Genealogy and Selection

One story for all the trees

Initial Reaction

• Evolution is well-supported by the data, as Huxley sees it.

• What do we mean by ‘evolution’ here?• ‘Descent with modification’ was Darwin’s phrase for it. • But natural selection is another matter: It seems to

Huxley to be a plausible view, but he’s not completely convinced. To many, including Huxley, it just wasn’t clear that NS could really do the job of explaining evolution by itself.

• This was a very typical reaction, though Huxley was quicker than most to appreciate the strength of Darwin’s case.

The species problem• The line between species was known to be fuzzy and often-

disputed.• What is the difference between two species and one species with

two ‘well-marked’ varieties?• For example, interfertility comes in different degrees, so there’s no

sharp distinction there. • The lines became harder to draw, not easier, as investigation

continued. Counts of species continually came up with different numbers. (cf. Hooker)

• The resistance to Darwin came from a few (Agassiz included) who preferred to invoke a direct separate creation for each species; anyone inclined to prefer secondary/intermediate causes that science could actually investigate soon agreed with Darwin on evolution– and this was just about everyone.

Species and locations

• Invasive new species showed that species weren’t specifically designed to suit only the location where they are naturally found.

• Darwin’s view had no trouble with invasive species– they hadn’t spread to their new locations because of accidents of geology (barriers to migration), not because they weren’t intended/designed for the regions they now were doing so well in.

• Hooker argued that the distribution of plant species fit with Darwin’s ideas beautifully.

Wallace

• Wallace’s line separating Bali from Lombok in the islands of southeast Asia is a striking instance of the kind of pattern involved.

• These two islands mark a sharp shift, from Tropical Asian to Australian forms of life.

• As we now know, Lombok is part of the Australian plate, while Bali is on the Asian plate. Australia is still moving northwards (the mountains of Papua New Guinea are, like the Rockies, the result of the plate collision).

Bali and Lombok

The Plates

More detail

• Wallace’s The Geographical Distribution of Animals (1876): Climate and vegetation (pace Buffon) don’t explain the distribution.

• Camels: Now in deserts across Africa and into Asia; but the closely related Llamas are in South America.

• However, fossil camels are common in North America, from where they could have spread to both South America and to Asia.

• Local ‘arising’ of species and groups together with migrations and the fossil record explains the presence or absence of animals in various areas.

An early sticking-point

• Minor changes to produce new species make perfect sense to Pictet.

• But major changes, enough to produce entirely new groups over time, were another matter.

• Soon, however, the evidence from ‘unity of plan’, taxonomy, the fossil record, biogeography etc. persuaded him to accept large-scale evolution as well.

Intermediate fossils

• These are very popular target of creationist arguments.

• Detailed transitions are rare– after all, we don’t have fossils of most species. One recent estimate says that we have not found fossils of 70% of dinosaur genera (and a far larger percentage of dinosaur species).

• But fossil intermediates at higher levels are not rare at all.

• Archaeopteryx is a wonderful example

An important intermediate

• We see feather imprints in the fine limestone.

• But there’s a long bony tail.

• The bones are solid.• There are teeth and

‘hands’ just like those of small dinosaurs.

• Similar dinosaurs (Compsognathus) are also known.

The horse

• Fossils, especially in North American, detailed the genealogy of the horse.

• It shows the branching pattern of groups within groups.

• Many intermediates join hyracotherium (eohippus) to modern Equus.

Cope and Marsh

• These were two great rivals in N. American fossil hunting.

• The mammalian fossils they found produced many candidates for distant ancestral forms and intermediates between them and familiar modern forms.

Haeckel

• Beyond the fossil record, we can try to identify likely ancestral forms by studying development.

• Haeckel emphasized recapitulation– the idea that species retrace their evolution as they develop from egg to adult.

• He introduced the terms ‘phylogeny’ and ‘ontogeny’, and a slogan: “Ontogeny recapitulates phylogeny”.

• This is not true– development does not proceed as a series of transitions between adult forms of ancestors.

• But von Baer’s more defensible branching account of development still makes development a critical clue to evolution (as are shared vestigial features like human and great ape tail-bones).

Humans

• Huxley: Apes and humans are anatomically closer than monkeys and apes – this applies in particular to the structure of our brains.

