Lec 01 & 2 Reading BIOB51: p. 73-97
Chapter
3: Evolution by Natural Selection
- Natural selection is sufficient
straightforward that at least two authors discovered it well before Darwin. WC
wells used it to explain how human
populations on different continents came to differ in their physical appearance
and resistance to disease
- Patrick Matthew discussed it on farming
trees for lumber with which to build ships.
- Alfred Russel Wallace discovered natural
selection independently while Darwin was incubating his ideas
- Biologists thought natural selection w/
greater skepticism than evolution itself
3.1
Artificial Selection: Domestic Animals and Plants
- Darwin studied the method breeders use
to modify their crops and livestock. His fave = pigeons
- We still study the tomato
o
Wild
tomatoes have tiny fruit and domestic tomatoes are huge as a result of
artificial selection
o
Tomatoes
carry a gene called fw2.2, which
encodes a protein made during early fruit development. Its job is to repress
cell division. The more of that protein the tomato has, the smaller it will be
o
Fas is another gene that influences fruit size by
controlling the number of compartments in the mature fruit. More compartments =
larger fruit
- Organisms with traits favoured by humans
would fare badly in the wild
- Small fruits are better because small
animals that would disperse the seeds more easily carry them?
- NA grey wolves have acquired a genetic variant
conferring a black coat that benefits individuals living in forests
3.2
Evolution by Natural Selection
Darwin’s 4 postulates in On the Origin of Species:
-
If all 4 postulates
hold, then the composition of the population inevitably changes from 1 generation
to the next
- Darwinian
fitness: individual’s
ability to survive and reproduce
o
Fitness
is relative bc it refers to how well an individual survives and reproduces
compared to other individual of its species
- Adaptations:
a trait that increases
fitness relative to others lacking it
- Each of the 4 postulates can verified
independently = the theory is testable
3.3
The Evolut’n of Flower Colour in an Experimental Snapdragon Pop.
- 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, were true as well
1. The snapdragon population varied in
flower colour. ¾ of the plants had flowers that were almost pure white, with
just 2 spots of yellow on the lower lip. The rest had flowers that were yellow
all over
2. Colour variation was due to differences
in the plants’ genotypes for a single gene. The gene has two alleles S and s. Genotypes with either SS
or Ss have white flowers w/ 2 spots of yellow. ss are yellow all over. 12 were SS 24 were Ss and 2 were ss.
3. Testing
postulate 3: Do individuals vary in their success @ surviving or reproducing? They let free bumblebees pollinate the
plants. They tracked the number of times bees visited each flower. The plants
showed considerable variation in reproductive success, both as pollen donors
and as seed mothers.
4. Testing
postulate 4: Is reproduction non-random? They found
that white flowers attracted twice as many bee visits as yellow flowers.
Testing
Darwin’s Prediction: Did the Population Evolve?
- The bumblebee selected particular
individuals in the target population and granted them high reproductive success
- White plants had higher reproductive
success than yellow, and since flower colour is determined by genes, then next
generation of snapdragons should have had a higher proportion of white flowers
- The next gen did have higher proportion
of white flowers
- 75% had white flowers and 77% amongst
their offspring had white flowers
- They did evolve as predicted
- Jones and Reithel’s experiment shows
that Darwin’s theory works
- Turn to research of finches to find out
whether the theory works in nature
3.4
The Evolution of Beak Shape in Galapagos Finches
- Finches are birds derived from a small
flock of dome-nested finches that invaded the archipelago, most likely from the
Caribbean, 2-3 million years ago
- Descendents of this flock comprise 13
species that inhabit Galapagos
- The deepest split on the evolutionary
tree separates 2 lineages of warbler finches
- Third-deepest split separtes two
lineages of sharp-beaked ground finches that are likewise considered a single
species
- All species of Darwin’s finches are
similar in size and coloration
o
Range
from about 4-6 inches in length and from brown to black in color
o
They
show a lot of variation in beak size and shape
- Warbler finches feed on insects, spiders
and nectar
- The vegetarian finch eats leaves and
fruit
- Medium ground finch (Daphne major)
o
Population
is small enough to be studies exhaustively
§ Since 1980, almost 100% of the
population has been marked
o
Location
and tiny size make it a superb natural laboratory
§ It is the top of a volcano, rises from
sea to max elevation of just 120 meters
§ Main crater and small secondary crater
§ Only 1 spot on the island is both flat
and large enough to pitch a camp
§ Climate is seasonal
§ Vegetation consists of a dry forest and
scrub along with several species of cactus
o
Generation
time is about 4.5 years
o
Primarily
seed eaters
- Beak size is correlated with the size of
the seeds harvested; larger beaks = larger seeds
Testing Postulate 1: Is the Finch population
variable?
