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Darwinian Natural Selection – Pt. 2
• Natural selection
– beak size in Darwin’s medium ground finch,
Geospiza fortis
• The nature of natural selection
Darwin’s finches
• Galàpagos (and Cocos) Islands are home to 14 (or 15?) endemic finch
species – collectively known as Darwin’s finches
• Similar in size (4 – 6 inches long) and color (brown to black)
• Genetic data support hypothesis that they represent an adaptive
radiation from a single ancestral species (i.e., they are a monophyletic
group)
• Closest relative is probably the grassquit (Tiaris obscura) which is
native to S. America
• Est. time of divergence from common ancestor 1.2 to 2.3 Myr ago (by
molecular clock on mtDNA)
• Galàpagos Is. Are 0.7 to 3.5 Myr old
• Major evolutionary modification has been to beak size and shape,
which differentiates the species by diet
Darwin’s Finches
Phylogeny of Darwin’s Finches
Sato, Akie et al. (1999) Proc. Natl. Acad. Sci. USA 96, 5101-5106
Copyright ©1999 by the National Academy of Sciences
Natural selection and evolution of beak size in
Darwin’s medium ground finch, Geospiza fortis
• Peter and Rosemary Grant and colleagues
• 1973 –
• Geospiza fortis on islet of Daphne Major
– island is small enough that the population of G. fortis can be
censused completely
– by 1977 more than half the population captured and banded
– since 1980 virtually 100% banded
– when birds are banded, they are also weighed and measured
– little migration to and from the island
– G. fortis is primarily a seed eater (grasp seeds at base of bill and
crack them by applying force)
Map of Galàpagos Is. and Daphne Major
Are G. fortis phenotypes variable? (Postulate 1)
Are G. fortis phenotypes heritable? (Postulate 2)
Heritability of morphological traits in G. fortis
Trait
Heritability (parent offspring regression)
Weight
0.85
Wing length
0.89
Beak length
0.67
Beak depth
0.82
Beak width
0.90
Estimating heritability
• Heritability estimates for morphological traits in
G. fortis range from 60 – 90%
• Roughly, this is the proportion of total phenotypic
variation in a trait that is due to genetic differences
among individuals (as opposed to environmental
causes of phenotypic variation)
• Estimation of heritability by parent-offspring
regression assumes that the only cause of
resemblance between parents and offspring is
shared genes (see Box 3.1 on p. 82 of your text)
Drought and bird mortality
• During 1976-77 a severe drought occurred
(about 20% of normal rainfall during the
wet season)
• 84% of G. fortis on Daphne Major
disappeared (some deaths confirmed by
direct observation, no missing birds
returned the following year)
– (Postulate 3: individuals vary in their success at
surviving or reproducing)
Population size of G. fortis on Daphne Major,
1975-78
Natural selection on G. fortis
• Was survivorship of birds during the
drought (and subsequent reproduction) nonrandom? (Postulate 4)
• Yes! Larger individuals were more likely to
survive the drought — larger birds were
naturally selected
Distributions of beak depth before and after
the 1976-77 drought
Note: this
change in
average
beak depth
occurred
within a
cohort of
individuals
Change in average size of G.fortis due to selective
mortality during the 1976-77 drought (mean ± 2 SE)
Trait
Before (N ≈ 640) After (N = 85)
Weight (g)
15.8 (0.12)
16.9 (0.34)
Wing chord (mm)
67.7 (0.19)
69.2 (0.49)
Tarsus length
18.8 (0.06)
19.1 (0.15)
Beak length
10.7 (0.06)
11.1 (0.16)
Beak width
8.7 (0.05)
9.0 (0.13)
Beak depth
9.4 (0.07)
10.0 (0.18)
Why did larger birds have a survival
advantage?
• The abundance of seeds declined during the
drought (mirroring closely the decline in the finch
population)
• Seeds that were available during the drought were
large and hard
• Small individuals could not eat the larger, harder
seeds (confirmed by observational studies)
Change in seeds during the drought
Did the population of G. fortis on Daphne Major evolve?
(natural selection + heritability = evolution)
Longer-term evolution in G. fortis
Pink bands represent 95% confidence intervals
for 1973 (1st year with complete data)
Some references on Darwin’s
Finches
• Book
– Weiner, Jonathan. 1994. The Beak of the
Finch: a Study of Evolution in Our Time
• Genetic relationships
– Sato, A., et al. 1999. Phylogeny of Darwin’s
finches as revealed by mtDNA sequences.
