Chapter 3 Natural Selection • Assigned reading chapter 3.
Evolution by Natural Selection • Darwin envisaged process similar to artificial selection that had produced organisms we see today. He called it Natural Selection.
Evolution by Natural Selection • Recall Darwin proposed evolution was the inevitable outcome of 4 postulates: • 1. There is variation in populations. Individuals within populations differ. • 2. Variation is heritable.
Evolution 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.’
Evolution by Natural Selection • If 4 postulates are true then the population will change from one generation to the next. • Evolution will occur.
Evolution by Natural Selection • Darwinian fitness: ability of an organism to survive and reproduce in its environment. • Fitness measured relative to others of its species
Evolution by Natural Selection • Adaptation is a characteristic or trait of an organism that increases its fitness relative to individuals that do not possess it.
Phenotype • An organism’s physical characteristics such as height, coloration, shape as well as its behavior are its phenotype. • Phenotype is the result of both the organism’s genotype and the influence of the environment acting on the genotype.
Natural Selection and coat color in the oldfield mouse: is there variation? • The oldfield mouse is widely distributed in the southeastern U.S. It is preyed upon by a variety of visually hunting predators such as hawks and owls. • The mouse displays considerable variation in coat color both within and between populations across its range.
Natural Selection and coat color in the oldfield mouse • Most populations of the mouse are dark colored, but populations on beaches and barrier islands have lighter colored coats. • Hoekstra et al. carried out a series of experiments to evaluate the hypothesis that natural selection favors a match between coat color and background color.
Is variation in coat color heritable? • There is considerable phenotypic variation in coat color in oldfield mice. • For natural selection to occur the variation must be heritable. Hoekstra et al. have shown that several genes affect coat color in these mice.
Genetics of coat color • The first gene is the melanocortin-1 receptor gene (Mc1R). This gene switches between producing a dark pigment (Eumelanin) and a light pigment (Phaeomelanin) depending on how it is affected by signals from other genes.
Genetics of coat color • If a protein called alpha-MSH binds to the McR1 gene then the dark pigment eumelanin is produced. • If alpha-MSH cannot bind to the Mc1R gene either because of the presence of a different protein called ASP (agouti-signaling protein) or because a mutation in the Mc1R gene so it cannot bind alpha-MSH then the light-colored pigment phaeomelanin is produced.
Genetics of coat color • Hoekstra et al. have shown in populations with large numbers of light-colored mice that two mutations are common: one that prevents alpha-MSH binding to Mc1R and the second a mutant agouti allele that produces much more ASP than usual. • Both mutant alleles result in light-colored mice. Thus there is a clear genetic basis for the observed variation in coat color.
Does variation affect fitness? • Does coat color affect the survival and ultimately reproduction (i.e. fitness) of oldfield mice? • Two experiments suggest it does.
Does variation affect fitness? • Kaufman (1974) carried out an experiment in which pairs of mice (one dark-coated, one light coated) along with an owl were placed in large cages located in habitats with different backgrounds (light or dark and with different vegetation densities).
Does variation affect fitness? • In all cases mice that better matched the background survived better than mice that matched less well.
Does variation affect fitness? • Kaufman et al. carried out a follow-up experiment in which they made silicone mouse models painted light or dark to mimic either the dark or light background. • They placed the models in different habitats and measured from beak and claw marks how often the models were attacked. They found clear differences in attack rates. Models that matched their background were attacked much less.
Natural Selection and coat color in the oldfield mouse • Thus for oldfield mice all 4 postulates are satisfied. There is (i) variation in coat color and it is (ii) heritable. • There is (iii) differential reproductive success (or in this case differential survival which is a necessary precursor to reproduction). • That differential reproductive success is (iv) related to the variation (different coat colors survive better in different habitats).
Another example of natural selection: Darwin’s finches • Evolution of beak shape in Darwin’s Finches. • Peter and Rosemary Grant’s (and colleagues) work on Medium Ground Finches Geospiza fortis • On Daphne Major since 1973.
Evolution of beak shape in Darwin’s Finches. • Postulate 1. Is the population variable? • Finches vary in beak length, beak depth, beak width, wing length and tail length.
Evolution of beak shape in Darwin’s 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.
Evolution of beak shape in Darwin’s Finches. • Peter Boag compared average beak depth of parents with that of their adult offspring. • Strong relationship between offspring and parent beak depths.
Evolution of beak shape in Darwin’s 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.
Evolution of beak shape in Darwin’s 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.
Evolution of beak shape in Darwin’s Finches. • Postulate 4: Are survival and reproduction nonrandom? • Do those who survive and reproduce have different characteristics than those that don’t?
Evolution of beak shape in Darwin’s 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.
Evolution of beak shape in Darwin’s 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.
Did the population evolve? • Chicks hatched in 1978 had deeper beaks on average than those hatched in 1976. • Population evolved.
Evolution of beak shape in Darwin’s 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.
Fig 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.
Fig 3.11 B Over same 30-year period birds evolved more pointed beaks and (next slide) significantly smaller body size.
Constraints on natural selection • There are limits to what natural selection can produce. • Natural selection cannot for example produce organisms that violate the laws of physics for example.