- Yes, they vary in wing and tail length,
beak width, depth, and length
- Variation among the individuals within
populations is virtually universal
Testing Postulate 2: Is some of the variation
amongst individuals heritable?
- Heritability of a trait is defined as
the fraction of the variation in a population that is due to differences in
genes
- It can take any value between 0-1
- Boag’s data revealed a strong
correspondence among relatives
- BMP4 is a signalling molecule that helps
sculpt the shape of bird beaks
- The genetic mechanisms responsible for
variation among individuals within species may or may not be the same as those
responsible for differences between species
Testing Postulate 3: Do Individuals vary in their
success at surviving and reproducing?
- 1977 there was a drought resulting in
plants making few flowers and seeds
- The medium ground finches did not even
try to breed
- Inferred that most died of starvation
- Only a fraction of the population
survived to reproduce in 1978
- In every natural population studied,
more offspring are produced in each generation than survive to breed
- In a population of constant size, each
parent, over its lifetime, leaves an average of 1 offspring that survives to
breed
- Elephants are the slowest known breeder
known among animals
- The only thing that saves us from being
buried in starfish and elephants is massive mortality
Testing Postulate 4: Are survival and reproduction
non-random?
- A non-random group of medium ground
finch survived the 1977 drought
- As the drought wore on, the number as
well as the types of seeds available changed
- Large hard fruits became a key food item
after the drought dried out all the soft seeds
- Only birds with deep narrow beaks were
able to crack and eat the Tribulus fruits
- In wet years, small birds with shallow
beaks survive better and reproduce more because they harvest small seeds more
sufficiently
- Natural selection is dynamic
Testing
Darwin’s Prediction: Did the population evolve?
- All 4 of the postulates were true for
the medium ground finch population on Daphne Major
- There was another drought in 2003-2004
but this time the medium ground finches had to compete with another species of
finches
- They were larger so they dominated and
ate the tribulus fruits that they had survived on before. As a result, the
finches died at an even higher rate
3.5
The Nature of Natural Selection
Natural
Selection Acts on Individuals, but its Consequences Occur in Populations
- None of the individuals in the
snapdragons or finches changed
- What changed was the characteristics of
the overall population
- The effort of cracking the tribulus
seeds did not make the beaks of individual finches grow larger nor did the
birds need more food, or their desire for bigger beaks make their beaks grow
- Exposure to particular environmental
circumstances changes the phenotypes of individuals
o
Ex.
spending time in the sun gives you tan but the changes are not passed onto the
offspring. A woman who sunbathes while pregnant does not give birth to a baby
with darker skin, but what she and the father pass to the baby is a heritable
tanning capacity
Natural
selection Acts on Phenotypes, but Evolution Consists of Changes in Allele
Frequencies
- Only when the survivors of selection
pass their successful phenotypes to their offspring, via genotypes does natural
selection cause populations to change from 1 gen to the next
Natural
selection is not forward looking
- Involve no conscious entity with
foresight. Evolving populations always lag at least a generation behind changes
in the environment
Although
Selection Acts on Existing Traits, New Traits Can Evolve
- Selection itself generates no new
genetic variation, adaptive or otherwise
- Study at University of Illinois:
o
162
ears of corn
o
In
the starting population, oil content = 4-6% by weight
o
After
100 gens of selection, the average oil content was about 20%
- Persistent natural selection can lead to
the evolution of new fxns for existing behaviours, structures, or genes
- Exaptation:
a trait that is used in
a novel way
o
Represent
happenstance, enhances an individual’s fitness a lot
- Secondary
adaptations: additional
modifications that arise during exaptation
o
A
trait may eventually be elaborated into a completely new structure bu selection
related to its new fxn
- Ex. venus fly trap with leaves modified
into snap traps, and monkey cups with pitfall traps that develop from tendrils
on leaf tips
Natural
Selection Does Not Lead to Perfection
- Evolution does not result in organisms
that are perfect
- Populations may face contradictory
patterns of selection
o
Ex.
