2 Populations And Evolution Flashcards
Define species.
A group of similar organisms that can reproduce to give fertile offspring. They can exist as one or more populations.
Define population.
A group of organisms of the same species living in a particular area at a particular time that can potentially interbreed.
Define gene pool.
The complete range of alleles present in a population.
Define allele frequency.
How often an allele occurs in a population
What is the Hardy-Weinberg principal?
A mathematical model that predicts that the frequencies of alleles in a population won’t change from one generation to the next.
What is the hardy-Weinberg principle used to predict?
- allele frequency
- genotype and phenotype frequency
- percentage of population that has certain genotype
What is the hardy-Weinberg equation to predict allele frequency?
p + q = 1
p= frequency of one allele, usually dominant one
q= frequency of other allele, usually recessive one.
What is the hardy-Weinberg equation to predict genotype and phenotype frequency?
p ² + 2pq + q ² = 1
p ² = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q ² = frequency of homozygous recessive genotype.
Explain stabilising selection.
Stabilising selection is where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. Occurs when environment isn’t changing, and it reduces the range of possible phenotypes.
Explain directional selection.
Directional selection is where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce. This could be in response to an environmental change.
Explain disruptive selection.
Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce. characteristics towards the middle of the range are lost. It occurs when the environment favours more than one phenotype.
Define variation.
The differences that exist between individuals.
Describe variation due to genetic factors. Mutations
sudden changes to genes + chromosomes may, or may not, be passed on to next generation.
What is the main source of genetic mutation?
- mutation eg. Changes in DNA base sequence lead to production of new alleles.
- Meiosis through crossing over of chromatids and independent segregation of chromosomes and ransoms fertilisation of gametes during sexual reproduction.
Define evolution
A change in allele frequencies in a population.
Explain evolution.
- individuals of same species vary as have different alleles, so some better adapted to selection pressures than others.
- selection pressures create struggle for survival
- different levels of survival and reproductive success in population. Individuals with phenotype that increases chance of survival are more likely to survive and reproduce and pass on their genes
- means greater proportion of next generation inherit beneficial alleles
- they in turn more likely to survive, reproduce and pass on their genes
- frequency of beneficial alleles in gene pool increases from generation to generation.
Describe selection pressures
Predation, disease and competition. These determine frequency of all alleles within the gene pool
Describe variation due to genetic factors. Meiosis.
This special form of nuclear division produces new combinations of alleles before they’re passed into gametes, all of which are therefor different.
Describe variation due to genetic factors. Random fertilisation of gametes.
In sexual reproduction this produces new combinations of alleles and offspring as therefore different from parents. Which gamete fuses with which at fertilisation is random process further adding to variety of offspring 2 parents can produce.
Describe variation due to environmental factors
Environment experts influence on all organisms. These influences affect way an organisms genes are expressed. The genes set limits but largely environment that determines where, within those limits, an organism lies. Environmental influence include; climate conditions, soil conditions, pH, and food availability.
Give an example of variation due to environment factors.
In buttercups, one plant may be determined by its genes to grow much taller than other plants. If however, the seed germinated in environment of poor light and low soil nitrate, plant may not grow properly and be short.
Explain the role of overproduction of offspring in natural selection.
Darwin realised that, in nature, populations rarely increased in size at such a rate. He concluded that death death rate of even most slow breeding species must be extremely high. High reproductive rates have evolved an many species to ensure sufficiently large population survives to breed + reproduce next generation. Compensates for high death rates from predation, competition for food + water, extremes of temp, and natural disasters.
What is the link between over-production of offspring and natural selection?
It lies in the fact that, where there are too many offspring for available resources, there’s intraspecific competition for limited resources available. Greater the numbers, greater this competition + the more individuals will die. These deaths not total random. Individuals in pop. best suited to conditions more likely to survive + reproduce, passing on more favourable allele combinations to next gen.
Explain the role of variation in natural selection.
Conditions change overtime and having a wide range of genetically different in population means some will have combination of genes needed to survive in almost any new set of circumstances populations showing little individual genetic variation often more vulnerable to new diseases + climate changes.
Larger pop is, more genetically varied individuals within it, greater chance 1 or more individuals will have combination of alleles that lead to advantageous phenotype. These individuals more likely to survive + reproduce. Variation provides potential for pop to evolve + adapt to new circumstances.
