Mechanisms of evolution Flashcards
Population
Group of organisms of the same species living together in a particular place at a particular time
Gene pool
Sum of all alleles in a given population, changes over time with allele frequencies increasing and decreasing
Causes of variation (5)
Random assortment, crossing over, non-disjunction, random fertilisation, mutations
Random assortment
Multiple combinations of chromosomes that can come from the mother and father as chromosomes are sorted randomly in daughter cells
Crossing over
when homologous chromosomes pair up with each other and exchange different segments of their genetic material forming recombinant chromosomes
Non-disjunction
when one or more chromosomes pairs fail to separate resulting in gametes with more or less chromosomes
Random fertilisation
chance decides which sperm meets which egg
Mutations
permanent change in the DNA of a chromosome that may result in a totally new characteristic in an individual, Random mutations case changes in allele frequency thus changing the gene pool, E.g- blue eyes, being able to drink milk
Allele frequencies
Measured in percentages, 95% of the population don’t carry the cystic fibrosis allele meaning the frequency of the cystic fibrosis allele is 5%, Chinese have a high allele frequency for straight dark hair
Evolution
Gradual change in phenotype thought to be caused by a change in allele frequency, Changes in allele frequency result in phenotypic changes
Gene pool changes
Causes of changes to allele frequencies
Mutations, Natural selection, Random genetic drift, Migration, Barriers to gene flow, Genetic diseases
Natural selection criteria (6)
Variation in the population, Competition between individuals, Selection pressures make some genetic traits more favourable for survival, Those with favourable traits survive, reproduce and pass it onto offspring, Those without the favourable trait die, Allele frequency of favourable trait increases
Natural selection
Directional- works towards the advantageous trait, Occurs in large and small populations
Random genetic drift
By chance, the allele frequency in a population changes (not because it is advantageous), Some random event (famine, natural disaster) that is not associated with an increased chance of survival changes the allele frequency, Non-directional, No selection pressures/not related to the environment, More likely in small populations, Chance occurrence
Examples of random genetic drift
Dunkers in Germany- small religious group who only intermarry within the population, allele frequencies of blood groupings, mid-digital hair, ear loves and handedness are different from the general population, Some Islander group populations have a high allele for A blood group and no B blood group whilst mainlanders are reverse
Bottleneck effect
results in genetic drift, When there is a disaster of some sort that reduces a population to a small handful that rarely represents the actual genetic makeup of the initial population
Founder effect
Often leads to genetic drift, Small group moves away from the original population to begin a new population, Allele frequency of the emigrating group just happens to be different form the frequencies of the original population, Eg Amish populations- descended from a few dozen people who migrated from Germany in the 1700’s, over the last 40 years 61 babies from 21 families were born with microcephaly (small cranium), all families descended from an Amish couple from nine generations before who had the genes for microcephaly
Migration
Genre flow from one population to another, As individuals join a population, they change allele frequencies, Large migrations have considerable impacts on allele frequencies
Barriers to gene flow
Stop interbreeding between populations, Isolated populations may be subjected to different environments with different pressures resulting in different gene pools
Geographical barriers
Ocean, mountain range, dessert
Sociocultural barriers
Race, religion, money, status
Genetic diseases
Can change allele frequencies in a population, Generally, it is expected that the frequency of a fatal allele will decrease in a population over time but this is not always the case (shown in Tay-Sachs disease and sickle-cell anaemia)
Tay-sachs disease
Recessive autosomal (non-sex chromosome) trait, Homozygotes recessive lack and enzyme and accumulate lipids in the nervous system, Causes fatty substances to build-up in the nervous system meaning people usually die at about 5 years old, Results in mental and physical disabilities such as blindness, deafness, inability to swallow, atrophied muscles, Some Jewish populations (religion barrier) form Eastern Europe have high frequencies for this allele, Being heterozygous for the trait gives you resistance to Tuberculosis (caused by bacteria) meaning in crowded ghettos, heterozygotes had a higher change of survival against Tuberculosis meaning they survive and reproduce, Frequency of the allele increased because of environmental pressures
Sickle-cell anaemia
Recessive trait, Causes flattening of erythrocytes (red blood cells) due to a mutation in a gene, preventing them from carrying oxygen, Fatal in homozygotes recessive, Allele frequency in unexpectedly high in some Black African populations, Symptoms- fatigue, shortness of breath, anaemia, swelling and inflammation of hands or feet, lung and heat injury/leg ulcers, Heterozygotes have resistance to malaria meaning they have greatest survival, Homozygotes for sickle-cell anaemia die from the disease, Homozygotes for health erythrocytes die from malaria
Speciation steps (5)
Variation- wide variety of genes present, Isolation- isolation of gene pool/barriers to gene flow, Struggle- Individuals that possess the suitable genes are more likely to survive and produce offspring meaning more offspring are produced that can survive, Selection- suitable genes are passed onto offspring whilst individuals without the suitable gene die off, Speciation- no longer to reproduce with others/new species, evolution brought about by an increase in the frequency of advantageous alleles and a decrease in the frequency of disadvantageous alleles
