Lecture 3: Natural Selection

3 Components of Natural Selection

1. Variation

2. Inheritance

3. Differential reproductive success

VID

Natural Selection

• process by which beneficial alleles increase in frequency over time in a population due to increased survival and reproductive success of individuals carrying those alleles

• sorts on phenotype, not genotype

  • acts on phenotypes, but evolution consists of changes in allele frequencies

• acts on individuals, but its consequences occur in populations

• acts on existing traits, but new ones can evolve

• faces several constraints, and does not lead to perfection

Mutations

• source of variation

• mutations occur randomly and independently of whether they will be favoured by natural selection

How to deduce whether natural selection occurs in the wild?

1. Variation

   • is variation present?

2. Inheritance

   • evolution via natural selection can only occur if variation is heritable.

     • this is tested by identifying the genes involved

3. Differential Reproductive Success

   • differences in fitness

   • measuring fitness is in measuring survival, after removing confounding factors

Trait Fitness

• expected reproductive success of an individual relative to other members of the population

• differential effect of trait on reproduction

• small differences in fitness → large differences in populations over time

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Adaptation

• an inherited trait that increases an organisms fitness, and is the result of natural selection for its primary function

• increases fitness to both abiotic and biotic environments

• to be considered an adaptation, a trait must have been shaped by natural selection to serve the same primary functions that make it beneficial today

  • ex. WVS in Echinoidea adapted for feeding functions (which it exhibits today), you would not say it adapted for reproduction if that isn’t something that it currently exhibits/benefits from (even if they did at one point)

Exaptation

• a trait that currently serves one function today, but which evolved from a trait that served a different function in the past

  • most complex traits will have undergone layering of adaptation and exaptation changes

• ex. feathers

  • todays function: flight

  • original function: thermoregulation, sun protection, defense?

• co-opted trait can be multi-purpose, and the original function may not be necessarily lost

Gene Duplication

• entire gene or region of the gene is duplicated

• second copy of a gene can undergo neofunctionalization

Hormone-receptor Pairs

• lock and key systems, and exhibit of gene duplication

• which came first?

  • does receptor evolve before signal exists

  • does signalling protein evolve before receptor

• gene duplication could allow second receptor to mutate, and bind to the new hormone

Gene Sharing

• protein recruited to serve second function elsewhere

How can a complex trait evolve?

1. co-opted from another trait

   • exaptation

   • intermediate steps also functional

   • complex trait serves new function

2. gradual, intermediate steps

   • each step itself adaptive

   • serve same function throughout

Evolution of Eyes

• gradual steps

• eyes at different levels of complexity exist today

• suggest complex eye could have evolved through series of intermediate stages

• each individual step is fully functional

  • complex focusing eye

  • simple photoreceptive layers of cells

Short term constraints on NS?

• lack of sufficient variation

Long term constraints on NS?

• physical constraints

• co-evolution

• lack of foresight

Physical Constraints NS

• constrained by physical and mechanical laws

• example: binocular vision

  • 2 extremes: almost 360 vision, but limited depth perception vs. full depth perception, but limited field of view.

  • can’t have both full field view and stereoscopic vision with only two eyes

Coevolution

• evolutionary change in one species can affect selective conditions for a second species

• ex. plants and pollinators

Evolutionary Arms Race

• form of coevolution where the species involved each evolve countermeasures to the adaptations of the other species

• basically, much of what is significant about an organisms environment is provided by other organisms, who are themselves evolving through natural selection

• ex. predator-prey interactions

NS Lacks Foresight

• selection favours traits that are immediately beneficial, not traits that may be useful in the future

• seen in the blindspot in human eyes

  • legacy of how our eyes evolved

  • cephalopods lack a blindspot

  • NS does not lead to perfection

Guppy NS

• life history:

  • reside in streams, which are physically close, but separated by waterfalls. the separated populations are under different selection pressures

• observed patterns:

  • female at upstream and low predation sites produced fewer offspring (but larger in size) than downstream

• explanation:

  • upstream predators are small, meaning that the larger offspring will be safer from predation

  • downstream predators are bigger, meaning that the better life strategy is to produce as many offspring as possible

• here, natural selection favours different life history strategies under different predation pressures

• however, when experimentally manipulated, populations rapidly evolved in response to selection

E. coli evolutionary experiment

• compared what happened to cell sizes across 12 genetically identical cell lines

  • would the same phenotypes evolve?

  • does NS happen in exactly the same way every time?

• cell size and fitness both increased in all cell lineages over time

  • phenotypically, populations evolved in a similar manner

• but despite starting with genetically identical cells, cell size increased more in some lineages than others. fitness was higher in some lineages than others.

  • NS operated in a similar way in all populations, but didnt take an identical path

NS and random events

• NS is a predictable, repeatable process, but it is also affected by random events that can significantly affect the course of evolution

  • random events may be specific mutations, or orders of mutations