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.

Figure 3.2 (part 1) The three components of natural selection. Evolution by natural selection occurs when there is variation, inheritance, and differential reproductive success among individuals in a population.

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.’

Figure 3.2 (part 2) The three components of natural selection. Evolution by natural selection occurs when there is variation, inheritance, and differential reproductive success among individuals in a population.

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.

Figure 3.3 Phenotype depends on the effects of both genotype and environment. Here we see how the height of a yarrow plant (Achillea millefolium) depends on its genotype and the altitude at which it is raised, as shown by populations of yarrow plants grown in gardens at three sites that were at different altitudes: high, medium, and low elevation. For example, the green screen behind the plants of genotype 1 shows that these plants grow tall at high and low elevations, but are short at medium elevation. The blue screen behind the plants of genotype 4 shows that these plants respond very differently to elevation. This genotype grows tallest at medium elevation and shorter at high and low elevations. Adapted from Clausen, Keck, and Hiesey (1940, 1948).

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.

Figure 3.5 Coat color variation in mice. Two color variants of Peromyscus polionotus: (A) the darker inland form, and (B) the lighter beach-dwelling form.

Figure 3.6 Variation in coat color and genotypes at the Mc1R locus. Peromyscus polionotus exhibits extensive coat color variation across localities in Florida. Red areas indicate the distribution of beach populations; gray areas denote the distribution of inland populations. Characteristic phenotypes for each population are indicated by the coat coloration sketches, but coat color varies within populations as well. The pie charts indicate that the Perdido Key, Santa Rosa Island, Choctawhatchee, and St. Andrew’s beach mouse populations had more than a single variant of the Mc1R locus associated with coat coloration. All populations shown here are considered part of a single species—Peromyscus polionotus. Adapted from Hoekstra et al. (2006).

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.

Figure 3.7 Genetics of coat color determination in mice. The protein Mc1R acts as a genetic switch, determining whether dark eumelanin or light phaeomelanin is produced. (A) α-MSH binds to the Mc1R receptor and triggers eumelanin production. (B) ASP binds to the Mc1R receptor, preventing α-MSH from binding and thereby causing the cell to switch over to phaeomelanin production.

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.

Figure 3.8 Two mutations contribute to light coat coloration in two different ways. (A) A mutation in the Mc1R protein reduces the ability of -MSH to bind, and thus limits eumelanin production. (B) A mutation in the regulatory region of the Agouti gene increases ASP expression and thus further inhibits eumelanin production.

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.

Figure 3.9 Early work on predation, coat color, and fitness in the oldfield mouse. In his experiments on predation, Kaufman exposed mice with light and dark coats to owl predators in three different environments: dark background with sparse vegetation (A), light background with sparse vegetation (B), and light background with dense vegetation (C). The percentile figures in each panel indicate the fraction of mice of each color captured by the owl. In all cases, owls captured a higher percentage of “color-mismatched” mice—namely, those with coat colors that failed to match their environments.

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.

Figure 3.10 Predation, coat color, and fitness in the oldfield mouse, using plastic models in the field. Hoekstra and colleagues placed light and dark silcone mouse models in light and dark environments to test predation rates. (A) The experimental sites: a light beach environment and a dark inland environment. (B) Proportion of attacks against light and dark mice in the light environment. (C) Proportion of attacks against light and dark mice in the dark environment. Adapted from Vignieri et al. (2010).

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.

FIG 3.7

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.

FIG 3.8b

FIG 3.8A

Fig 3.8c

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.

FIG 3.9

Did the population evolve? Chicks hatched in 1978 had deeper beaks on average than those hatched in 1976. Population evolved.

Fig 3.10

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.

Fig 3.11 C

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.

Constraints on natural selection For example, the size of unicellular organisms is limited because they depend on diffusion across the cell membrane to obtain their essential requirements. Because the volume of a cell increases as a cubic function of linear dimension, but surface area only increases as a square function, above a certain size, a cell becomes too big for diffusion to supply its needs.

Constraints on natural selection A lack of genetic variation also constrains natural selection. Natural selection depends on mutation and sexual reproduction to produce variation, but if a trait is not produced by these processes it cannot be selected for.

The nature of Natural Selection Many misconceptions about how selection operates and evolution occurs. Points to remember about natural selection

Natural 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.

Natural selection acts on individuals, but its effects accumulate in populations During drought individual finch’s beaks did not change, but average beak dimensions changed because more small-beaked birds died than large-beaked birds.

Evolution 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.

Natural 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.

New 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.

New 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.

Fig 3.12

New traits evolve even though selection acts on existing traits. Natural selection can also modify existing features over time for a new purpose e.g. Panda’s 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.

Natural selection does not produce ‘perfect’ solutions Panda’s thumb not a perfect solution.

Natural 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.

Natural 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.

Natural 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.

Selection 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.