Flashcards in Chapter 16 - Evolutionary Mechanisms Deck (54)
A species is a group of individuals that share many characteristics and are able to interbreed under natural conditions to produce fertile offspring.
A population is a group of organisms of the same species living together in a particular place at a particular time.
When studying populations, what do geneticists (scientists who specialise in the study of inheritance) prefer to do?
They prefer to consider the characteristics of the population as a whole and not those of the individuals that make up the population.
What is Gene pool?
-Geneticists find it convenient to pool the genotypes of all the individuals capable of reproducing and refer to this as the gene pool.
- Thus the gene pool is the sum of all the alleles in a given population.
Define Allele frequencies.
-In studying a population, geneticists are interested in how often each allele of a gene occurs in the gene pool for that population.
-Knowing how often a characteristic occurs in a population, geneticists are able to work out the frequency of each of the alleles for a particular gene.
How does the gene pool in different populations differ?
-Populations that differ in the characteristics they posses are likely to be different in the frequencies of the various alleles of a gene in their respective gene pools.
-Thus, any two populations having differing characteristics are likely to have different gene pools.
What are some factors that may cause the frequency of particular alleles in a population to change over time?
- chance events
eg. mutation that alters the expression of a gene or more natural means, where changes in the environment may result in variations to the alleles frequencies.
What are mutations?
Characteristics which occur when new variations in offspring, which show no resemblance to either parent, may occur quite suddenly and purely by chance.
What is a mutant?
It is an organism with a characteristic resulting from a mutation.
What are the two main types of mutation?
1. Gene Mutations - changes in the DNA of a single gene so that the traits normally produced by that gene are changed or destroyed.
2. Chromosomal Mutations - all or part of a chromosome is affected.
Explain Somatic Mutation?
-Mutations can occur in the body cells or the reproductive cells of a person.
-A mutation in the body cells, also called somatic cells, is called a somatic mutation.
-Only the characteristics of the individual with the somatic mutation are affected.
-Reproductive cells are not affect, so when the individual dies, the mutation is lost.
-Somatic mutations are often the cause of many cancerous growth.
Explain Germline (or germinal) mutation.
-Mutations that affect the reproductive cells can result in the gametes containing the changed genetic information that can then be passed on to the next and subsequent generations.
-In such cases the person in whom the mutation occurs is not usually affected, but that individual produces gametes with changed DNA.
-A germline mutation may, by chance, result in characteristics that improve the survival chances of the individual.
-Such a characteristic is likely to be passed on to future gnerations resulting in a change to the allele frequences of the gene pool.
Define Natural Selection.
The process whereby organisms better adapted to their environment tend to survive and produce more offspring.
When does natural selection take place ?
-When such a situation occurs, nature is favouring one set of alleles at the expense of others.
-Natural selection is major cause of changes to allele frequencies.
Is Natural Selection random?
-Nope; Alleles are not passed on randomly to the next generation.
- The alleles passed on tend to be the ones that enhance survival and reproduction of the individual and therefore of the species.
What is a random genetic drift?
-A phenomenon which occurs in small populations, where there is often a random, non-directional variation in allele frequencies.
-AKA Sewall-Wright efft, after the man who first recognised its significance in causing changes to allele frequencies.
-It is a purely chance occurrence, a change in allele frequencies that may occur where a population is very small.
What is another phenomenon similar to genetic drift?
-This effect occurs when a small group moves away from its homeland to a totally new area and establishes a community, which later expands.
-The migrant group, being such a small sample of the original population, is usually not genetically representative of them.
-This new community, therefore, generally shows features that are not typical of the original homeland population.
What is Achromatopsia?
It is an inherited form of total colour blindness.
How can migration cause changes in allele frequencies in a gene pool?
Geneticists describe migration as gene flow from one population to another.
-Therefore, if immigrants to a certain country bring alleles that are not already in the population, the frequencies for the alleles of that gene will be altered.
Why are populations often kept apart by barriers that inhibit the amount of interbreeding between them?
-As no two environments are exactly the same, the environmental pressures on one population will be different from the pressures on the other.
-Over time, the allele frequencies of each gene pool will change, depending on which characteristics are favoured for survival.
Which is the most common barriers to interbreeding in the early human populations?
What are Geographical barriers?
-Include oceans, mountain ranges, large lake systems, deserts and expensive ice sheets.
For eg. The original inhabitants of Australia were isolated for thousands of years by ocean barriers that formed as sea levels rose.
What caused Sociocultural barriers?
As human populations developed and became culturally more complex, other types of barriers to interbreeding such as religious groups began to form.
What are some examples of Sociocultural barriers today?
Economic status, educational background and social position are barriers to interbreeding.
Genetic disease results from...
Changes to allele frequencies in a gene pool.
What happens when there is an allele causing an inherited, fatal disease ?
It would be expected to be gradually eliminated from a population because people with the allele would die and would not pass it on to the next generation.
What is Tay-Sachs disease?
TSD is a hereditary disorder of lipid metabolism that occurs most frequently in individuals of Jewish descent from Eastern Europe (the so-called Ashkenazi Jewish population).
What causes Tay-Sachs disease?
-Tay-Sachs is caused by a missing enzyme that results in the accumulation of a fatty substance in the nervous system.
-Death usually occurs by the age of 4 or 5.
Why is the fatal disease (Tay-Sachs) much higher amongst the Ashkenazi Jews?
-Genetic Drift; Jewish populations have tended to be small and isolated, factors that increase the chances of genetic drift.
-Observations on individuals with 1 Tay-Sachs allele, who are heterozygous would be able to resist the disease Tuberculosis (TB).
>But individuals with 2 normal alleles are more susceptible to TB, thus they are more likely to die from it.
>Individuals with 2 Tay-Sachs allele, who are obviously homozygous would die early in life.
>Therefore, heterozygotes would have a survival advantage and would be more likely to reproduce and pass their alleles on to the next generation.