How do we know populations are evolving? We will discuss what evolutionary changes are, what examples of evolutionary change have been observed in various lines of evidence, and what causes evolutionary change.
What are evolutionary changes?
Evolution is a gradual and cumulative change in the heritable traits of a population of organisms over time. This change takes place over the course of many generations. Patterns of evolutionary change can be observed through different lines of evidence, including fossils, homology, molecular biology, and direct observations.
These observations show that lifeforms change over time.
What are examples of observed evolutionary changes?
In this section, we will discuss some examples of evolutionary changes.
Darwin’s finches
Darwin’s finches show us evolutionary changes that lead to the emergence of many descendant species with adaptations to their specific environment.
In his travels to the Galapagos Islands, Darwin found over a dozen species of finches that were very similar in size and color, but had varied beak shapes. Darwin thought their unique beaks gave them a higher chance of survival. For instance, some finches had broad, blunt beaks which helped them crack nuts and seeds while other finches had long, pointed beaks which helped them snatch insects.
Darwin theorized that all of the different finch species on the Galapagos Islands came from one parent species that first colonized the islands millions of years ago. Darwin explained that as populations of the parent species spread from one uninhabited island to the next, they adapted to different ecological niches and rapidly evolved into many descendant species.
Figure 1: The parent species of finch rapidly formed several new species of finch with different beak shapes and feeding habits. Source: National Human Genome Research Institute's Talking Glossary, Public Domain, via Wikimedia Commons
Peppered moths
We can observe evolutionary changes in the coloration of peppered moths as an effect of their environment.
Peppered moths have evolved to be either very light or very dark. In rural areas, they are very light because darker-colored moths are easily seen so they are more vulnerable to predators. In industrial areas, they are very dark because their surroundings are dark so lighter-colored moths are more vulnerable to predators.
Few medium-colored moths are left because they had less chance of survival in either area.
Cetaceans
The fossil record documents broad evolutionary changes that caused new groups of organisms to emerge.
For example, fossils show that the pelvis and hind limb bones of extinct cetaceans ancestors became smaller over time, eventually disappearing completely and developing into flukes and flippers.
By studying the fossil record, we can observe the evolutionary changes that caused cetaceans (an order of marine mammals that includes whales, dolphins, and porpoises) to branch off from terrestrial mammals like hippopotamuses, pigs, and cows.
Are there direct observations of evolutionary change? Evolutionary changes can also be observed directly in species with fast reproductive cycles such as bacteria. For example, when bacteria are exposed to antibiotics, individuals with no resistance quickly die off. Individuals with resistance to the antibiotic are able to survive and reproduce. Then, resistant traits are passed on to more individuals in the population. Eventually, the population becomes more resistant.
What are the agents or causes of evolutionary change?
If at least one of five conditions is met, a population would depart from the Hardy-Weinberg equilibrium, which is the state in which a population is not evolving. These conditions are described in the following section.
Mutation
A mutation is a change in the sequence of genes in DNA.
It is the ultimate source of new alleles (variants of a gene), which determine the phenotype (observable traits) of an organism. While it is the ultimate source of genotypic and phenotypic variation, the mutation rate tends to be low for most organisms. Mutations can have one of three possible outcomes:
Depending on its impact on the evolutionary fitness of the population, the mutation may be favored or selected against. As mutations accumulate, the population evolves.
Non-random mating
Non-random mating could change the population by altering the frequencies of homozygous and heterozygous genotypes.
Non-random mating occurs for several reasons.
One reason could be that the organism prefers mates with particular traits.
For example, peahens prefer to mate with peacocks with larger and more colorful tails.
Another reason for non-random mating could be that the organism prefers partners with phenotypic similarity to themselves.
For example, Heliconius butterflies prefer to mate with peers that have similar color patterns.
Figure 2: Mating between two Heliconius butterflies with similar color patterns. Source: Holger Krisp, CC BY 3.0, via Wikimedia Commons.