• Lyell: evidence of stone tools, associated with ‘extinct quadrupeds’.

• Fossil humans: 1856 ‘Neanderthal man’ discovered.

• Very distinctive (head shape, brow ridges,…)

Neanderthals and us

• There are many differences here: height of forehead, length of skull, size of nasal foramens, robustness of limb bones, shape of rib cage…

Upshot

• Even humans fit in with the rest of nature, in an evolutionary picture– no structure or feature is unique to us, we have co-existed with extinct species, we have near-relatives in the great apes: we are as much a part of the overall scheme of nature as any other animal.

• But natural selection was still not widely accepted as the cause of evolution.

Unification and explanation

• Common descent ties the different trees of biology– taxonomic, developmental and paleontological– together into a single pattern.

• More recently biochemical trees have been shown to reflect the same pattern.

• The unity provided by a single explanation that applies to all these at once is a striking feature of evolution.

• Clearly enough, NS is not essential to this; if other processes were involved, the trees would still all get the same explanation via common descent.

What is an explanation?

• An answer to a why question!• Think of examples:

– Causal explanations: Why did that happen (what led it to come about)? The orbit of the moon, the fall of an object, the spread of a disease (John Snow & the Broad Street pump), etc.

– Intentional explanations: Why did he do that (with what purpose, for what reason)?

– Unification: why is this important? The risk of danglers and ‘dormative virtue’ explanations.

Batesian Mimicry

• The form and distribution of mimicry in butterflies makes a strong case for evolution.

• Local conditions lead to selection for resemblance to common, bad-tasting forms of butterfly.

• Variation in degree & intermediates show it didn’t arise at ‘one go’.

Sexual selection

• Adornment and competition for mates.

• Colourful caterpillars? Warning colours, says Wallace (and it’s testable, and it passes the tests)!

A source of creativity?

• One common objection to NS was that, while it would remove failed ‘experiments’, it didn’t seem to have the potential to ‘create’ anything really new.

• Lyell said that, while it could play the roles of ‘Shiva’ (the destroyer) and ‘Vishnu’ (the preserver), it couldn’t play the role of ‘Brahma’ (the creator).

Building useful structures

• For Darwin there was no limit here– subtle variations can be selected for one reason, and later provide a base for new variations that are selected for others (this is called exaptation or co-option).

• But Mivart argued that the ‘incipient stages of useful structure’ could not be supported/ built by natural selection. (Examples: the giraffe’s long neck, baleen plates, also known as whale bone, detailed mimicry in insects; wings not yet ready for flight.)

Darwin’s reply

• Intermediates for many such traits are found in nature, so selection apparently favours them in some way; it doesn’t matter how…

• For the insects, a very slight initial resemblance could make a small difference, and be the basis for further selection refining the resemblance.

• These replies are pretty good, but many still thought that another mechanism must also be involved (an innate force that tends to produce helpful variations) and rejected gradualism (important adaptations arise suddenly, jumping the gap between their present state and a useful form of the new trait).

Variation enough

• Natural selection does nothing without variation to work on.

• Careful study showed that there was indeed a lot of variation in natural populations.

• Generally the degree of variation was fairly slight (favouring Darwin’s gradualism); as Wallace later showed, up to 20% of the mean value in many traits.

• Darwin saw no way to link what variations occur to the conditions that favour (or disfavour) them– in this sense, Darwin believed variations are undirected.

• Wallace emphasized the ubiquity of variation, while Darwin retained aspects of an earlier view that saw variation as abnormal, produced by some sort of tension between the organism and its environment.

Pangenesis

• But this brings us to the big gap: What is this variation based in, and how is it inherited (as it must be, for natural selection to work)?

• Darwin proposed a theory of his own, pangenesis.(1868)

• Hereditary particles are produced by all parts of the body (gemmules or pangenes).

• They circulate to the reproductive organs and contribute to the traits passed along to offspring.

• This will allow inheritance of acquired characteristics.

Galton

• Galton used rabbits and blood transfusions to test this account– and it failed– no trace of the donors’ colours could be found in the progeny. Galton concluded that hereditary material was passed on without change from ancestors to descendants.