4.4: Variation and evolution

Variation

Variation

Differences in phenotypes within a species. Can be caused by the environment or differing alleles. In asexual organisms, this is only produced via mutations, but in sexual organisms this can be caused by independent assortment, crossing over during meiosis and the combing of alleles during fertilisation.

Non-heritable

Phenotype differences which are not passed down to offspring. Often environmental changes.

Epigenetics

The study of how behavioural and environmental factors can change the way individual’s genes work. These changes can be heritable.

Discontinous

Type of variation where there is only a small amount of variation, with no intermediate types and discrete characteristics. It is monogenic. An example is blood types.

Monogenic

A factor controlled by one gene, which is discontinuous variation. An example is blood types.

Continous

Type of variation where there are many possible values which often show a normal distribution. It is polygenic, and the environment has a considerable impact on it’s expression. For example, plants may have tall alleles, but cannot be tall without sunlight.

Selection

The pressure, caused by environmental factors, which causes animals with the most advantageous phenotypes to survive and pass on their alleles to their offspring. These alleles are therefore ‘selected for’. This varies between environments.

Competitive advantage

Individuals with better adapted phenotypes have this, allowing them better chances of survival and reproduction to pass on their alleles. These alleles then increase in proportion, until they are the average phenotype. This is the process of evolution.

Adaptations

A type of evolutionary product where structures are used in an original form, such as a bat larynx for echolocation signals.

Exaptations

A type of evolutionary product where structures have their original use modified, such as feathers for body heat being used for flight.

Stabilising

When selective pressure means the average phenotype has a competitive advantage. A curve will begin to have a higher peak and smaller SD. An example is birth weight.

Directional

When selective pressure means an extreme phenotype has a competitive advantage. The curve will shift to the left or right, changing the average, meaning speciation can occur. Examples include speed to escape predators.

Speciation

The creation of a new species due to adaptation. Occurs with directional or disruptive selection. Can be caused by pre or post zygotic isolation.

Disruptive

When selective pressure means the average phenotype does not provide an advantage and is selected against. The curve will become bimodal, with two peaks either side of the average. This changes the average, meaning speciation can occur. Examples include salmon size for reproduction.

Gene pool

All alleles within a population at one time.

Genetic drift

Chance variation in allele frequency. Can eliminate certain alleles, especially in small populations.

Founder effect

Where isolated populations that colonise a new area have a different allele frequency to the original population. This can create two completely different gene pools.

Bottleneck

A term for an effect where there is sharp reduction in population, therefore in genetic diversity. This could leave the population unable to adapt, and therefore go extinct.

Hardy-Weinberg

Principle that states in ideal conditions allele frequencies will remain constant throughout generations. Requires diploid, sex consistent, sexually reproducing organisms and the population to have no mutation, migration, selective pressure and to be large.

Hardy-Weinberg equilibrium

Term for when a population has the same proportion of alleles over generations. If this is not true, there must be some factor affecting this, such as selective pressure.

p and q

The representation of the dominant and recessive alleles in the Hardy-Weinberg equation.

Deme

Subgroups within a population that breed more often with each other than the rest of the population. If separated for a long period of time, speciation often occurs.

Pre-zygotic

Reproductive isolation that prevents gametes from ever fusing, so no zygote is formed.

Post-zygotic

Gametes fuse and a zygote can form, but is either prevented from developing, growing or reproducing.

Allopatric

Speciation occurs as demes are separated into other areas. Occurs during geographical isolation.

Geographical

Type of prezygotic isolation that occurs due to a physical barrier, such as a river or mountain. This is allopatric speciation.

Sympatric

Speciation while demes share an environment.

Behavioural

Type of prezygotic isolation that occurs due to mating rituals isolating demes, such as grasshopper songs. This is sympatric speciation.

Morphological

Type of prezygotic isolation that occurs due to a structural barrier from mating, such as insect exoskeletons. This is sympatric speciation.

Gametic

Type of prezygotic isolation that occurs due to barriers preventing gamete fusion, such as molecules in the membranes, specific chemoattractants, and male gamete dying in oviduct. This is sympatric speciation.

Seasonal

Type of prezygotic isolation that occurs due to differing sex organ maturation times. This is sympatric speciation.

Inviability

Type of postzygotic isolation where cross species gametes can be fertilised, but the hybrid embryo cannot develop, living for only a day in some cases. This is sympatric speciation.

Sterility

Type of postzygotic isolation where a hybrid embryo can develop, but chromosomes are not similar enough to pair at prophase 1, preventing meiosis. This prevents them from breeding. This is sympatric speciation.

Breakdown

Type of postzygotic isolation where a hybrid offspring is fertile, but the second generation is not. Suspected to be due to incompatibility between nuclear genes, genes in the mitochondria and chloroplasts. This is sympatric speciation.