Title: Points to remember about Natural Selection
1Points to remember about Natural Selection
- Populations evolve, but individuals do not.
I.E., Natural selection acts on the level of the
individual, but populations are the smallest unit
that can evolve. - Natural selection only works on heritable
variations, not acquired traits. - Natural selection can only work with what its
given. Variations are produced by different
genetic mechanisms. - Natural selection is situational to a given
environment in a given time and place. - No ideal organism.
- Fitness of a given trait changes.
2Population Genetics Microevolution
3The historical context of Darwins life and ideas
4Evolution is.
- For Darwin (1859) Evolution is gradual change of
heritable traits in a population across
generations, eventually generating species over
time. - For the Modern Evolutionary Synthesis (early 20th
c.) Evolution is a change in allele frequency
over time
5Population Genetics - Terms
- Population Localized group of individuals of the
same species - Species Group of populations whose individuals
can potentially interbreed - Gene Pool Total aggregate of genes in a
population at one time - Allele an alternative form of a gene
- Homozygous have identical alleles for a given
trait (dominant or recessive) (e.g. AA or aa in a
diploid) - Heterozygous have gt1 different alleles for a
given trait (e.g. Aa or aA in a diploid) - Why arent all individuals of one species
identical?
6Genetic Diversity
1
1
7Genetic Diversity is the product of
- Sexual reproduction
- Genetic recombination
- Mutation- key form of genetic variation in
asexual reproduction, substrate for new genetic
material - Provides alternative alleles that may or may not
be useful with changes in environment raw
material for evolution - R.A. Fisher's Fundamental Theorem of Natural
Selection the rate of natural selection is
proportional to the degree of variation within a
population
8The Modern Evolutionary Synthesis in action
- Hardy-Weinburg Equilibrium
- p2 2pq q2 1
9Figure 23.3a The Hardy-Weinberg theorem
10Figure 23.3b The Hardy-Weinberg theorem
11The Modern Evolutionary Synthesis in action
- Hardy-Weinburg Equilibrium
- p2 2pq q2 1
- Assumptions
- 1. Large population size
- 2. No gene flow
- 3. No mutation
- 4. Random mating
- 5. No selection
12The Modern Evolutionary Synthesis in action
- Microevolution (shift in allele frequencies)
happens when the H-W assumptions fail ! - Small population size gt Genetic Drift
- Migration gt Gene Flow
- Mutation
- Natural Selection
- Nonrandom mating
13Genetic Drift random changes in allele (and
genotype) frequencies from generation to
generation due to sampling error
14Genetic Drift, part A The bottleneck effect
Alleles in original population
Alleles remaining after bottleneck
15Cheetah
Bottleneck effect
www.petsdoc.com/pics/funpages/ wildlifephotos/chee
tah.jpg
16Northern Elephant Seal
Bottleneck Effect
- http//www.icsi.berkeley.edu/dbailey/gallery/imag
e/elephant.jpg
- Reduced to 20 individuals in 1896
- Now 30,000 individuals, with no detectable
genetic diversity
17Genetic Drift, Part B The founder effect
Change in allele frequencies when a new
population arises from only a few
individuals e.g., only a few fish are introduced
into a lake e.g., only a few birds make it to an
island
18www.fishbase.org
Scorpaenidae Lionfish Pterois volitans
Founder Effect
- New Atlantic population, maybe from only 10
individuals
19Gene Flow
addition or removal of alleles due to
individuals entering or leaving a population from
another population
20- On average, two humans differ by 0.1 (1 in 1000
bp). - Is race a genetic reality?
- NO 90 of genetic variation occurs within any
single "race". LOTS OF OVERLAP - YES only 10 of the variation can be explained
by Asian, African, or European origin.
21Gene Flow
22Bluefin Tuna
23http//www.chambers-associates.org/Big-Marine-Fish
/map_bluefin_spawning_areas.html
24Mutation
random, heritable changes in DNA that
introduces new alleles into a gene pool
25 random, heritable changes in DNA sequences that
introduce new alleles into a gene pool
Mutation
- members.tripod.com/Alphacentaur/ X-men.JPG
26Modes of natural selection
27Modes of selection
28Directional selection
Beak size selection in a Galápagos population of
the medium ground finch
29Diversifying selection
30Diversifying selection
Small bill size for soft seeds, large bills for
hard seeds selection against intermediate sized
bills which crack both types of seeds
inefficiently
31Stabilizing selection
32Stabilizing selection
http//www.hummingbirds.net/gallery0.html
33What maintains genetic variation?
- Diploidy
- Balancing selection
- Heterozygote advantage
- Frequency-dependent selection
- Neutral variation
34Sicle cell anemia is a good example of
heterozygote advantage
35Sex
36The two-fold disadvantage of sex
BUT, genetic diversity allows adaptation to
changing environments
37Sexual Selection
- non-random mating where individuals have sexually
dimorphic traits that make them more attractive
to mates
Broad-billed hummingbirds
- http//www.hummingbirds.net/
38Sexual Selection - how can this be favored?
39Sexual selection and the evolution of male
appearance
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