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Mechanisms of evolution and their effect on populations. SBI3U Section 9.1 Pg 350-359. Allele Frequencies. The gene pool of a population consists of all the alleles of all genes of each individual in that population
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Mechanisms of evolution and their effect on populations SBI3U Section 9.1 Pg 350-359
Allele Frequencies • The gene pool of a population consists of all the alleles of all genes of each individual in that population • The percentage of each allele of any given gene present in the population determines the genetic characteristics of that population
Genetic Variation • Most traits in a population vary from one extreme to another (eg. Height, weight)
Allele Frequency • How often a particular allele appears in a population. 30% of us have the brown phenotype Homozygous Individuals carry two copies of the allele, while heterozygous individuals carry one.
Mutations • A mutation is a change that occurs in the DNA of an individual and a heritable mutation takes place in the gametes of an individual and can be passed on to its offspring • Mutation can effect an entire gene pool and thus allele frequencies especially if the mutation gives organisms a selective advantage for survival
Gene Flow • Gene flow is the net movement of alleles from one population to another due to the migration of individuals • Can change the allele frequency in both populations • Can also spread new alleles that arise in one population
Gene Flow Migration between habitats Population: 5 blue (25%) 15 Green (75%) Population: 7 blue (35%) 13 Green (65%) Population: 15 blue (75%) 5 Green (25%) Population: 13 blue (65%) 7 Green (35%)
Non-Random Mating • Non random mating is mating among individuals on the basis of mate selection for a particular phenotype or due to inbreeding • Random mating would be like drawing from a hat which males and females mated • An example of non random mating is male caribou that spar with each other to be able to mate with a female in the herd, this prevents the individuals that lose the fights from mating • Another example of non random mating is inbreeding which occurs when closely related individuals mate • Non random mating increases the proportion of homozygous individuals in a population but does not affect the frequencies of alleles
Genetic Drift • Genetic Drift – The gradual change in allele frequencies due to chance events in a breeding population. Especially strong in small populations • Genetic drift could occur if there is a natural disaster that wipes out certain individuals in a population, certain individual don’t produce offspring or a portion of the population migrates to a new area
Genetic Drift Random events cause the death of some individuals Population 17 blue (85%) 3 green (15%) Population 15 blue (93.75%) 1 green (6.25%)
Genetic Drift: The Founder Effect • Founder Effect – A change in a gene pool that occurs when a few individuals start a new isolated population • When these individuals start a new population they will carry some but not all of the alleles of the original population thus limiting diversity • The alleles carried by the migrating individuals may not be the alleles that most represent the original population
Founder Effect Access to new habitat Some individuals migrate Genetic makeup of that group populate new habitat Population 13 Blue (82%) 3 Green (18%) Population 15 Blue (75%) 5 Green (25%) Population 8 Blue (50%) 8 Green (50%) Population 2 Blue (50%) 2 Green (50%)
Founder Effect • In the 1680s Ariaantje and GerritJansz emigrated from Holland to South Africa, one of them bringing along an allele for the mild metabolic disease porphyria. • Today more than 30000 South Africans carry this allele and, in every case examined, can trace it back to this couple — a remarkable example of the founder effect
Genetic Drift: The Bottleneck Effect • The bottleneck effect is changes in gene distribution that result from a rapid decrease in population size • Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population’s genetic variation by a great deal
Natural Selection • If a single allele gives even a slight selective advantage, the frequency of the allele in the population will increase from one generation to the next at the expense of less favourable alleles • Natural selection therefore cases changes in allele frequency in a population
Stabilizing Selection • Favours an intermediate phenotype and acts against extreme variants thus reducing the variation
Directional Selection • Natural selection that favours the phenotypes at one extreme over another, resulting in the distribution curve of phenotypes shifting in the direction of that extreme
Disruptive Selection • Natural selection that favours the extremes of a range of phenotypes rather than the intermediate phenotypes; this type of selection can eliminate intermediate phenotypes
Sexual Selection • Sexual selection occurs in two ways: through contests and through choice • Contests are competitions between members of the same sex for access to the other sex • Choice involves competition for attention from the opposite sex
Speciation: How species form SBI3U Section 9.2 Pg 360-373
Species • Species - A group of actually or potentially interbreeding natural populations that ordinarily do not interbreed with other such groups even when there is opportunity to do so.
Speciation • Speciation is the formation of new species from existing species • The formation of two or more species often requires geographical isolation of subpopulations of the species. Only then can natural selection or perhaps genetic drift produce distinctive gene pools. • Two populations can become reproductively isolated over time (become two species) if there is little or no gene flow between them.
