Chapter 18 - Populations And Evolution Flashcards
What do the terms population, gene pool and allelic frequency mean?
Population - a group of organisms of the same species in a particular place at a particular time + have the potential to interbreed
Gene pool - all the different alleles of a particular gene for all the individuals in a population at a given time
Allelic frequency - number of times a single particular allele occurs in a gene pool
What is the study of population genetics?
Looking at how the allelic frequencies for different alleles change over time
What is the Hardy-Weinberg Principle and when does it apply?
A mathematical equation used to calculate the frequency of alleles for a particular gene in a population
Only applies:
- when looking at one gene controlling one characteristic
- when one gene only has two different alleles for it
What is the first equation of the Hardy-Weinberg Principle? Explain it
p + q = 1.0 (100%)
p = probability of allele A
q = probability of allele a
- looks at the frequency of each allele (dominant or recessive) for a gene in the population
What is the second equation of the Hardy-Weinberg Principle? Explain it
p² + 2pq + q² = 1.0
- looks at the frequency of different genotypes in a population
To use the Hardy-Weinberg equations, what assumptions must be made?
- No mutations arise
- Population is isolated - no movement of alleles into or out of the population (population is not interbreeding with another population)
- No allelic selection occurs (both alleles are equally as likely to be passed onto the next generation)
- Large population
- Mating within the population is random not controlled
What causes variation within a population of a species?
Random mutations = results in new alleles of a gene which may or may not be passed onto the next generation
What are the 3 genetic factors influencing variation?
Random mutations, meiosis, random fertilisation of gametes
What are the two types of variation in population?
Continuous variation and discontinuous variation
Explain the properties and examples of continuous variation
Properties:
- no distinct categories
- no limit of the value
- tends to be quantitative
Examples:
- height, weight, heart rate, finger length, leaf length
What is continuous variation controlled by and how is it represented?
Controlled by:
- a lot of genes and environments causing a range of phenotypes between 2 extremes (height in humans)
Represented by a line graph
Explain the properties and examples of discontinuous variation
Properties:
- distinct categories
- no in-between categories
- tends to be qualitative
Examples:
- tongue rolling
- finger prints
- eye colour
- blood groups
What is discontinuous variation controlled by and how is it represented?
Controlled by:
- a few genes which limit number of phenotypes so no intermediates (A,B,AB,O blood groups)
Represented by a bar graph
What is an advantageous allele vs a disadvantageous allele?
Advantageous allele = makes it more likely for organisms to survive to reproduce so pass on allele to next generation
Disadvantageous allele = less likely to survive to reproduce so don’t pass on allele to next generation because they die
Describe the process of natural selection [4]
- There is variation present within a population due to random mutations producing new/ different alleles of a gene.
- Selective pressures arise due to competition for limited resources (food, living space, water, mates)
- Individuals with an advantageous allele are better adapted to their environment, giving them a selective advantage and are therefore more likely to survive (survival of the fittest).
- Those that survive can reproduce so pass on advantageous allele to offspring. This results in an increase in frequency of advantageous allele in population and so species becomes better adapted for environment
What are the 3 types of selection that affect a population?
- Stabilising selection - selection for the individuals around the mean and against individuals at either extremes (birth weights)
- Directional selection - changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population (antibiotic resistance)
- Disruptive selection - favours individuals with extreme phenotypes rather than those with phenotypes around the mean
What does species mean?
A group of individuals that can interbreed and produce fertile offspring
What does speciation mean?
The evolution of new species from existing ones (occurs over many years and has resulted in huge diversity)
- it occurs when a population of a species becomes somehow isolated from other populations of the same species
What are the 2 types of isolation?
- Pre-zygotic isolation
- Post-zygotic isolation
What are the kinds of pre-zygotic isolation?
- geographical = isolated by physical barriers (oceans, rivers)
- ecological = inhabit different habitats in same area
- temporal = breeding seasons don’t coincide
- behavioural = mutations cause variations in patterns of courtship so females don’t respond to males
- mechanical = physically impossible for penis to enter vagina
- gametic = gametes prevented from meeting due to genetic/ biochemical incompatibility
What are the types of post-zygotic isolation?
Hybrid sterility - hybrids formed from the fusion of gametes from different species are often infertile as cannot produce viable gametes
- e.g. in cross between horse (2n=64) and a donkey (2n = 62), the resultant mule has 63 chromosomes so mule is infertile
What are the 2 types of speciation?
Allopatric speciation and sympatric speciation
What is allopatric speciation?
Results from 2 populations becoming geographically isolated (physical barriers)
What is the key example of allopatric speciation?
Darwin’s finches on Galápagos Islands
- large number of finches on these islands which were similar to those on mainland South America
= a small random population of finches became geographically isolated from mainland (founder event)
What does the term genetic drift mean?
Change in the frequency of existing alleles in an existing population due to random sampling of organisms
- the loss of genetic variation
Explain the genetic drift that occurred using the finches on Galápagos Islands example
- some birds from founder event cross and inhabited the Galápagos Islands
- the environmental conditions differed e.g. food sources available
- created a selection pressure resulting in natural selection occurring
- this resulted in a change in allele frequencies in the populations on different islands
- if enough change occurs in populations, this may lead to reproductive isolation (so new species that can no longer interbreed)
Give the model answer for allopatric speciation [6]
- Geographical isolation of populations = allopatric speciation
- Therefore there was reproductive isolation between these two populations as no interbreeding could occur
- There is variation in each population due to random mutations + genetic drift
- Different environmental conditions in different locations = different selective pressure
- Advantageous alleles in each population are selected for by natural selection changing frequency of alleles in different populations
- Eventually each populations may change so much that they can no longer interbreed between the populations and become different species
What is sympatric speciation?
Speciation that results within a population in the same area (no physical separation) but something else isolating them
What was the key example of sympatric speciation?
Anoles - types of lizard
Give the model answer for the Anoles example of sympatric speciation [7]
- There is variation in a population of anoles due to random mutations (different limb length, toe pad size and dulap colour)
- The anoles are not geographically isolated as are found in same environment
- A species living in the same area compete for limited resources so members move in to different habitats in same area to access and use resources not available to others
- Leads to reproductive isolation
- Different environmental conditions in different locations = different selective pressures
- Advantageous alleles in each population are selected for by natural section (survive , reproduce and pass on alleles) which changes the frequency of alleles in different populations
- Eventually each population may change so much they can no longer interbreed between the populations so have become different species
What is a type sympatric speciation in plants called?
Polyploidy
Outline what polyploidy is in plants
- pollen from plants anther to stigma on same plant or on another plant
Species A (2n=4) x Species B (2n=6)
Hybrid zygote = 5 (uneven number so produces sterile offspring)
- however a mitotic error can occur in which spindle fibres do not form properly so a diploid hybrid zygote forms (2n=10)
- all homologous pairs go into one cell rather than split into two
- then self fertilisation produces a fertile hybrid plant = a new species
