Natural Selection

Define natural selection
A process in which events that befall individuals alter the collective properties of populations, requiring us to think statistically
What is Darwins favourite domestic organism? What studies did he use this organism for?
The pigeon. He bred pigeons himself to learn the experts techniques

What was the example used to demonstrate evolution under domestication?

Explain the process of this experiment and its results

Domestic tomato – Solanum lycoperscium which has been bred to a larger size. Disparity in fruit size between wild versus domestic tomatoes is largely due to genetic differences. Tomatoes carry a gene fw2.2 which codes for the protein made during early fruit development.

Proteins job is to repress cell division. The more of the protein a plant makes, the smaller its fruit. Changes in the nucleotide sequence in the fw2.2 promoter – the genes on-off switch – alter the timing of production and the amount of repressor made.

Because farmers preferred larger tomatoes, year after year they planted their fields with seeds from the largest fruit of the previous crop.

Bin Cong and colleagues (2008) identified a second gene, fas, that influences fruit size by controlling the number of compartments in the mature fruit.
Fewer compartments = common in wild types
More compartments = domestic varieties

Results infer that the vast majority of cases traits selected under domestication are disastrous in nature.

What are Darwins four postulates?

1. Individuals within a population differ from one another

2. The differences are, at least in part, passed from parents to offspring (variation is inherited)

3. Some individuals are more successful at surviving and reproducing

4. The successful individuals are not merely lucky; instead they succeed because of the variant traits they have inherited and will pass to their offspring (reproduction is nonrandom: successful individuals succeed because of the useful trait)

Define Darwinian fitness
An individuals ability to survive and reproduce
Define adaptation
A trait that increases an organisms fitness relative to individuals lacking it
Define adaptive
A trait that increases an organisms fitness

What experiment did Kristina Niovi Jones and Jennifer Reithel (2001) want to know?

How was their experiment set up?

Wanted to know whether natural selection y choosy bumblebees would drive the evolution of flower color in snapdragons

To find out, they established an experimental population of 48 individuals in which they made sure Darwin’s postulates 1 and 2 were true then they monitored the plants and their offspring to see whether postulates 3 and 4 and the predicted consequence were all true

Kristina Niovi Jones and Jennifer Reithel Experiment:

Explain how Postulate 1: Individuals differ from one another was supported

The snapdragons in Jones and Reithels population varied in flower color.

3/4 of the plants had flowers that were almost pure white with just two spots of yellow on the lower lip. Rest had flowers that were yellow all over

Kristina Niovi Jones and Jennifer Reithel Experiment:

Explain how Postulate 2: Variation is inherited was supported

Variations in color among jones and Reithel’s plants was due to differences in plants genotypes for a single gene.

These genes were passed on to present 12 SS, 24 Ss and 12 ss plants

Kristina Niovi Jones and Jennifer Reithel Experiment:

Explain how Postulate 3: Do individuals vary in their success at surviving or reproducing? was supported

To guage the plants success at reproducing by making seeds, researchers counted the seeds produced from each fruit. Consistent with Darwin’s third postulate, the plants showed considerable variation in reproductive success, both as pollen donors and as seed mothers

Kristina Niovi Jones and Jennifer Reithel Experiment:

Explain how Postulate 4: Is reproduction nonrandom? supported

Reproductive success through seed production was less strongly associated with color than was success through pollen donation

Nonetheless, the white plants were somewhat more robust than the yellow plants and so produced, on average, slightly more seeds per fruit

Consistent with Darwin’s fourth postulate, reproductive success was not random. At both pollen export and seed production, white plants beat yellow

Who was involved in the Galapagos Finch studies?
Peter Grant, Rosemary Grant and colleagues who started studying finches on the Galapagos Archipelago since 1973
Name the species of finches under study and their food source

Warbler finches – Feed on insects, spiders and nectar

Woodpecker and Mongrove finches – use twigs or cactus spines as tools to pry insect larvae or termites from dead wood

Ground finches – Pluck ticks from iguanas and tortoises in addition to eating seeds

Vegetarian finch (Platyspiza crassirotris) – Eat leaves and fruit

What did Grant and Grants study conclude?
That both within and across finch species, the beak size is correlated with the size of seeds harvested. In general, birds with bigger beaks eat larger seeds, and birds with smaller beaks eat smaller seeds. This is because birds with different beak sizes are able to handle different sizes of seeds more efficiently

Grant and Grant:

Testing Postulate 1: Is the finch population variable?

-Weighed each bird and measured its wing length, tail length, beak width and depth

All characteristics measured correlate with there being variation among the individuals within populations

Grant and Grant:

Testing Postulate 2: Is some of the variation among individuals heritable?

