Chapter 20 - Genes within Populations Flashcards
Basis of evolutionary change
Genetic variation
Evolution
change in the genetic makeup of a population over time
Jean Baptist Lamark (1802)
Inheritance of acquired characteristics - use and disuse
Theory: variation is acquired
Ex. Proposed ancestor of giraffes has characteristics of modern-day okapi. The giraffe ancestor lengthened its neck by stretching to reach tree leaves, then passed the change to offspring.
Charles Darwin
Descent with modification “Natural Selection”
* Organisms produce more offspring than will survive - resources are limited, the more fit individuals will survive
* Populations are highly variable
* Variability is inherited - not acquired, genetic make-up of the population will change
* Resources are limited so there is a struggle to survive - if there are unlimited resources, there is no driving force for change, if carrying capacity is reached, the fight to survive is reached
* Animals best adapted survive and reproduce
* Inherited traits become more prevalent in the population - change in the genetic make-up overtime
Theory: variation is inherited
Ex. Some individuals born happen to have longer necks. Over many generations, longer-necked individuals are more successful, perhaps because they can feed on taller trees, and pass the long-neck trait to their offspring.
How does genetic variation arise?
- Mutation - ultimate source of variation - can happen spontaneously, chemicals, or just random
- Crossing over and random assortment in meiosis
- Sexual reproduction results in new genetic combinations - bringing haploid together and forming new genetic combinations, faster evolution than asexual reproduction
Population genetics
study of properties of genes in a population
Gene pool
sum of all the alleles of all genes in a population
Alleles
alternate form of a gene
If a gene has two alleles then what is the sum of the frequency of the alleles?
1
Hardy – Weinberg Principle
helps determine allele frequencies by using the following equation:
- p2 + 2pq + q2 = 1 ; p + q = 1
- p2 = homozygous dominant
- 2pq = heterozygous
- q2 = homozygous recessive
Hardy – Weinberg Principle
helps determine allele frequencies by using the following equation:
- p2 + 2pq + q2 = 1 ; p + q = 1
- p2 = homozygous dominant
- 2pq = heterozygous
- q2 = homozygous recessive
gives us a base line to measure change in a population
Hardy – Weinberg Principle conditions
- Large population size
- Random mating occurs
- No mutation
- No gene flow
- No selection
5 agents of evolutionary change
Mutation Gene flow Nonrandom mating Genetic Drift Natural Selection
Mutation
new forms of DNA occur - increase in variation - alleles will change
Gene flow
exchange of genes between populations - (same species) population A and population B exchanging, movement of species
Nonrandom mating
mate selection - selecting mates depending on traits wanted or needed, inbreeding
Genetic Drift
change in allele frequency of small populations - small populations are greatly affected by movement, likelihood of extinction, inbreed, complications
Founder effect
Sometimes one or a few individuals disperse and become the founder of a new, isolate population at some distance from their place of origin. Common in islands, migration from one location to another because of some event, starting a new “colony”
Bottleneck effect
Even if organisms do not move from place to place, occasionally their populations may be drastically reduced in size. Result from flooding, drought, disease, etc. small population, chance event, genetic make-up becomes limited
Natural Selection
- Variation in populations - natural selection works by favoring individuals with some traits over individuals with alternative traits
- Differential reproductive success of individuals - some individuals are more successful than others in producing offspring
- Variation is inherited
Fitness
- Reproductive success, leaving behind the most offspring
* Is a combination of survival, mating success, and # of offspring/mating
Mutation vs. natural selection
mutation rates are rarely high enough to overcome the effect of natural selection - doesn’t have high rate of mutation to beat natural selection but it can happen
Genetic drift vs. natural selection
drift may by its nature reduce or remove adaptive genes from a population - rare genes or abundant genes can be removed
Gene flow vs. natural selection
gene flow may be a constructive or constraining force on natural selection
Frequency
Natural selection’s role in maintaining variation
dependent selection
Fitness of a phenotype depends upon its frequency
Negative frequency
dependent - rare phenotypes are preyed upon less, variation is maintained - animals, predators have search images, they look for what they usually eat and ignore the odd, the odd survives
Ex. search image
Positive frequency
dependent selection has the opposite effect. Rare phenotypes eaten, reducing variation - rare doesn’t usually survive, can be easily preyed upon
Ex. oddballs, attracts attention
Oscillating Selection
selection that favors one phenotype at one time, and another phenotype at other times
Ex. Galapagos finches, drought (more large seeds)
Heterozygote Advantage
- individuals with copies of both alleles are favored
Ex. Sickle cell anemia - malaria infects red blood cells, sickle cells protection from malaria, heterozygote have an advantage
Forms of Selection
How selection changes a population depends on which genotypes are favored
Disruptive Selection
diversifying selection, large and small are favored but not medium
Ex. bird beaks, small and large seeds only
Directional Selection
moving towards one direction
Stabilizing Selection
extreme are not favored, selecting middle
Ex. Babies birth weight, mortality
