populations and evolution Flashcards
gene pool definition
all of the alleles of all the genes in a population at one time
allele frequency definition
he proportion of an allele in a gene pool
evolution definition
a change in the allele frequencies in a population over time.
why do individuals within a population of a species often show a wide range of variation in their phenotypes.
this variation is due to both genetic and enviromental factors
why is detetcting this variation difficult
Although variation is the result of genetic factors and environmental influences, it is rarely entirely due to one or the other but rather a combination of both. It is very hard to distinguish between the effects of the many genetic and environmental influences that combine to produce differences between individuals.
As a result, it is very difficult to draw conclusions about the causes of variation in any particular case.
Within a population, all members have the same genes.
Genetic differences occur as members of this population will have different alleles of these genes.
why does GENETIC variation occur within a population
1) meiosis - nuclear division produces new combinations of alleles before they are passed into the gametes, all of which are therefore different.
Crossing over between homologous chromosomes
Independent segregation of homologous chromosomes
2)MUTATIONS - sudden changes to genes and chromosomes may, or may not, be passed on to the next generation, a main source of variation.
3) RANDOM FERTILISATION OF GAMETES - during sexual reproduction, this 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.
here variation is very largely the result of genetic factors, organisms fit into a few distinct forms and there are no intermediate types - discontinuous data.
A character displaying this type of variation is usually controlled by a single gene.
This variation can be represented on a bar chart or pie graph.
Environmental factors have little influence on this type of variation.
what do enviromental factors do
These affect the way the organism’s genes are expressed.
what are enviromental influences
-Climatic conditions (e.g., temperature. rainfall. and sunlight)
-Soil conditions
-pH
-food availability/diet
-Presence of mutagenic agents
what are the enviromental factors that limit the population of a species called
selection pressures
what are some examples of these enviromental pressures
predation, disease and competition.
what do these selection pressures determine
These selection pressures determine the frequency of all alleles within the gene pool.
what is the process of natural selection
-Variation within a population due to mutations.
-Selection pressure e.g. predation, disease, competition create a struggle for survival
Some organisms have phenotypes providing selective advantages (due to favourable allele(s))
-These organisms are more likely to survive and reproduce, producing more offspring and passing on their favourable allele(s) to the next generation i.e. differential levels of survival and reproductive success.
-Allele frequencies in gene pool change over many generations (time) - evolution.
what is differential reproductive success
not all individuals are equally likely to reproduce, and this results in changes in allele frequencies within a gene pool.
extra information
Charles Darwin appreciated that all species have the potential to increase their numbers exponentially but in nature, populations rarely increase at this rate.
Death rate of even the most slow-breeding species must be extremely high.
So 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 etc.
Some species have evolved lower reproductive rates along with a high degree of parental care.
The lower infant death rates that result help to maintain their population size.
what is the link between overproduction and natural selection
Link between overproduction and natural selection: where there are too many offspring for the available resources, there is competition amongst individuals (intraspecific competition) for the limited resources available.
The greater the numbers, the greater this competition and the more individuals will die in the struggle to survive. therefore high mortality rate .
These deaths are not totally random as best suited to conditions will be more likely to survive so more likely to breed and so pass on their more favourable allele combinations to the next generation and increase allele frequency.
The population will have evolved a combination of alleles that is better adapted to the prevailing conditions.
This selection process depends on individuals of a population being genetically different from one another, variation.
describe stabilising selection
Favours - the mean phenotype.
Eliminates / selection against - extreme phenotypes.
Occurs when - the environmental conditions are constant over long periods.
The range of alleles decreases
describe directional selection
avours - phenotypes that vary in one direction, extreme.
Eliminates / selection against - one extreme phenotype.
Occurs when - environmental conditions change causing the optimum phenotype for survival to change too.
describe disruptive selection
Favours - extreme phenotypes leading to 2 different species that are suited to 2 different environmental conditions.
Eliminates / selection against - intermediate phenotypes.
Occurs when - environmental factors take two or more distinct forms.
This is the most important type of selection in bringing out evolutionary change as it increases genetic variation.
Continued disruptive selection can ultimately lead to speciation.
what is polymorphism
where a species has two or more distinct forms, they are genetically different but can interbreed.
Peppered moth (Biston betularia)
An example of selective predation and directional selection.
It existed mostly in natural light form.
Mutation occurred, forming a melanic black allele.
The black variety of moths was subjected to greater predation due to being less camouflaged.
However, during the Industrial Revolution, soot covered buildings so the black moths then became less conspicuous.
The natural light moths became subjected to predation.
The process of natural selection occurred in this species to increase the frequency of the selective advantage black allele in the population leading to evolution.
Environmental changes therefore affect the probability of an allele being passed on in a population and hence the
number of times it occurs within the gene pool.
It must be emphasised that environmental factors do not affect the probability of a particular mutant allele arising, they simply affect the frequency of a mutant allele that is already present in the gene pool.
Evolution by natural selection is a change in the allelic frequencies within a population.
what is speciation
Speciation is the evolution of new species from existing ones, the process that results in the creation of new species.
what are the two different ways that populations can become reproductively isolated:
- geographically(allopatric)
-changes in reproductive mechanisms(sympatric)
what is the process of speciation
This occurs when one original population of the same species becomes reproductively isolated.
The different populations undergoes different mutations and will become genetically different.
Each of the populations will experience different selection pressures because the environment of each will be slightly different.
Natural selection will then lead to changes in the allelic frequencies in the populations.
The different phenotypes each combination of allele produces will be subject to selection pressure that will lead to adaptive radiation where each population becomes adapted to its local environment.
This results in changes to the allele frequencies (evolution) of each population so each population evolves.
This isolation means that there are now two populations of the same species, but they cannot breed together so there is no gene flow.
As a result of genetic differences in their gene pools, if the two populations were no longer physically separated then they would be unable to interbreed successfully to make fertile offspring, and are therefore classed as two different species.
allopatric speciation definition
Where populations become separated geographically leading to reproductive isolation.
what are some examples of allopatric speciation
Geographical separation may be the result of any physical barrier between two populations which prevents them interbreeding/ reproduction - no gene flow
Oceans, rivers, mountain ranges and deserts. What proves a barrier to one species may be no problem to another.
what is symatric speciation
Where populations become reproductively isolated due to differences in their behaviour.
how does sympatric speciation occur
This could be because a random mutation within the population could impact reproductive behaviour.
For example, it may cause individuals to perform a different courtship ritual or for individuals to be fertile at different times of the year.
These individuals will not reproduce together and there will be no gene flow between the two groups within the populations.
This leads to reproductively isolated populations.
Each population will accumulate different mutations
Their DNA becomes so different that they cannot interbreed to create fertile offspring so are therefore classed as two different species.
define genetic drift
a change in a populations allele frequencies that occurs due to chance rather than selective pressures
What must you remeber about hardy Weinberg
Only ever use hardy Weinberg if they tell you too , it’s an estimate so if it asks for the ‘actual frequency ‘ in a population make sure to not use hardy Weinberg
why does genetic drift affect small populations rather than large ones
the gene pool is smaller so there are less alleles available and any change in alleles may result in a change in allele frequencies
