ecosystem may lead to speciation

Evolution (in genetic terms)

A change in allele frequencies in a population over time.

Gene pool

All the alleles of all the genes in a population.

Allele frequency

The proportion of a particular allele among all alleles of that gene in the population.

Mechanism of natural selection on allele frequencies

Individuals with advantageous alleles survive and reproduce more, so those alleles become more common.

Importance of genetic variation in evolution

Without variation, there are no different phenotypes for selection to act upon.

Three main sources of genetic variation in sexually reproducing populations

Mutation, independent assortment/crossing over in meiosis, and random fertilisation.

How mutation creates new alleles

It changes the base sequence of DNA, sometimes creating a new version of a gene.

Contribution of meiosis to genetic variation

Independent assortment and crossing over produce new combinations of existing alleles.

Contribution of random fertilisation to genetic variation

Which sperm fertilises which egg is random, combining different alleles each time.

Differential reproductive success

Differences in reproductive success between individuals in a population.

Reason some individuals produce more offspring than others

They possess phenotypes better suited to the environment, so they survive longer and reproduce more.

How differential reproductive success alters allele frequencies

Advantageous alleles are passed on more often, so their frequency increases in the gene pool.

Reason evolution lacks a predetermined direction or goal

Selection acts on existing variation and changing conditions, not towards a pre-set target.

Selection pressure

An environmental factor that affects survival and reproduction.

Examples of selection pressures

Predation, disease, competition, climate, and food availability.

Why selection pressures determine survival and reproduction of phenotypes

Individuals with traits better suited to pressures survive and leave more offspring.

Directional selection

Selection that favours individuals with phenotypes at one extreme.

Environmental conditions for directional selection

When the environment changes in a consistent direction (for example, a colder climate).

Effect of directional selection on phenotypic mean

The mean value of the trait shifts towards the favoured extreme.

Effect of directional selection on phenotypic range

Phenotypes on one side of the distribution become more common, while others become less common.

Stabilising selection

Selection that favours average phenotypes and acts against extremes.

Environmental conditions for stabilising selection

Stable, unchanging environments.

Effect of stabilising selection on mean phenotype

The mean phenotype remains roughly the same.

Effect of stabilising selection on phenotypic range

The range becomes narrower, with fewer extreme phenotypes.

Disruptive selection

Selection that favours individuals at both extremes and acts against intermediates.

Environmental conditions for disruptive selection

Environments where conditions vary in different ways within the same area or favour extremes.

Effect of disruptive selection on extreme phenotypes

Individuals at the extremes have a selective advantage and increase in frequency.

Effect of disruptive selection on intermediate phenotypes

Individuals close to the mean are at a disadvantage and decrease in frequency.

Role of disruptive selection in speciation

It can split a population into distinct groups, serving as a step towards speciation.

Species (biological species concept)

A group of similar organisms that can interbreed to produce fertile offspring.

Speciation

The formation of new species from existing ones.

Importance of reproductive isolation in speciation

Different species must be reproductively separated to remain distinct.

Role of allele frequency divergence in speciation

Differences in allele frequency lead to differences in phenotype that prevent successful interbreeding.

Allopatric speciation

Speciation that occurs after populations become geographically isolated.

Type of barrier in allopatric speciation

A physical (geographical) barrier that prevents interbreeding.

Examples of geographical barriers leading to allopatric speciation

Mountains, rivers, oceans, deserts, and ice sheets.

Why geographically separated populations experience different selection pressures

Each environment has a different climate, predators, competitors, and resources.

Action of natural selection in different environments

Different traits are favoured in different environments, so selection acts differently.

Changes in allele frequencies in isolated populations

They change in different directions, reflecting local selective pressures.

Selection of mutations in isolated populations

Different beneficial mutations arise by chance in each isolated group.

How time and continued selection result in new species in allopatry

Over many generations, accumulated differences become large enough to prevent interbreeding, forming new species.

Sympatric speciation

Speciation occurring within the same geographical area.

Mechanism of sympatric speciation without geographical isolation

Populations become reproductively isolated without physical separation.

Barriers involved in sympatric speciation

Behavioural, ecological, temporal, or mechanical barriers.

Role of behavioral changes in sympatric speciation

Changes in courtship behaviour or mate preference may prevent interbreeding.

Role of seasonal (temporal) isolation in sympatric speciation

If groups breed at different times, they do not mate with each other.

Role of polyploidy in plant sympatric speciation

A sudden increase in chromosome number can produce a new fertile plant type that is reproductively isolated from its parents.

Reproductive isolation

Barriers that prevent gene flow between populations.

Pre-zygotic isolating mechanisms

Barriers that prevent mating or fertilisation from occurring.

Examples of pre-zygotic isolating mechanisms

Different mating seasons, different courtship behaviour, mechanical incompatibility of reproductive organs, and gamete incompatibility.

Post-zygotic isolating mechanisms

Barriers that act after fertilisation.

Examples of post-zygotic isolating mechanisms

Hybrid inviability, hybrid sterility, or hybrid breakdown in later generations.

Effect of reproductive isolation on gene flow

Alleles can no longer be exchanged, allowing populations to diverge independently.

Genetic drift

Random changes in allele frequencies, especially in small populations.

Impact of genetic drift on small vs. large populations

Chance effects have a proportionally bigger impact in small populations.

Contribution of genetic drift to speciation

Drift can fix different alleles in different isolated populations, contributing to divergence.

Difference between genetic drift and natural selection

Selection is non-random and favours advantageous alleles, whereas drift is random and not necessarily adaptive.

Founder population

A small number of individuals that form a new population isolated from the original.

How the founder effect alters allele frequencies rapidly

By chance, the founder individuals may carry allele frequencies very different from the original population.

How the founder effect sets the stage for speciation

Isolation and strong drift/selection in the small population can rapidly produce a distinct gene pool.

Reason populations in different environments diverge over time

Different selection pressures, mutation events, and drift operate in each population.

Why previously identical species populations lose interbreeding ability

Accumulated genetic and behavioural differences eventually prevent successful mating or fertilisation.

Why reproductive isolation is key to defining speciation

It shows that the populations no longer share a gene pool and evolve independently.

Fossil evidence supporting evolution of new species

Fossils show changes in forms over time and the appearance of new groups not present in earlier rocks.

Punctuated equilibrium explanation of fossil record

It suggests long periods of little change interrupted by rapid speciation events, matching sudden fossil appearances.

Gradualism vs. punctuated equilibrium

Gradualism suggests slow, continuous change, whereas punctuated equilibrium suggests bursts of change separated by stasis.

Advantage of high genetic variation for future evolution

It provides a wider range of traits that may be advantageous under future environmental changes.

Why stable environments favour stabilising selection

The same conditions favour the same intermediate phenotypes, so there is little divergence.

Why changing or fragmented environments promote speciation

Changing conditions create new selection pressures in different places, promoting divergence and speciation.