Changes to Allele Frequencies And the consequences
The Gene Pool The total collection of genes in a population at any one time.
Allele Frequencies in a Gene Pool • When all genotypes in a population are known, we can directly count the numbers of each kind of allele & calculate their frequencies. • These values are known as allele freqencies • An allele frequency can have any value between 0 and 1
Calculating Allele Frequencies • Each individual carries 2 alleles for a particular trait. • The total number of alleles comprises 100% (1.0) of the total alleles. • p is used to denote the dominant trait. • qis used to denote the recessive trait • p + q = 1 • Therefore: • Freq (p) = 1 – freq (q) • Freq (q) = 1 – freq (p)
What are the allele frequencies of this ladybird population? • 50 beetles: • 12 black with red spots homozygous (rr) • 26 heterozygous red with black spots (Rr) • 12 homozygous red with black spots (RR)
Hardy-Weinberg Principle Freqencies of alleles in populations remain constant, generation after generation provided that some conditions are met: 1. Very large population 2. Isolation from other populations 3. No net mutations 4. Random mating 5. No natural selection Under these circumstances a population is said to be in Hardy-Weinberg Equilibrium
Biological fitness • Biological fitness is the ability to reproduce and pass on genes to the next generation. • Organisms of higher fitnessare more adapted to a particular environment so are able to survive, reproduce and make a greater contribution to the gene pool of the next generation and are said to be “at a selective advantage”. • less fit are “selected against”.
Allele Frequencies • Non-evolving Populations • Allele frequencies stay constant if; • There is random mating • Population is isolated • There are no mutations • There is no migration • The population is large • Evolving Populations • Allele frequencies are subject to “change agents”.
Change Agents • Selection Pressure • Gene Flow (migration) • Chance events
Selection Pressure • The limiting factor acts as a selection pressure. • The agent which causes the difference to occur in the phenotype is called the selecting agent. eg. Predators Insecticides
Selection Pressure I think that I will be selected against in this environment • Selection factors can be: • Natural • Artificial
Natural Selection • The action of natural agents in the wild is termed as natural selection. • Individualswith favorabletraits are more likely to produce more offspring better suited for their environment.Alsoknown as “Differential Reproduction” • Example: English peppered moth (Bistonbetularia) - light and dark phases
Natural selection is a mechanism for gradual change over time. Environmental change Dark phenotype no longer has a selective advantage. Allele frequency of light phenotype will increase over time.
Artificial Selection • Selecting agent is human • This happens when selective breeding of domesticated plants and animals are done by humans. • Less variation happens and population become monomorphic
Gene Flow • Gene flow is the movement of genes through genetic exchange between populations. Immigration may increase the variety of alleles for a particular trait Emigration may decrease the variety of alleles present in the population if the emigrant group is not representative of the whole population.
Genetic Drift • Chance events can dramatically change allele frequencies, particularly in small populations. BANG!!!
Genetic DriftThe bottleneck effect • Occurs when there is a severe reduction in the population caused by intense natural selection or disaster • Surviving members may be urepresentative sample of the origninal population The Great Flood
Plight of the Tasmanian Devil A victim of a genetic bottleneck – caused by bounty hunting in the late 19th Century and a program of poisoning in the early 20th Century
Genetic DriftThe founder effect • A small unrepresentative sample leaves to colonise a new region. • Captive breeding programs in zoos represent a founder effect. Care must be taken to avoid inbreeding and to maximise genetic diversity. well, we’re less crowed but do we have enough genetic diversity