Transcript Natural Selection

Natural Selection
A cause of evolution
Forces of evolutionary change
• Natural selection
– traits that improve survival
or reproduction will accumulate
in the population
• adaptive change
• Genetic drift
– frequency of traits can change
in a population due to
chance events
• random change
Essence of Darwin’s ideas
 Criteria for natural selection
variation exists in populations – important in
a dynamic environment
overproduction of offspring
 more offspring than the environment can support
competition for limited resourses
 for food, mates, nesting sites, escape predators
differential survival
 successful traits = advantage
differential reproduction
 successful traits become more
common in population
Natural Selection
• Survival and reproduction of the fittest
• Evolutionary fitness is measured by
REPRODUCTIVE SUCCESS!
• Selection acts on any trait that affects survival or
reproduction
– predation selection, physiological selection, sexual
selection
• Predation selection
– act on both predator & prey
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behaviors (ex. safety in numbers)
camouflage & mimicry
speed
defenses (physical & chemical)
Physiological Selection
• Acting on body functions (physiological)
– disease resistance
– efficiency at using oxygen, food, etc.
– biochemical versatility (ex. able to use multiple food
sources)
HOT STUFF!
Some fish had the
– protection from injury
variation of producing
anti-freeze protein
5.5 mya
The Antarctic Ocean freezes over
Sexual Selection
• Acting on reproductive success
– attractiveness to potential mate
• Example: bird coloration
– fertility of gametes
– successful rearing of offspring
Survival doesn’t matter
if you don’t reproduce!
Sexual selection
It’s FEMALE CHOICE, baby!
The lion’s mane…
• Females are attracted to males
with larger, dark manes
• Correlation with higher
testosterone levels
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better nutrition & health
more muscle & aggression
better sperm count / fertility
longer life
• But imposes a cost to male
– HOT! Is it worth it??
Effects of Selection
• Deviations from the normal distribution of a
population
• Evolution is NOT goal oriented – different
variations can be selected for in different
environments
• Environment affects which way evolving
population will deviate and how fast
• If environment changes, it can change deviation
• 3 ways a population can deviate:
Distribution Histograms
• Y axis – number of organisms
• X axis – some measurable trait
Stabilizing Selection
• Stabilizing selection –
natural selection
favors the average
individual
– Ex. Plant height
• Too short – can’t
compete for sunlight
• Too tall – sustains wind
damage
Directional Selection
• Directional selection – natural selection favors
one extreme or the other
– Seen often in changing environments
– Ex. Giraffe necks
• Short-necked cannot reach taller plants
Disruptive Selection
• Disruptive selection – natural selection favors
both extremes (selects against average)
– Occurs often when there is 2 microenvironments
– Example: limpets growing on light and dark rocks
Peppered moth
• What was the selection factor?
– early 1800s = pre-industrial England
• low pollution
• lichen growing on trees = light colored bark
– late 1800s = industrial England
• factories = soot coated trees
• killed lichen = dark colored bark
– mid 1900s = pollution controls
• clean air laws
• return of lichen = light colored bark
Year
1848
1895
1995
% dark
5
98
19
% light
95
2
81
Long Term Effects of Selection
• Some variations significant increase, decrease,
or disappear
• Adaptation – variation that is favored by
selection and provides an advantage in a
particular environment
Artificial selection
• Selective breeding can use variations in
populations to create vastly different
“breeds” & “varieties”
“descendants” of wild mustard
“descendants” of the wolf
Why does evolution matter now?
Applications of Evolution
1.Antibiotic-resistant bacteria (ex. MRSA)
• Bacteria reproduce and mutate at a very high
rate.
• Some bacteria’s DNA has mutated to be
resistant to many known antibiotics.
• When the resistant bacteria reproduce, they
pass this ability onto their offspring.
• Bacteria can also pass the resistance to other
bacteria through conjugation.
• To help prevent the development of antibiotic
resistant bacteria,
– don’t overuse antibiotics (they do not kill viruses!)
– use a variety of antibiotics (not just the same one
over and over again)
– finish the whole dose of antibiotics
Applications of Evolution
2.Pesticide-resistant insects
• Same mechanism as above.
• To help prevent this,
– don’t overuse pesticides (or don’t use them at all)
– raise/buy organic food
– use biocontrols (such as lady bugs)
Natural selection in action
• Insecticide &
drug resistance
– Insecticide/drug didn’t
kill all individuals
– resistant survivors
reproduce
– resistance is inherited
– Insecticide/drug
becomes less & less
effective
Fig. 22-UN2