Proc. Nat. Acad. Sci. 96:5101-5106
– Sato, A., et al. 2001. On the origin of Darwin’s
finches. Molec. Biol. Evol. 18:299-311
The nature of natural selection – 1
• Natural selection and evolution are logically
distinct
• Evolution without selection?
– Yes, random genetic drift
• Selection without evolution?
– selected character(s) not heritable
– stabilizing selection (rather than directional
selection)
The nature of natural selection – 2
• Natural selection acts on individuals but its consequences
occur in populations
– drought did not change the size of individual G. fortis, and AZT
does not alter the amino acid sequence of individual reverse
transcriptase molecules
– The population of G. fortis changed because larger birds survived
better and left more offspring, and the population of HIV in a host
evolves to become resistant to AZT because virions with reverse
transcriptase variants that are better at synthesizing DNA in the
presence of AZT leave more offspring
• Although the environment may modify the phenotype of
an individual, such acquired characteristics are not
heritable and cannot result in evolution.
The nature of natural selection – 3
• Natural selection acts on phenotypes but evolution
consists of changes in the genetic composition of
populations
– This is just another way of saying that non-heritable
traits cannot evolve (i.e., variation in a trait must have a
genetic basis in order for the trait to evolve)
– What about changes in average height in human
populations during the last 100 - 200 yrs?
The nature of natural selection – 4
• Natural selection is not forward looking
– Natural selection is a function of current environmental
conditions and current phenotypic variation in a
population, it cannot anticipate future environments or
which phenotypes will be adaptive in the future
– “Evolution is always a generation behind any changes
in the environment” (F. & H., p. 88)
The nature of natural selection – 5
• Natural selection and evolution are not purposeful
• Natural selection acts on individuals (those that
are better adapted are more likely to survive and
reproduce), not for the good of the species –
although the consequence may be survival of a
species in the face of environmental change
• Similarly, species do not evolve to fulfill some
higher purpose, or plan of nature, or to ensure the
“balance of nature” (the teleological fallacy)
The nature of natural selection – 6
• Natural selection is non-random, but it is not
progressive
• There is no inexorable trend toward more
advanced forms of life
• Be careful when you use terms like “higher” and
“lower” in describing taxonomic groups. These
terms are meaningless as far as adaptedness is
concerned
The nature of natural selection – 7
• What is fitness?
– a basic definition of the fitness of an individual is the number of
offspring produced by that individual
• What are fitness components?
– survivorship (viability), mating success, fecundity, age of first
reproduction, etc.
• Is Darwinism circular (tautological)?
– “survival of the fittest”: the fittest will survive but we can only
identify the fittest as those who have survived. There is no
independent criterion by which we can determine fitness.
– In effect, “fit” is being used in two different senses: (1) how well
and organism is fit (or adapted) to its environment = “fittest”; and
(2) number of offspring = “fitness”
The nature of natural selection – 8
• Darwinism is not circular
– those individuals who are better suited to their environments will
be more likely to survive and reproduce and thus have higher
fitness on average
– a distinction is made between variation which is responsible for
non-random survival (= natural selection) and fitness which is
number of offspring produced
– in order for natural selection and evolution to occur, it is not
necessary to specify in advance which variations will be most
adaptive
– in fact, we often test whether or not a trait is adaptive by looking to
see if it affects fitness (or a fitness component)
The nature of natural selection – 9
• Natural selection does not produce perfect adaptation
– It simply does the best it can with the range of phenotypic and genetic
variation that is available in the population
• Deeper analysis of the selection event on G. fortis during 1976-77
indicated that an optimal evolutionary response would have been to
have birds with deeper and narrower beaks, that is, for beak shape as
well as size to evolve. But beak depth and beak width are positively
correlated (both phenotypically and genetically) which means that
evolution of deeper beaks also means evolution of wider beaks (at least
in the short term)
• This is a form of genetic constraint — alleles that make deeper beaks
also make wider beaks (pleiotropy)
• The average phenotype of a population will almost always be a
compromise resulting from competing environmental demands and
genetic constraints