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

Copyright ©1999 by the National Academy of Sciences

Sato, Akie et al. (1999) Proc. Natl. Acad. Sci. USA 96, 5101-5106

Phylogeny of Darwin’s Finches

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) non-random? (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