Chapter 24 (Population Genetics and Epigenetics) Flashcards
Natural selection steps
- In a given population there is natural variation.
- Some individuals will have characteristics which give them an advantage over others in the population. They have a selective advantage
- These individuals will be more suited to their environment as they are better able to cope with the selection pressures in their habitat.
- These individuals are more likely to reach reproductive age and breed.
- They will pass on the advantageous allele for the beneficial characteristic to the next generation.
- Over time the advantageous allele frequency will increase.
- This continues over many generations
(Survival of the fittest –> natural selection)
What is population genetics
The study of genetic variation within populations
What is a population’s gene pool
All variants of a gene in a population
Natural selection role
Increases frequency of advantageous alleles - alters gene pool.
Factors affecting allele frequencies
Natural Selection
Genetic bottleneck
The Founder effect
What is a Genetic Bottleneck
A drastic reduction in population numbers, caused by natural disaster ( eg, volcanic eruption, habitat destruction, hunting by humans)
How does a genetic bottleneck cause a decrease in genetic diversity?
The proportions of alleles in the surviving population could be very different to those in the original population.
Survival is due to chance - some alleles may disappear if individuals with those alleles don’t survive.
Decreases genetic diversity and causes a reduction in gene pool.
What is the founder effect?
When a small group of individuals break away from original large population and form a new colony.
Can occur as result of migration –> geographical separation.
Examples of founder effect in human populations (Ellis-van Creveld syndrome)
- Amish people migrated to Pennsylvania in 1744, and 2 members possessed recessive allele for Ellis-van Creveld syndrome
- After years of inbreeding, the Ellis-van Creveld allele increased in frequency.
Symptoms of Ellis-van Creveld syndrome
dwarfism, extra fingers, short ribs, cleft palates.
Examples of founder effect in human populations (Blood Group distribution)
- Human global migration. over 10,000 years meant many small populations established.
- Each new population has potential to filter gene pool and change proportion of each allele present.
- in South America, mainly blood group O, but blood group differences meant allele for IB was passed on more often and it became more common.
What is genetic drift
A reduction in the genetic diversity of a population just because some alleles weren’t passed on to the next generation.
What is the Hardy Weinberg principle
States that the proportion of alleles will remain the same from one generation to the next provided that the following conditions are met
Assumptions of Hardy-Weinberg principle
- Large population size (ensure no sampling error from one generation to next)
- Random mating (no mating by genotype)
- No new mutations (no genetic change)
- No natural selection for or against alleles
- No migration in or out of population (no movement of alleles)
Definition of a species
A group of individuals which can interbreed with each other to produce fertile offspring.
Definition of speciation
Formation of a new species
What causes speciation to occur
A form of isolation must exist
What causes speciation to occur
A form of isolation must exist
Steps of allopatric speciation
Geographical isolation
- A physical barrier separates members of the same species
- Separated populations experience different selection pressures, so evolution is driven in different directions
- A new species develops and if two populations are reintroduced, they wont be able to interbreed successfully.
- 2 separate species, 2 separate gene pools.
Steps of sympatric speciation?
Reproductive isolation
Reproductive isolation eventually occurs in geographically isolated populations
New species which are unable to interbreed successfully and can’t reproduce.
Geographical distributions may still overlap
Mechanisms for reproductive isolation
Temporal - Different mating seasons in animals, different flowering seasons in plants
Behavioural - Different mating rituals
Mechanical/anatomical isolation - incompatible reproductive systems
Post-mating barrier means gametes can’t meet - eg, sperm is destroyed by chemicals in female’s reproductive system
Examples of reproductive isolation in primate speciation
Guenon species.
- many species of Guenon monkeys in Africa, and several species have overlapping geographical ranges but don’t interbreed.
Distinctive visual features prevent breeding and reinforce reproductive isolation, so reduces risk of producing infertile offspring.
Examples of reproductive isolation in primate speciation
Hominids
Chimpanzees and bonobos evolved from common ancestor so have similar genomes, thought to have been separated by the Congo river over 1 million years ago
Will mate in captivity to produce fertile hybrid offspring.
Behavioural and anatomical differences insufficient to stop them mating and no post-mating barriers evolved.
What are epigenetic changes
Epigenetic changes leave the structure of a gene intact, but genes are switched on or off - the extent of gene expression is altered.
Epigenetics is the study of this.
Epigenetic studies in human populations (Dutch Winter Studies)
Children conceived during wartime winter famine in Netherlands have inc. risk, of diabetes, obesity, heart disease.
Malnourishment of their mothers in early pregnancy - affect their. whole live and grandchildren’s’ lives.
DNA methylation information
Methyl groups added to DNA
Usually added to cytosine nitrogen-containing bases
Decreases transcription
Decreases gene expression
Histone Modification information
DNA is wrapped around histone proteins in eukaryotes
Chemical modification of histone proteins occurs by addition of: acetyl, methyl or phosphate group.
Modification activates or deactivates gene, making gene more or less accessible to soluble transcription factors
What are transcription factors
Proteins that control the rate of transcription
Epigenetic studies in human populations (Norrbotten studies)
People of Norrbotten experienced famine and feast years.
Descendants of women who experienced famine when they were foetuses had dec. life expectancy.
Descendants of men who were over-nourished during. puberty had dec. life expectancy.
so epigenetic changes were passed onto subsequent generations
Epigenetic studies in human populations (Twin Studies)
Identical twins have same DNA sequence, so any differences in them will be due to environmental factors if separated early in childhood.
They would experience different environmental factors
Shown that identical twins raised apart show few epigenetic differences in early years, but significant epigenetic differences in middle age.
Definition of a deme
A group of interbreeding members of one species.
Why are there different types of haemoglobin?
DNA mutation -> different primary structure of ppc -> different secondary and tertiary structure -> altered function of protein. Many different gene variants exist for Hb ppc’s.
Type of Hb
Haemoblobin S
Haemoglobin C
Haemoglobin H
Info on Haemoglobin S
Mutation - substitution in B ppc. gene
Valine replaces glutamic acid
Changes to protein function - sickle cell disease (Hb clumps at low oxygen levels.)
Info on Haemoglobin C
Mutation - Substitution in B ppc gene.
Lysine replaces glutamic acid
Effect is less severe than SCA - can cause some RBC to break down
Info on Haemoglobin H
Severe mutations to A ppc genes.
Unstable Hb formed from 4 B ppc due to lack of A ppc.
Higher affinity for 02 than Hb A - less 02 transported to and offloaded in respiring tissues.
Malaria and Hs allele
Those with HbA HbA are unlikely to survive and reach reproductive age, but those with HbA HbS have sickle cell trait which provide some resistance to malaria. The HbS allele provided an advantage and natural selection maintains it at a high frequency in the population.
Sickle Cell trait info
SCA offers some resistance to malaria, and smaller amounts of abnormal haemoglobin are produced. The Hs allele increases carbon monoxide production during the breakdown of red blood cells, which prevents development of the disease.
