Types of Selection

Natural Selection Natural selection leads to adaptation – an increase in the fitness of a population in a particular environment. Natural selection works because some genotypes are more successful in a given environment than others. Successful (adaptive) genotypes become more common in subsequent generations, causing an alteration in allele frequency over time and a consequent increase in fitness.

Types of selection Stabilizing selection Occurs when an intermediate phenotype is favoured over the two extremes. Example: birth weight of human infants – death rates are highest for those extremely low (1 kg or 2.2 lb) & those extremely high (5-6 kg or 11-13 lb)

Stabilizing Selection for the Sickle Cell Allele In heterozygous form, the sickle cell allele of beta globin confers resistance to malaria. Therefore, the allele is maintained, even though its harmful in homozygous form.

Directional selection Occurs when an extreme phenotype is favored & the distribution curve shifts in that direction. Examples: Industrial melanism with moths (peppered moths) Pesticides & antibiotics selecting for “super” insects & bacteria

Directional Selection Hominid Brain Size

Disruptive selection (Diversifying Selection) Two or more extreme phenotypes are favored over any intermediate phenotypes British land snails have wide habitat range– Light banded survive in one area, dark banded in another, mixtures of the bands get consumed

Disruptive Selection Disruptive selection is rare. A west African species of finch shows two beak types, large or small, without intermediate beak sizes. Disruptive selection is rare.

This is an example of disruptive selection. Cichlid fish Cichlids live in African lakes. Males set up territories which they defend against other males, while they also try to attract as many females to lay eggs in their nest. This sounds very straightforward, but there is a male polymorphism (more than one type of male). Some males are very small and when dissected most of their body cavity is taken up by giant testis. How do these males fertilize female eggs when they are too small to hold territories? Well these males are called 'sneak' males. They wait until the moment that the big male and a female release their sex cells and rush in and release their sperm too. Not only do they father some of the offspring, but they do not expend energy and time looking after their offspring because the big male does this. This is an example of disruptive selection.

Sexual Selection Sexual dimorphism is the term used to describe distinctly male and female phenotypes or secondary sexual characteristics. Sexual selection is based on any trait that gives the individual a competitive edge in mating and producing offspring. Many times the males compete for the privilege of mating with certain females but at other times the females are the agents of selection when they pick their mates. Through nonrandom mating, alleles for preferred traits increase. This leads to increased sexual dimorphism Run away sexual selection….led to the extinction of Irish Elk! (not really Irish, best fossils found in Ireland, almost 3m tall and 4 m antler span.)

Beetles and beer bottles. A species of beetle is on the verge of extinction in Australia because of beer bottles thrown out of car windows. Male beetles like big female beetles because big female beetles have lots of eggs. The bigger the females that a male mates with, the more offspring he will have. Unfortunately for the continuity of this species of beetle, males are so enthusiastic about the size of females that they often fail to realize that what they mating with is an empty beer bottle.