Table of Contents
Overview
Evolution does not happen “by itself.” It is driven by concrete biological processes that change the genetic makeup of populations over generations. The synthetic theory of evolution (often called the “modern synthesis”) brings these processes together into a unified explanation of how evolution works in real populations.
In this chapter, the focus is on the main evolutionary factors and how they interact:
- How new genetic variation arises
- How that variation is sorted by the environment
- How chance and geography shape gene pools
- How new species can form from existing ones
The goal here is not to give all details (these appear in the subchapters) but to show clearly what each factor does and how their combined effects produce evolutionary change.
Populations, Gene Pools, and Evolution
Evolutionary factors act on populations, not on isolated individuals.
- A population is a group of individuals of the same species living in the same area and potentially interbreeding.
- The gene pool is the sum of all alleles (gene variants) present in that population.
- Evolution, in the sense of population genetics, can be defined as a change in allele frequencies in a population over generations.
If $p$ is the frequency of one allele and $q$ the frequency of another at the same gene locus (with $p + q = 1$ for two alleles), evolution means that $p$ and $q$ change over time.
The evolutionary factors described in this chapter are the mechanisms that actually cause those changes.
The Baseline: No Evolution Under Ideal Conditions
To understand how evolutionary factors work, it is helpful to know how a population behaves without them. Under certain idealized conditions, allele frequencies do not change from generation to generation. This is described by the Hardy–Weinberg principle.
These ideal conditions are:
- Very large population size (no genetic drift)
- Random mating (no sexual selection or mating preferences)
- No mutation
- No migration (no gene flow into or out of the population)
- No natural selection (all genotypes have equal chances of survival and reproduction)
Under these conditions, the genotype frequencies for two alleles $A$ and $a$ with frequencies $p$ and $q$ are given by:
$$
p^2 + 2pq + q^2 = 1
$$
where:
- $p^2$ is the frequency of genotype $AA$
- $2pq$ is the frequency of genotype $Aa$
- $q^2$ is the frequency of genotype $aa$
This “equilibrium state” is a reference model. Real populations almost never meet all these conditions, so their gene pools change. Each violation of a Hardy–Weinberg condition corresponds to one or more evolutionary factors.
Main Evolutionary Factors
1. Mutation and Recombination: Sources of New Variation
Evolution cannot proceed without genetic variation among individuals. Two key processes generate and rearrange this variation.
Mutation
- Mutation introduces new alleles into the gene pool.
- Most mutations are neutral or harmful; a few may be beneficial in a given environment.
- On its own, mutation usually changes allele frequencies only slowly, but it is the ultimate source of all new hereditary variation.
Mutation is, in effect, what keeps “fueling” evolution with new raw material for other factors to work on.
Recombination
- Recombination does not usually create new alleles, but it creates new combinations of existing alleles in offspring.
- It results from processes like independent assortment of chromosomes and crossing-over during meiosis.
- Recombination greatly increases the number of possible genotypes, even with a limited number of alleles.
Together, mutation and recombination produce the genetic diversity on which selection, drift, and other factors can act. Without them, a population would eventually lose variability and evolutionary change would stall.
2. Adaptive Selection: Sorting Variation by Fitness
Once variation exists, the environment “filters” it. This is the role of adaptive (natural) selection.
- Individuals differ in traits that affect their fitness (their contribution to the next generation).
- If certain alleles confer higher survival or reproductive success under given conditions, they become more common.
- Selection is systematic: it consistently favors some alleles over others in a given environment.
Selection has characteristic effects:
- It increases the frequency of beneficial alleles and combinations.
- It can lead to adaptations: traits that are well-suited to the current environment.
- It tends to reduce genetic variation at the loci under strong selection (by eliminating less-fit variants), while mutation and recombination constantly reintroduce or reshuffle variation.
Selection can act in different patterns (e.g. stabilizing, directional, disruptive), which strongly influence how populations evolve. These patterns are addressed more specifically in the subchapter on adaptive selection.
3. Genetic Drift: Chance Changes in Small Populations
Where selection is about non-random differences in survival and reproduction, genetic drift is about random fluctuations in allele frequencies.
