of Causes Evolution
small population non-random mating recurrent mutation migration selection
These two processes involve introduction of new genetic variation
{
Origin of genetic variation
Genetic change
Mutation Recombination
Sexual organisms: meiosis Bacteria: DNA uptake from environment and/or exchange of species-specific plasmids (circular DNAs)
Gene Flow (migration)
Causes of Evolution
small (effective) population size non-random mating recurrent mutation migration selection
EFFECTIVE population size
Refers to the number of individuals who successfully reproduce This number may be substantially smaller than the total number of individuals in the population
In a sexually reproducing species, there may be 50% males and 50% females in the population
BUT that does not mean that every individual reproduces
Genetic Drift
Fig 23.7
Genetic Drift
Fig 23.7
Genetic Drift
Fig 23.7
Genetic Drift: Bottleneck Effect
Fig 23.8
Increase in frequency of blue allele and loss of yellow allele occur by chance.
Bottleneck Effect
Non-random mating: Inbreeding
100%
AA
25%
AA
Aa
Result: excess of homozygotes (See textbook Fig. 22.11 for example of how some plants avoid inbreeding with flower structures)
Aa aa
50% 25%
aa
100%
e.g. "selfing" in plants
Causes for Evolution small population non-random mating recurrent mutation migration selection
Adaptation
A heritable trait that increases the fitness of the individual with the trait relative to others that lack the trait.
Improves performance in that environment
Usually thought of as response to environmental conditions Although sexually selected traits are also adaptations - to sexual selection
Modes of Selection
Life 8e, Figure 22.12
Sexual Selection"?
Originally proposed by Darwin Selection favoring mating success of an individual/genotype relative to other individual/genotypes in that population Can oppose natural selection
How?? (keep this question in mind during next slide)
Classic experiment by Malte Andersson on long-tailed widowbirds (Euplectes progne) in Africa
http://www.hlasek.com/fo to/euplectes_progne_ad63 72.jpg
What about natural selection on this trait?
Selection can also maintain genetic variation
One mechanism:Any kind of selection that favors heterozygotes over homozygotes will help to maintain polymorphism
heterozygote advantage or heterosis This kind of selection will violate HardyWeinberg expectations because there will be an excess of heterozygotes
A real world example: sickle cell anemia
Maps to gene encoding chain of hemoglobin -over 100 alleles have been found in humans (most of these are rare) -3 common alleles in West Africa, where sickle cell anemia is common: HbA, HbC, HbS S/S = severe anemia (low red cell count) abnormal tertiary structure of globin, red blood cells sickle under shaped low O2 tension, hemoglobin forms long crystals S allele carries heterozygote advantage, due to increased resistance to Plasmodium-mediated malaria
Maintenance of genetic variation: Sickle cell anemia
Fig 23.13
Causes of evolution
1. Variation in traits (mutation, migration) 2. Small population (genetic drift) 3. Non-random mating (inbreeding, sexual selection) 4. Selection: Fitness and adaptation natural selection sexual selection
What is evolution?
Darwin
Transformation of species from one form to another
Never really got to the actual Origin of species
Branching relationships of all living and extinct species on Earth from a common ancestor
20th century neo-Darwinians
Agree with Darwins concepts but reduced to its essence: Evolution is the change of allele frequency in a population over time.
This is our modern definition but Darwins above definition is also valid
Origin of species - still one of our major questions
What is a species?
Most basic way to define species:
Linnaeus (Morphological species concept) Still the most common species concept
There are many other species concepts
For geneticists, we need to think about populations
What is a species?
Lets first address: what is a population?
What is a population?
A group of interbreeding individuals living in the same area at the same time. Share the same gene pool.
Successful Reproduction
Parents Gametes
Mating
Geography Habitat Time Behavior
Mating: Behavior
Fig 24.3
Galapagos Blue-footed boobies
Successful Reproduction
Parents Gametes Zygote
Mating
Fertilization
Geography Habitat Time Behavior
Recognition Compatibility
Successful Reproduction
Parents Gametes Zygote Progeny
Mating
Fertilization
Development
Geography Habitat Time Behavior
Recognition Compatibility
Viability Fecundity
Barriers to Reproduction
Parents Gametes Zygote Progeny
Mating
Fertilization
Development
Geography Habitat Time Behavior
Recognition Compatibility
Viability Fecundity
Pre-zygotic
Post-zygotic
reproductive isolation
If reproductive isolation becomes permanent
There is no longer One group of interbreeding individuals A single gene pool A single population Speciation: formation of distinct biological species
Biological Species Concept
A group of individuals with potential to interbreed in nature and produce viable, fertile offspring, but which do not interbreed with other species in nature. Ernst Mayr 1942
Biological Species
Individuals of two populations:
If they can interbreed, they belong to the same species If they cannot interbreed, they belong to different species
Modes of Speciation
Allopatric speciation geographical isolation Sympatric speciation
Allopatric speciation
Speciation
Divergence may be due to selection genetic drift Reproductive isolation a byproduct of divergence