What Is a Species?
We've come up with some interesting ideas about how a flying hamster could have evolved from a non-winged one. We've hypothesized that long ago a hamster may have developed an extra limb, which then developed into a rudimentary wing. If this rudimentary wing gave the hamster a better chance of surviving in its environment, the increased fitness would have selected for individuals in the population that could glide or float with these rudimentary wings. A question we haven't addressed yet, though, is whether or not flying hamsters are a completely different species than regular hamsters. To answer this question, first we need to ask 'what is a species?'
We refer to different kinds of organisms as species. But what does that mean really?
Historically, scientists have used the morphology of organisms - that is, the size and shape of body parts - to classify organisms. For instance, a cat and dog clearly look different, and we call them different species. However, we could also say that a Chihuahua looks significantly different than a German Shepherd, yet scientists categorize both as dogs. It's more accurate to think of a species as an independent evolutionary unit of organisms.
The gene pool of dogs does not overlap with the gene pool of cats because genetic information can't flow between those two populations. Members of the same species either do or could mate with each other given the opportunity. For instance, dogs of different breeds can mate and produce what we call mutts. In contrast, members of different species, like cats and dogs, do not mate, or the matings are unsuccessful in some way. We will see later how the process of classifying organisms into species works. For now, let's consider how species are formed.
Speciation is the process by which an ancestral species splits into two or more new species. It's sometimes difficult to determine when speciation has occurred because it is often a gradual process, and not all evolutionary changes result in a new species. For speciation to occur, members of an ancestral species must become isolated from each other.
Dividing these organisms into two populations results in two separate gene pools. These gene pools can be separated through geographical or biological mechanisms. In addition, altered reproductive mechanisms can further isolate two populations of organisms. Let's first consider how gene pools can become isolated.
Usually, speciation involves division of a population into two new populations by a physical barrier. This is called allopatric speciation. 'Allo' means 'different' and 'patris' means 'country,' so 'allopatric' literally means 'different country.' It is also referred to as geographic speciation.
Allopatric speciation is considered the dominant mode of speciation among most groups of organisms. It's easy to see how things like the appearance of a mountain range, a canyon or a body of water could isolate populations of organisms. Once two separate populations are established, different selective forces could alter these populations enough to establish new species. Since we haven't seen any normal hamsters in the area during our field studies, allopatric speciation may have played a role in the speciation of flying hamsters.
However, sometimes speciation can occur without geographical isolation. This is known as sympatric speciation. Not surprisingly, the root 'sym' means 'together,' since this form of speciation occurs without any physical separation of the new species. A news species evolves while living in the same location as the original one.
The most common means of sympatric speciation is polyploidy. Polyploidy is a characteristic of a cell or organism with more than two complete sets of chromosomes. Consider the following example:
We've said that most organisms are diploid. What if an error in cell division created an organism that is tetraploid rather than diploid? Rather than two complete sets of chromosomes, a tetraploid has four sets. Mating between a diploid and tetraploid would introduce all kinds of problems. The diploid parent would produce haploid gametes, whereas the tetraploid parent would produce diploid gametes. If a haploid and a diploid gamete fused, the resulting cell would be triploid. Even if the resulting triploid organism survived to reproductive age, having three sets of chromosomes introduces an impossible problem during meiosis. How do you divide an odd number of homologous chromosomes evenly? The answer is that you can't, so triploid organisms are usually sterile.
Since our flying hamsters are diploid, sympatric speciation probably didn't play a role in the evolution of flying hamsters. However, sympatric speciation has played a major role in the evolution of many plant species.
In summary, a species is an independent evolutionary unit of organisms. Members of a species either do or could mate with each other given the opportunity. In contrast, members of different species do not mate, or matings are unsuccessful in some way.
Speciation is the process by which an ancestral species splits into two or more new species. For speciation to occur, a population's gene pool must become divided. Typically, this isolation is caused by either a physical or biological barrier. Allopatric speciation is speciation that results when a population is separated by a physical barrier. It is also referred to as geographic speciation. Sympatric speciation is speciation that occurs without physical separation of members of the population.
Polyploidy is a characteristic of a cell or organism with more than two complete sets of chromosomes.
After watching this lesson, you should be able to:
- Define species, speciation and polyploidy
- Explain the difference between allopatric speciation (or geographic speciation) and sympatric speciation
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