To determine whethere at least part of the variation among finch beaks is genetically based and thus passed from parents to offspring, Peter Boag estimated the heritability of beak depth

Boag compared the average beak depth of families of G.fortis young after they had attained adult size to the average beak depth of their parents. Boags data reveal a strong correspondence among relatives. Parents with shallow beaks tend to have chicks with shallow beaks and parents with deep beaks tend to have chicks with deep beaks

This is evidence that a large proportion of the observed variation in beak depth is genetically based and can be transmittted to offspring

Describe the BMP4 experiment and its significance

Campas and colleagues focused on a growth factor bone morphogenic protein 4 (BMP4) which is active during embryonic development and helps sculpt the shape of bid beaks for each of the six species of ground finch. Researchers treated three embryos of a particular developmental age with a fluorescent probe that binds to mRNA made by B,p4, gene encoding for BMP4.

It was concluded that species with higher Bmp4 expression have deeper beaks. Abzhanov and colleages suggested that the different species of ground finches harbor alternate veersions of one or more of the genes that determine when, where and how strong the gene encoding BMP is activated

Define heritability
Heritability of a trait is definesd as the fraction of the variation in a population that is due to differences in genes

Grant and Grant:

Testing Postulate 3: Do individuals vary in their success at surviving and reproducing?

Mortaility rates showed that 89% of Geospiza conirotris individuals die before they breed. Trevor Price and coworkers determined that 19% and 25% of the G.fortis on Daphne Major died during droughts in 1980 and 1982

Every natural population studeid had more offspring produced each generation than survive to breed. In a population of constant size, each parent over its lifetime leaves an average of one offspring that survives to breed

Grant and Grant:

Testing Postulate 4: Are survival and reproduction nonrandom?

Survivors had deeper beaks, on average, than the pre-drought birds (and, we can infer than the birds that starve). Because beak depth and body size are correlated and because large birds tend to win fights over food, the survivors had large body sizes too

When El Nino hit, smaller bodies were selected for

Smaller bodies are selected for in small birds, larger birds are favored in drought years

Grant and Grant:

Did the population evolve?

The average bird in 2001 had a smaller body and a sharper beak and was significantly smaller than the average bird in the mid 1970s

Who does natural selection act on? Who does its consequences affect?


Acts on individuals but its consequences occur in populations

The effort of cracking Tribulus seeds did not make the beaks of individual finches grow larger nor did the birds need for more food or their desire for bigger beaks make their beaks grow. instead the average beak depth in the finch population increased due to the simple fact that more small-beaked finches died

What does Natural selection act on?

What does evolution consist of?


Acts on phenotypes

Evolution consists of changes in allele frequencies

Only the survivors of selection pass their successful phenotypes to their offspring via genotypes that help determine phenotypes. The variation in finch phenotypes that selection acted on had a genetic basis. As a result the new phenotypic distribution seen among survivors persisted into the next generation

What does it mean that natural selection is not forward looking?

There is no “pre-adapatation”. A trait will be useful (but at present is not) will not be selected

There is however expectation. A trait that originally served one function but shifts under selections to perform a different function

Although selection acts on existing traits, explain how new traits can evolve


Selection doesn’t generate new genetic variation. However new traits can evolve under selection through mutation and recombination

Persistent natural selection can lead to the evolution of new functions for existing behaviors, structures or genes

Example: Carnivorous plants such as the butterwort which captures small insects in a sticky liquid exuded in droplets from glandular hairs, or trichomes covering its leaves

Define exaptation
A trait that is used in a novel way and enhances an individuals fitness fortuitously, not because natural selection is concious or foresighted.
Define secondary adaptations
When a trait may eventually be elaborated into a completely new structure by selection related to its new function.

Explain what is meant by natural selection does not lead to perfection


Evolution doesn’t result in organisms that are perfect. The reason is that popultions may face contradictory patterns of selection
-Selection for one function may decrease fitness with respect to another
-Selection acts on existing variants in the population

ie. Male mosquitofish whose anal fin is modified to serve as a copulatory organ or gonopodium. Females prefer males with larger gonopodia but when predators attack a big gonopodium slows the males escape.

Describe what is meant by natural selection is nonrandon, but is not progressive


Evolution doesn’t move towards a predetermined goal
-Mutation and recombination, the processes that generate genetic variation are random with respect to the changes tey produce in phenotypes but natural selection, the automatic sorting among variant phenotypes and genotypes is the opposite of random

-Crocodilians are poikilotherms but have a number of features typically associated with homeotherms for example a 4 chambered heart

What is meant by fitness is not circular? Bad simplifcation of the argument for natural selection?


“Survival of the fittest” is a tautology but Darwin’s argument is not
-Basically says survival of the survivors which is pointless
-Key to resolving the issue is to recognize that “survival of the fittest” is an oversimplified and misleading characterization of Darwin’s theory
-Darwinian fitness is not an abstract quantity. It can be measured in nature
Explain what it means that selection does not act for the “good of the species”

Infanticide in prides is not for the good of the pride it’s so that the alpha male can have children with the females of the pride by getting rid of their cubs which therefore increases the individual males fitness since the females become feertile again sooner and will conceive his offspring by the new males

Behavior like this obviously doesn’t exist for the good of the species. Rather infanticide exists because under certain conditions it enhances the fitness of individuals who perform the behavior relative to those who don’t