Causes of Microevolution

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Department
Biology
Course
BIOL 1113
Professor
Mary- Susan Potts- Santone
Semester
Spring

Description
1/10/13 Causes of Microevolution • Causes of Microevolution: o Genetic Mutations:  The raw material for evolutionary change; the original source of  genetic variability   Source of new alleles; new combinations of alleles  Source on which other evolutionary forces can act   Not goal­directed; Not a result of environmental necessity   Random events (good, bad, neutral) ­ depending on environmental  conditions o Gene Flow – Gene Migration:  Movement of alleles between populations when: • Gametes or seeds (in plants) are carried into another  population • Breeding individuals migrate into or out of population  Continual gene flow reduces genetic divergence between  populations o Nonrandom Mating ­ when individuals do not choose mates randomly o Assortative Mating ­ Individuals select mates with their phenotype and  reject mates with differing phenotype o Sexual Selection:  Males compete with each other for the right to reproduce  (intrasexual)  Females choose males possessing a particular phenotype  (intersexual) o Genetic Drift:  Occurs by disproportionate random sampling from population • Can cause the gene pools of two isolated populations to  become dissimilar • Some alleles are lost and others become fixed (unopposed)  Stronger effect in small populations • Genetic Drift: o Bottleneck Effect:  A random event prevents a majority of individuals from entering  the next generation  Next generation composed of alleles that just happened to make it  Loss of genetic variability o Founder Effect:  When a new population is started from just a few individuals   The alleles carried by population founders are dictated by chance  Formerly rare alleles will either: • Occur at a higher frequency in the new population • Be absent in new population • Natural Selection: o Major cause of microevolution  o Adaptation of a population to the biotic and abiotic environment  Abiotic: • Climate, water availability, minerals  Biotic: • Competition, predation, sexual selection o Requires:  Variation ­ The members of a population differ from one another   Inheritance ­ Many differences are heritable genetic differences   Differential Adaptiveness ­ Some differences affect survivability   Differential Reproduction – Some differences affect likelihood of  successful reproduction o Results in:  A change in allele frequencies in the gene pool  Improved fitness of the population • Types of Selection: o Heterozygote Advantage (i.e. sickle cell gene) 1. Directional Selection:  Extreme phenotype is favored  Curve shifts in that direction  Ex: size of modern horse, DDT­resistant mosquitoes, antibiotic­ resistant Bacteria 2. Stabilizing Selection:  The peak of the curve increases and tails decrease  Intermediate phenotype is favored  Ex: human babies with low or high birth weight are less likely to  survive 3. Disruptive (Diversifying) Selection:  Two or more extreme phenotypes are favored over intermediates;  bimodal distribution  Ex: Cepaea snails vary because a wide geographic range causes  selection to vary • Maintenance of Variations: o Genetic Variability:  Populations with limited variation may not be able to adapt to new  conditions  Maintenance of variability is advantageous to population  Only exposed alleles are subject to natural selection  Population evolves as gene frequencies change
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