Bi 312 Lecture
Guide: Chapter 3 Winter 2010
Darwinian Natural Selection
•
Natural selection is
…
•
The process that
produces a …
– Descent with modification (AKA Evolution) AND
– Pattern in nature
•
Terms:
•
____________________
____________________ : is the ability of an individual to survive and
reproduce in its environment (i.e., fitness is the extent which an individual
contributes genes to future generations)
•
____________________
: refers to a trait of an organism that increases its fitness relative to
individuals lacking the trait
Darwin’s 4 Postulates
(which apply to populations)
•
Individuals within
species are variable
•
Some of this
variability is passed on to offspring
•
In every generation,
some individuals are more successful at surviving and reproducing than others
•
The survival and
reproduction of individuals are NOT random
–
The individuals who
survive and go on to reproduce, or who reproduce the most, ...
–
Those
____________________ are naturally selected
•
As a result of this
process …
•
This
is …
•
____________________
____________________
•
Evolution via
natural selection is …
• ____________________
Finches of the Galápagos: A Case Study in
Natural Selection
•
Finch Beaks
–
–
•
Primary feeding tool
that reflects the diversity of food types. As such, it has …
–
–
Figure 3.6
Are Populations Variable?
• Organisms vary: no two living things are exactly alike. This is the
result of:
–
–
Recombination during
sexual reproduction
•
•
•
into novel diploid genotype
•
Is the Geospiza
fortis population on Daphne Major variable?
•
____________________–
especially beak depth, but also other structural (morphological) traits (Figure
3.9)
•
Note that variation
is ____________________ ____________________ (e.g., morphology,
biochemistry, DNA)
Is Some of this Variability Passed on to
Offspring?
•
Variation is
heritable:
–
Some differences
among organisms are due to differences in their environments, ...
–
Parents pass their ____________________
to their offspring – so
____________________
among adult organisms in any generation will result in differences in
their offspring
•
–
genes
–
environment (in this
case, body and beak size are correlated with the amount of food a chick gets,
e.g.)
–
both genes &
environment
•
Is variation in a
specific trait(s) heritable?
•
One approach is to
measure the similarity between relatives:
–
if variation is
heritable, ...
•
Heritability varies
between 0 and 1
–
in Galapagos
finches, heritability in beak size is high (Figure 3.10)
• Variation in most traits can be due to both
...
– selection can operate as long as some of the variation
is due to ...
– heritability is substantial in most traits measured in
most organisms
Do Individuals Vary in Their Success at
Surviving and Reproducing?
•
In every generation,
far more offspring are born than ever survive to reproduce. This is due to:
–
Abiotic Interactions
–
Biotic interactions,
including:
•
•
overproduction:
– more offspring are born into the environment every
generation ...
– leads to ____________________ within
and among species
•
A series of “natural
experiments” on Daphne Major beginning with a drought in 1977:
–
rainfall decreased
from 130 mm to 24 mm during rainy season
–
~ 84% of the
population of finches died over 20 months (Figure 3.12a)
–
correlated with
strong decline in seed availability (Figure 3.12b)
–
correlations with
food “characteristics” (Figure 3.12c)
•
Overproduction is
typical in most populations
–
–
____________________
sizes stay ± same
(Table
3.1)
Are the Survival and Reproduction of
Individuals Nonrandom?
• Fitness varies among individuals based on
____________________
____________________
– Survival and reproduction is non-random: some
individuals have traits that allow them ...
– Individuals with the most favorable traits will ____________________
____________________
____________________ than do others in the population
• Survival and reproduction must be
non-random ...
– some trait or traits make certain individuals better
able to survive and reproduce than others
• How to test this?
–
• survivors had deeper (larger) beaks than
did non-survivors (Figure 3.13)
• survival was ____________________
• Why did individuals with larger beaks
survive “better”?
– As drought progressed, small, softer seeds were eaten
first, leaving mostly large hard fruits of a less desirable plant
– Birds with the deepest beaks were able to crack and
eat those fruits
– The largest, “deep-beaked” birds were better able to
defend food sources
• What happened when drought ended?
• In 1983 there was 57 times the amount of
rain as in 1977
– super-abundance of soft seeds
• This led to strong selection for small
birds with shallow beaks
• This demonstrates that natural selection
is
____________________!
Did Evolution Occur in This Example?
•
The result of these
four factors (i.e., Darwin’s 4 postulates) is natural selection:
–
favorable traits
will ...
•
Another example:
–
when AZT is present,
the frequency of AZT-resistant HIV virions ...
•
__________
–
a non-random subset
of the G. fortis population were better able to survive and reproduce
than were others
–
large-beaked
individuals increased in frequency in the population
•
However, evolution
requires a change in ____________________
____________________in populations from one generation to the next
The Nature of Natural Selection
Natural selection acts on individuals, but
its consequences occur in populations
•
Selection does not
change individuals – e.g.:
–
AZT doesn’t produce
AZT-resistant mutations
–
drought (and
cracking large seeds) didn’t produce larger-beaked birds
•
Selection acts to ...
•
Populations change
as a result of selection
–
specifically, the
distribution of traits (and their underlying genes/alleles) changes over time
–
Selection doesn’t
cause adaptive changes to ____________________ ; it
just
causes adaptive traits to become _________ ____________________
• Selection acts on phenotypes, but
evolution is a change in ...
