4: Evolution for A-Level Biology (OCR B

In a population, what describes the number of different alleles present for genes within that population?

Genetic diversity

High genetic diversity increases the range of traits present in a population.

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Why does a population with higher genetic diversity have a greater potential to survive environmental change?

Greater range of alleles increases chance of advantageous traits.

Some individuals may already possess alleles suited to new conditions.

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What evolutionary process causes populations to become better adapted to their environment over generations?

Natural selection

Individuals with advantageous traits reproduce more successfully.

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Fill in the blank:

New alleles can arise when random ______ occur in DNA.

mutations

Mutations change nucleotide sequences and can create new alleles.

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Why are most mutations unlikely to increase an organism’s chances of survival?

Most mutations are either harmful, or have no effect.

Many mutations disrupt protein structure or function.

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How can a mutation sometimes increase reproductive success in a specific environment?

It produces an advantageous allele.

The allele may improve survival or reproductive success.

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What must happen for a beneficial allele to spread through a population?

It must be inherited by offspring.

Natural selection acts on inherited genetic variation.

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Fill in the blank:

Over many generations, an advantageous allele tends to ______ in frequency within a population.

increase

Individuals with the allele produce more offspring, some with the advantageous allele.

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True or False:

Natural selection acts on individuals but results in evolutionary change in populations.

True

Individuals survive or reproduce differently, but allele frequencies change in populations.

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What term describes the change in allele frequencies within a population over time due to differential reproductive success?

Evolution

Evolution occurs through processes such as natural selection.

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What type of selection occurs when one extreme phenotype is favoured and the population mean shifts?

Directional selection

This type of selection drives traits in a particular direction.

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What example commonly demonstrates directional selection caused by human activity in medicine?

Antibiotic resistance in bacteria

Resistant bacteria survive treatment and reproduce.

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True or False:

Antibiotic resistance occurs because bacteria intentionally mutate to survive antibiotics.

False

Mutations occur randomly before, or during exposure to antibiotics.

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What type of selection favours individuals with intermediate phenotypes and reduces extremes?

Stabilising selection

This type of selection maintains the status quo for a trait.

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Which human trait is commonly used as an example of stabilising selection?

Birth weight

Very low or very high birth weights have higher mortality.

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Fill in the blank:

Adaptations that involve body structures, such as limb shape, are called ______ adaptations.

anatomical

Adaptations can also be physiological or behavioural.

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What type of adaptation involves internal processes such as enzyme activity or metabolism?

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Physiological

Physiological traits improve function under certain conditions.

What type of adaptation involves the way an organism acts or responds to its environment?

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Behavioural

Examples include migration or mating displays.

True or False:

Natural selection contributes to the diversity of living organisms.

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True

Different environments favour different adaptations.

Why can bacterial populations evolve antibiotic resistance very rapidly?

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Large population size and rapid reproduction

Large numbers increase mutation chances and selection acts quickly.

Why can individuals within the same species show a wide range of phenotypic variation?

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Genetic and environmental factors

Genetic variation (e.g. mutations, meiosis) and environmental influences both affect phenotype.

What is the primary source of genetic variation in populations?

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Mutation

Mutations create new alleles and are the ultimate source of genetic variation.

How does meiosis increase genetic variation in sexually reproducing organisms?

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Independent assortment and crossing over

These processes shuffle alleles during gamete formation, producing genetically different gametes.

Why does sexual reproduction generate more genetic variation than asexual reproduction?

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Random fertilisation of gametes

Fusion of genetically different gametes produces unique allele combinations in offspring.

# True or False: Predation, disease and competition for resources can act as **selective pressures** in **populations**.

True ## Footnote These factors affect survival and reproduction, driving natural selection.

What process describes the **differential survival and reproduction** of individuals with **advantageous phenotypes**?

Natural selection ## Footnote Individuals with beneficial traits are more likely to survive and reproduce.

What happens to **favourable alleles** in a population when individuals with **advantageous phenotypes** reproduce more successfully?

Increase in allele frequency. ## Footnote Successful individuals pass their alleles to offspring, changing the gene pool.

What term describes the collection of all **alleles** present in a **population**?

Gene pool ## Footnote Evolution involves changes in allele frequencies within this pool.

How is **evolution** defined in **genetic terms**?

Change in allele frequencies in a population. ## Footnote Evolution occurs over generations within populations.

What type of **selection** occurs when individuals with the **average phenotype** have the highest fitness?

Stabilising selection ## Footnote Reduces variation and favours intermediate phenotypes.

What type of **selection** occurs when one **extreme phenotype** is favoured over others?

Directional selection ## Footnote Causes the population mean to shift toward one extreme.

What type of **selection** occurs when both **extreme phenotypes** are favoured over the intermediate phenotype?

Disruptive selection ## Footnote Can increase variation and potentially split populations.

# True or False: **Natural selection** can lead to changes in both **phenotype frequency** and allele frequency in a population.

True ## Footnote Selection acts on phenotypes but changes underlying allele frequencies.

What process occurs when **populations** become unable to **interbreed** and produce fertile offspring?

Speciation ## Footnote This marks the formation of new species.

What is required before two **populations** can evolve into separate **species**?

Reproductive isolation ## Footnote Isolation prevents gene flow between populations.

What type of **speciation** occurs when populations become **geographically separated**?

Allopatric speciation ## Footnote Physical barriers such as mountains or oceans isolate populations.

What type of **speciation** occurs when new species arise without **geographic separation**?

Sympatric speciation ## Footnote Often due to ecological, behavioural or genetic isolation.

Why can **isolated populations** gradually develop different **gene pools**?

Accumulation of different mutations and selection pressures. ## Footnote Without gene flow, allele frequencies change independently.

# True or False: Members of different **species** can always produce **fertile offspring** together.

False ## Footnote Species are defined by their inability to interbreed to produce fertile offspring.

What process can cause **random changes** in **allele frequency** in a population?

Genetic drift ## Footnote Changes occur due to chance rather than selection.

Why is **genetic drift** more significant in **small populations** than large ones?

Chance has a greater effect on allele frequencies. ## Footnote Random events can drastically change allele frequencies in small populations.

How can **evolutionary change** over long periods contribute to **biodiversity**?

Accumulation of speciation events ## Footnote Repeated divergence leads to many different species over time.

4: Evolution Flashcards

Explain how natural selection leads to adaptation and speciation. (42 cards)