Changes in Population Gene Composition and the Formation of Species

Section 3: Changes in Population Gene Composition and the Formation of Species

I. Changes in Population Gene Composition

Natural selection acts directly on the individuals of a species, specifically on their phenotypes. However, in nature, no individual lives forever, and their phenotypes disappear with their death. The genes that determine these phenotypes, however, can be passed on through reproduction and spread within the population. Therefore, when studying biological evolution, it is not enough to only examine individuals and their phenotypes; it is also necessary to study changes in the gene composition of populations.

Population and Gene Pool

A population refers to the collection of all individuals of the same species living in a specific area. For example, all the macaques in a forest make up a population (Figure 6-7), and all the dandelions in a meadow also constitute a population. The individuals within a population are not just mechanically grouped together; a population is essentially a reproductive unit where male and female individuals can pass on their genes to their offspring through reproduction.

As a population reproduces, individuals come and go, but genes are passed down through generations. For example, many insects have a lifespan of less than a year (such as locusts), and all locusts die in the autumn. Some individuals successfully reproduce, leaving fertilized eggs buried in the soil before they die (Figure 6-8). In the following spring and summer, some of these fertilized eggs successfully develop into locusts. Compared to the locust population of the previous year, what changes might occur in the gene composition of the new locust population? You may try to make your own predictions based on what you’ve learned about heredity, variation, and natural selection.

The total set of genes contained in all individuals within a population is called the gene pool of that population. In a population’s gene pool, the proportion of a specific gene relative to the total number of alleles is known as the gene frequency. For example, in a certain insect population, the gene that determines green wing color is A, and the gene that determines brown wing color is a. If 100 individuals are randomly sampled from this population, and the genotypes AA, Aa, and aa are found in 30, 60, and 10 individuals respectively, then, for these alleles, each individual can be considered to carry 2 genes (Figure 6-9). Therefore, these 100 individuals possess a total of 200 genes. From this, we can deduce:

• The number of A genes is 2×30+60=1202 \times 30 + 60 = 1202×30+60=120;
• The number of a genes is 2×10+60=802 \times 10 + 60 = 802×10+60=80;
• The frequency of the A gene is 120÷200=60%120 \div 200 = 60\%120÷200=60%;
• The frequency of the a gene is 80÷200=40%80 \div 200 = 40\%80÷200=40%.

Will the gene frequency of this population change after several generations of reproduction?

Changes in Population Gene Frequency

You already know that gene mutations are common in nature. Gene mutations generate new alleles, which can alter the gene frequency in a population.

Darwin clearly pointed out that heritable variations provide the raw materials for biological evolution. Modern genetic research has shown that heritable variations originate from gene mutations, genetic recombination, and chromosomal variations. Among these, gene mutations and chromosomal variations are collectively known as mutations.

We know that the spontaneous mutation rate in organisms is very low, and many mutations are harmful. So why can mutations still serve as the raw materials for biological evolution? This is because populations are composed of many individuals, and each individual’s cells contain thousands of genes. As a result, a large number of mutations occur in each generation. For example, a set of chromosomes in fruit flies contains approximately 1.3×1041.3 \times 10^41.3×104 genes. If we assume that the mutation rate for each gene is 10−510^{-5}10−5, then for a medium-sized fruit fly population (about 10610^6106 individuals), the number of gene mutations that occur in each generation would be: 2×1.3×104×10−5×106=2.6×105 mutations2 \times 1.3 \times 10^4 \times 10^{-5} \times 10^6 = 2.6 \times 10^5 \text{ mutations}2×1.3×104×10−5×106=2.6×105 mutations

Additionally, whether a mutation is harmful or beneficial is not absolute, as it often depends on the organism's environment. For example, wingless or short-winged mutations occasionally appear in winged insects, which would typically struggle to survive under normal conditions. However, on wind-swept islands, these insects might survive better because they cannot be blown into the sea and drowned.

Alleles produced by gene mutations can form various genotypes through genetic recombination during sexual reproduction, leading to a variety of heritable variations within a population.

Mutations and recombinations are random and non-directional, so is the change in population gene frequency also non-directional?

Under natural selection, individuals with favorable variations are more likely to produce offspring, increasing the frequency of corresponding genes in the population. Conversely, individuals with unfavorable variations have fewer opportunities to leave offspring, decreasing the frequency of corresponding genes in the population. Therefore, under the influence of natural selection, the gene frequency of a population undergoes directional changes, leading organisms to evolve in a certain direction.

II. The Role of Isolation in Species Formation

Different populations of the same species may experience changes in gene composition in different directions due to differences in mutations and selection factors, leading to morphological and physiological differences between populations. However, this does not necessarily mean that these populations will diverge into different species. So how do we determine whether two populations are of the same species?

The Concept of Species

In the study of genetics and evolutionary biology, a species is defined as a group of organisms that can naturally interbreed and produce fertile offspring. Different species typically cannot interbreed; even if they do, they cannot produce fertile offspring. This phenomenon is known as reproductive isolation. For example, although horses and donkeys can mate, their offspring—the mule (Figure 6-10)—is sterile, indicating reproductive isolation between horses and donkeys, and therefore they are considered two different species.

Isolation and Its Role in Species Formation

Due to geographical barriers such as mountains, rivers, deserts, or other obstacles, each species is divided into one or more populations of varying sizes. These populations are the different groups within a species, such as the carp in two separate ponds representing two different populations. When a species is divided into different populations due to geographical barriers, preventing gene flow between populations, this phenomenon is known as geographical isolation.

Both geographical isolation and reproductive isolation refer to situations where individuals from different groups are unable to freely exchange genes under natural conditions. This is collectively referred to as isolation. So, is there any connection between geographical isolation and reproductive isolation? The following hypothetical scenario (Figure 6-11) can help you imagine and think about this.

The finches of the Galápagos Islands are a famous example of new species formation through geographical isolation. The ancestors of these finches belonged to the same species and migrated from the South American mainland, gradually spreading to different islands. Due to differences in mutations and genetic recombination, the gene pool of one population did not influence that of another. As a result, the gene frequencies of different populations changed differently. Since the food and habitat conditions on each island were different, natural selection affected the gene frequencies of different populations differently: in one population, certain genes were preserved, while in another population, different genes might be preserved. Over time, the gene pools of these populations developed significant differences, leading to the gradual emergence of reproductive isolation. Once reproductive isolation is established, the finches that originally belonged to the same species will become different species. Thus, isolation is a necessary condition for species formation.