–
The likelihood of an
individual finch (or strain of HIV) surviving and reproducing depends on its
particular traits – and it doesn’t matter whether variation in those traits is
genetic or environmental
–
Selection will
result in ...
•
those generations
will be genetically different (although only slightly so) than their
predecessors
–
Those underlying
genetic differences will, in turn, produce differences in the distribution of
traits in subsequent generations
•
Selection is
“backward-looking”, not “forward-looking”
–
The organisms that
best succeed in surviving and reproducing are more fit because their traits ...
–
The distribution of
traits among offspring populations are always the result of selection acting ...
–
No mechanism exists
...
For example,
•
Finch beaks didn’t
get smaller a generation or two before the unusually high precipitation of 1983
•
AZT-resistant
virions won’t decrease in frequency before an individual stops taking AZT
• New Traits can Evolve
–
·
An example, the panda’s thumb (Figure 3.19)
–The “thumb” is their highly
modified radial sesamoid bone (a component of the wrist in other closely
related species)
–___________________
and ___________________ selection over many generations has led
to an increase in the sesamoid bone length and hence, a novel characteristic
–The radial sesamoid of the
ancestral panda is a ___________________
–A preadaptation improves an
individual’s fitness fortuitously, not because natural selection is conscious
of forward looking
• Selection is not “perfect” – i.e.,
selection cannot result in “perfect adaptation” because of several types of
constraints or limitations:
• Time lags:
– every generation of organisms is adapted to the conditions
that existed in ...
– selection can act very slowly relative to the rate of
____________________
____________________
– The result = organisms may not be “perfectly adapted”
to
____________________
____________________
–
Evolution is not
“progressive”
•
Selection has
“produced” organisms that are more complex, organized, and specialized than the
earliest forms of life on earth
•
Increasing
complexity is NOT universal:
(e.g.,
the loss of eyes in cave-dwelling fish)
•
Evolution is
____________________ ____________________” in the sense of
leading to some predetermined goal (e.g., the evolution of human intelligence,
e.g., was not inevitable)
•
Note: because all
extant organisms are the result of the same ± 3.5 billion years of evolutionary history, no organism is ...
” than any other!
•
Genetic/developmental
“links” between traits, for example,
–
In G. fortis populations,
individuals with narrowest beaks survived drought better than did individuals
with broad beaks
•
so “perfect”
adaptation would be birds with narrow, deep beaks
•
but beak width is
correlated with both body size and with beak depth:
–
birds with large
bodies and deep beaks also have wide beaks
•
A particular genetic
variation may produce an adaptive change in one trait, but a deleterious
(“harmful”) change in another, simply because of the complex
genetic/biochemical/developmental relationships among traits
•
The result is often
a ...
•
Selection ...
–
Nonrandom selection
is free of conscious intent!
–
The “random” element
in natural selection is genetic variation, which is random with respect to the
environment
•
the environment does
not cause adaptive variation to arise
•
e.g., just because
AZT resistance is useful for HIV virions in people taking AZT didn’t mean that
the trait would arise – it was a random mutation that produced the trait
–
:
the
characteristics of the environment “specify” which variants will be more likely
to survive and reproduce
•
Fitness is NOT
Circular
•
Evolution via
Natural Selection is often criticized as ____________________
–
i.e., circular in
its reasoning
–
“Of course
individuals with favorable variations are the ones that survive and reproduce,
because the theory defines favorable as the ability to survive and reproduce”
•
The word “____________________
” is misleading
•
The only requirement
is that selection for particular variants be non-random
•
As long as a
non-random subset of the population survives “better” and leaves more
offspring, natural selection has occurred
•
____________________ is not an abstract quantity,
is can be measured in nature
–
Count the offspring that individuals produce during
their lifetime OR
–
Observe the ability of individuals to survive a
selection event AND
–
Compare each individual’s performance to that of
other individuals in the population
–
These are independent, measurable, and objective
criteria for assessing fitness
•
Selection acts on individuals, not groups
•
It is a mistake to think that selection can act on
traits ...
–
this is especially the case when we see behaviors
that would seem to decrease fitness, for example,
•
prairie dogs or other social animals giving alarm
calls
•
mother lions nursing offspring other than their own
•
Selection always acts to ____________________
____________________
____________________ : if a trait benefits another individual at its bearer’s
expense, it will be selected against
•
When we see apparently “altruistic” traits, we’re
usually seeing something more complex than meets the eye – usually, the trait
either
–
benefits a ____________________
____________________ (which, as we’ll see later, increases individual fitness)
or
–
is reciprocated – so it doesn’t represent a
“____________________”
Summary/Conclusions
•
Natural selection is
a mechanism that produces evolutionary pattern
•
Evolution via
natural selection is the logical outcome of 4 facts (Darwin’s postulates):
–
individuals vary (in
their traits)
–
some of this
variability is genetically based and can be passed on to offspring
–
more offspring are
born than can survive and some of those are more successful at reproducing
–
those individuals
that reproduce the most are more fit
•
This selection
process results in changes in the genetic makeup of population over time. This
is …
____________________