How Humans Affect Diversity of Other Species (College Board AP® Biology)

Artificial Selection

• Artificial selection is the process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of these desirable traits over time and many generations

• This practice is also known as selective breeding

• Humans have been selectively breeding organisms for thousands of years, long before scientists understood the genetics behind it

• Knowledge of the alleles that contribute to the expression of the desired traits is not required because individuals are selected by their phenotypes, not their genotypes

• As the genetics is not always understood, breeders can accidentally enhance other traits that are genetically linked to the desirable trait

  • These other traits can sometimes negatively affect the organism's health

• Examples of artificial selection include:

  • Increased milk yield from cattle

  • Faster racehorses

  • Disease-resistant crops

• There are always biological limitations to how extreme a trait can become in an organism

Principles of selective breeding

1. The population shows phenotypic variation - there are individuals with different phenotypes / traits

2. Breeder selects an individual with the desired phenotype

3. Another individual with the desired phenotype is selected. The two selected individuals should not be closely related to each other

4. The two selected individuals are bred together

5. The offspring produced reach maturity and are then tested for the desirable trait. Those that display the desired phenotype to the greatest degree are selected for further breeding

6. The process continues for many generations: the best individuals from the offspring are chosen for breeding until all offspring display the desirable trait

Artificial selection in racing horses

• Selective breeding has been a major part of the horseracing industry for many years

• Breeders have found that horses tend to have one of the three following phenotypes:

  • Good at sprinting short distances

  • Good endurance over long distances

  • All rounder

  

Racehorses are selectively bred for their speed or stamina

• If a breeder wanted to breed a horse for a sprinting event they are likely to do the following:

  • Select the fastest sprinting female horse they have

  • Select the fastest sprinting male horse they have

  • Breed the two selected horses

  • Allow their offspring to reach maturity and test their sprinting speeds to find the fastest horse (male or female)

  • The breeder could then use this horse for racing, or they could continue the process of selective breeding by breeding this horse with another horse that is fast or descended from fast sprinters

  • Over several generations, it would be hoped that the offspring are all fast sprinters (but remember there are biological limitations to this)

Convergent Evolution

• Analogous structures are characteristics with similar form and function, but with different evolutionary origin

  • Such structures have historically caused some confusion for scientists working in the field of taxonomy

• While homologous structures provide evidence of shared ancestry, analogous structures come about as the result of convergent evolution

  • Analogous structures provide evidence for the passing on of advantageous characteristics during natural selection

• Convergent evolution can occur when two distantly related species live in habitats with similar selection pressures, meaning that similar characteristics provide a survival advantage

  • Advantageous characteristics evolve separately, rather than as the result of a single mutation

• Examples of similarities that have arisen due to convergent evolution include:

  • Dolphins and sharks

    • These are both groups of aquatic animals that share a similar body shape, but they in fact belong to different classes

      • Dolphins are mammals and sharks are fish

    • Their streamlined body shapes evolved separately rather than originating in one common ancestor

  • Cacti and euphorbia

    • These are two groups of desert plants recognisable by their spiny leaves and branching, succulent stems

    • They belong to different orders of plants

      • Cacti are found in the deserts of the Americas, while euphorbias are found in Africa

    • They evolved separately, but adapted to similar environments

Analogous Structures Diagram

Analogous structures, such as body shape in sharks and dolphins, and wings in butterflies and bats, occur as the result of convergent evolution