PART FOUR Evolution
Macroevolution: Evolution of Species
The process of speciation can only occur if there is a separation that divides a gene pool.
If microevolution is the change in the gene pool of a single population, then macroevolution is the creation of entirely new species. Macroevolution is a huge process that created all life-forms on Earth today and is defined as the rise of two or more species from an existing species (the process of speciation). In order to create a different species from a current population, there needs to be some sort of separation within the population. It is our must know to understand that the process of speciation can only occur if there is a barrier that divides the ancestral species’ gene pool. The two groups of individuals are no longer able to mate and exchange genes, and the gene flow is interrupted; you now have two separate gene pools.
This, however, isn’t enough to say speciation has occurred! Just because the populations are separated doesn’t mean they’re different species. If you recall, a species is defined as a group of individuals who are able to interbreed and produce viable offspring. The isolated gene pools need to change enough so that if brought back together, the members can no longer interbreed. This can happen just by random genetic drift, as we learned about in the previous chapter.
Furthermore, what if the two populations lived in slightly different environments? Natural selection would take over! Random mutations would be selected for or against (depending on the environment) and soon enough, different adaptations would accumulate. Would your two separate groups now be considered different species? Once again, it all has to do with their ability to mate and produce offspring. Eventually, the adaptations prevent mating between individuals of these two separate groups (even if they did come back together). Now speciation has occurred!
The process of speciation
An adaptation that prevents the production of offspring is referred to as a pre-zygotic barrier or a post-zygotic barrier. Recall that a zygote is the cell that forms immediately after fertilization. A pre-zygotic barrier would, therefore, stop the egg and sperm from fusing and creating a zygote in the first place. If fertilization occurs, the process of creating an offspring can still be stopped by a post-zygotic barrier. In this case, the zygote is created, but it may not develop further, or the offspring may not be able to reproduce (it’s sterile). Following are a few examples.
■ Temporal isolation (pre-zygotic)
The term temporal means “time,” so an example of this type of isolation would be a difference in mating seasons. If two species’ mating seasons don’t align, breeding can’t occur. For example, some species of frog may live in the same habitat, but because they reach sexual maturity at different times in the spring, they won’t interbreed.
■ Behavioral isolation (pre-zygotic)
Two closely related species can have significantly different courting behavior, and thus the other species just would not look very attractive. For example, different species of firefly will use a different flash pattern to attract a mate. Wrong light signals, no interest!
■ Mechanical isolation (pre-zygotic)
Mechanical isolation means that the reproductive bits just don’t fit together. Many species of dragonfly are mechanically isolated because they have incompatible structures used in mating and things just can’t line up properly.
■ Offspring viability (post-zygotic)
In this case, mating can occur, but the offspring do not survive, or are themselves sterile. A horse and a donkey can mate, they can produce an adorable little offspring (a mule), but that mule is itself sterile.
A liger is the offspring of a male lion and a female tiger, and a tigon is created by a male tiger and a female lion (yes, they’re different critters). These offspring were long thought to be sterile, but there have been documented cases of the offspring producing babies of their own. This shows how the concept of “species” can be a bit muddled and not as clean-cut as what we learn. Other interesting hybrids: wholphin (whale-dolphin), beefalo (bison and cow), and a pizzly (polar bear and grizzly).
1. Imagine a hypothetical population of beetles that are suddenly isolated from one another (for example, an earthquake creates a fault that the insects cannot traverse). One side of the fault has a lot of dark soil and the other side of the fault is covered mostly by grass. Using natural selection, explain how speciation could occur between these two isolated groups of beetles.
2. One species of flower can be fertilized by another species of flower, but once the egg and sperm fuse, their differing chromosome numbers prevent the formation of a viable zygote. This is an example of ________________, a type of ________________ barrier that prevents the production of offspring between two different species.
3. About 200 years ago, an ancestral population of flies laid their eggs on tree fruits called hawthorns. Once domestic apple trees were introduced, some flies in the population chose to lay their eggs on apples, instead. The maggots that developed in the hawthorn fruit grew into adult flies who preferred to mate with other hawthorn-reared flies; maggots from apples developed into flies who preferred to mate with other apple-grown flies. This is an example of ________________, a type of ________________ barrier that prevents the production of offspring between two different species.
4. Compare and contrast microevolution with macroevolution.
5. In order for speciation to occur, there needs to be some sort of barrier that divides the current population’s ________________, thus interrupting the gene flow between the two resulting groups. Next, the isolated gene pools need to ________________ in a way that renders them unable to interbreed if brought back together.
6. One species of field cricket mates in the spring, whereas a second species of field cricket mates in the fall. This is an example of ________________ isolation, a type of ________________ barrier that prevents the production of offspring between two different species.