D4.1 Natural Selection

basic tenets of natural selection

• populations produce more offspring than the environment can support

• genetic variation in the population (mutation, random fertilization, independent assortment, crossing over)

• selective pressures cause struggle

• differential survival and reproduction - individuals with the most favorable variation will be most likely to survive and reproduce to pass on their genes

• over generations, features that aid survival are disproportionately inherited by successful offspring, favorable features increase in frequency in the population

causes of variation in populations

• mutations - new alleles produced by mutation enlarge gene pool of a population

• meiosis - produce new combination of alleles through crossing over and independent orientation of bivalents

• sexual reproduction - offspring have a combination of alleles from two individuals

epigenetic tags

chemical markers reversibly added by a cell to its chromosomes, establishing pattern of gene expression

• small proportion of tags passed on in gametes to offspring, only patterns of gene expression altered

John Endler’s guppies

controlled experiments to determine effect of predation on physical traits of guppies in Trinidad and Tobago

• females prefer colorful males for mating

• coloration in shallow water makes guppies more visible to predators