Natural Selection

Evolutionary response

heritability x selection

How can we predict the amount of evolution we expect from an amount of selection?

The breeder’s equation

What is the breeder’s equation and what do the variables mean?

R = h² x S

R - evolutionary response, h² - narrow-sense heritability, S - selection differential

Selection differential

measures the extent to which selection favors an increase or decrease in the trait mean within a generation

Evolutionary response

measures the extent to which the trait increases or decreases in the next generation

The same magnitude of selection can result in very different evolutionary responses, why?

The magnitude of heritability 

Why does evolution not keep pushing strongly in one direction? For example, why do Darwin’s finches not evolve larger and larger beaks? Hint: three possible reasons

selection was only that strong that year

environmental variation creates fluctuating patterns of selection

not consistent in favored traits

How do we measure S (selection differential)?

change in mean after a period of selection (after - before)

How do we determine S on a graph?

the slope of relative fitness compared to any phenotype

Practice question: Which of the following is not a potential way to measure the strength of selection, S, on a quantitive phenotype?

Directional selection

favors an increase or decrease in phenotypic mean, referred to as positive or negative

Stabilizing selection

favors a decrease in phenotypic variance

Disruptive selection

favors an increase in phenotypic variance

Components of fitness (hint: three answers) that add up to net fitness

survival, mating and fertilization, and fecundity

Opportunity for selection (I)

variance in fitness/ mean absolute fitness squared

High variability doesn’t matter without any — — —

difference in fitness

Practice question: In which case is the opportunity for selection the greatest?

Agents of selection

 one or more aspects of the environment that creates selection

Extended phenotype

a structure produced by an organism that is not part of the organism, but whose properties influence the fitness of the organism and reflect the genotype

Three examples of an extended phenotype

burrows, nests, galls

How do humans act as agents of selection? Hint: three answers

introduction of new species, killing undesirable species, hunting/harvesting

Breeding value

an estimate of the phenotype that an individual would pass on due to additive genetic effects (breeding value of mom + dad)

How did selection affect fisheries?

selectively harvested the largest fish for decades, who were also the oldest, which led to a strong selection that favored small body size and early age for maturity

Artificial selection

breeders/scientists only allow individuals with a particular phenotype to reproduce, creating selection

How has artificial selection been used?

Domestication of crops, livestock and pets

Dogs are estimated to have — — — — — — —.

the most phenotypic variation of any species

How did artificial selection affect dogs?

rapid artificial selection swept many deleterious mutations

purebred dogs suffer from inordinate frequency of genetic maladies as a result of inbreeding

What is the fundamental unit of selection, the gene or the organism?

Both!

Gene’s point of view

the organism is just a way to preserve and propagate DNA

Selfish genes

distort Mendelian segregation and increase their own representation in gametes above the normal 50%

What is always true of a selfish gene?

it always has some transmission advantage, but it will come at a cost for the individual or other alleles

Selection acts on

genes, individuals, families, and other groups

Kin selection

selection arising from the indirect fitness benefit of helping relatives, based on inclusive fitness

Inclusive fitness

an individual’s combined fitness arising from its own reproductive success and the success of its relatives, direct + indirect fitness

Coefficient of relatedness r

proportion of genes shared by two individuals

r for parents/offspring and siblings/siblings

0.5

r for cousins

0.125

Altruism

an individual helps another in a way that benefits the other at a cost to the helping individual

How does altruism affect haplodiploidy organisms?

a female would rather help a sister (r=0.75) than make a daughter (r=0.5) and males would rather make a daughter (r=1) than help a brother (r=0.25)

What is haplodiploidy?

Females are diploid, while males are haploid. Males pass on 100% of genes (r=1) to their offspring, while females pass on 50% (r=0.5). Females can have offspring on their own (haploids), which all would be male. Mating between a male and female would have to occur to produce females (diploids), and all offspring would be female.

Coefficient of relatedness for haplodiploid:

mother/daughter, father/daughter, mother/son, sister/sister, sister/brother

m/d = 0.5, f/d = 1 for dad, 0.5 for daughter, m/s = 0.5 for mom, 1 for son, s/s = 0.75, s/b = 0.25

Eusocial species

species with complete reproductive division of labor (some individuals will never reproduce)

Why do social mammals give alarm calls?

Despite the cost of attracting the attention of a predator, they give calls to warn other members. Males tend to not warn, as they are typically not related to anyone in the colony, but females will call to save family members (indirect fitness). 

Why do some birds breed cooperatively?

some birds don’t breed well in their first year, so they have a better fitness if they help raise siblings (relatedness coefficient is the same for offspring and siblings, but older birds will produce more offspring, raising fitness for the younger bird)

Selection is inherently —

multilevel