The Species Concept
What is a species?
It is a group of living things that are similar enough to mate and have babies that can also have babies.
What is a biological species?
It’s a group of animals that can have babies together, and those babies are healthy enough to have their own babies later.
What is a deme?
It is a small, local group of the same species that mostly mates with each other—like all the squirrels living in one specific park.
What is a ring species?
It is a group of animals that live in a big circle around a barrier (like a mountain). Neighbors can have babies together, but the two groups at the very ends of the circle cannot
What is allopatric speciation?
It is when one species splits into two because they are physically separated by something like a river, mountain, or ocean.
Can a dog and a cat be the same species?
No, because they cannot mate with each other to have offspring.
What is the "test" for this concept?
If two animals meet in the wild and naturally decide to mate, they are the same species.
What is a cline?
It is a slow, gradual change in a trait (like height or color) as you move from one environment to another.
How does the "ring" work?
Imagine a chain: link A can mate with link B, B can mate with C, and C can mate with D. But link A and link D have become too different to mate with each other.
How does the split happen?
A group of animals gets divided. Because they can no longer reach each other to mate, they start to change independently in their own environments.
What is the easiest way to tell two species apart?
By looking at them! If they look very different (like a bird and a fish), they are usually different species.
What if the babies can’t have babies?
Then the parents are different species. For example, a lion and a tiger can have a baby (a liger), but because ligers usually can't have babies, lions and tigers are separate species.
How do they differ?
A deme is about a specific group of individuals; a cline is about how a feature changes over a long distance.
What makes them change?
As the animals move around the circle, they slowly change to fit their environment. These tiny changes add up until the start and the end of the chain no longer match.
What counts as a "barrier"?
Anything that keeps them apart! It could be a new highway, a rising sea level that creates islands, or even a glacier.
Do all members of a species look exactly the same?
No. Just like humans look different from one another, animals in the same species can have different colors or sizes.
Do they have to live in the same place?
Yes, usually. If two groups of the same animal are separated for a long time and stop mating with each other, they might eventually become two different species.
What causes a cline?
Usually the environment. For example, animals might gradually get larger as you move from a warm area to a cold area because being big helps keep them warm.
Are they one species or two?
That’s the tricky part! Because they can all mate with their neighbors, they seem like one species. But because the ends can't mate, they act like two different species.
Do they become different right away?
No. It takes a very long time. Over many years, they develop different colors, behaviors, or sizes to survive in their specific spot.
Why do scientists group animals into species?
It helps them organize and study the millions of different living things on Earth so they know which ones are related.
Who does this rule not work for?
It doesn't work for bacteria (which don't mate) or dinosaurs (because we can't see them mate anymore).
Can members of different demes still mate?
Yes. Even if they live in different ponds or parks, they are still the same species and can have babies if they meet.
What is a famous example?
Salamanders in California. They live in a ring around a valley. The ones at the "top" of the valley are the ancestors, and the ones at the "bottom" meet up but are now too different to have babies.
When are they officially two different species?
When the barrier is removed (like a river drying up), but the two groups have changed so much that they won't or can't have babies together anymore.