Heliconius pachinus
Heliconius cydno
10.3 Gene Pools and Speciation
Gene pools change over time.
Essential idea
10.3 Gene Pools and Speciation
Vocabulary
| Statement | Guidance |
10.3.U1 | A gene pool consists of all the genes and their different alleles, present in an interbreeding population. | |
10.3.U2 | Evolution requires that allele frequencies change with time in populations. | |
10.3.U3 | Reproductive isolation of populations can be temporal, behavioural or geographic. | |
10.3.U4 | Speciation due to divergence of isolated populations can be gradual. | |
10.3.U5 | Speciation can occur abruptly. | Punctuated equilibrium implies long periods without appreciable change and short periods of rapid evolution. |
10.3.A1 | Identifying examples of directional, stabilizing and disruptive selection. | |
10.3.A2 | Speciation in the genus Allium by polyploidy. | |
10.3.S1 | Comparison of allele frequencies of geographically isolated populations. | |
10.3 Gene Pools and Speciation
Syllabus Reference
10.3 Gene Pools and Speciation
10.3.U1 A gene pool consists of all the genes and their different alleles, present in an interbreeding population.
Allele frequency
Allele frequency is the proportion of all copies of a gene that is made up of a particular gene variant (allele).
Gene pool
The total collection of different alleles in an interbreeding population.
Example
Say if a recessive allele h made up 2% of the total in a human population…
…then the dominant allele H would make up 98%.
The frequency for h would be expressed as 0.02 and for H 0.98
Recessive allele frequency + dominant allele frequency = 1
(for characteristics determined by two alleles)
10.3 Gene Pools and Speciation
10.3.U1 A gene pool consists of all the genes and their different alleles, present in an interbreeding population.
New combinations of alleles lead to new phenotypes that can then be selected for or against by the environment.
10.3 Gene Pools and Speciation
This leads to evolutionary change in the species
10.3.U2 Evolution requires that allele frequencies change with time in populations.
Rabbit image: http://cliparts.co
Key
Evolved population
Original population
Selective pressure
If the selective pressures do change then the population will evolve, but how it evolves depends on which phenotypes are experience the greatest pressure.
10.3 Gene Pools and Speciation
If the selective pressures applied to a population do not change then the population will not evolve.
10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.
Medium ground finch
Beak shape and size in Geospiza fortis
Selective pressure: during dry years small seeds are not abundant.
Result: Birds with larger tougher beaks become more frequent
Example from 5.2.A1 Changes in beaks of finches on Daphne Major.
10.3 Gene Pools and Speciation
10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.
Selective pressures: Babies of low weight lose heat more quickly and get ill from infectious diseases more easily. Babies of large body weight are more difficult to deliver through the pelvis.
Result: Medium weight babies have a much lower mortality and hence the frequency of medium weight babies increases.
Mayumi Paine (aged 1 day) – photo by Chris Paine
10.3 Gene Pools and Speciation
Human birth weight
10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.
Selective pressure: Oysters living on rocks and rockpools
Result: Light-coloured oysters would blend into the rocks in the shallows, and the darkest would blend better into the shadows.
10.3 Gene Pools and Speciation
Disruptive selection
10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.
Gene Flow
10.3 Gene Pools and Speciation
Summary of Gene flow!
10.3.U2 Evolution requires that allele frequencies change with time in populations.
Allopatric speciation is speciation that results when a population is separated by a physical barrier. It is also referred to as geographic speciation.
10.3 Gene Pools and Speciation
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
Sympatric speciation is speciation that occurs without physical separation of members of the population.
10.3 Gene Pools and Speciation
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
Sympatric speciation is speciation that occurs without physical separation of members of the population.
10.3 Gene Pools and Speciation
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
The circumstances preventing different species from interbreeding are known as reproductive isolating mechanisms
Really good introduction to speciation and reproductive isolation* by Bozeman Science
Your syllabus focuses on three ways in which populations can be isolated to prevent reproduction:
*This video also looks at other aspects of the topic including polyploidy, but remember it is not an IB course specific resource so make sure that you know what is relevant to you.
10.3 Gene Pools and Speciation
Extra Information
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
The reproductive isolation only promotes selection in sexually reproducing organisms: it doesn’t apply to single-celled organisms.
Rats!
10.3 Gene Pools and Speciation
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
10.3 Gene Pools and Speciation
Why does reproductive isolation occur?
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
MAX
Pinus radiata (Monterey Pine)
Pinus attenuata (Knobcone pine)
Pollen Production
Pinus radiata and Pinus attenuata are prevented from hybridising because they have separate pollination times.
They can be made to hybridise by pollinating them manually.
Month
10.3 Gene Pools and Speciation
Temporal isolation
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
The two species are in the same area, but live in different habitats
I love me some CaCO3 in my soil
Blechhh!
Acidic soils are more my thing
Viola arvensis
Viola tricolor
10.3 Gene Pools and Speciation
Ecological isolation
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
Animals exhibit courting behaviour (song, dance etc.) or release pheremones to attract mates. Individuals are only attracted to, and will only mate with, members of the opposite sex who perform the appropriate ritual or release the correct chemical.
10.3 Gene Pools and Speciation
Behavioural isolation
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
PanI is a gene in cod fish that codes for an integral membrane protein called pantophysin.
