BIOB51 Extra Notes UTSC

C) Sexual selection

(a subset of natural selection)

•differential reproduction due to phenotypic differences among individuals in their ability to obtain mates

Affects mating success, so directly affects gene flow

Sexual selection can play a role in speciation

e.g., Monarch flycatchers, Solomon Islands

Variation in plumage colour across sub-species

Behaviour: Birds are most responsive to birds that look similar to siblings/parents/social group

Visual recognition important in sexual selection:

•Male defense of territories

•Males soliciting copulations

•Females choosing among males

Intersexual selection can play a role in speciation

Affects mating success, so directly affects gene flow

• Speciation often begins when populations are spatially separated in a way that stops or decreases gene flow (migration)

= allopatric speciation

• Mate preferences can make hybridization less likely & maintain or increase divergence

3. Reproductive isolation

•A) divergent traits prevent interbreeding or fertilization if groups have secondary contact

•B) hybrids do poorly, strong selection for mechanisms that maintain genetic isolation Reinforcement

Hybrid zones = area of where diverging populations come into contact (sympatry), mate, and may produce offspring

Frequent result of hybridization:

Introgression = movement of genes from one species into gene pool of another because hybrids mate with parental species (backcrossing)

Can occur despite reinforcement and maintenance of distinct species

How do new species form?

Speciation is a slow process Gene pools may contain alleles from other species, or from extinct species

e.g. Evolution of humans: Homo sapiens

“African replacement” model: -

Modern humans are descendants of a small group of homo species from Eastern Africa

- Migrated out into Europe – Asia – Australasia - North America

 - Other Homo species were already in these locations, evolved from earlier migrations

Speciation requires the loss/reduction of gene flow between populations

•occurs most frequently with physical separation (allopatry )

 • New species can be maintained despite secondary contact

• Pre-zygotic isolating mechanisms

• Reinforcement maintains isolation

 •can occur without physical separation (sympatry, parapatry )

 •Clinal variation in selection

•polyploidization (genetic isolation)

 •disruptive selection: genetic or habitat/resource specializations How do new species form? Speciation is an ongoing process

• e.g., Incipient species, hybrid zones, introgression

BIOB51 – Lecture 2 Natural Selection:
Big Questions in Biology:
• Adaptation: is the good fit of organisms to the environment. For example,
o The desert rat doesn’t drink water because it gets water intake from seeds
o A star nosed mole has tentacles around its nose that allows it to detect chemical
stimuli.
o Harverous aunts use silk to make a nest in the community it lives in.
• Before Darwin:
o The idea before him regarded the historical special creation

• ▪  Various religious traditions in the 1800s.
  
• ▪  This theory states that all species were created independently at the same
  time
  
• ▪  They didn’t change through time, but they were created recently (about 10
  000 years ago)
  
• ▪  Mechanism: created by supernatural force
  
• ▪  Darwin refuted this idea because he didn’t think what he learnt was
  consistent to what he was learning currently.
  
• ▪  The species that were created initially is what is seen now although time
  has passed.
  

o Observations suggested that species change over time in the form or shape of
organisms.
▪ Differences in time can be seen
o Species can become extinct
o Time-series of species appearance
o Environments had changed over time
▪ This is evidence from people who studied erosion
▪ They saw fossils on mountains from organisms that were not from the
mountainious area.
o Relatedness of species (similarity within diversity ‘tree of life’)
Alfred R Wallace:
• He wondered why organisms still adapt to the environments if all the organisms were created at one time, but the environment has changed since then
Before Darwin:
• Historical special creation was then considered as not a good representation for life, diversity, and adaptation
Evolution: the development of new types of living organisms from pre-existing types • This was a very early definition

• It still didn’t explain adaptation Mechanisms:

• John Baptiste de Lamark proposed that organisms can inherit acquired traits.
  
• He said organisms constantly changed over time and developed traits that can help them
  succeed in their environment
  o He said then the offspring can inherit traits that were developed by their parents
  
• For example, shorebirds have long legs
  
• Independent progression: individuals evolve independently of other species
  

o Individuals change and pass the traits to their offspring
o For example, single cell organisms change as their habitat changes
o Some organisms become birds because they are striving to succeed in another
way due to the change in their habitat
• Problems with mechanisms:
o Changes that happen to the parents in the somatic cells cannot be passed to their
offspring
Diversity of Life:
• Mammals all have similarities when compared to birds, etc Problems with Lamarck’s Mechanism:

• The clusters/patterns of similarities suggested that the organisms are related.
  
