Natural selection means that individuals with useful traits are more likely to reproduce than individuals with harmful traits. Individuals do not evolve, populations evolve. Population : a local, interbreeding group of individuals of the same species.
Gene pool : all the genes in a population.
The Darwin awards
Hardy-Weinberg theory : random mixing of alleles alone does not affect the frequency of alleles in the gene pool. In other words the percentage of individuals with a dominant allele should stay the same from one generation to the next. See Fig 19-1.
In a population with 2 alleles, where p is the frequency of one allele, and q is the frequency of the other allele, the frequency of genotypes in the population is : p2 + 2 p q + q2
Frequency Frequency Frequency
of AA of Aa of aa
The Hardy-Weinberg theory requires :
1) No natural selection
2) No mutations ( mutations produce new alleles )
3) Random mating
4) Very large population size ( at least several thousand )
5) Isolation from other populations
The Hardy-Weinberg theory is used for :
A) Predicting the percentage of carriers for genetic disease
B) Detecting evolution
Evolution of Populations
Populations can evolve by breaking any of the 5 rules listed above.
1) Gene flow - individuals, or gametes, sometimes move from one population to another
- immigration and emigration eg lions ( Panthera ) males leave the pride when they become nature, females stay in the pride they are born into.
2) Genetic drift - changes in the gene pool of a small population due to chance
a) bottleneck effect eg whooping cranes ( Grus ), which dropped to a total population of less than 20 birds.
Humans also went through a genetic bottleneck:
Human evolution
b) founder effect eg Galapagos finches ( Camarhynchus ), which spread to the Galapagos islands from South America ("founding" a new population), and are now different species.
3) Mutations - changes in genes ( DNA ) are usually harmful, but rarely produce a useful new allele which can be passed on to future generations. For example most humans in northern Europe have inherited a mutation that allows them to digest milk. Milk
4) Natural selection - some genotypes help the individual to survive and reproduce, others are harmful. Butterfly evolution
5) Non-random mating : plants often mate with nearby plants. Animals usually select their mate. This is called sexual selection.
Courtship and Sexual selection
Birds and mammals do not mate at random. Typically females select their mate (sexual selection) because females produce fewer offspring than the most successful males. Males court the females to persuade females to mate with them.
Birds: 90 % of species are monogamous (meaning one male mates with one female at least for the season) because it takes both parents to raise the young. Male birds often have elaborate plumage eg the tail of a peacock ( Pavo ).
Brightly colored male birds are chosen by females. Birds
- the widow bird ( Vidua ) in Africa, males have long tails to attract females.
- bower birds ( Sericulus ) in Australia, males decorate a bower on the ground to court females. Bower birds.
- Several species have lek mating, where males congregate to display to, and mate with, females. Lek
Mammals : 90 % of species are polygynous (meaning one male mates with many females, each female only mates with one male) because females feed the young milk, males typically do not help raising the young. Males are usually larger than females, with antlers, manes, tusks, etc for display in male to male competition. The dominant male mates with most females eg Gorillas ( Gorilla ) have one dominant male in each group.
Fish and gazelle antelope ( Gazella ): females select daring males that approach predators.
Elephants ( Loxodonta ): long tusks in males indicate fewer parasites.
Mice ( Mus ) pheromones indicate lack of parasites. Pregnant females may abort if they smell a new male pheromone. Smell
What evidence is there for pheromones in humans?
a) women who live together tend to menstruate at the same time. Link
b) women are attracted to the smell of men with different immune systems from their own. Scent