Natural Selection, Fitness, & Adaptation Flashcards
Natural selection
Process by which adaptive structures are evolved and maintained
What is regarded as the main cause of change in organisms in relation to their environment?
Natural selection
Natural selection depends on ______
genetic variations, which originate idiosyncratically by mutation and recombination
The biased reproduction of certain genetic alternatives based on an organism’s characteristics and interactions
Natural selection
Conditions for natural selection to operate
1.) The individuals or units of selection must vary
2.) Some individuals must be more fit than others (differential fitness)
3.) There must be a correlation between the fitness of parents and offspring (heritability)
5 Observations in Origin
1.) Populations have the potential to increase exponentially
2.) But they generally maintain a stable size
3.) Resources are limited
[Struggle for existence]
4.) Variation in all species
5) Variation is heritable
[Variation makes some individuals more successful than others in the struggle; those that survive and reproduce will leave offspring that have those same variants]
*Evolution by natural selection
Darwin and Wallace’s mechanism for evolution
Natural selection
The phenomenon of ________ variation leading to a better chance of survival and reproduction
favorable
Fitness
A measure of how good an organism is at survival and reproduction (“reproductive success”)
- a number
An example of natural selection in finches
Variation in beak size and shape in a single species of ground finch
Factors: annual rainfall and number of cactus flowers by year
Variation in beak shape
- long and thin to short and thick
Beak shape affected food resource possibilities
- nectar of flowers
- bugs inside cactus paddles
El Niño event in 1983
- most cactuses fell over and rotted, causing beak size to evolve from long and thin to SHORT and THICK
Favorable
= higher relative fitness
Variation is not inherently “good” or “bad”
A variant that has high fitness in one environment might not have high fitness in another different environment
- the fitness value associated with a phenotype/genotype is ENVIRONMENTALLY DEPENDENT!
Selected for
Higher relative fitness
Selected against
Lower relative fitness
How is fitness a weighting factor?
Determines how quickly evolution will happen
Adaptation
The phenomenon of an organism being a good “fit” to its environment
Natural selection produces…
adaptation
Key points about adaptation
1.) Trait that increases the reproductive success of an organism
2.) The process of genetic change of a population, owing to natural selection, whereby the average state of a character becomes improved with reference to a specific function, or whereby a population is thought to have become better suited to some feature of its environment
3.) Improvement in function
4.) The process of successful interaction between a population and an environment
5.) An anatomical physiological or behavioral response of organisms or populations to the environment
The 3 different patterns of selection
1.) Directional selection
2.) Stabilizing selection
3.) Disruptive selection
Directional selection
Relative fitness increases toward one extreme
Stabilizing selection
Fitness is low at both extremes
Disruptive selection
Relative fitness low at intermediate phenotype/genotype; high at both extremes
- relative fitness is high at both extremes but low at the initial average
Cavet of selection
Sickle Cell Disease
Why doesn’t natural selection eliminate the HbS allele if it causes Sickle Cell?
Heterozygote advantage
Heterozygote advantage
Fitness of the heterozygotes is greater than either homozygote
- explains how a seemly deleterious allele can be maintained in a population
The heterozygote that causes Sickle Cell has an advantage in…
Malaria-endemic areas
Malaria
Common and deadly disease passed to humans through an insect vector
- the protozoan Plasmodium falciparum has a lifecycle partly in mosquito bodies and partly in red blood cells
The HbA HbS heterozygotes are more resistant to developing malaria than the HbA HbA homozygotes
Red blood cells of heterozygotes are less hospitable for the parasite that causes Malaria
The greater relative fitness of the HbA HbS heterozygotes PREVENTS the HbS allele form being eliminated…
Even though the HbS HbS condition is associated with a severe disease state and low fitness
The hetozygote advantage can create a state of balanced polymorphism, where what happens?
Where multiple alleles reach a steady state distribution
- sometimes called balancing selection
What happens when the heterozygote advantage disappears?
The HbS allele should be “selected against” because of the lower fitness of the HbS homozygote
- It takes a long time to eliminate a deleterious allele if reduced fitness is exhibited only by the rare deleterious allele homozygotes
- There are some studies that suggest the frequency of the HbS allele is decreasing in the US populations (because Malaria is not too common in the US so we don’t need the heterozygote advantage)
The evolutionary history of sickle cell provides a good example of human behavior modifying evolution
Human populations shift to agricultural practices
- humans create malarial breeding grounds
- human population density increases
Because there is more malaria, heterozygote has even greater relative fitness
- the deleterious allele become more common in the population
Bicultural feedback
- human behavior alters environment
- New environment causes new selective pressure
- new selective pressure alters genotype frequency
- More people survive, population grows
The Modern Synthesis
Period 1930s-40s marking assimilation of Darwin’s evolution (emergence of contemporary evolutionary theory)
- Re-define evolution in the framework of genetics
- creation of population genetics
- combination of mutation and natural selection causes evolution
- but there are other causes of evolution as well!
Genetics and statistics
Ronald Fisher
JBS Haldane
Sewall Wright
Theodosius Dobzhansky
Taxonomy and Systematics
Ernst Mayr
Paleontology
G.G. Simpson
Evolution is…
The transformation of species over time
- shift frequency of different variants in populations
- and the appearance of new species
a change in allele frequency in a population over time
Population genetics
Field concerned with detecting and explaining how phenotype, genotype, and allele frequencies change over time (generations)
- sometimes called microevolution
Microevolution
Within a species, changes in allele frequency from one generation to the next
Evolution is defined by what?
Population frequencies
- Individuals do NOT evolve
The 4 forces of evolution!
1.) Mutation
- source of all variation
2.) Natural selection
- shifts pattern of variation due to fitness differences
- produces adaptation
3.) Genetic drift
4.) Gene flow
