Selection

“This preservation of favourable individual differences and variations, and the destruction of those that are injurious, I have called Natural Selection….”

Charles Darwin

The Origin of Species 1859

Remember the idea….

….more organisms born than can survive and reproduce

….variation among individuals in the population

….some of this variation is heritable

….variation in form leads to variation in survival.

Therefore, over generations, the population changes in response to the environment.

Some call it “survival of the fittest”.

Darwin called it natural selection.

The environmental factors leading to selection may be either…..

…biotic

…or abiotic.

The term “survival of the fittest” was actually introduced by Herbert Spencer in a biology text he wrote in 1864 (after reading Darwin).

It’s important to note that “survival” alone isn’t enough to lead to adaptation. Evolutionary success requires reproduction.

Herbert Spencer

A key concept….

Fitness – the relative genetic contribution of an organism to future generations. Determined by survival and reproduction.

A gazelle that does not survive, can not contribute to future generations….

…but survival alone does not assure a genetic contribution. The gazelle must reproduce.

Artificial Selection

Exactly the same process as natural selection, except that WE make the choices about which individuals are allowed to contribute.

Pigeons

Brought into domestication very early.

Aristotle describes five different breeds.

All are descended from the wild rock dove.

• Artificial selection has produced numerous breeds of dogs, which diverged from wolves about 15,000 years ago.

The first signs of cat domestication show up in Mediterranean culture some 7500 years B.C.

Like dogs, cats first were symbiotic with humans. Then, capture led to artificial selection for coexistence with humans and control of vermin.

Agriculture

Artificial selection has had a tremendous effect on agriculture in providing enhanced varieties of food and other crops.

Corn has been developed from the ancestral wild grass, teosinte.

Artificial selection has produced the male tassel and female ear of modern corn. Ears are larger in modern corn, and tassels and ears are on different parts of the plant compared to the ancestral teosinte.

• From left to right: popcorn, sweet corn, flint corn, dent corn, and pod corn.

• Through selective breeding, tomatoes with different shapes have been produced; within the rose family, different colors and flower structures have been artificially selected.

• Artificial selection of begonias for flower shape, size, and color has produced distinct varieties.

• Tulips come in a variety of colors and stripes, but never in a deep color of completely black. This variety does not exist, at least so far, within the species.

Natural Selection

Organisms less suited to their environment die, or contribute fewer offspring. Better suited individuals are more likely to survive and reproduce.

There is no “guarantee”. It is a statistical phenomenon.

Natural selection acts on the phenotype.

Ultimately, this acts on the genotype.

Where can we find examples of natural selection?

The moth occurs in two color phases, peppered and melanic. (a) Both phases are displayed against an unpolluted, lichen-covered tree. (b) Both phases are displayed against a dark tree, on which the lichen were killed by pollution.

Peppered moth – Biston betularia

In 1996, cliff swallows returning to nesting sites in North America were met by cold, rainy weather. Many died.

Researchers found significant differences between survivors and the ones that died.

Survivors were larger, and had a more symmetric body form.

Cepaea

This land snail shows significant variation in its shell color.

The background colors are brown, pink, and green.

Researchers found that brown background coloration was common in woodland-beech habitats, while green was prevalent in meadows.

Birds, such as this song thrush, hunt snails and break their shells open against “anvil rocks” where debris collects. The shards of shells at the anvil rocks indicated that the green snail were being eaten in the woodland habitat, while the brown shells were being taken in the meadows.

The researchers further demonstrated that, in deciduous woodlands, the frequency of the color phases changes over the year.

Nerodia sipedon

The northern water snake (Nerodia sipedon) occurs throughout eastern North America. It is darkly patterned, except on some islands in Lake Erie where many are light colored.

The Lake Erie water snake is unbanded, and is usually pale grey or brownish.

Scored color phases A (light) to D (banded). In young born on the islands, most are C or D (dark and banded). But by the time the snakes reach adult stage, most are A or B (light colored).

On the islands, predaceous gulls feed on young snakes, usually spotting and eating the more conspicuous banded snakes, producing differential survival of mostly unbanded snakes. Occasional immigrants from the mainland return some of the genes for banded color.

Types of Selection

We can look at the evolution of resistance to insecticides as an example of natural selection

DDT is very effective in its first use. Over time, however, DDT-resistant mosquitoes begin to appear. Can we calculate the fitness of the resistant strains?

Assume that resistance to DDT is the result of a single dominant allele that we will call R, with the susceptibility allele being designated r.

Resistance of Culex to permethrin is due to a resistance allele, R, which acts in a semi-dominant fashion.

Sickle cell anemia is an example of a polymorphism with a heterozygous advantage.

Sometimes the selective value of a trait is determined by complex factors.

Sickle cell anemia kills about 100,000 people per year.

About 80% of the homozygotes die without reproducing, yet the gene frequency is almost 10% in some populations.

Why?

The incidence of SC coincides closely with the occurrence of malaria.

Malaria is probably the deadliest disease in history, currently killing some 3.5 million people per year, mostly children aged 1-4 years.

It is caused by protozoans in the genus Plasmodium.

Evolution: Sickle Cell Anemia

It appears that individuals that are heterozygous for the sickle cell gene have an increased resistance to malaria, providing them with a heterozygous advantage.

The fitness of the heterozygote is higher than either homozygote

Batesian mimicry

Scarlet king snake

Coral snake

In some cases, the fitness of a genotype may depend on its frequency.

This is true in the case of mimicry.

Mullerian mimicry

Monarch

Queen

Sexual Selection

Why do we often find major differences among individuals?

In particular, why do we find major morphological differences between the sexes within the same species.

Secondary Sexual Characteristics

Male Stellar’s sea lion

females

How do we explain the selection of individuals as mates who seem to have traits that are disadvantageous.

One possibility:

Zahavi’s Handicap Principle

Females may choose to mate with healthy, unparasitized males

Bird species subject to higher degrees of parasitism tend to have more brightly colored males.

Brown creeper

Subfamily Certhiinae

Painted bunting

Subfamily Emberizinae

Another possiblity:

The Runaway Principle