Another reason could be geographic separation. Populations can be spread out over a large area, making some mates more accessible than others.
Whether non-random mating counts as an agent of evolution is up for debate because the allele frequencies in the gene pool stay the same. Allele frequencies can change if those with certain traits are able to reproduce at a higher rate, but in that case, natural selection will also be an agent of evolutionary change.
Gene flow
Gene flow is the movement of genes from one population to another.
This can occur when organisms migrate and reproduce with a different population or when pollen or seeds are dispersed to a population that is geographically separated. When organisms or gametes enter a population, they may bring with them new alleles or existing alleles, but in varying amounts compared to those already present in the population.
Figure 3: How gene flow can occur through the movement of individuals from one population to another. Source: Tsaneda, CC BY 3.0, via Wikimedia Commons.
For example, the mainland populations of the Lake Erie water snake (Nerodia sipedon) tend to be banded, while its island counterparts tend to be un-banded or have intermediate banding. The banding coloration allows snakes to hide in marshes, but it puts snakes that live on islands at a disadvantage.
We might expect that, as a result of natural selection, the entire population of the island Lake Erie snake would be un-banded. But this is not the case because, every year, several snakes swim from the mainland to the islands, bringing with them alleles for banded coloration. This has prevented the complete removal of the alleles for banded coloration from these island populations. Gene flow can be a strong agent of evolution. It has the potential to change the gene structure of the population and introduce new phenotypic and genotypic variations to populations in different geographic areas.
Genetic drift
Genetic drift refers to chance events that cause changes in allele frequencies.
Unlike natural selection, where individuals with traits that help them survive in their environment are able to have more offspring because of those traits, genetic drift causes allele frequencies to change at random.
We will discuss two sample scenarios that could result in genetic drift.
In the first scenario, let’s say a storm causes a few members of a population to be blown away and become isolated on a different island, where they establish a new population. This happens indiscriminately; meaning, individuals–and their alleles–that are transported are selected at random. This new island population will have a gene pool that is different from the original population. This is called the founder effect.
Figure 4: How the founder effect alters the allele frequencies of a population. Source: Professor Marginalia, CC BY-SA 3.0, via Wikimedia Commons.
In the second scenario, let’s say a fire drastically reduces the size of a population. This can lead to a loss of genetic variation, and alleles can be disproportionately represented in the surviving population. This is called the bottleneck effect.
Figure 5: How a bottleneck event alters the allele frequencies of a population. Source: Tsaneda, CC BY 3.0, via Wikimedia Commons.
In both scenarios, alleles can be overrepresented or underrepresented in the next generation.
Genetic drift can occur in any population size, but its impact is more significant in small populations.
Founder effect: When a chance event causes a few members of a population to become geographically isolated and they establish a new population.
Bottleneck effect: When a chance event drastically reduces the size of a population.
Natural selection
Natural selection is the major mechanism of evolution. It occurs when phenotypic variations (or variations in traits) lead to different survival and reproduction rates among offspring. Individuals with traits that are more adapted to the environment have better chances of survival and reproduction, and thus are able to pass on these traits to their offspring. As these traits accumulate, the population evolves.
Evolutionary Changes - Key takeaways
- Patterns of evolutionary change can be observed through different lines of evidence, including fossils, homology, molecular biology, and direct observations. These observations show that life forms change over time.
- If at least one of five conditions are met, a population would depart from the Hardy-Weinberg equilibrium or the state in which a population is not evolving. These conditions are:
- Mutation: a change in the sequence of genes in DNA
- Non-random mating: preference for mates with particular traits or phenotypic similarity to themselves
- Gene flow: movement of genes from one population to another
- Genetic drift: chance events that cause random changes in allele frequencies
- Natural selection: individuals with traits that are more adapted to the environment have better chances of survival and reproduction, and thus are able to pass on these traits to their offspring
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