- Part 1: Introduction and the Introduction
- Part 2: Darwin Didn’t Know About Chromosomes!
- Part 3: Darwin 1, Old-Timey Creationists 0
- Part 4: All Your Evidence Are Belong To Us
- Part 5: An Admittedly Weak Chapter
- Part 6: Jeanson’s Fulcrum Fails
- Part 7: TBA
- Part 8: TBA
- Part 9: TBA
- Part 10: TBA
We’re now embarking on Part II of the book, entitled “All Things (re) Considered”. After laying some of the foundations of genetics in Part I, now Jeanson reviews Darwin’s evidence that he presented in On the Origin of Species to argue for evolution. Over the forthcoming 3 chapters, Jeanson will tackle geography, ancestry, and timescales, asking the question ‘how could Darwin have talked about the origin of species and convinced the scientific community at the time that his ideas were correct without genetic evidence?’
Chapter 4 – The Riddle of Geography
First, using an analogy and examples, Jeanson establishes that inductive reasoning is the core of the scientific method. Data are collected and scrutinised in order to come up with a model that explains them, and then competing models are eliminated by employing further tests and gathering more data. He acknowledges that Darwin employed this strategy meticulously:
In On the Origin of Species, Darwin carefully employed the method of inductive reasoning. As we observed in chapter 1, Darwin’s ideas represented a paradigm shift from the century prior. In the 100 years leading up to 1859, competing explanations — creationist ideas — for the origin of species dominated scientific thought. Consequently, Darwin took great pains in his seminal work, not only to undergird his own ideas with evidence, but also to reveal the shortcomings of creationist theories.
Their elimination was critical to Darwin’s success.
Of course, the “creationist theories” Jeanson is talking about here doesn’t include his “modern” creationism but refers to the creationist ideas that were around in Darwin’s day.
At this point, the primary topic of this chapter is revealed. As Jeanson briefly noted in the introduction, in the early 19th century the idea was that species had always lived in their “matching” habitats because that’s how they were created in the beginning. Lions were created to “match” savannahs, so that’s why they’re only found there, polar bears were created to “match” Arctic tundras, so that’s why they’re only found there, an so on. This immediately implies that species are “fixed”. But as more of the world was explored, it became clear that this isn’t the whole story. Lions aren’t found in every savannah, just those in Africa, even though they could presumably thrive just as well in, say, the Brazilian savannah. This trend held true for huge numbers of species – they were endemic to particular regions, not just to particular habitats.
In particular, observations of island biodiversity contained important clues. Islands like Hawaii only hosted mammals that were capable of swimming or flying there like seals and bats. This suggested that the resident mammals of Hawaii really had got there by migrating there – they weren’t created on the island.
Jeanson notes that Darwin and Wallace synthesised a compelling argument for all this endemism: barriers to migration exist (e.g. oceans, deserts) that prevents a species from moving to another favourable environment. Even though lions could probably thrive in the Brazilian savannahs, they would have to swim across the entire Atlantic ocean to get there, which seems unlikely. While migration explains the overall pattern of biodiversity, there is of course some nuances: geological events change landscapes and sea levels, changing the barriers to migration over time, and there is also an element of stochasticity. Most barriers to migration aren’t 100% effective, they just reduce the statistical probability that a species will cross them. The more effective the barrier, the fewer species should be found to have traversed them, for example.
This raised the question of why each habitat seemed to host a range of morphologically similar species, relative to other near-identical habitats. For example, there are species of flightless birds in both Africa (2 species of ostriches) and South America (3 species of rheas). According to the idea of species fixity, all 5 species would have been created independently, and all independently migrated to their respective continents. But why would it be that the 5 species sorted themselves by morphological similarity: rheas with rheas and ostriches with ostriches? That was a difficult question to answer, but luckily (my emphasis):
Darwin proposed another explanation. Though he knew nothing about the relationship between DNA and heredity, he was able to draw rudimentary principles about the process of inheritance.
Though common ancestry preserves biological similarity, some level of change happens each generation. Darwin called this principle descent with modification. Though the identification of DNA as the substance of heredity was still 100 years in the future, this simple principle was all that Darwin needed to make his point — and eliminate competing explanations.
In other words, descent with modification explained the distribution by proposing that the 3 species of rheas stemmed from a single common ancestor in South America, and the 2 species of ostriches stemmed from another common ancestor in Africa. This model explained global patterns of biodiversity. Jeanson is correct in his recounting of history here, but notice what I highlighted in bold. He spent a good deal of Chapter 2 lamenting that Darwin was clueless about inheritance and now he’s admitting that Darwin could actually come to some rudimentary conclusions based on his observations. This isn’t strictly contradictory because in Chapter 2 he was basically teasing this answer, but it still comes across to me as strange to emphasise a rhetorical question so much only to answer it in a throwaway sentence in a later chapter.
Thus, Darwin successfully eliminated the creationist hypothesis of the fixity of species’ geography. Furthermore, as we observed in the preceding sections, the fact of migration also leads to the conclusion of common ancestry among some species. In other words, Darwin’s geographic observations successfully eliminated the view of species fixity.
Thus, by virtue of the nature of the scientific method and despite lacking genetic data, Darwin could successfully argue for the scientific merits of his thesis. At least, he could do so in 1859.
Aside from that little caveat at the end of the passage (my emphasis) which leads into further chapters which attack Darwin’s thesis from other angles, not from biogeography, there’s little in this chapter to object to, so unfortunately there hasn’t been much for me to add this time. Jeanson describes how Darwin used biogeography and the principle of descent with modification to provide a superior explanation for global species distribution than the creationists of the day, and that’s what did happen. Jeanson doesn’t argue with this because he’s not arguing for the “old” creationist position of species fixity, but for the shiny, “modern”, version which accepts speciation and limited common ancestry.
The next 2 chapters have a lot more baggage to be unpacked, so I’ll cover them separately in upcoming blog posts, leaving this one a little short.
Comments and queries are welcome.