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Table of Contents
What is a Species, Anyway?
What is a Species, Anyway?
Urry, L., Cain, M., Wasserman, S., Orr, R., & Minorsky, P. (2020). Campbell Biology in Focus, AP Edition (3rd ed.). Pearson Education.
Usually it is easy for us to distinguish between species visually, but not always. Initially you might think these species are the same, but the eastern meadowlark (Sturnella magna, left) and the western meadowlark (Sturnella neglecta, right) are distinct biological species because their songs and other behaviors prevent interbreeding. It can be especially difficult to differentiate species from afar while in the field using only visual observations.
Furthermore, there can also be significant diversity within a species. Think about humans. You may have similarities to your siblings, but you look very different from most humans. In fact, humans are not particularly diverse by many measures. Other organisms have similar variation - we are simply not evolved to notice it in them.
This shows one of the limitations of defining a species using morphological features. Thus, we typically will use the biological species concept, which states that members of a species are all reproductively compatible.
Speciation
Speciation
Causes
Causes
Essentially all that is needed to form a new species is a lack of gene flow and time. If a population genetically splits into two (by lack of gene flow), eventually little changes will accumulate within each group until they are no longer the same species and will not successfully breed any longer.
There are essentially two ways by which gene flow can be stopped: geographic (allopatric) isolation or reproductive (sympatric) isolation.
Urry, L., Cain, M., Wasserman, S., Orr, R., & Minorsky, P. (2020). Campbell Biology in Focus, AP Edition (3rd ed.). Pearson Education.
A. Geographic Isolation [Allopatric Speciation]
A. Geographic Isolation [Allopatric Speciation]
The most common way by which gene flow is cut off is via geographic isolation. This is referred to as allopatric speciation. The causes of this geographic isolation can include a change in the landforms, a moving population, and even severe weather.
There have been many cases of this over Earth's history due to its landforms that constantly change via plate tectonics, for example. However, these changes are often so slow that you cannot notice them within a human lifespan. Rather, they are usually measured using genetic history and fossil data.
One such case is exemplified here with the rising of the Isthmus of Panama 3.5 million years ago. When this isthmus breached sea level, it effectively cut a population of fish into two. Now, 3.5 million years later, we have two distinct species that have obvious morphological differences: the porkfish and the Panamic porkfish.
One extreme example of this speciation via geographic isolation occurred in beavers. North American beavers (Castor canadensis) and Eurasian beavers (Castor fiber) originated from the same ancestor, but have been isolated for so long that they no longer have the same number of chromosomes. Thus, even if they successfully mated if brought together, the offspring likely would not be viable because the number of chromosomes in the gametes will not match.
B. Reproductive Isolation [Sympatric Speciation]
B. Reproductive Isolation [Sympatric Speciation]
Populations do not have to be physically separated to have no gene flow, however. Anything that prevents individuals from successfully mating will have the same effect. This can include morphological differences (i.e. their reproductive parts are no longer compatible) as well as behavioral differences.
Many organisms rely on behaviors to court members of the opposite sex and reproduce. If some individuals are acting in a way that is not seen as advantageous or 'attractive' to members of the opposite sex, they will not mate with those individuals. These behaviors can be types of song, courtship display or even nuptial gifts provided.
Speciation on Phylogenies
Speciation on Phylogenies
To summarize, this lack of gene flow eventually leads to two populations that one would consider to be distinct species because they will no longer successfully mate (according to the biological species concept).
Often you will see these relationships represented on a cladogram, or evolutionary tree. Any time you see one line split into two (at a spot known as a node), that is a speciation event. We will learn to represent these evolutionary relationships in much more detail later.
Extinction
Extinction
Extinction vs Extirpation
Extinction vs Extirpation
When the word extinction comes to mind, people often first think of the dinosaurs. This is certainly an example of extinction at a large scale, but extinction really is just the death of a single species. So within that one extinction of the dinosaurs were many extinctions of individual species, genera, and families.
Populations die out all the time - perhaps a toxic chemical leeched into a pond and killed all the fish there or a drought wiped out all of the corn in a given population. These are examples of extirpation, or the death of a population. We only consider it an extinction when that is the last living population of a given species. The final extirpation for a population means extinction has occurred.
Causes
Causes
It is tempting to say that extinctions should not occur because evolution should cause the population to adapt to changing conditions. However, remember how slowly evolution typically occurs and how quickly environmental change often occurs. In the last 200 years or so since the Industrial Revolution, we have altered global conditions incredibly quickly. If evolution cannot keep up, or if mutations that allow for survival do not occur (remember, they're random), those species will not be able to keep up with the changes and will die.
Historical Extinctions & Punctuated Equilibrium
Historical Extinctions & Punctuated Equilibrium
According to historical biological evidence from long before humans evolved, scientists have determined that there have been five major mass extinction events in Earth's history. These are cataclysmic events that result in the vast majority of all species on Earth going extinct.
There is still extinction going on other times - that is normal. It is the rate at which the extinction occurs that determines whether a period of time is a mass extinction.
Normally, we hope that the rate of speciation is higher than the rate of extinction in order to increase biodiversity in the world. But speciation is usually slow, and extinction is far too rapid during mass extinction events for speciation to compensate.
We describe this as punctuated equilibrium - long periods of stability followed hy brief periods of sudden change.
To put into perspective just have disastrous these events are on global biodiversity, the Permian Extinction about 250 million years ago wiped out about 95% of all living species. The only thing keeping Earth from becoming a rock devoid of life once again was 1/20 of the species from the time.
The asteroid impact that led to the extinction of the dinosaurs 65 million years ago was also massively destructive, but only wiped out 76% of all living species (according to most estimates).
For a gorgeous representation of historical mass extinctions and the organisms impacted, see here.
Rapid Recovery & Adaptive Radiation
Rapid Recovery & Adaptive Radiation
The good news is that mass extinctions leave in their wake huge amounts of open niche space, or roles to be filled in ecosystems. This means that there might now be a plethora of some resources that were previously very sparse. These resources could include space, nutrients, and more.
These abundant resources can lead to rapid diversification and speciation. So often mass extinctions are followed by mass speciation events. Keep in mind that we are discussing this in terms of geological and biological time, so rapid is relative and typically still means millions of years.
We see this not only in the wake of extinction, but also when a small population colonizes a new island. Take, for example, the Hawaiian honeycreepers, a diverse group of birds with various feeding techniques and morphologies that rapidly diverged from one ancestral population that landed on the islands.
This explosion of diversity from a small ancestral population is known as adaptive radiation, so named because adaptations radiate from the ancestral population quickly.
Rapid diversification does not always follow extinction, however. A classic example of rapid diversification occurred 535-525 million years ago (long before any mass extinctions) in the Cambrian explosion. Despite its incendiary name, this was an explosion of diversity, not extinction. This burst of evolutionary change included the emergence of the first large, hard-bodied animals.
Sixth Great Extinction?
Sixth Great Extinction?
A growing proportion of scientists argue that we are now living in the sixth mass extinction event, brought on primarily by humanity's anthropocentric use of resources and resulting effects on climate and habitat destruction.
This is not a baseless claim, as there is significant evidence to back this up, and we have seen many species entering decline and eventually extinction within a single human lifespan.
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