The Common Descent Podcast

Listen to Episode 98 on PodBean, Spotify, YouTube, or elsewhere!

The study of life, past and present, revolves around the evolution, diversity, and extinction of organisms. But exactly how we group organisms and how we understand how new groups arise is a complex topic with much terminology and occasional turmoil. In this episode, we discuss Species and Speciation.

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Possible sensory structures on the tail of the dinosaur Juravenator

What is a Species?

“No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.”
Charles Darwin

The concept of a species seems pretty straightforward: a group of organisms that fit together, united by their ancestry, their features, their reproduction, and various other attributes. Species are often treated as the basic level of classification and as a basic unit of evolution. Scientists identify, study, and compare species to understand life evolution, diversity, and extinction.

But things get tricky when we try to write down a universal definition. This trickiness is often called the “species problem.”

The most popular definition – the one you probably know – is that a species is a group of organisms that can reproduce (the Biological Species Concept). But that doesn’t work for organisms that reproduce asexually, or that hybridize, or which we simply can’t observe mating.

Many species, living and fossil, are identified based on their physical features, the Morphological Species Concept. But that can run into trouble with variation, sexual differences, ontogeny, and metamorphosis.

More than 20 definitions have been formally proposed, emphasizing genetics, ecology, evolutionary history, and more. No single definition works in every situation, and none totally satisfies everybody. Some scientists even argue that the whole idea of a species is arbitrary and unnecessary, while others contend that its messy but useful. In practice, different definitions are often used by scientists studying different types of life in different scenarios.

If you’d like to dive into the details of the debate, check out:
Species Concepts, by John Hawks (non-technical)
Ernst Mayr and the modern concept of species, by Kevin de Queiroz (technical)
A paleontological perspective on species, by Mary Silcox (technical and paywalled)

Speciation: the Origin of Species

Speciation is the process that produces new species from old species. It was a major focus of the work of Charles Darwin when he introduced his theory of natural selection.

Speciation can be as diverse as species themselves, but the basic concept is that, given time and the right conditions, a species will build up enough differences – through natural selection, mutation, genetic drift, or otherwise – to become different from what it was before. Sometimes one species gives rise to another; other times part of one species might split off into something new; yet other times one species might split into two new species.

Often, this process involves some sort of barrier developing between populations. These barriers can be physical (mountains or oceans), behavioral (ecology or breeding behavior), genetic (changes to chromosomes), or anything else that sends one population on a different trajectory than the other. The classic example might be one species that becomes separated by a river into two populations which stop interbreeding and continue evolving separately until they’ve become too different to be considered the same species any more.

Studying speciation in fossils has its benefits and challenges. On the one hand, fossils allow us to track change over time, giving us a great opportunity to watch speciation in action; but on the other hand, the fossil record is often incomplete, and we lack a lot of the genetic, behavioral, or geographic information that helps us identify speciation in action.

For more details into how speciation occurs, look at:
Speciation, National Geographic (non-technical)
Speciation: The Origin of New Species (semi-technical)
Speciation: Introduction (technical, paywalled)

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 44 – Hybridization
• Episode 28 – Charles Darwin
• Episode 54 – Alfred Russell Wallace
• Episode 10 – The Tree of Life

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