Chapter 18 Flashcards
(21 cards)
Gene pool
all the alleles of all the genes of all the individuals in a population at a given time
Allelic frequency
the number of times an allele occurs within the gene pool
What are the Hardy-Weinberg equations
p + q = 1
p^2 + 2pq + q^2 = 1
- p = dominant allele
- q = recessive allele
- 2pq = heterozygous
What are the genetic causes of variation?
- mutations - sudden changes to genes/chromosomes are able to be passed on = main source of variation
- meiosis - all gametes produced are genetically varied
- random fertilisation of gametes - offspring are different from parents as which gametes fuse with which is a random process
What are the environmental influences on genetic variation
- climatic conditions
- soil conditions
- pH
- food availability
these play a major role in determining where on the continuum an organism actually lies
Describe a normal distribution curve
bell-shaped curve
What are selection pressures
the environmental factors that limit the population of a species
What factors does natural selection depend on
- organisms produce more offspring than can be supported by the ecosystem
- there is genetic variety within the populations of all species
- a variety of phenotypes that selection operates against
Intraspecific variation
competition between individuals of the same species
How does overproduction of offspring play a role in natural selection
- too many offspring = more competition between offspring
- individuals that die due to competition are the ones that aren’t best adapted for survival
- individuals that have the more advantageous features are the ones that survive and breed = more favourable allele combination passed onto next generation = different allele frequency in new population
- depends on individuals of a population being genetically different from one another
What role does variation play in natural selection?
- populations with little genetic variation are often more vunerable to new diseases and climate change
- the larger the population the more genetically varied it is = greater chance the an individual will have the advantageous allele combination that leads to best chance of survival
- variation therefore provides the potential for a population to evolve and adapt to new circumstances
What are the types of selection?
- stabilising selection
- directional selection
- disruptive selection
Explain what stabilising selection is
- eliminates the extremes of the phenotype range within a population and with it the capacity for evolutionary change
- tends to occur where environmental conditions are constant over long periods of time
- so if the environmental conditions fluctuate year on year both extremes will survive as there will be years where they are most advantageous
- if the environment is constant individuals at the extremes will never be advantageous
Explain what directional selection is
- in a population there is a range of genetically different individuals in respect of any one phenotype
- continuous variation amongst these individuals forms a normal distribution curve
- theres a mean in the curve that represents the optimum value for phenotypic character under the conditions
- if environmental conditions change the optimum value for survival also changes
- directional selection results in one extreme variation being selected against in favour of the other extreme
Explain what disruptive selection is
- the opposite of stabilising selection
- it favours the extreme phenotypes at the expense of the intermediate phenotypes
- the most important in bringing evolutionary change
- occurs when an environmental factor takes two or more distinct forms
- could result in two seperate species of the mammal
Speciation
the evolution of new species from existing ones
Species
a group of individuals that have a common ancestry so share the same genes but different alleles and are capable of producing fertile offspring
Genetic drift
something that can take place in small populations
Allopatric speciation
- geographical separation
- may be the result of any physical barrier - prevents interbreeding
- if environmental conditions either side of the barrier vary then natural selection will influence the separate populations so each will evolve differently
- ultimately resulting in reproductive separation
Sympatric speciation
- means same country and describes a form of speciation that results within a population in the same area leading to them becoming reproductively separated
- e.g. one nocturnal species and one normal species
- results species in being unable of successfully breeding with one another and resulting in two separate species
What are the forms of isolating mechanisms
- geographical
- ecological
- temporal
- behavioural
- mechanical
- gametic
- hybrid sterility
