Lec #11: Natural Selection

Darwin said

it is not the strongest that survives, it is one that can best change

Evolutionary fitness

(Darwinian Fitness)

The contribution an individual makes to the gene pool (offspring) of the next generation, relative to the contribution of other individuals in that population/species

How much am I passing on more than you

Compare success to someone else

Relative fitness

The contribution of a genotype to the next generation compared to the contribution of alternative genotypes for the same locus

Division of the highest producer

Measure of Darwinian Fitness; more offspring, more fit variant

How fit in terms of the most fit

Selection for the dominant allele (more fit than the recessive)

if the relative fitness values remain the same, a allele is not lost, just very low frequency because it is less expressed (aa)

As (a) becomes rare a smaller fraction of pop. are found as homozygotes and express the traits, so it is weakly selected against.

- selective pressure decreases

- rarely expressed so not going to select against it (covered in heterozygotes)

- will always have residual in pop

Selection for the recessive allele

if the relative fitness values remain the same, a allele is increased very slowly because (a) allele starts out rare and there is low expression.

Lag period, threshold, then expressed in high numbers

Once a threshold value is reached, expression is large enough in population to be selected for, which rapidly increases the frequency of (a). (Can result in loss of (A) because always expressed) (natural selection will select against for recessive)

Balancing Selection: Selection for the Heterozygote: (Heterozygote Advantage)

Decrease in (AA) and (aa) genotypes

Increase in heterozygosity (Aa)

Because (Aa) genotype is beneficial it is selected for and there is high expression. Can result in a balanced polymorphism

Ex: sickle-cell genotype in Africa

Balanced polymorphism

(maintain 2+ alleles in a pop)

- non-identical alleles for a trait are maintained at frequencies > 1%

- frequencies may shift but over time often return to the same value

Heterozygosity is the advantageous condition

Context dependency

Ex: sickle cell anemia; S is mutant form of hemoglobin

SS: ill, stroke, die to sickle cell, but not malaria

AA: move thru pipes quick, normal, good at gas transfers, not noticing parasite (damage/burst cells; catches malaria), parasite can reproduce quick and efficiently

AS:

• maladaptive to have the trait

• carrying this allele is high in Africa where Malaria is high

• slows the disease from replication (allows immune system to catch up

• the shape makes it hard for the parasite to infect the cell

• spleen gets rid of the damaged cells quickly, including the sickle cell carrying malaria parasite

• selected advantage against malaria

• only slight decreased fitness; it is neutral or a disadvantage in other areas

• condition is adaptive

Balancing Selection: 2. Selection for the most common phenotype

Frequency-Dependent Selection

• positive: favoring common

• negative: favoring rare phenotype; against common

  • bright birds are easier to hunt down, so it is selected against

Fitness is varied based on freq of expression in environ

If selection is reducing alleles, how is genetic variation maintained in populations?

1. Recessive alleles are only lost very slowly (less and less expressed, so less selecting force)

2. Alleles may be neutral in current environment (no selecting force on them)

3. Balancing Selection:

- Heterozygote Advantage

- Frequency Dependent Selection (back and forth)

(most common phenotype is selected for (positive) or against (negative)

4. Environment may vary across different landscapes (promotes retention of alleles) (context dependent)

• big impact on selection

• diff alleles can be adaptive, maladaptive, or neutral (so don’t lose alleles)

• creates buffer w environ change

Natural selection acts on phenotypes not genotypes (just what are passed on)

- The relative fitness of an allele depends on the entire genetic and environmental context in which it is expressed

- Many traits are polygenic (determined by multiple genes)

Reproductive success: Survival and Fecundity

1. Survival does not guarantee reproductive success

2. Organisms must be fecund (fertile - producing viable offspring)

Mean fitness of population

Mean fitness of population

The average reproductive success of members of a population

Compared to someone else in population

• who is more fit and more offspring

• greater vs. less contribution to the pool of genes in the next gen

+ and - convey reproductive success

+ = greater contribution (well adapted, high fitness)

- = less contribution (not well adapted, low fitness)

• not negative fitness, but less than/fit

Types of Natural selection: Polygenic Traits 1. Stabilizing Selection

Intermediate forms of traits are favored and alleles on genes that

specify extreme forms are selected against

- reduces variation by counteracting effects of sexual recombination,

mutation, and migration (of allels in and out of populations)

- favors survival & reproduction of intermediate phenotypic characters

Ex: height

Basic conditions: LONG time scale

1. environment remains stable over time

2. organisms have obtained a high state of adaptiveness for particular environ

Individuals that are well adapted cluster around the mean

Pinches over time; less contributions (individuals) on the ends

Types of Natural selection: Polygenic Traits 2. Directional Selection

Favors survival and reproduction of phenotypes at one extreme or the other of phenotypic distribution; not at the same time

- Allele frequencies in a range of phenotypic characteristics shift

in one direction or another

- Individuals that deviate from the average are favored (selected for)