Pre-Zygotic Isolating Mechanisms • Are barriers that impede mating between species or prevent fertilization of eggs if individuals of different species attempt to mate • Temporal isolation- Individuals of different species do not mate because they are active at different times of day or in different seasons (e.g. Two similar species my be present in the same habitat but mate or flower at different times of the day) • Habitat isolation- Individuals mate in their preferred habitat, and therefore do not meet individuals of other species with different ecological preferences (e.g. two similar types of species may live in the same area but one group might live closer to water while the other may prefer a dryer area) • Behavioural isolation – Species that have different mating signals or behaviours not likely to mate (e.g. two birds that are similar may have different mating calls) • Mechanical isolation – Species may attempt to mate but fail to achieve fertilization due to anatomical incompatibility (e.g. male and female genitals may not fit together)
Pre zygotic Isolating Mechanisms • Gametic Isolating Mechanisms – If gametes (egg and sperm) from different species do meet, gametic isolation ensures that they will rarely fuse to form a zygote. This can occur due to inability of sperm to survive within the female reproductive tract, in the case of species in which eggs are fertilized within a female. Gametic Isolation can also occur in plants where pollen grains of one species fail to germinate on the stigma of another species thus preventing fertilization.
Post zygotic Isolating Mechanisms • Barriers that prevent hybrid zygotes from developing into viable fertile individuals. • Hybrid Inviability – Genetic incompatibility of the interbred species may stop development of the hybrid zygote during its development (e.g. embryos between sheep and goats die in early development before birth) • Hybrid Sterility – Species can mate and produce offspring but the offspring are not fertile (e.g. a female horse and male donkey produce a mule which is infertile) • Hybrid Breakdown – The first generation of hybrids are fertile but their offspring are sterile or weak (e.g. different species of cotton plants can produce fertile hybrids but the offspring of the hybrids die as seeds or in early development)
Sympatric Speciation • Occurs when populations that live in the same geographical area become reproductively isolated • This type of speciation occurs if there are: i) chromosomal changes (in plants) thus creating a different species of plant ii) non-random mating (in animals), since these animals are choosing mates based on certain traits thus producing offspring with the same traits • Sympatric speciation is more common in plants than in animals
Example of Chromosomal Changes Causing Sympatric Speciation • Polyploidy is a condition occurring mainly in plants that is caused by non-disjunction in meiosis resulting in an extra three or more sets of chromosomes • Non-disjunction occurs when chromosomes don’t separate properly during anaphase I or anaphase II of meioses, producing gametes with either too many or too few chromosomes • These gametes can then fuse to produce organisms with extra chromosomes • An organism that has an odd number of homologous chromosomes (3n, 5n,…..) will be sterile • An organism that has an even number of chromosomes will be fertile
Allopatric Speciation • Occurs when a population is split into two or more isolated groups by a geographical barrier such as a body of water, canyon, mountain etc. • The gene pool of the spit population becomes so distinct that the two groups are unable to interbreed even if they are brought back together • An Ancestral population of Ensatina Salamanders in the mountains of northern California were separated by the San Joaquin valley • The subspecies on the same side of the valley can breed with each other however the western end species can’t interbreed with the eastern end species • This type of species is referred to a ring species where interconnected species on one side of the ring can breed but species that are separated can’t breed
Adaptive Radiation • Is a form of allopatric speciation, it is the diversification of a common ancestral species into a variety of differently adapted species. • An example are the Finches that Darwin observed on the galapagos islands
Darwin’s Finches • Darwin noticed varieties of finches on the Galapagos • Kept specimens, but didn’t record which islands they came from • Back in England, recognized they were closely related, but different species • Finches evolved from an ancestor that came from mainland South America
Divergent Convergent and Divergent Evolution Convergent Pattern of evolution in which similar traits arise between species because different species have independently adapted to similar conditions E.g. both birds and bees have wings yet have different ancestors • Pattern of evolution in which species that were once similar to an ancestral species diverge, or become more distinct • Occurs when populations change as they adapt to different environmental conditions
Speed of Evolutionary Change Gradualism • Big evolutionary changes occur through the accumulation of many small gradual changes Punctuated Equilibrium • There are drastic evolutionary changes when a species first diverges from its parent species followed by long periods of very little evolutionary change
Mass Extinctions • Large scale dying out of a large percentage of all living organisms in an area over a relatively short period of time • There have been 5 global mass extinctions