Evolution II
How does it work? What does it
mean?
topics
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Patterns
Mechanisms
Speciation
Macroevolution
trends
• Evolution and how it
works
• Evidence:
– Fossils
– Homology
– Embryology
• Mis-conceptions
Example 1: beetles on a diet
% of a population of beetles is green;
a % is brown
One year, a drought limits food supply
As a result of this environmental stress,
the next generation of beetles are smaller
After another generation, the % of green and
brown beetles changes
Is this an example of evolution? NO! Why?
All changes -even those from environmental stress or
limited resources - are not evolution. What do you
need to see to know you are looking at evolution?
• Change in the “gene pool” - the total
genetic makeup of a population (the
actively interbreeding portion of a species; a
species consists of interbreeding or
potentially interbreeding individuals)
• So if changes in the gene pool define
evolution, what causes change in this?
Changes in the gene pool (the genetic
composition of a population) are caused by:
1. Mutation
2. Gene flow, or migration
3. Genetic drift
4. Natural selection
Introduction of genetic variation:
1. Mutation - changes in DNA, usually
from a “copying error” but also from
external causes (ex, radiation). A single
mutation can have a large effect, but
usually accumulation of mutations over
time produces change. Most mutations
are neutral. Mutations are random
2. Gene flow - introducing new
genes to a population from
emigrants, including:
sexual recombination - shuffling
genes during fertilization and
cell division
What is genetic drift?
In every generation some
individuals leave behind
slightly more offspring (and
their genes) than other
individuals. As a result, the next
generation has more of the genes
of the reproducing ancestors than genes
of the ancestors with fewer
offspring. As these descendents
in turn create offspring, so more of
their genes will be transmitted to
the next generation than those
ancestors who had fewer offspring. No
adaptations are involved, it’s just a
matter of statistical change over time.
Natural Selection
Variation in traits; green and brown beetles
Variation in reproductive success. If one color of
beetle is preferentially eaten by a predator, more of the
other color will remain to reproduce
Heredity: more of the brown beetles will pass their
genes on to offspring
End result: as these 3 steps continue, greater and
greater numbers of brown beetles will appear in
successive generations; the composition of the gene
pool changes over time = evolution
Some questions:
• does the beetle in our example need to
change color in order to survive?
• Is a brown beetle “better”?
• How is our green beetle/brown beetle
example of natural selection different from
“inheritance of acquired characteristics” of
Lamarck?
An important feature of natural selection is “fitness”
What does this mean?
A genotype’s fitness is defined by its ability to successfully pass on
its genes to the next generation.
What defines reproductive success?
survival to reproduce
acquire a mate
successful reproduction
behavior (in mate acquisition and parenting)
gentoo
“survival of the fittest” does not necessarily mean big, strong,
mean, etc, etc. It may refer to protective coloration, metabolic
needs, lengthy parenting, etc, etc.
What is “adaptation”?
• Modification of the phenotype that enhances
fitness or reproductive success
• There are limits on the amount of adaptation in a
population…rates on the processes that produce
genetic variation are low, and there are also
constraints on the phyletic history. Ex, a lobster
can’t adapt by losing it’s exoskeleton…that’s
“hard wired” into what defines an arthropod.
• Every generation that reproduces IS adapted; there
aren’t “losers”.
• Adaptation doesn’t happen FOR a reason…it can
only be viewed historically; there is no genetic
modification to produce a “goal” of adaptation
What is microevolution?
• The change in gene frequency in a population
which may lead to speciation (creation of a new
species)
• Ex: our green and brown beetles: if you
determine the genetic makeup of the beetle
population in successive years and notice a change
in ratios of genes, you are studying microevolution
This important question is,
“How did this happen?”
Mechanisms for microevolution:
Mutation (although it’s not likely that
you would see this after only 1
generation)
Gene flow - introduction of
new genes into the
population by emigration or
sexual recombination
genetic drift - random changes in
the % of various genes
Natural selection - brown beetles escape
predation and reproduce more frequently
Speciation
species = a population of
interbreeding or potentially
interbreeding individuals
Speciation = a lineagesplitting event that
produces 2 species
Speciation happens through geographic
isolation
The gene pool for a species becomes
geographically separated and due to subtle
environmental differences between the two
regions, the gene pool diverges over time.
Geographic isolation can also happen without
a physical barrier. If the geographic distribution
of a species is very wide, those populations on
one “side” won’t actively interbreed with
populations on the other “side,” and over time,
genetic drift will result in their divergence.
Do we see speciation happening today? 3 examples:
The 1995 Hurricane Marilyn washed several
tree logs, and resident lizards, to a new Caribbean
island (Anguilla). Iguana iguana had not been
recorded on Anguilla before. Biologists are waiting to
see if these new emigrants survive and reproduce,
and how, over time I.iguana’s gene pool may
change (= evolution) and if that will result in
a new species of iguana. Stay tuned!
The spotted owl, Strix occidentalis, is
widespread in the western U.S. The
populations in the south are slightly
different from those in the Pacific NW.
Current studies on owl genetics
suggests that the gene pools are
diverging. Speciation appears to be
happening.
Experimental results…..
Drosophila populations physically
separated are fed different foods.
After several generations they are
reintroduced to one another to see if they
mate or are reproductively isolated.
Fruit flies fed one food source preferred
to mate with other fruit flies fed the same
food (the food source altered their
feeding behavior). Studies continue to
see if their gene pools have diverged (if
speciation has occurred yet).
What speciation would look
like in the fossil record:
Species accumulate morphologic
change over time, much of which is
represented in unconformities and
bedding planes. Sometimes the
entire sequence of intermediate
forms IS preserved
The “gaps” or missing
intermediary steps are real, reflecting
the very rapid burst of evolution that
creates a new species, followed by
long periods of stasis.
How phyletic gradualism happens:
Incremental morphologic change over time.
results from changes in the gene pool over time.
The graph represents an imaginary distribution
of genes in a species (think of the human
genome project for all humans!).
If an environmental change applies a stress
on a species, those having certain genes
that enable them to adapt will
successfully reproduce and individuals with
more of those genes will appear in subsequent
generations. Over time the entire gene pool will
accumulate these changes and the distribution will
change.
This type of gradual change in the genotype will
result in incremental changes in the
phenotype.
Another model for speciation:
The slow change in the genetic makeup of a population as a result of
reduced gene flow.
Perhaps this happens as a result of a population exploiting a new
niche or food source, changing its behavior as a result, and diminished
interbreeding. Continued over time, reduced gene flow will isolate
the populations.
Are we seeing this type of speciation now?
Apple maggot flies have historically
eaten/laid eggs in apples. More recently they
have also started to eat/reproduce in
hawthorne apples. Since insects tend to mate
with other insects that eat the same food,
apple maggot flies are interbreeding with
other apple maggot flies and not with
hawthorne maggot flies. Over time will
these gene pools isolate?
Another mode of speciation: “punctuated
equilibrium”
• Morphology (and gene frequency) stays relatively constant
over long periods of time followed by rapid change and the
appearance of a new species.
• Populations living in the geographic limits of the species
develop slightly different genotypes because they are
living in less than ideal environments (temp extremes,
water depth extremes, etc).
• Environmental change stresses the populations living in
the optimal environments..they have difficulty successfully
reproducing. If the geographically distant populations are
more successful reproducing under these new conditions,
their genotype will allow for more successful reproduction.
Thanks to the Berkeley Museum
website for these illustrations
http://evolution.berkeley.edu/evosite/evohome.html