• We can best represent the similarities in a phylogenetic tree
  Darwin’s Contribution:
  

• His contribution was the theory of evolution by natural selection o It is falsifiable
  o Can find evidence across the natural world
  o It was consistent
  o Descent with modification: evolved from one or a few original species
  ▪ Consistent with evidence in the natural world
  ▪ Phylogenetic tree
  o Species arise over long periods of time
  o The mechanism that results in all of the above adaptation is natural selection
  
• He provided evidence that evolution occurred
  
• He also proposed natural selection
  
• He gave evidence from across different fields of science for evolution that occurred by
  natural selection
  Evolution by Natural Selection:
  

• The logic of Darwin’s theory of evolution by natural selection is based on 4 postulates
o Postulates: an assertion proposed to be true as the basis for an interference 

▪ Test 4 ideas to find out if natural selection worked

1. Individuals within populations are variable
   o Variation exists in almost every measurable characteristic in every species
   ▪ For example, the stripes on a zebra
   
2. Some of these variations are passed onto offspring (are heritable)
   
3. Many more offspring are produced than can survive and reproduce
   o Like the aphids
   o Observation – physical (abiotic) factors:
   ▪ Climate
   ▪ Catastrophic events
   ▪ Habitat/food limitation
   
4. Survival and reproduction aren’t random
   o Individuals with favorable traits will produce more offspring over their lifetime
   than individuals with less favorable traits, which is natural selection
   ▪ Favourable traits: are adaptations, which are traits that increase the
   

ability of an individual to survive and reproduce relative to others without the trait
• Adaptations are traits that lead to increased fitness

• Kaufman released two types of mice in different types of soil
  
• For visually hunting predators, the soil made a difference for who survived.
  

o For light coloured mice on a dark soil, it was easier for them to be killed
o Oldfield mice with a camouflaged (cryptic) fur colour are less likely to be killed
by predators
Darwinian Fitness: is the ability of individuals to survive and reproduce relative to others in the same population in their environment
Relative: lifetime reproductive success or expected reproductive success Textbook Definition (FLAGGED):
• TEXTBOOK DEFINITION:
o Adaptation: inherited traits that makes an organism more fit in its environment,
and has arisen as a result of natural selection for its primary function
▪ Adaptations must serve the same function now as they did when they first
evolved
o Exaptation: a trait that serves a different function today than it did when it first
evolved.
• RIGHT DEFINITION:
o Adaptation: is an inherited trait that makes an organism more fit in its
environment

• ▪  An adaptation may have a different function now than it did in the past (or it may have the same function)
  
• ▪  For example, feathers for flight
  o Exaptation/co-opted trait: one type of adaptation is a trait that served a different
  function in the past from its current function.
  
  • ▪  Is a type of adaptation
    
  • ▪  Evolutionary biologists agree and say that exaptation has declined over
    time
    
  • ▪  For example, feathers started off as something else but started being used
    as flight
    Natural Selection: differences in average reproduction of individuals with different phenotypes in a population
    

• Natural selection favours traits that maximize: o Fitness (lifetime reproductive success) o Favourable traits or adaptations
  
• For example, green colouration is naturally selected
  

o In a population of snakes, there are green and yellow snakes
o The green snakes on green grass are less likely to be killed, which means the
surviving green snakes can reproduce more than the yellow snakes
o The population goes from 50% green and 50% yellow to 66% green and 33%
yellow
o Thus, the population has evolved
o The heritable traits that lead to the production of the most offspring will become
more common over time
o As the environments change over time, the heritable traits present in a population
will change, which is evolution
Alfred R Wallace: both him and Darwin agree that evolution by natural selection explains why organisms are adapted despite the changes in the environment over time
After Darwin:
• Natural selection was consistent with life, diversity, and adaptation Darwin’s 5 Postulates:
• If the 4 postulates hold:
o The distribution of traits in the population will change as environments change o The organisms will be adapted to their environment
Evolution by Natural Selection:
• Natural selection is a process
o Differential reproduction of different phenotypes o Mechanism of evolutionary change

o Can explain adaptation The Modern Synthesis:

• Evolutionary theory was updated to incorporate the knowledge of genetics and inheritance
  
• This included Darwin’s insights that were consistent with genetics
  

o Genetic variation is inherited in the same way whether phenotypic effects are
large or small
o Small genetic changes (changes in allele frequencies) can accumulate gradually in
population
o Some traits arise from the interaction of numerous genes, so small changes in
allele frequencies can lead to large changes in phenotypes
o Major change between taxa (macroevolution) can be explain by the action of
natural selection with..
Modern Definition of Evolution: changes in allele frequencies for a given trait in a population over time