populations AND evolution year 2 Flashcards
variation in phenotype
Variation within individuals of a population is often due to both genetic and environmental factors. Genetic differences occur as members of populations have different alleles of genes. Genetic variation rises as a result of:
- mutations
- meiosis- crossing over and independent segregation
- random fertilisation of gametes - which gametes fuse with which is completely random, adding to the variety of offspring parents can produce
When variation is largely the result of genetic factors organisms fit into a few distinct forms and there are no intermediate types. eg in the ABO blood grouping system- there are four distinct blood groups: A, B, AB or O. A character displaying this type of variation is usually controlled by a single gene. This is discontinuous variation and can be plotted on a bar chart
variation due to environmental influences
- environment exerts an influence on all organisms. These influences affect the way the organisms genes are expressed. The genes set limits, but largely the environment that determines where, within those limits an organism lies. Environmental influences include climatic conditions (eg temperature, rainfall and sunlight), soil conditions, pH and food availability
Some characteristics of organisms grade into one another, forming a continuum. In humans eg height and mass. Characters that display this type of variation are not controlled by a single gene but by many genes (polygenes). Environmental factors play a major role in determining where on the continuum an organism actually lies. eg individuals who are genetically predetermined to be the same height actually grow to different heights due to variations in environmental factors such as diet. This is the product of polygenes and the environment. If you plotted number of individuals against height, would obtain a normal distribution curve
In most cases variation is due to combined effects of genetic differences and environmental influences. Very hard to distinguish between the effects of the many genetic and environmental influences that combine to produce differences between individuals. As a result very difficult to draw conclusions about the causes of variation in any particular case
natural selection
Every organism is subjected to a process of selection, based on its suitability for survival under the conditions that exist at the time. The environmental factors that limit the population of a species are called selection pressures. These include predation, disease and competition. They determine the frequency of all alleles within the gene pool.
gene pool
Total nuber of all the alleles of all the genes of all the individuals within a particular population at a given time
