. Now there is, now you're not at a disadvantage for spending the energy that give the ability to eat oil, now you've a major advantage while those who are covered with oil and can't eat it off, they die. So that's the idea of punctuated equilibrium, where you have the seven branchings and this explains why you don't find a lot of transitional fossils it's because you were in one condition and then something happened in that quick period of time you branched off from this species into species a and b.
Now you got to be careful here when I say quick I'm talking quick in the geological time scale, quick could be 30,000 years might have been quick, whereas long periods of time that could be millions of years. So be kind of careful that you don't caught up in this geological time scale.