Title: Influence of geological thinking on Darwin
1Influence of geological thinking on Darwin
- By time of The Beagle voyage idea that Earth
was young was being challenged. - Opposition based on principle of
Uniformitarianism. - Idea that geological processes happening today
are the same as have operated in the past.
2Influence of geological thinking on Darwin
- Uniformitarianism contrasted with Catastrophism
which proposed that current geological formations
had resulted from catastrophic events (such as
biblical flood) which occurred on scale unknown
today. - Uniformitarianism first proposed by James Hutton
and championed by Charles Lyell.
3Influence of geological thinking on Darwin
- Hutton and Lyell inferred Earth must be very old
based on measurements of rate of ongoing rock
forming processes (e.g. deposition of mud and
sand). - These developments in geology focused Darwin on
the potential importance of gradual change in
shaping structures.
4Darwinian Natural SelectionArtificial Selection
- Artificial Selection. Humans have selectively
bred for desirable traits in domestic animals and
plants for millenia. - Process has produced our crop plants, garden
plants, pets, and domestic animals. - Darwin closely studied pigeon breeding.
5Artificial Selection
- Cauliflower, broccoli, kale, brussels sprouts all
descended from wild cabbage. - All these crops can be crossed and produce
fertile offspring. - Cauliflower edible bit is the inflorescence or
flower stalk.
6Artificial Selection
- Cauliflower has large dense infloresence. This
results from mutant loss of function alleles of
two genes that affect flower structure and
infloresence density.
7Artificial Selection
- Early farmers choosing among their crops selected
those with largest infloresences. Process has
resulted in cauliflowers that are homozygous for
both loss of function alleles.
8Evolution by Natural Selection
- Darwin envisaged process similar to artificial
selection that had produced organisms we see
today. He called it Natural Selection.
9Evolution by Natural Selection
- Darwin proposed evolution the inevitable outcome
of 4 postulates -
- 1. There is variation in populations.
Individuals within populations differ. - 2. Variation is heritable.
10Evolution by Natural Selection
- 3. In every generation some organisms are more
successful at surviving and reproducing than
other. Differential reproductive success. - 4. Survival and reproduction are not random, but
are related to variation among individuals.
Organisms with best characteristics are
naturally selected.
11Evolution by Natural Selection
- If 4 postulates are true then the population will
change from one generation to the next. - Evolution will occur.
12Evolution by Natural Selection
- Darwinian fitness ability of an organism to
survive and reproduce in its environment. - Fitness measured relative to others of its species
13Evolution by Natural Selection
- Adaptation is a characteristic or trait of an
organism that increases its fitness relative to
individuals that do not possess it.
14Testing the postulates
- Evolution of beak shape in Darwins Finches.
- Peter and Rosemary Grants (and colleagues) work
on Medium Ground Finches Geospiza fortis - On Daphne Major since 1973.
15Evolution of beak shape in Darwins Finches.
- Postulate 1. Is the population variable?
- Finches vary in beak length, beak depth, beak
width, wing length and tail length.
16(No Transcript)
17Evolution of beak shape in Darwins Finches.
- Postulate 2 Is variation among individuals
heritable? - Variation can be a result of environmental
effects. - Heritability proportion of the variation in a
trait in a population that is due to variation in
genes.
18Evolution of beak shape in Darwins Finches.
- Peter Boag compared average beak depth of parents
with that of their adult offspring. - Strong relationship between offspring and parent
beak depths.
19FIG 3.7
20Evolution of beak shape in Darwins Finches.
- Postulate 3 Do individuals differ in their
success at survival and reproduction? - 1977 drought 84 of G. fortis individuals died,
most from starvation. In two other droughts 19
and 25 of the population died.
21Evolution of beak shape in Darwins Finches.
- Seed densities declined rapidly during drought
and the small soft seeds were consumed first. - Average size and hardness of remaining seeds
increased over the course of the drought.
