# Factors Affecting Evolution ## Selection Pressures - Environmental factors that influence which organisms survive and reproduce are called **selection pressures** - Think of a drought selecting for cacti with deep roots, or competition for food selecting for faster cheetahs - These pressures drive changes in populations over time - Selection pressures can have different effects on **allele frequencies** within populations through natural selection - The frequency of advantageous alleles increases - The frequency of disadvantageous alleles decreases

Selection pressures are environmental factors that affect an organism's chance of survival and reproduction

## Types of Selection - There are two main types of natural selection: - **Stabilising selection** - maintains the status quo - **Directional selection** - drives evolutionary change ### Stabilising Selection - Stabilising selection keeps allele frequencies **relatively constant** across generations - Things stay as they are unless environmental conditions change - The population remains well-adapted to its current environment - Selection acts **against extremes** on both ends of the phenotypic spectrum - This maintains intermediate phenotypes - The "average" individuals have the highest fitness

Stabilising selection preserves the traits that already work well for a population

#### Example: Human birth weight - Newborn babies show variation in birth weight - Both very low and very high birth weights are selected against: - Very small babies face higher mortality risks - Very large babies can cause complications during delivery - This results in selection favouring **intermediate birth weights** - The optimal birth weight range is maintained generation after generation The chart below shows how survival rate varies with birth weight - notice the peak at intermediate values:

### Directional Selection - Directional selection produces a **gradual change** in allele frequencies over multiple generations - The population shifts in a particular direction - One extreme phenotype becomes more common

Directional selection is natural selection that changes allele frequencies progressively over time

- This usually happens when: - There is a **change in the environment** (new selection pressures) - A **new allele appears** in the population that is advantageous #### Process of directional selection: 1. **Phenotypic variation exists** within the population 2. A **selection pressure favours** a particular phenotype 3. The phenotype is produced by **specific alleles** 4. Individuals with the favoured phenotype are **fitter** and more likely to survive and reproduce 5. They **pass on the advantageous alleles** to their offspring 6. Those without the advantageous alleles are less likely to survive and reproduce 7. Over time and multiple generations, the **frequency of the advantageous allele increases** while other alleles decrease #### Example: Climate change and fish body size - Rising ocean temperatures are affecting fish populations in certain habitats - Warmer water creates a selection pressure favouring **smaller body sizes** **Why smaller is better:** - Warmer seas cause fish metabolism to **speed up** - This increases their oxygen demand - But oxygen levels are **lower in warmer water** - Larger fish have greater metabolic needs than smaller fish - They feel the effects of temperature increase more acutely - Temperature affects enzyme activity, which is crucial for metabolism **Evolutionary response:** - Fish with smaller body size are fitter in warmer conditions - Body size is determined by genetic and environmental factors - Smaller fish are more likely to reproduce successfully - They pass on alleles associated with smaller size to their offspring - Over generations: - Frequency of "small body size" alleles **increases** - Frequency of "large body size" alleles **decreases** The chart below illustrates how the frequency of fish with different body sizes shifts over generations due to directional selection:

Notice how the distribution shifts left (towards smaller sizes) from Generation 0 to Generation 10. ## Other Factors Affecting Evolution - Beyond natural selection, several other processes can alter allele frequencies in populations - These factors can significantly impact the evolution of a species ### Genetic Drift - **Genetic drift** occurs when **chance** (rather than environmental selection) determines which individuals survive, breed, and pass on alleles

Genetic drift is the gradual change in allele frequencies in a small population due to chance, not natural selection

- This effect is most significant in **small populations** - Chance events can have major impacts - Some alleles may be lost or favoured purely by random events - In **large populations**, genetic drift has minimal effect: - Chance variations in allele frequencies even out across the whole population - Natural selection plays a much more influential role #### Example: Flower colour in a small plant population - Consider a small population of wildflowers growing beside a car park - Half the plants carry alleles for purple flowers - Half carry alleles for white flowers **A chance event:** - By random chance, most seeds from purple-flowered plants land on the tarmac (where they cannot germinate) - Meanwhile, seeds from white-flowered plants land on soil (where they successfully germinate and grow) **Result:** - Over several generations, the alleles for purple flowers may **disappear** from this population - This is entirely due to **chance**, not because white flowers are "better" - The genetic composition has changed randomly, not through adaptation The chart below shows how allele frequency can fluctuate randomly in a small population due to genetic drift:

Notice the random fluctuations and overall decline - not due to selection, but pure chance. ### Genetic Bottleneck - A **bottleneck effect** occurs when a large population suffers a dramatic decline in numbers - This is similar to the founder effect but affects an existing population

Population bottlenecks drastically reduce genetic diversity, making species vulnerable

**What happens:** - A major environmental event (disease, natural disaster, climate shift) kills most individuals - Only a small number survive - This massively **reduces genetic diversity** as many alleles are lost - The survivors must breed with close relatives - **Inbreeding** becomes common **Consequences:** - Reduced genetic variation in the population - Increased risk of harmful recessive alleles being expressed - Lower adaptability to environmental changes - Decreased overall fitness #### Example: Cheetah population bottleneck - Modern cheetahs show remarkably low genetic diversity - Evidence suggests a severe bottleneck occurred roughly 10,000-12,000 years ago - At the end of the Ice Age, dramatic climate changes killed off most cheetahs **The bottleneck:** - The surviving cheetahs were isolated in small, scattered groups - Extensive inbreeding occurred - Many alleles were permanently lost from the gene pool **Modern consequences:** - Today's cheetah population has serious lack of genetic variation - This is problematic for conservation: - Genetic variation increases a species' ability to respond to environmental changes - With low variation, the species is vulnerable to disease and other threats - Recovery efforts are complicated by limited genetic diversity The chart below illustrates a population bottleneck - notice how population size crashes and recovers, but genetic diversity remains permanently reduced:

### Founder Effect - The **founder effect** occurs when a small number of individuals from a large parent population establish a new population elsewhere

The founder effect is when a new population is established by a few individuals carrying only a subset of the original population's genetic variation

**Key characteristics:** - The new population contains only **some alleles** from the parent population - Not all of the gene pool is represented - Many alleles are left behind in the parent population - Which alleles end up in the founding population is determined by **chance** - The new small population may evolve in a **different direction** from the larger parent population #### Example: Island lizards and scale colour - Lizards inhabit Caribbean islands and can travel between islands on floating vegetation or debris - The individuals that arrive on a new island (and their alleles) are determined by **chance** **Scenario:** - On the original mainland island, lizards show a range of scale colours from cream to orange - Two individual lizards with cream scales happen to float to a new, uninhabited island - These two founders carry only alleles for cream scales **Outcome:** - The entire population that develops on the new island may only have cream-scaled individuals - They only inherited cream colour alleles from the founders - The original mainland population retains both cream and orange individuals - The difference between these populations is entirely due to **chance** - Not because cream scales are advantageous on the new island **Genetic implications:** - The founding population has a **reduced gene pool** compared to the parent population - Allele frequencies in the new population may differ greatly from the original - The new population may follow a different evolutionary trajectory ## Summary Table: Processes Affecting Allele Frequencies | Process | Result | |---------|--------| | **Natural Selection** | Selection pressures produce a gradual change in allele frequencies over several generations | | **Founder Effect** | Changes in allele frequencies occur in a different direction for the newly isolated small population compared to the larger parent population, due to chance | | **Genetic Drift** | Gradual change in allele frequencies in a small population due to chance, not natural selection | | **Bottleneck Effect** | Reduction in the gene pool of a population due to a dramatic decrease in population size |

All these processes reduce genetic diversity except directional selection, which can maintain or increase it if new advantageous alleles arise through mutation

## The Role of Mutations - Both natural selection and random processes (drift, bottleneck, founder effect) require **mutations** to provide raw material for evolution - Without mutations, there are no new alleles for any process to act upon - Mutations create the genetic variation that: - Natural selection can favour or select against - Chance events can randomly increase or decrease in frequency - May be lost in bottlenecks or founding events

New alleles from mutations may be advantageous, disadvantageous, or have no apparent effect on phenotype due to the degenerate nature of the genetic code

- New alleles don't always appear immediately in phenotypes: - They can remain hidden (not expressed) for several generations - Recessive alleles may be masked by dominant alleles - They contribute to phenotypic variation only when expressed