- occur when mutations appear and are adaptive

Basic conditions: LONG time scale

1. Environmental and/or biological conditions are changing over time

2. Species can not be completely adapted to the changing conditions

Mean shifts left or right

Ex: wet then dry environ for bears; becomes big and small based on climate

Ex: peppered moth morphology ranges from light gray to nearly black; balanced but shifts over time

• environmental driver of bird predation; depends on environ context

• quick evolution

• Experiment: Kettlewell (1950’s)

  - used mark-release-recapture method

  - released light and dark moths in polluted and unpolluted areas

Ex: Pesticide Resistance: artificial

- Chemical pesticide in agriculture has resulted in directional selection (many cycles)

- If pest resistance is heritable, it becomes more common in next generation

- Chemicals are agents of selection favoring the most resistant form

Pest Resurgence: pesticides also kill natural predators of the pests (double-edged sword)

Antibiotic Resistance

- Overuse & misuse of antibiotics has resulted in directional selection

- If resistance is heritable, it becomes more common in next generation

- Antibiotics are agents of selection favoring the most resistant forms

- Evolution of antibiotic resistant bacteria is fast

Coevolution

The reciprocal evolutionary change between interacting species or genes (reciprocal selection; also directional)

- Each species evolutionary changes exert selection pressure on the other species

- Drives the phenotypic characteristics in both species, mutual adaptation

- Forms: Host-parasite, Predator-prey, and mutualistic

Where does it end? The back and forth ends

Red Queen Hypothesis: ”keep running to stay in place” Organisms must constantly adapt and evolve to survive in an evolutionary arms race.

Morphological, behavioral, physiological limits: Example human response to infection (fevers; burning pathogens, but pathogens have better tolerance and 104 degrees is death)

Types of Natural selection: Polygenic Traits 3. Disruptive Selection:

Phenotypic characters at opposite extremes of character distribution are favored at the same time and intermediate forms are selected against

- Positive selection tends to affect the extremes not the mean

- The number of organisms is often reduced

- bimodal curve

Ex: small and big beaks finches for soft and large seeds based on wet and dry seasons

• Intermediates sized bills are inefficient at cracking both types of seeds (1 food source) (Have lower relative fitness than small or large billed birds)

Basic conditions: SHORT time scale

1. Fitness values for a genotype are higher in one environment and lower in a different environment

2. Occurs in diverse and shifting environments (seasonal, physical, unstable, fluctuating, fast, natural disasters)

Asexual advantage

fast, little E and resources, less specialization; high reproduction, accumulate mutations, need stable environ

Sexual advantage

slow, hope its passed on, some variation and modifications, high E, 2 sexes, high biomass, unstable environment

Pros and Cons

1. Faster Evolution (Red Queen Hypothesis)

2. Clearing deleterious alleles (Mullers Ratchet)

Red Queen: ”keep running to stay in place”

Organisms must constantly adapt and evolve in order to survive in an evolutionary arms race.

Mullers Ratchet:

- in an asexual lineage mutations occur and accumulate (and there is no way backwards); no checks, lose alleles

- In sexual lineage mutations will occur but can be added or lost (possible to go backward); natural selection can eliminate deleterious trait

- May create an upper size limit for the genome in asexual lineages,

Sexual reproduction may over come this size limitation

Isogamy

gametes (sperm and egg) of equal size (Fungus and algae)

not special bc produce a lot of gametes and no travel problem; easy to meet another gamete

Anisogamy

gametes of unequal size (Plant and Animals)

hard to find egg, transport is difficult, low odds of finding egg

leads to asymmetry of sex

- Female fitness limited by ability to gain resources required to produce

eggs and rear young

- Male fitness limited by the ability to attract mates

non-random mating – intersexual (between male and female) and intrasexual selection (males and females competing for mates amongst themselves)

advantage in reproduction but decrease in survivability trade-off

Ex: sexual selection and selection balance of Guppies in Trinidad experiment of evolution; fast reproductive cycle

• in low predator environs: fish mature later, reproduce less often, & produce fewer, larger offspring

• Rapid evolutionary change in guppies due to balance of natural selection via predation pressure and sexual selection

• context-dependent

males at risk because they are the disposable sex (1 male can reproduce with many females)

usually females are agents of selection

Sexual dimorphism

maximizes reproductive success

Individuals of most sexually reproducing organisms have a distinct male or female phenotypes (common in mammals and birds)

Artificial Selection

Selection and breeding of individuals with desired traits by humans

1 plant, same genetic material with wild type with variations

push towards speciation

Ex: mustard plant with kale, broccoli, cauliflower, cabbage, brussel sprout

Natural Selection

1. Selection can only edit existing variations: selects fittest/not best phenotypes currently in the population, may not be ideal

• natural selections operates on mutations

2. Evolution is limited by historical constraints: does not build new complex structures from scratch, takes existing structures and adapts them to new situations

3. Adaptations are often compromises (trade-offs): give up smth to build smth else

4. Chance (most fit gets hit by a bus), natural selection (weed out the poor variance) & the environment interact (environ is never stable, so traits are not always adaptive or maladaptive or neutral)