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Transcript
EVOLUTION- a change in the gene frequency of a population
Two important consequences of evolution for ecology (the first is
important for understanding life history patterns, the second for
understanding of community interactions)
1.
MECHANISMS OF EVOLUTION
1. mutations
2. migration
3. genetic drift
4. natural selection
1. mutations - mistake in replicating genetic information. e.g. single
nucleotide deletion. A change in genetic instructions.
-RANDOM (no matter how the env changes, will not determine type of
mutation), whether beneficial or harmful depends on the environment.
- only way that new alleles arize, i.e. the original source of variation
2. migration - immigration of new individuals either carrying new alleles or a
different frequency of alleles. How much populations move around is very
difficult to determine. Can not follow every individual. Larger animals
can be tagged.....
3. genetic drift - random changes in the frequency of genes due to small
population size. If only 6 individual (3 AA and 3 aa) and there is an equal
chance of death before reproduction and three die, then a significant
chance one allele will go extinct. If 1000 AA and 1000 aa, and half die,
very unlikely that all AA will die.
4. natural selection - the only mechanism that explains how organism adapt
to their environment. Is a non-random process but is effected by random
processes of mutation and environmental change. -predictable
Proposed by C. Darwin and A. Wallace; 3 conditions:
(both recognized diversity of earth and geologic record in voyages to
tropics)
1) Traits are Inherited e.g..... Certainly can observe heritability of
traits in nature.
2) Variation in Traits e.g.....
Variation occur by mutation, recombination, (sexual reproduction),
migration and hence is random.
However, variation is limited (a population can only carry so many
types of genes and mutations are random not produced when
needed) so that the potential directions of evolution for a given
population are limited
3) Differential survival and reproduction of heritable traits depending on the environment and interacting organisms, some traits
increase the chance of survival and hence reproduction. Population
have the potential to grow fastest enough to outstrip food.
i.e. N.S. is differental survival and reproduction of genes
Higher survival increase chance of reproduction (blueprint)
The more copies of the blueprint passed on by one type of individual, the
more prevalent those blueprints become in the population.
Environmental change directs evolution. Environment may change randomly,
but organism will adapt to change through natural selection (very nonrandom) i.e. explains correlation of environment with adaptation.
(adaptation- an existing structural, physiological, or behaviorial of an
individual that promotes survival and reproduction under prevailing
conditions).
No perfecting trend (e.g. parasite evol.)
Is larger size adaptive?
Can not say a trait (or a gene) is adaptive unless you consider the
evironment it is in. e.g. a gene that causes thicker body fur will only be
adaptive in a colder environment. Selection is not survival of the
strongest (fittest is not the most fitness in the exercise sense).
"Natural selection is not an active agent with physical properties, much less
a mind. It is no more than a statisical measure of the difference in
survival and reproduction among entities that differ in one or more
characteristics. Selection is not caused be differential survival and
reproduction; it is differential survival and reproduction, and no more"
Dawkins - The Blind Wacthmaker
i.e. no way that N.S. can anticipate future change.
Modes of selection- Env determines direction of selection.
STABILIZING SELECTION birth weights, horseshoe crabs
DIRECTIONAL SELECTION peppered moths, pest resistance, antibiotics
and sphyllis
DISRUPTIVE SELECTION multi-phenotype mimics
If same selection pressure is applied for 40 generation, number of AA and
Aa become less than 1% of population
If selection pressure is only a 1% difference in survival, applied for 40
generation, number of AA is 20% of population, down 5%
Small differences in selection pressures can make significant evolutionary
changes because selection is cumulative because living systems
replicate
(Other systems undergo selection, but do not reproduce. Hence selection is
not cumulative in nonbiological systems. In biological systems selection is
cumulative, systems can be complex because changes build on previous
changes)
Natural selection is the ability to pass traits on to future generations, not
just the next generation. If a trait becomes more common in the next
generation, but those individuals can not survive, the trait will never be
selected for in the long run because selection is cumulative.
