Chapter 8 Natural Selection: Empirical studies in the wild  Assigned reading chapter 8.

 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.

 Recall -- 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. Evolution by Natural Selection

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 lots of phenotypic variation in coat color in oldfield mice.  For natural selection to occur the variation must be heritable. Several genes affect coat color in these mice.

Genetics of coat color  Melanocortin-1 receptor gene (Mc1R). This gene produces either a dark pigment (Eumelanin) or a light pigment (Phaeomelanin) depending on signals it receives from other genes.

Genetics of coat color  If a protein called alpha-MSH binds to the Mc1R gene then the dark pigment eumelanin is produced.  If alpha-MSH cannot bind then a light- colored pigment phaeomelanin is produced.

Genetics of coat color  In populations with many light-colored mice two mutations are common:  (1) mutant that prevents alpha-MSH binding to Mc1R  (2) mutant allele that produces an excess of a protein called ASP that competes with alpha-MSH to bind to the MC1R.  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) experiment  Pairs of mice (one dark-coated, one light coated) and 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. made silicone mouse models painted light or dark.  Placed the models in different habitats and measured how often the models were attacked.  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.

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  Population evolved.

Strong association between parent and offspring beak sizes. Hence narrow-sense heritability is high. There is a difference in beak dimensions (selection differential) between breeders and original population. Response to selection in that beak dimensions increased in the offspring.

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.

Agents of selection operating in opposite directions– gall flies. Gall flies induce plants to produce galls in which the larva develops in a protected environment.

Gall diameter is variable. Some individuals produce large galls and others small ones. Relatives produce similar size galls and there is heritable variation in gall size.

Stabilizing selection on gall size  There are two major predators of larvae in galls – birds and parasitic wasps.  Parasitic wasps cannot reach larvae enclosed in very large galls, but birds spot large galls more easily and consume the larvae. There is thus stabilizing selection on gall size with intermediate sized galls favored.

Milk drinking: evidence for natural selection  Milk contains the sugar lactose and young mammals produce an enzyme, lactase, to break it down. Most humans (about 70%) stop producing lactase after weaning, but many western Europeans retain the ability to digest lactose into adulthood.

Milk drinking: evidence for natural selection  Humans began to domesticate cattle in NW Europe about 10,000 years ago and this new food source favored individuals able to digest milk into adulthood.  The frequency of alleles for lactose tolerance are highest in NW European populations and lowest in SE Europe the in populations furthest from the origin of cattle domestication.

Milk drinking: evidence for natural selection  A similar pattern is found when comparing animal herding societies with nearby non- herding populations.  The herders have much higher tolerance for lactose than their non-herding neighbors.

Humans as agents of selection  Humans act as strong agents of selection.  This has occurred through deliberate choice (artificial selection for desired traits in crops and domesticated animals) and inadvertently through environmental change.

Artificial 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.  Recall: Darwin closely studied pigeon breeding as a process analogous to natural selection.

Artificial 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.

Artificial Selection  Cauliflower has large dense infloresence. This results from mutant ‘loss of function’ alleles of two genes that affect flower structure and infloresence density.

Artificial 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.

Pesticides and herbicides act as agents of selection

Resistance to pesticides  Insects and plants treated with chemicals designed to kill them have rapidly developed resistance.  Heavy spraying creates an environment in which any mutations that offer resistance are strongly selected for and spread rapidly.

Resistance to pesticides in houseflies Inverted triangle indicates first occurrence of resistance and R indicates when most Populations were resistant. Bar width indicates extent of the pesticides use.

Rapid evolution of herbicide resistance

Resistance to pesticides  Farmers are now using evolutionary biology to reduce rate of evolution of resistance.  Resistance frequently comes with a cost and in pesticide-free environments non- resistant pests may have an advantage and outcompete resistant forms.

Resistance to pesticides  To maintain non-resistant genes in pest populations farmers are now setting aside pesticide free refuges that are not sprayed.  For example farmers using BT-corn (corn containing a gene that produces a natural pesticide) must set aside 20% of their plantings as non-BT corn

Resistance to pesticides  States in which large areas of refuges were used have shown much slower rates of BT-resistance in pests than states where smaller areas of refuges were set aside.

Hunting and fishing as agents of selection  Humans have intensively fished all the world’s oceans and that fishing pressure has resulted in fish populations evolving in response.  For example, because under fishing pressure few individuals survive to breed late in life, fish such as cod today mature much younger and at smaller sizes than they did 20 years ago.

Hunting and fishing as agents of selection  In a similar fashion selective shooting by trophy hunters of males with larger horns has led to the evolution of smaller horns in hunted populations.

Evolution of shorter male horns due to hunting