populations and evolution- chapter 18 Flashcards
(26 cards)
what is a population
a group of organisms of the same species that occupies a particular space at a particular space at a particular time and that can potentially interbreed
what is a gene pool
All the alleles of all the genes of all the individuals in a population at a given time
what is the allelic frequency
The number of times an allele occurs within the gene pool
what is the Hardy-Weinberg principle and what assumption does it make
- Provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population.
- The principle makes the assumption that the proportion of dominant and recessive alleles of a gene in a population remains the same from one generation to the next
what 5 conditions have to be met for Hardy-Weinberg principle
- No mutations arise
- The population is isolated (no flow of alleles into or out of the population)
- There is no selection (all alleles are equally likely to be passed to the next generation)
- The population is large
- Mating within the population is random
why do individuals within a population of a species show a range of variation within their phenotypes
due to both genetic and environmental factors
what are 3 reasons for genetic variation arising
- Mutations- sudden changes to gene and chromosomes may, or may not, be passed onto the next generation.
- Meiosis- produces new combinations of alleles before they are passed into the gametes, all of which are therefore different.
- Random fertilisation of gametes- produces new combinations of alleles and the offspring are therefore different from parents. Which gamete fuses with which at fertilisation is a random process further adding to the variety of offspring two parents can produce.
what environmental influences are there
climatic conditions (temperature, rainfall and sunlight), soil conditions, pH and food availability
what are polygenes
- Some characteristics of organisms grade into one another, forming a continuum
- Characters that display this type of variation are not controlled by a single gene, but by many genes called polygenes
what are selection pressures
- The environmental factors that limit the population of a species
- These vary and determine the frequency of all alleles within the gene pool
-These include predation, disease and competition
what factors affect the process of evolution by the means of natural selection
- Organisms produce more offspring than can be supported by the available supply of food, light, space, etc
- There is genetic variety within the population of all species
- A variety of phenotypes that selection operates against
what is the role of overproduction of offspring in natural selection
- All species have the potential to increase in numbers exponentially although this rarely happens.
- High reproductive rates have evolved in many species to ensure a sufficiently large population survives to breed and produce the next generation. This compensates for high death rates from predation, competition for food and water, extremes of temperatures, natural disasters and diseases.
- Some species have evolved lower reproductive rates along with a high degree of parental care.
- When there are too many offspring for the available resources, there is competition amongst individuals (intraspecific competition) for the limited resources available.
- Those individuals in a population best suited to prevailing conditions will be more likely to survive than those less well adapted, they will be more likely to breed and so pass on their more favourable allele combinations to the next generation.
what is the role of variation in natural selection
- Conditions change over time, so having a wide range of genetically different individuals within a population means that some will have the combination of genes needed to survive in almost any new set of circumstances.
- Populations showing little individual genetic variation are often more vulnerable to new diseases and climate changes. It is also important that a species is capable of adapting to changes resulting from the evolution of other species.
- The larger a population is, and the more genetically varied the individuals within it, the greater the chance that one or more individuals will have the combination of alleles that lead to a phenotype which is advantageous in the struggle for survival.
- Variation therefore provides the potential for a population to evolve and adapt to a new circumstances.
what do environmental factors help create in a population
variation
what are the 3 main types of selection that affect the characteristics of a population
- Stabilising Selection- preserves the average phenotype of a population by favouring average individuals, in other words, selection against the extreme phenotypes
- Directional selection- changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population, in other words, selection for one extreme phenotype.
- Disruptive selection- favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population.
explain stabilising selection
- Tends to eliminate the extremes of the phenotype range within a population and with it the capacity for evolutionary change.
- It tends to occur where the environmental conditions are constant over long periods of time.
- However, if the temperature is constantly 10C, then individuals at the extremes will never be at an advantage, so they are selected against in favour of those with average fur length. The mean remains the same but fewer individuals at either extremes
explain directional selection
- Within a population there will be a range of genetically different individuals in respect of any one phenotype. The continuous variation forms a normal distribution curve.
- This curve has a mean that represents the optimum value for the phenotypic character under the existing conditions.
- If the environmental conditions change, so will the optimum value for survival.
- Some individuals, either to the left or the right of the mean, will possess a combination of alleles with the new optimum for the phenotypic character.
- As a result there will be a selection pressure favouring the combination of alleles that results in the mean moving to either the left or the right of its original position.
- Directional selection therefore results in one extreme of a range of variation being selected against in favour of the other extreme or even the average.
explain disruptive selection
- The opposite of stabilising selection.
- Favours the extreme phenotypes at the expense of the intermediate phenotypes
- Although the least common form of selection, it is the most important in bringing about evolutionary change.
- Disruptive selection occurs when an environmental factor, such as temperature, takes two or more distinct forms.
what is polymorphism
Some species of organisms have two or more distinct forms. They are different forms are genetically distinct but exist within the same interbreeding population
how is the allelic frequency affected by selection
- is due to environmental factors
- Environmental changes affect the probability of an allele being passed on in a population and hence the number of times it occurs within a gene pool.
- Environmental factors do not affect the probability of a mutant allele arising, they simply affect the frequency of a mutant allele that is already present in a gene pool.
what is speciation
- Is the evolution of new species from existing ones.
- A species is a group of individuals that have common ancestry and so share the same genes but different alleles and are capable of breeding with one another to produce fertile offspring.
how are new species formed (adaptive radiation)
-individuals tend to only breed within their population they are capable of breeding with the other.
- These populations get separated and undergo different mutation becoming genetically different.
- Each of the populations will experience different selection pressure because the environment of each will be slightly different.
- Natural selection leads to change in the allelic frequency of each population.
- Different phenotypes each combination of alleles produces will be subject to selection pressure that will lead to each population becoming adapted to its local environment. This is called adaptive radiation, and results in evolution.
- Each population is now a different species with its own gene pool
how are new species formed (genetic drift)
- Something that can take place in small populations because relatively few members posses a smaller variety of alleles so their genetic diversity is less.
- As these few individuals breed, the genetic diversity of the population is restricted to those few alleles in the original population. As there are only a small number of different alleles there is not an equal chance of being passed on.
- Those that are passed on will quickly affect the whole population as their frequency is high.
- Any mutation to one of these alleles that is selectively favoured will also more quickly affect the whole population because its frequency will be high.
- The effects of genetic drift will be greater and the population will change relatively rapidly, making it more likely to develop into a separate species.
- In large populations the effect of mutant allele will be diluted because its frequency is much less in the much larger gene pool.
what is allopatric speciation
- Where two populations become geographically separated.
- This may be the result of a physical barrier between two populations which prevents them interbreeding.
- The barriers will be different depending on the species.
- If environmental conditions either side of the barrier vary, then natural selection will influence the two populations different and each will evolve leading to adaptations to their local conditions.