male mosquitofish who anal fin is modified to serve as a copulatory organ, or
gonopodium
§ Females prefer males with larger
gonopodia, but whenpredators attack, a big gonopodium slows them down, no male
can be both
- Natural selection leads to adaptation,
not perfection
Natural
Selection is Nonrandom, but It Is Not Progressive
- Evolution by natural selection is not a
random or chance process
- Mutation and recombination is random
- Definition: non-random superiority at
survival and reproduction of some variants over others
- Completely free of conscious intent
- Darwin regretted using the phrase
“naturally selected” because it lead to some people thinking that it was a
decision made by a sentient entity
- Evolution is not progressive in the
sense of moving toward a predetermined goal
- They only improve in that their average
adaptation to the environment increases
- There is no inexorable trend toward more
advanced forms of life
- “Never use the worlds higher or lower”
when discussing evolutionary relationships
Fitness is not
circular
- “the survival of the survivors”
- Survival of the fittest is a misleading
phrase
- The essential feature of natural
selection is that certain heritable variants do better than others
- As long as a non-random subset of the
population survives at higher rates and leaves more offspring, evolution will
result
- Darwinian fitness is not an abstract
quantity, it can be measured in nature
o
Done
by counting the offspring that individuals produce, or by observing their
ability to survive a selection event, and comparing each individual’s
performance to that of others in the population
Selection acts
on individuals, not for the good of the species
- Common misconception: individual
organisms preform actions for the good of the species
- Altruistic acts do occur in nature
- Traits cannot evolve by natural selection
unless they increase the fitness of the genes responsible for them relative to
the fitness of other genes
o
Ex.
when the beneficiaries are kin or can be counted on to repay the favour
- Ex. lions doing things for the good of
the species in their prides
o
Killing
cubs to increase the new alpha’s fitness because pride females become fertile
again sooner and will conceive offspring by the new males
o
Infanticide
is widespread in animals
3.6
The Evolution of Evolutionary Biology
- Darwin’s theory as one o the great ideas
in intellectual history
- The theory was not accepted until 70
years after it was initially proposed
- 3 serious problems
Variation
- Darwin didn’t know about mutations and
how variability was generated in populations
- Was not until 1900s when Morgan began
experiments with fruit flies
o
Showed
that mutations occur in every generation and in every trait
Inheritance
- Darwin had no idea how variations are
passed to offspring
- Biologists didn’t understand inheritance
until Mendel’s pea
- Blending
inheritance: favourable
variants would merge into existing trants and be lost
o
Ex. when people w/ light skin land on an island full
of a people w/ dark skin. Even tho it might be advantageous to have light skin
in that population, even their offspring will have brown skin. No one will ever
have fully light skin
- Lamarck: offspring inherit phenotypic
chances acquired by their parents (WRONG AF)
Time
- William
Thomson: Estimated the
age of Earth at 15-20 million years
o
Based
on measurements of the Sun’s heat and temp of the earth
o
Because
fire was the only known heat source, he assumed that the Sun was burning a
giant lump of coal and slowly burning out
- Geologists and physicists believed that
the surface of the Earth was gradually cooling
- Earth was changing from a molten state
to a solid one by radiating heat to the atmosphere – a view supported by
measurements of higher temps deeper down in mineshafts (this data allowed Thomson
to calculate the rate of radiant cooling)
- The window that Thomson calculated was
too small for the gradual changes of Darwinism to accumulate and supported a
role for special creation in explaining adaptation and diversity
- Calculations were correct, assumptions
were wrong
o
The
Earth’s heat is from nuclear fusion, not combustion
The Modern
Synthesis
- Between 1932 and 1953, a lot of books
integrated genetics with Darwin’s 4 postulates and led to a reformation of the
theory of evolution, known as the modern synthesis was a consensus grounded in
two propositions
1. Gradual evolution results from small
genetic changes that rise and fall in frequency under natural selection
2. Origin of species and higher taxa, or
macroevolution, can be explained in terms of natural selection acting on
individuals, or microevolution
- The original 4 postulates were
reinstated:
- Outcome: alleles associated w/ higher
fitness increase in frequency from 1 generation to the next
- “Natural selection has been the main but
not exclusive means of modification” – Darwin
o
We
now think of evolution in terms of changes in the frequencies of alleles
responsible for traits
- Darwinian view of life: a competition
between individuals w/ varying abilities to survive and reproduce has been
proven correct in almost every detail
No comments:
Post a Comment