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BIOC54H3 Chapter Notes - Chapter 1: Medium Ground Finch, Lactose Intolerance, House Mouse

Biological Sciences
Course Code
Kamini Persaud

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BIOC54H Chapter 1 Tharsini Sivananthajothy
January 20, 2012
Darwinian Theory and Ultimate Hypotheses
- Darwin’s theory of evolution by natural selection is based on the premise that evolutionary
change is inevitable if just three conditions are met:
o Variation: such that members of a species differ in some of their characteristics
o Heredity: with parents able to pass on some of their distinctive characteristics to their
o Differences in reproductive success: such that some individuals have more surviving
offspring than others in their population, thanks to their distinctive characteristics
- if there is hereditary variation within a species and if some of the variation allows some to
reproduce more successfully than others, then large number of the successful type will change
the makeup of the species
- species will evolve as it becomes dominated by individuals that possess the traits associated with
successful reproduction
- the process that causes evolutionary change is natural so Darwin called it natural selection
- although Darwin knew little about heredity, we now know that variation among individuals
within a species arises because of differences in their genes, the segments of DNA that faithfully
encode the information needed for the synthesis of proteins
- genetic variation within a species occurs when a given gene exists in two or more forms or alleles
- the different alleles
o different alleles affect the nature or abundance of the protein coded for by the gene so
genetically different individuals transmit different instructions for protein manufacture
to their offspring
- if some alleles are superior in their ability to make individuals reproductively successful, then
those alleles will get themselves passed on from generation to generation
o will become more common over time
o genetic variation + differential reproduction = evolutionary change at the genetic level
o alleles will spread in proportion to how well they help build bodies that are unusually
good at reproducing
Darwinian Theory and the Study of Behaviour
- the conditions required for natural selection apply to nearly every organism, which means that
almost every species has probably been shaped by natural selection in the past
- evolution would happen when people created the conditions needed for selection to occur as
they domesticated certain useful animals and plants
- test Darwinian theory by attempting to generate evolution in the laboratory, starting with a
population that exhibits hereditary variation in attributes that affect the reproductive success of
- artificial selection experiment done by Carol Lynch with house mice which build nests of soft
grasses and other plant materials in natural but will happily accept cotton as nesting material in
the laboratory
o the amount of cotton a mouse collects can be quantified as the number of grams it pulls
into a nest cage over a 4-day period
o in the starting generation, individual mice moved 13 18 grams of cotton in to their cage
from an external supply
o Lynch worked with this variation on the assumption that the differences were hereditary
o She attempted to evolve a high line by interbreeding males and females who collected
large amounts of cotton and a control line which crossed males and females at random
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BIOC54H Chapter 1 Tharsini Sivananthajothy
January 20, 2012
o Offspring produced by these crosses were reared under the same conditions their parents
were, eliminating environmental variation as a possible cause for difference in behavior
o The new generation was allowed to collect cotton and the most avid cotton collectors
were permitted to breed, creating a second selected generation
o Least eager collectors were low line
o These procedures were repeated over 15 generations and resulted in the high line mice
gathering about 50 grams of cotton for their nests on average while low-line mice
brought in about 5 grams on average and control brought in 20 grams
o Evolution occurred in the lab under the conditions that should have resulted in
evolutionary change if the theory of natural selection is correct
- Another example includes lactose tolerance
o People began rearing milk cows successfully only 5000 years ago in northern Europe
because this area had few deadly contagious diseases of cattle
o Once dairying originated, a mutant gene spread naturally through this human
population, a gene that contributed to the ability of adults to digest milk sugar
o Although children can digest lactose, most lose this ability when they are no longer being
nursed by their mothers, after which they become lactose intolerant
o Natural selection acting on variation in the ability of adults to absorb milk sugar resulted
in the rapid spread of lactose tolerance in adults within the northern European cattle-
herding culture and also in a number of African groups where dairy cattle provide milk
for children and adults
- Another example includes bill size of ground finch in the Galapagos who undergo evolutionary
change annually
o In drought years, when the smaller seeds eaten by medium ground finch were scarce
relative to the large seeds, selection favoured relatively large billed individuals
o Birds with the hereditary tendency to develop larger than average beaks survived and
reproduced because of their ability to crack open big tough seeds while smaller-beaked
cousins were starving to death rather than reproducing
o In rainy years, the situation reversed as birds with smaller bills handled the then
abundant smaller seeds better than larger-beaked finches
o Temporary feeding advantage of smaller-billed finches translated into a reproductive
advantage, leading to the evolution of somewhat smaller-billed population
o Annual changes in selection caused the average bill size of the finches to fluctuate
ANNUALLY in response to the changes in the resources available
- Other examples include the evolution of bacterial resistance to antibiotics, change in
overwintering behavior of some songbirds in the last few decades, the spread and decline of
melanic wing coloration in moths, evolutionary of Australian black snakes of an average to a
highly poisonous, very recently introduced toad
- If we assume that individuals are given alleles that have spread because they were better than the
alternative at getting their bearers to reproduce successfully, then every hereditary attribute of
almost every species probably has something to do with reproductive success
- When biologists wish to understand the ultimate reasons why an animal does something, they
almost always try to come up with a working hypothesis that is consistent with natural selection
- Some traits do not look especially likely to increase an individual’s reproductive success
o Male Hanuman langurs expend much energy and time trying to kill the infants of the
females that they live with in bands in northern India
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