Evolution below the species level results from variation and from natural selection
and the action of other forces upon this variation.
Selection occurs whenever the chances of leaving offspring differ among
genotypes.
Microevolution is evolution below the species level.
It results from:
- Variation, brought about by mutation, chromosomal changes, and
genetic recombination through mating. Variation is a prerequisite for selection.
- Selection, genetic drift, and other forces that act upon variation.
- Gene exchange within and between populations.
- Restricted gene exchange, including reproductive isolation.
Darwin's On the Origin of Species (1859)
suggested natural selection as a
mechanism for evolutionary change.
Natural selection:
- All living species tend to over-reproduce.
- Most seeds, eggs, or hatchlings die without reproducing.
- All living species are extremely variable.
- Many of these variations are inherited.
- Inherited differences in survival and reproductive ability
(natural selection) bring about change in each generation.
Darwin quotes: selection
"workmanship"
Selection - examples
Evidence for natural selection as a cause of evolution:
- All living species are highly adapted to their way of life.
- Many adaptations cannot be explain by environmental influence alone.
Examples:
- Unrelated but ecologically equivalent species live on different continents.
- Some embryonic structures (e.g., a flap in the human heart that
seals closed at birth) develop before they become useful
- Some behavior (like bird migration or nest building) occurs
in advance of its usefulness.
- Natural selection can explain mimicry (see below).
- Some adaptations are less than perfect, contrary to Paley's earlier theory
that used perfect adaptation to prove divine creation.
Review: Adaptation
- Structural adaptations
- Biochemical adaptations
- Color & pattern adaptations:
camouflage, industrial melanism, mimicry
- Natural selection has repeatedly been documented (e.g., among
peppered moths in England), and has resulted in changes over time
in natural populations.
Camouflage & Industrial melanism
- Artificial selection by animal and plant breeders has produced many new
adaptations, some of them similar to adaptations occurring naturally.
Mimicry and camouflage:
- Many species gain protection against predators by resembling
their background (camouflage) or by falsely resembling other
species (mimicry).
- In Batesian mimicry, a palatable
species resembles a distasteful or harmful one.
- Mullerian mimicry
is resemblance among distasteful or harmful species.
- Other types: Aggressive mimicry ("wolf in sheep's clothing"); Reproductive mimicry (copulation dummies)
- Mimicry works only when certain models are present,
a fact explained easily by natural selection, but not by Lamarckism
or similar theories, nor by theories of special creation.
- Mimicry may vary geographically, with the same mimic species
resembling different models in different places. Natural selection
can explain this; Lamarckism cannot.
Mimicry
Types of selection: In all types of selection, genotypes contribute genes
unequally to the next generation,
either by differences in mortality and survival, by differences in mating
success, or by differences in fertility and fecundity (leaving offspring).
- Natural selection is differential contribution by natural processes.
The peppered moths of England, selected by predators (birds), are an example.
- Artificial selection is selection of captive species by humans.
- Sexual selection is selection based on success in mating.
- Selection against a dominant trait can eliminate the trait rapidly.
- Selection against a recessive trait works very slowly and becomes
much less effective once the recessive allele becomes rare.
- Selection against heterozygotes can result in either allele becoming
lost and the other taking over 100% of the gene pool.
- Selection favoring heterozygotes over both types of homozygotes
results in balanced polymorphism in which both alleles persist
indefinitely. Sickle-cell anemia is an example of this situation.
- Directional selection shifts the population mean.
- Disruptive selection increases population variance.
- Centripetal or stabilizing selection (very common) reduces variance.
Selection - examples(2)
REVIEW:
Study guide and vocabulary
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