- Slides: 27
Gene Flow, Gene Pools and Genetic Drift
What is a gene pool? • A gene pool consists of all genes and their different alleles, present in an interbreeding population. • A population is the smallest level at which evolution can occur. • The gene pool is the reservoir of the genes. • It supplies the genetic variation for evolution. • Sexual recombination – meiosis and fertilisation, provide variety in offspring. • Mutations and sexual recombination – random and cannot be predicted. • While an individual’s genotype is fixed from birth, the gene pool of a population changes with every birth and death.
“Fixed alleles” • • • When the frequency of one allele approaches 1. Every member contains at least 1 copy of the allele. The population moves toward homozygosity Every member of the population is homozygous for the allele. Task: BZ pg 107 -108 Heterozygous advantage
Microevolution and Macroevolution Microevolution is evolution on a small scale (within a single population). • Generation to generation change in the allele frequency within a population. • This change is due to four different processes: mutation, selection (natural and artificial), gene flow and genetic drift. • Refers to the alteration in a gene pool of the population over time, resulting in small changes of an organism in the same species Macroevolution happens on a scale that transcends the boundaries of a single species. • Refers to the alteration in organisms, and these changes gradually give rise to completely new species, which is different from their ancestors.
The Hardy-Weinberg theorem In population genetics, the Hardy–Weinberg principle, also known as the Hardy– Weinberg equilibrium, model, theorem, or law, states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. What is the Hardy-Weinberg Equilibrium? • We can use the HWE to work out IF a population is evolving. • Describes populations in which the gene pool does not change. • Such populations have constant allele frequencies; are not evolving (the frequencies of alleles and genotypes in a population will not vary from one generation to the next). • Sexual recombination during meiosis and fertilisation has no effect on the frequency of the alleles in a population. • If this is the case then there is no evolution. • Rare for populations to remain in H-W equilibrium. • Serves as a no-change baseline. Hardy Weinberg theorem explained
Hardy Weinberg Theorem • Let p = frequency of the dominant allele • Let q = frequency of the recessive allele • There are only two alleles –p+q=1 p 2= the frequency of homozygous dominant allele (AA) 2 pq = the frequency of the heterozygote (Aa) q 2 = the frequency of the homozygous recessive allele (aa) • p 2 + 2 pq + q 2=1 AA + Aa + aa =1
The Hardy Weinberg Theorem in Action • Dominant allele = 0. 7 Rare alleles are “only” found as heterozygotes, hardly ever as homozygotes • Recessive allele = 0. 3 • • • P = 0. 7 and q = 0. 3 Frequency of AA = p 2= (0. 7)2 = 0. 49 Frequency of Aa = 2 pq =2(0. 7 X 0. 3) = 0. 42 Frequency of aa = q 2 = (0. 3)2 = 0. 09 Total = 1. 0 In a population 49% would be homozygous dominant
Criteria for Hardy Weinberg Theory Holds true: • There is a large population • It is isolated from other populations • No immigration/emigration • There are no mutations • Mating is random • No natural selection If any of these conditions are NOT met then there is microevolution.
What is gene flow? • Gene flow — also called migration — is any movement of individuals, and/or the genetic material they carry, from one population to another. • Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. • If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene version for brown colouration moves from one population to another.
Gene flow versus Genetic drift • Gene flow, not to be confused with genetic drift, is what happens when organisms move into or out of a population. Gene flow either eliminates or introduces new alleles to the gene pool. • In essence, Genetic drift is the changes in allele frequency in a population due to chance events.
Changes in the gene pool • 2 main factors: • Genetic drift • Natural selection
The Bottleneck Effect (a form of genetic drift) • Natural disasters can reduce a population to only a few individuals. – Fire, floods and earthquakes • Deaths are random and survivors are not a good representation of the genetic diversity in the original population. – Some important alleles can be lost completely • Chatham Island Robin – Rare genes can increase in frequency – Leaves populations less able to adapt to further changes.
The Chatham Island Robin • In 1972 wildlife officers could find only 18 black robins living on Little Mangere Island. In 1976 there were only seven birds left. These were all moved to Mangere Island where 120, 000 trees had been planted to provide better shelter. By 1980 a further two birds had died, and none had bred. • There were only five black robins in the world in 1980, with just a single breeding pair left. The outlook was bleak, but a dedicated team of New Zealand Wildlife Service staff took the daring step of cross-fostering eggs and young to another species to boost productivity. • The last breeding pair named Old Blue (female) and Old Yellow (male), and a foster species, the Chatham Island tits, ended up saving the black robin from extinction. • In early 2013, the black robin population was around 250. Numbers remain stable. • Attempts made to establish another population in a fenced convenant on Pitt Island have failed, possibly due to competition for food with introduced mice.
Bottlenecks in action • Ashkenazi Jews – Population underwent a bottleneck in the middle ages – One of the surviving members contained a rare mutation that causes Tay-Sachs disease. (Children with Tay-Sachs disease lack a vital enzyme, hexosaminidase A (Hex-A). Hex-A is needed for the body to break down a fatty waste substance found in brain cells. Without Hex-A, this substance accumulates abnormally and causes progressive damage until the nervous system can no longer sustain life). • Enzyme deficiency disease that leads to blindness, idiocy and early death in infants homozygous for the mutation. • 1/27 Ashkenazi Jews is a heterozygote carrier of the disease.
• Bottleneck ppt – NZ example • On weebly
Examples of the Founder Effect The Dunkers Read pgs 200 -211 Ei. B 1. Where did the Dunkers emigrate from, and how many original families were there? 2. What characteristics were chosen for this study and why? 3. Summarise the findings in Table 20. 1 4. What are the two consequences of the Founder effect? 5. List other examples of the Founder effect. 6. Why can inbreeding be a problem?
Non-random mating • Most organism do not mate randomly. – Most mate with organisms that are nearby. – In extreme cases this can lead to inbreeding. • The net result is that organisms become homozygous/heterozygosity is lost. • Greatly increases the chance of seeing recessive traits.