Key features:
- Drift occurs because reproduction is a probabilistic process: not all individuals leave offspring, and not all alleles are passed on according to their exact expected frequencies.
- In small populations, chance events can have large effects, leading to:
- Loss of alleles (especially rare ones)
- Fixation of one allele (its frequency reaching 1, others disappearing)
Important consequences:
- Drift can reduce genetic diversity, making populations more similar internally but more different from one another.
- Drift can cause harmful alleles to increase in frequency or even become fixed purely by chance.
- Two special cases are often emphasized:
- Bottleneck effect: a drastic temporary reduction in population size that randomly removes alleles.
- Founder effect: when a new population is founded by a small number of individuals carrying only a subset of the original gene pool.
Drift is always present but is most powerful in small populations; in large populations, its effects are usually weaker compared to selection.
4. Isolation and Speciation: Splitting Gene Pools
Evolution is not only about change within a single population; it is also about how new species arise. A crucial requirement for speciation (the formation of new species) is isolation between populations.
Isolation
Isolation prevents or limits gene flow between populations. It can be:
- Geographical (allopatric): physical barriers such as mountains, rivers, oceans, or climate zones separate populations.
- Ecological or behavioral (sympatric/parapatric): even in the same region, different habitats, activity times, or mating signals can separate groups.
- Various forms of reproductive isolation can act before fertilization (prezygotic) or after fertilization (postzygotic).
Speciation
Once populations are isolated:
- Mutation, recombination, selection, and drift act independently in each group.
- Over time, differences accumulate in their gene pools.
- If reproductive barriers become strong enough that individuals from different populations can no longer produce fertile offspring (or do not attempt to mate), they are considered different species.
Thus, isolation does not create new variation itself but divides an original gene pool into separate evolutionary “branches,” each subject to its own combination of evolutionary factors.
Interaction of Evolutionary Factors
In real populations, evolutionary factors do not act in isolation. Their combined action determines both the tempo and direction of evolution.
Some common interactions:
- Mutation + Selection
Mutation introduces new alleles; selection quickly increases the frequency of beneficial ones and removes harmful ones. The balance between mutation introducing disadvantageous alleles and selection removing them can create a mutation–selection balance. - Drift + Selection
In small populations, drift can overpower weak selection. Slightly beneficial alleles may be lost by chance; slightly harmful ones can drift to high frequency or fixation. In large populations, selection typically dominates over drift for alleles with noticeable fitness effects. - Gene Flow vs. Local Adaptation
(Gene flow is often considered an additional evolutionary factor.) When individuals migrate between populations and interbreed, they introduce alleles from other gene pools. - Gene flow can counteract divergence and thus slow or prevent speciation.
- At the same time, it can introduce beneficial alleles that selection can then favor in the new environment.
- Isolation + Drift + Selection in Speciation
When a small group becomes isolated (e.g. founders on an island), drift can rapidly change allele frequencies, and selection refines adaptations to the local environment. Together, these processes can accelerate the formation of new species.
Microevolution and Macroevolution
The evolutionary factors described here operate at the level of populations and species:
- Microevolution refers to changes in allele frequencies and traits within a species over relatively short time scales (generations to thousands of years).
- Over longer time scales, the same processes (mutation, recombination, selection, drift, isolation) lead to the splitting of lineages and the formation of higher-level patterns, such as radiations, extinctions, and major shifts in body plans. These large-scale patterns are referred to as macroevolution.
The synthetic theory of evolution emphasizes that macroevolutionary patterns can be understood as the long-term, cumulative outcome of microevolutionary processes acting over immense spans of time.
Summary
- Evolution can be defined as a change in allele frequencies in populations over generations.
- Mutation and recombination generate and rearrange genetic variation.
- Adaptive selection non-randomly increases the frequency of alleles that improve survival and reproduction.
- Genetic drift randomly changes allele frequencies, especially in small populations, and can lead to loss of diversity.
- Isolation splits gene pools; combined with other factors, it can result in speciation.
- The synthetic theory of evolution integrates these processes into a unified framework, showing how their interplay produces both the small-scale changes observed within species and the large-scale patterns of biodiversity through Earth’s history.