Variation and evolution Flashcards
What is variation?
The result of a combination of genetic and environmental factors
What methods of creating variability does the sexual process have?
1) The mixing of two different genotypes where cross-fertilisation occurs
2) The random distribution of chromosomes during metaphase I of meiosis
3) The crossing over of homologous chromosomes during Prophase I of meiosis
What is continuous variation?
Most characters are controlled by a number of genes and the difference in the character are not clear cut. A character within a population showing a gradation from one extreme to another shows continuous variation. For example height- if an individual has inherited a number of alleles for tallness from the parents, that individual has the potential to grow tall.
What are largely responsible for continuous variation?
Environmental influences and nutrition during the growth period. If organisms of identical genotypes are subject to different environmental influences they show considerable variety.
What is discontinuous variation?
Characters that are clear cut and easy to tell apart are controlled by a single gene. For example, light and dark forms in the peppered moth, the ABO blood grouping system, where the gene has more than two alleles.
How can the environment influence in determining phenotypic variation?
They affect the way genes are expressed as in humans they may include diet and exercise whereas plants are affected by temperature, light and available nutrients.
What are the two types of competition?
1) Intra-specific- competition between individuals of the same species.
2) Inter-specific- competition between individuals of different species competing for the same resources
Why is population increase not only reliant on whether the same species can breed?
Because it’s the environmental influences that limit the numbers and the organisms must fight for limited resources.
What does variation mean?
Variation within a population of organism means that some will have characteristics that give them an advantage in the “struggle for survival”. E.g. if all rabbit survived and then reproduced successfully the rabbit population would increase rapidly. Eventually, when the increasing amount of rabbits eat more and more of the vegetation such as grass the food supply becomes short. Overcrowding would take place resulting in disease spreading easier . Predators (foxes) would increase. These environmental factors act to reduce the rate of growth of the rabbit so population growth is slow.
What determines a animal to survive?
Only certain individuals are reproductively successful and so pass on their alleles. For example a small number of rabbits may be homozygous for recessive allele which gives a white coat. This means they stand out more so more likely to be killed by a predator so allele is likely to be passed on so less rabbits are white. Whereas if it lived in the artic it would be camouflaged with snow so the allele passed on for white coat would increase in population.
What is selection pressure?
It’s an abstract force that alters the frequency of alleles in a population due to mutation, natural selection and genetic drift. Potential sources of selection pressure may include availability of prey, presence of predators, environmental stresses, competition with other species, and intra-specific competition.
What’s selection in terms of environment?
It’s the process by which organisms that are better adapted to their environment survive and breed, while those less well adapted fail to do so. These better adapted organisms are more likely to pass their characteristics to succeeding generations. The organisms environment exerts a selection pressure and this determines the spread of any allele within the gene pool.
What is allele frequency?
Determining the relative proportions of the various genotypes present in a population from which can be calculated the relative proportions of alleles in the population. When environment changes and not stable then gene pool isn’t stable and some phenotypes will be advantageous so be selected. This is why gene pools are constantly changing some alleles becoming more frequent and others less frequent. Some may be completely lost from the gene pool.
What is Hardy-Weinberg principle?
In a large, randomly mating population, assuming the absence of migration, mutation and selection, the gene and genotypes frequencies remain constant. that is the proportion of dominant and recessive alleles of a particular gene remains the same. It is not altered by interbreeding.
How can Hardy-Weinberg principle be used?
Calculate allele and genotype frequencies in a population. It can therefore be used to predict the number of defective individuals in a population.
