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Evolution Note #14 SBI3U – Evolutionary Change without Selection Natural selection – The way nature selects for some ____________ over others Selection is __________ random. BUT Evolutionary change doesn’t always happen this way! Change without Selection - Changes within a gene pool can occur without the ______________ ____ ___________ ____________ These changes tend to reduce genetic diversity within a population Change without Selection occurs through ____________ ____________, the __________________ _____________, the _____________________ ______________________. Allele - ________________________________________________________________________ Genetic Drift Genetic drift – the ________________________ shifting of the ___________________ make-up _____________________________________ - This shifting is entirely due to __________________ So some generations may see more of a particular feature, and others may see less of that feature Genetic Drift and Population - ______________ ___________________ has a major effect on genetic drift Genetic Drift and Large Populations - Large populations tend to produce a ___________________ ______________________ on alleles So in large populations, we expect to see a lot of __________________ _________________ One trait may be more common in a generation, and less common in another generation Overall though, genetic variability remains fairly ________________________ Genetic Drift and Small Populations - A particular trait is more likely to become very common or to disappear in a _______________ _________________ So small populations tend to have less _______________ __________________ Certain features become more common within that population, and certain features are lost Evolution Note #14 Variety and Large Populations - So in order to maintain a high level of genetic variability, a ____________ ______________ is best Features remain in the population at a fairly constant level Genetic Bottlenecks Bottleneck – a _______________ __________________ in population size, usually resulting in significant genetic drift, due to __________________________ How does it happen? A population reduction can occur in any number of ways: - _________________________ Habitat loss _________________________ The surviving population is likely to have a smaller gene pool than the initial population These particular traits will be passed along In this way, certain traits become __________________________, while certain alleles disappear If the population is allowed to recover, that population will be limited to ___________ __________ _____________ _____________ as a result of the bottleneck. Results of the Bottleneck - Overall, a bottleneck reduces the size of the gene pool, and genetic variety This is generally _________________ to the remaining population o Increased vulnerability to disease o Low ____________________ _________________________ o Increased infant mortality Evolution Note #14 Examples: Cheetahs - The cheetah species suffered a severe bottleneck about 10 000 years ago As few as 7 cheetahs survived, and all cheetahs today are descendents of these 7 cheetahs Elephant Seals - Population reduces to as few as 20 in the 1890s Today over 125 000 exist, but all are genetically very similar The Founder Effect The founder effect – occurs when a small number of individuals __________________ _____________ __________ _____________ ____________________ and establish a new population - The founding population would have a unique mix of alleles different than the larger or original population By chance, some alleles that are less common in the larger population will be _____________ ___________ in the founding population Frequency Differences - Perhaps a rare variant of a gene occurs once in every 10 000 people in the large population But in the founding population, that trait occurs in 1 of the 20 founders Original population = 1/10 000 = 0.1% Founding population = 1/20 = 5% This represents a 50x increase in the frequency for that variant Reduced Frequencies - The founder effect leads to a population with a ___________, ______________, __________ ___________ We can expect ________________ __________________ within the new population Example: Galapagos Tortoise - The entire population was created from one pregnant female tortoise from South America Her genes, and those of her offspring, formed the entire gene pool for the new Galapagos Tortoise population The Amish Community - Closed off religious community that shuns the modern world Isolation has created a smaller gene pool within the Amish community Bottleneck vs. the Founder Effect Effect on Gene Pool Results in Genetic Drift? How does it happen? Examples Bottleneck Founder Effect Smaller Smaller Evolution Note #14 A Mathematical Model of Evolution - We can think of evolution as the change in the ________________ _________________ of a population We can also think about evolution from a math perspective o The ___________________ of each allele in the population Can we predict the relationship between allele frequency and the chances of those frequencies remaining constant? The Hardy-Weinberg Principle The Hardy-Weinberg principle – states that in large populations where only random chance is at work, alleles should stay ___________________ ______________________ ______________________ - Other factors will result in evolution, as these will lead to changes in variant frequencies o ______________ ________________ o Small population size o ______________________________ o ______________________ ____ _____________________________ o Transfer of new alleles from a different species Predicting Change - By considering these factors, we can predict which populations are most likely to undergo the most evolutionary change Applying the Principle to Different Species - - Some species will undergo evolutionary change faster than others A species with o _______________ __________________ _______________ o _____________ ________________________ Will respond to natural selection more quickly This is why bacteria develop antibiotic resistance so quickly
