Higher Biology Unit 1 1 7 Evolution Selection

Higher Biology Unit 1: 1. 7 Evolution - Selection

Learning Intentions • To be able to describe how selection processes drive evolution • To be able to use numeracy skills to analyse graphs and explain what is shown

Success Criteria • I can give examples of selection that take place within species • I can explain how selection methods drive evolution • I can describe and explain changes in population graphs

Selection • Selection is the process by which the frequencies of some DNA sequences increase within a population • The sequences that increase are said to be selected for and those that decrease are selected against • This occurs in a non random manner

Types of selection • Continuous variables such as height, weight and seed size are controlled by many genes • They are said to be polygenic

Natural selection • Natural selection states that only the organisms with the characteristics best suited for their environment will survive and reproduce • The genes that control these characteristics will then be passed onto their offspring

Natural Selection • Organisms whose characteristics are not well suited to the environment are more likely to be predated upon or die before reaching reproductive age • The genes for the poorly suited characteristics are therefore lost • The process of natural selection occurs generation after generation and will continue to occur as the environment changes

Sexual Selection • Females and males have different investments in reproduction • Males produce lots of sperm and are capable of fertilising many females in a short period of time • Females spend more energy producing fewer eggs and tend to show more parental care than males • Males and females have therefore evolved strategies to try and maximise their reproductive potential

Sexual Selection • Sexual selection is a non random process that results in the increase in frequency of sequences of DNA that increase the reproductive success • Reproductive success can be classes as successful matings as well as number of surviving offspring

Male to male competition • One form of sexual selection is male to male competition • Males compete for territory and access to females • Examples include deer and elephant seals

Deer competition One way male deer compete with each other for mates is by locking horns. The male who wins in this competition is the “fittest” and therefore has the strongest genes to pass on. Aggressive competition between males can result in serious injury or death, some animals have devised other ways to compete to reduce this risk.

Elephant seals competing for mates

Female choice • In some situations it is the female who makes the choice of mate • Females select a male to mate with who will give their offspring the best chance of survival • This is often based upon males performing a display for the female • Examples include the red capped manakin and peacocks

Peacocks The peacock is much more brightly coloured and noticeable than the female peahen. Males attract females by displaying their bright plumage. Females select a male who has the best display and plumage.

Sexual selection • In both male to male competition and female choice their has been a cost in the evolution of reproductive strategies • Males have had to evolve structures to allow them to attract females • By singing and displaying male birds are putting themselves at increased risk of attracting predators

Red capped manakin video

Normal distribution • When quantitative data is collected for continuous variables in a large population we would expect to see a normal distribution that looks like a bell shaped curve

Increasing number of population Normal distribution Increasing value of inherited characteristic (height, weight)

• Natural selection can affect the distribution of characteristics within a population in 3 different ways

Stabilising Selection • This form of selection exerts pressure against the extreme variants of a characteristic • This leads to a reduction in genetic diversity • It operates in an unchanging environment and maintains the status quo for the best adapted organisms

Directional selection • Most common during a period of environmental change • Favours a version of the characteristic that was originally less common • Results in a directional change in the mean value • Examples include average size of bears increasing during each ice age

Disruptive Selection • Extreme forms of a characteristic are favoured instead of the intermediates • Results in a population being split into 2 distinct groups • Occurs when two different habitats or types of resource become available • Selective breeding is a form of disruptive selection

Success Criteria • I can give examples of selection that take place within species • I can explain how selection methods drive evolution • I can describe and explain changes in population graphs