As I understand it, various animal traits have to evolve gradually, but what happens to the species that are "neither here nor there"?
To put it differently, if a species evolved from another, it did so because it's somehow better, right? So why are there examples of the original species not being extinct?
What factors determine weather some species "stick"?
Answer
Short answer
Why are there species rather than a long continuum?
Three important reasons I could think of are sex, non-uniform adaptive landscape and ancestry.
Long answer
I am not sure I'll answer your question so let me know if I miss your point or if I help!
To start with, you might want to read this answer on the semantic difficulties behind the concept of species
What factors determine whether some species "stick"?
Natural selection is nothing but differential fitness (fitness is a measure of both reproductive success and survival) among genotypes within a population. Individuals having greater fitness will leave more offsprings and therefore the genes of these individuals increase in frequency in the population. There are few generalities to be made about what phenotypic traits are beneficial in a given population. For example, "white fur" is a very good trait for a polar bear but would highly deleterious for a mealworm.
However, there is a thing called species selection wherein a given lineage at least, it is possible to identify specific traits that seem to either reduce the extinction rate or increase the speciation rate. This is, for example, the case for polyploidy in angiosperms (Whitton and Otto, 2000)
if a species evolved from another, it did so because it's somehow better, right?
If you observe different extant species you cannot say that any of these species evolve from any other one you can today observe. The correct way of looking at two species is that they share a common ancestor in a given past. Therefore, looking at a cat and a blue tit you cannot say that one species evolved from the other one but you can only say that these two species share a common ancestor (just like any other pair of species) that was neither a cat nor a blue tit. The example is obvious because cats and blue tits are "not so closely related" (everything is relative) but the same logic holds for any pair of species.
Why are there species rather than a long continuum?
Sex
The simplest and most obvious reason why there are species within which individuals are more similar compared to each than to individuals from other species is due to the definition (the most common definition because different definitions exist!) itself of a species. A species is a group of individuals that can interbreed. See this for more info on the concept of species.
Take two originally different groups of individuals and allow them to interbreed. Their traits will mix up and you won't be able to tell two different groups apart. All individuals within the new mixed group are a mixture of the individuals from the two previous groups (under some circumstances this process has been sometimes called "reverse speciation"). If now you take one single group of individuals. You split them into two groups in the sense that you don't allow individuals from group 1 to mate with individuals from group 2. You will see that after some evolutionary time, the individuals of group 1 will tend to resemble much more to individuals of group 1 (its own group) than to individuals of group 2. If you wait long enough so that these two groups of individuals become different enough so that they can't interbreed any more because they diverged too much, then you have what is called a reproductive isolation and under the common definition of species, you can say that a speciation (You may want to have a look to the wiki article for "speciation") occurred and therefore you have two new species instead of one ancestral species.
why the two groups tend to diverge through times?
You may wonder "But why the two groups tend to diverge through times?". There are several processes that explain that divergence:
- Mutations
- Different mutations occur in the different groups (just by chance)
- Natural selection
- The environment differs and the selection pressures differ selecting for different traits in the two species. Also, the accumulation of different mutations affects the selection pressure at other loci.
- Genetic drift
- Shortly speaking genetic drift is due to random events. Different random events occur between the two populations. For more info about genetic drift, see this post
If you are not very familiar with these concepts I recommend that you have a look at Understanding Evolution (UC Berkeley).
Adaptive landscape
Note also that there are other reasons for explaining this pattern. One other reason is "Because the adaptive landscape is not a flat function". What this means to the layman is that there are some combinations of traits that cannot really be beneficial.
Ancestry
Also, individual phenotypes are not independent of each other and not only for ecological reasons but also because of shared ancestry. If you consider two families, you will easily accept no to see a continuum of phenotypes but two distinct groups (maybe in one family curly hair is common while in the other they all have straight hair).
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