Transcript Genealogy and selection

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 oftendisputed.
• 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 nearrelatives 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.