22FIG 3.8b
23FIG 3.8A
24Fig 3.8c
25Evolution of beak shape in Darwins Finches.
- Postulate 4 Are survival and reproduction
nonrandom? - Do those who survive and reproduce have different
characteristics than those that dont?
26Evolution of beak shape in Darwins Finches.
- As drought progressed small soft seeds
disappeared and large, hard Tribulus seeds became
a key food item. - Only birds with deep, narrow beaks could open
them.
27Evolution of beak shape in Darwins Finches.
- At end of the 1977 drought the average survivor
had a deeper beak than the average non-survivor
and also a larger body size.
28FIG 3.9
29Did the population evolve?
- Chicks hatched in 1978 had deeper beaks on
average than those hatched in 1976. - Population evolved.
30Fig 3.10
31Evolution of beak shape in Darwins Finches.
- Variation in weather from year to year on Daphne
Major over 30 years has led to variation in the
traits that are favored by selection. - Population has evolved over time.
32Fig 3.11 A
Over the course of 30 years (1970 to 2000) beak
size evolved. Rose sharply during drought (red
line) then declined to pre-drought dimensions.
33Fig 3.11 B
Over same 30-year period birds evolved more
pointed beaks and (next slide) significantly
smaller body size.
34Fig 3.11 C
35The nature of Natural Selection
- Many misconceptions about how selection operates
and evolution occurs. - Points to remember about natural selection
36Natural selection acts on individuals, but its
effects accumulate in populations
- Individual finches live or die during a drought
(the selection event). - But change occurs in the characteristics of the
population, not in individuals.
37Natural selection acts on individuals, but its
effects accumulate in populations
- During drought individual finchs beaks did not
change, but average beak dimensions changed
because more small-beaked birds died than
large-beaked birds.
38Evolution causes changes in allele frequencies
- Evolution only occurs when traits have a genetic
basis. - If beak dimensions were environmentally induced,
no evolution could take place. After drought,
frequencies of phenotypes in next generation
might have been the same as before.
39Natural selection does not plan ahead.
- Each generation is result of selection by
environmental conditions of the previous
generation. - Evolution always one generation behind
environmental changes.
40New traits evolve even though selection acts on
existing traits.
- This occurs because
- 1. mutation produces new alleles.
- 2. In sexually reproducing organisms meiosis and
fertilization recombine existing alleles to
produce new genotypes.
41New traits evolve even though selection acts on
existing traits.
- Artificial selection for oil content in corn.
- After 60 generations oil levels were well above
starting values.
42Fig 3.12
43New traits evolve even though selection acts on
existing traits.
- Natural selection can also modify existing
features over time for a new purpose e.g. Pandas
thumb. - Trait used in novel way and eventually developed
into a new structure referred to as a
preadaptation. This does NOT mean there is
pre-planning by natural selection.
44Natural selection does not produce perfect
solutions
- Pandas thumb not a perfect solution.
45Natural selection does not produce perfect
solutions
- On Daphne Major during drought finches with
narrow beaks survived better than those with
wider bills. - At end of drought, however, selection for deeper
bills and bigger body size resulted in wider
beaks even through deeper narrower beaks would
have been a better solution. -
- Presumably same genes control all three traits.
And solution is not perfect.
46Natural selection does not produce perfect
solutions
- Similarly, many characteristics of organisms are
the result of compromises between different
selection pressures. - The wings of various species of auk (seabirds
that dive and swim) are a compromise between the
need to fly (i.e. swim) underwater and in the
air, two very different media.
47Little Auk
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Razorbill
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48Natural selection is nonrandom, but not
progressive
- There is no goal of natural selection.
- Evolution makes organisms better adapted to their
environments, but there is no trend towards being
more advanced. - E.g. Tapeworms have no digestive tract. They are
simpler than their ancestors.
49Selection does not act for the good of the
species
- Apparently altruistic acts (e.g. giving an alarm
call) are favored because they enhance relatives
survival. - Infanticide in lions benefits individual male
lions not the species as a whole.