If gene A causes larger body size than gene B (assuming no heterozygotes,
e.g. asexual) which increases reproduction but decrease survival (e.g can
not hide)..
Gene A, results in 1.0 survival (100% survive to reproduce) and 1.0 offspring
per individ.
Gene B, results in .5 survival (50% survive to reproduce) and 3.0 offspring
per individ.
then
100 individual with gene A X 1.0 X 1.0 = 100 offspring
100 individual with gene B X 0.5 X 3.0 = 150 offspring
How will the population change? (which gene will become more common?)
Sexual selection demonstrates that the success of reproduction can
outweigh cost of increased mortality. However, excessive reproduction
no good if at the cost of low parent and offspring survival. Must not only
be able to reproduce but survive till reproduction and produce offspring
that are able to survive.
Natural selection is not always survival of the strongest or most
physically fit, but the ability to pass genes on to the next generation
(instruction that increase survival and reproduction will become more
common in the next generation)
Thus, success among individuals (and thus ultimately among populations) is
measured only in terms of successful offspring. When to reproduce, how
often, how many at each time, how long to live is physically constrained.
Natural selection should optimize these strategies depending on the
environment. This optimization of traits to maximize offspring
represents the LIFE HISTORY PATTERN
Limit to how much energy available, so must be partitioned between repro
and surv. (and further partioned within each; e.g. age of reproduction,
brood size....). How it is done depends on env. and strategies of
competitors, predators, etc.
Selection based on individual (those genes that are more likely to be passed
on increase),
Lamarck- env causes change in genes, Darwin - genes determines whose
genes surv/repro., random mutation changes genes. Again, variation is
limited because new adaptation are not produced just because they
are needed so that the potential directions of evolution for a given
population are limited.
Thus ind. selected causing evolution of population, but don't think
populations are selected:
Species are not selected, individuals are. Traits that favor the survival of
the species but not the individual are less likely to be passed on (the good
of species). Cheater's gene win under most conditions. Some behavoirs
may appear alturistic, but usually can be shown to improve individual
survival (e.g. crow calls scatter flock, confusing predators).
Wynn-Edwards: populations managed themselves, overpopulation prevented
because individuals limited reproduction or increased mortality
(lemmings). NO MECHANISM (Ockham's razor, atfter the 14th century
friar William of Ockham: it is in vain to do by more what can be done by
less - the simplest explanation is the best).
Explanation for decreased reproduction at higher populations density:
Wynn-Edwards - individuals limit reproduction for good of species.
Darwian - as population size increases those with fewer young are
better able to feed their young and those survive and pass on
genes for fewer offspring
Alturistic behaviors can evolve under Darwian natural selection in kin
selection. An individual shares a certain number of genes with siblings so
more likely to take risks saving them. (does not mean selfness is best,
only that we can understand biological basis. As humans we can think
through the consequences of selfness). (Also, some models of group
selection proposed but require very specialized conditions).
molecule
genes
cells
individuals
family groups
popul.
comm.
ecosys.
Heritability
and Repro.
Variability
-------decreasing----------------->
/--------none----------/
Another consequence decreasing heritablity with increasing levels of
organization is that ecosystems (and communities) are not likely to be
similar. Thus making generalizations about ecology difficult.
MAIN POINTS ON NATURAL SELECTION:
-natural selection is differential survival and reproduction of inheritable
traits. Ecological strategies must be considered not only in light of
survival, but also reproduction. (the number of genes passed onto
future generations depend both on survival of parents, number of
offspring, and survival of offspring). The resultant optimization of
reproduction and survivorship to maximize offspring is the life history
strategy. (answer questions like why is one species more abundant
than another, or why has a population evovle such and such life history
patterns?)
-the direction of natural selection depends on the environment which
results in individuals being selected causing populations to evolve.
Populations are regulated by external factors; they do not
regulate themselves
-variation is limited so that the potential directions of evolution for a
given population are limited