Two alleles of the gene, PanIA and PanIB, code for versions of pantophysin.
Samples of cod fish were collected from 23 populations in the north Atlantic and tested to find the proportions of the alleles in each population.
The proportions of alleles in a population are called the allele frequencies. The frequency can vary from 0.0 to 1.0 with the total frequency of all alleles always being 1.0.
Key
12
PanIA
PanIB
Population #
Source: RAJ Case et al. 2005. “Macro- and micro-geographic variation in pantophysin (PanI) allele frequencies in NE Atlantic cod Gadus morhua.” MEPS. Vol 301. Pp 267–278. Figs 1 and 3.
10.3 Gene Pools and Speciation
Use the information and charts to answer the questions on the following slides…
10.3.S1 Comparison of allele frequencies of geographically isolated populations.
Key
12
PanIA
PanIB
Population #
1 and 2
7/11
PanIA 0.75 and PanIB 0.25
Graph and questions from IB Questionbank
10.3 Gene Pools and Speciation
Comparison of allele frequencies
10.3.S1 Comparison of allele frequencies of geographically isolated populations.
Key
12
PanIA
PanIB
Population #
any population from 14 – 23 and any population from 1 – 4
5. Give Suggestions why the PanIB allele is more common in population 13 than population 22. [2]
Cooler water temperature favours PanIB;
Interbreeding with icelandic/more northern populations;
Graph and questions from IB Questionbank
10.3 Gene Pools and Speciation
10.3.S1 Comparison of allele frequencies of geographically isolated populations.
Species did not change for long periods of time but were in stasis until events punctuated (disrupted) the equilibrium (balance)
10.3 Gene Pools and Speciation
10.3.U5 Speciation can occur abruptly.
Punctuated equilibrium was first proposed by palaeontologists Niles Eldredge and
Stephen Jay Gould in 1972.
Gradualism
10.3 Gene Pools and Speciation
The rate of speciation varies
10.3.U4 Speciation due to divergence of isolated populations can be gradual. AND 10.3.U5 Speciation can occur abruptly.
Punctuated Equilibrium
10.3 Gene Pools and Speciation
The rate of speciation varies
10.3.U4 Speciation due to divergence of isolated populations can be gradual. AND 10.3.U5 Speciation can occur abruptly.
Darwin recognised however that not all species evolve at the same rate all of the time
"I think case must be that one generation should have as many living as now. To do this and to have as many species in same genus (as is) requires extinction . Thus between A + B the immense gap of relation. C + B the finest gradation. B+D rather greater distinction. Thus genera would be formed. Bearing relation" (next page begins) "to ancient types with several extinct forms"
10.3 Gene Pools and Speciation
10.3.U4 Speciation due to divergence of isolated populations can be gradual.
TOK - Find out more:
10.3 Gene Pools and Speciation
Richard Dawkins is a prominent critic of the theory
10.3.U5 Speciation can occur abruptly.
10.3 Gene Pools and Speciation
So far you’ve learnt that cells contain two homologous sets of chromosomes.
Well….. that isn’t always the case.
Nature of science: Looking for patterns, trends and discrepancies - patterns of chromosome number in some genera can be explained by speciation due to polyploidy. (3.1)
Polyploidy is much more common in plant species - they lack separate sexes and are capable of asexual reproduction (self-pollination)
Adders tongue (Ophioglossum reticulatum)
10.3 Gene Pools and Speciation
So far you’ve learnt that cells contain two homologous sets of chromosomes.
Well….. that isn’t always the case.
Nature of science: Looking for patterns, trends and discrepancies - patterns of chromosome number in some genera can be explained by speciation due to polyploidy. (3.1)
When non-disjunction occurs during meiosis in humans, an individual can end up with an extra chromosome or missing chromosomes (e.g. An extra chromosome 21 means Downs syndrome).
Total non-disjunction, is when one of the two cells produced during Meiosis I gets all of the chromosomes.
This results in two (2n) daughter cells from meiosis instead of the usual four (n) daughter cells.
10.3 Gene Pools and Speciation
How polyploidy happens
Nature of science: Looking for patterns, trends and discrepancies—patterns of chromosome number in some genera can be explained by speciation due to polyploidy. (3.1)
There exist few polyploid animals species (examples include salamanders, goldfish and salmon).
Polyploidy often leads to increased size, resistance to disease and overall vigour.
However, polyploidy is a great source of speciation amongst plants.
Many agricultural plants are polyploid (e.g. wheat) due to having bigger fruits, seeds and storage organs
10.3 Gene Pools and Speciation
Polyploidy in animals and plants
Nature of science: Looking for patterns, trends and discrepancies—patterns of chromosome number in some genera can be explained by speciation due to polyploidy. (3.1)
10.3 Gene Pools and Speciation
Chromosome number in genus Allium
10.3.A2 Speciation in the genus Allium by polyploidy.
Onion (A. Cepa), 16 Chromsomes
English Leek (A. Cepa), 32 Chromosomes
Many species of this genus commonly reproduce asexually and if polyploidy confers an advantage a new species may arise.
10.3 Gene Pools and Speciation
Chromosome number in genus Allium
10.3.A2 Speciation in the genus Allium by polyploidy.